Transcriptomic Analysis to Understand the Nitrogen Stress Response Mechanism in BNI-Enabled Wheat

A comparative transcriptomic analysis was conducted for the nitrogen-efficient (BNI-Munal) and derivative parent Munal wheat genotypes to unravel the gene expression patterns across four nitrogen levels (0%, 50%, 75%, and 100%). Analyzing the genes of BNI-enabled wheat helps us understand how they are expressed differently, which heavily influences BNI activity. Grain yield and 1000-grain weight were higher in BNI Munal than in Munal. All the other traits were similar in performance. Varying nitrogen dosages led to significant differences in gene expression patterns between the two genotypes. Genes related to binding and catalytic activity were prevalent among molecular functions, while genes corresponding to cellular anatomical entities dominated the cellular component category. Differential expression was observed in 371 genes at 0%N, 261 genes at 50%N, 303 genes at 75%N, and 736 genes at 100%N. Five unigenes (three upregulated and two downregulated) were consistently expressed across all nitrogen levels. Further analysis of upregulated unigenes identified links to the NrpA gene (involved in nitrogen regulation), tetratricopeptide repeat-containing protein (PPR), and cytokinin dehydrogenase 2. Analysis of downregulated genes pointed to associations with the Triticum aestivum 3BS-specific BAC library, which encodes the NPF (Nitrate and Peptide Transporter Family) and the TaVRN gene family (closely related to the TaNUE1 gene). The five unigenes and one unigene highlighted in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were validated in Munal and BNI Munal. The results obtained will enhance our understanding about gene expression patterns across different nitrogen levels in BNI wheat and help us breed wheat varieties with the BNI trait for improved NUE.

Dominant role of water-extractable soil chemicals in modulating N₂O emissions relative to soil bacteriome

Soil nitrous oxide (N₂O) emissions are influenced both by soil chemical properties and microbiome composition; however, their relative contributions remain unclear. We used soil-water extracts (SW), and cell extracts (bacteriomes) from two contrasting soils, black soil (BS) and fluvo-aquic soil (FS), to evaluate how water-extractable soil chemicals and bacteriomes directly impact N₂O emissions, as well as how SW influences bacteriome composition. Results show that SW chemistry, particularly pH, plays a dominant role in regulating denitrification dynamics, while bacteriome effects are less significant. In native BS water extract (BSW, pH 6.5), cell extract from BS (BB bacteriomes) exhibited high N₂O emissions (N₂O index = 0.669), but their denitrification efficiency improved in FS water extract (FSW, pH 8.2), reducing the N₂O index to 0.0491. Conversely, cell extract from FS (FB bacteriomes) in native FSW (pH 8.2) demonstrated efficient denitrification (N₂O index = 0.006), but exposure to BSW increased N₂O emissions (∼ 100 µmol vial⁻¹, N₂O index = 0.295). Bacterial community analysis revealed that high pH fostered diverse denitrifiers, including napA-harboring Pseudoxanthomonas and Lysobacter, and nosZ Clade II Chitinophaga, which are linked to N₂O reduction. In contrast, low pH favored narG-harboring incomplete denitrifiers like Klebsiella and Enterobacter. In the BB bacteriome, BSW promoted Rhodanobacter, which hindered complete denitrification, while FSW enriched complete denitrifiers like Cupriavidus and Ensifer. Conversely, BSW negatively impacted the enrichment of complete denitrifier Acidovorax in the FB bacteriome. This study contributes to the growing evidence of the critical roles of soil physicochemical properties and bacteriome composition in determining N₂O fluxes from agricultural soils.

Multispecies swards enhance animal performance in a co-grazing cattle and sheep production system

Grazing multispecies swards can have multiple benefits for the productivity and environmental sustainability of ruminant production systems. However, few studies have determined the effect of different sward-type systems on co-grazed cattle and sheep performance. The objective of this study was to determine the effect of different sward type systems: (1) a sown Lolium perenne (PRG) sward (receiving 170 kg of N/ha per year); (2) a pre-existing permanent pasture (PP) (receiving 135 kg N/ha per year); (3) a sown 6 species sward (6SP) with Lolium perenne, Phleum pratense, Trifolium pratense, Trifolium repens, Cichorium intybus and Plantago lanceolata (receiving 70 kg N/ha per year); and 4) a sown 12 species sward (12SP) with Dactylis glomerata, Lotus corniculatus, Onobrychis viciifolia, Achillea millefolium, Petroselinum crispum, Sanguisorba minor in addition to the 6SP listed above (receiving 70 kg N/ha per year); on the animal performance of co-grazed cattle and sheep. In 2020 and 2021, each sward-type system (9 ha) was rotationally co-grazed from April to November by dairy cross heifers (n = 20 per treatment per year, turned out at 395 ± 15 days of age, mean ± SD) and ewes (n = 22 ewes per treatment per year) plus lambs. Heifer live weight was recorded monthly, and heifers were drafted for slaughter when their estimated fat class on the EUROP grid scale reached 3-. Lamb live weight was recorded fortnightly, and lambs were drafted for slaughter at 42 kg (lambing to weaning), 44 kg (weaning to September) and 46 kg (after 1 September) to obtain a target carcass weight of 21 kg. Average daily gain (ADG) from turnout to slaughter was higher for heifers grazing the 6SP sward (1.09 kg/day) compared to all other sward types (12SP (0.99 kg/day), PRG (0.92 kg/day), and PP swards (0.92 kg/day); P < 0.001)). Lamb ADG from birth to slaughter in the 6SP (393 g/day) and 12SP (363 g/day) were greater than the PP (305 g/day; P < 0.001) or PRG swards (292 g/day; P < 0.001). Carcass dressing percentage was higher for lambs grazing the 6SP (48.5%) and 12SP (48.6%) compared to lambs grazing the PRG (44.7%; P < 0.001) and PP swards (44.6%; P < 0.001). Lambs grazing the 6SP and 12SP had a reduced number of grazing days from turnout to slaughter (82 and 93 days respectively) compared to the lambs grazing the PPG and PP swards (133 and 127 days respectively; P < 0.01). Overall, co-grazing multispecies swards improved heifer and lamb production performance at lower nitrogen fertilisation rates, potentially mitigating the environmental impact of beef and sheep production systems.

Machine learning reveals dynamic controls of soil nitrous oxide emissions from diverse long-term cropping systems

Soil nitrous oxide (N2O) emissions exhibit high variability in intensively managed cropping systems, which challenges our ability to understand their complex interactions with controlling factors. We leveraged 17 years (2003-2019) of measurements at the Kellogg Biological Station Long-Term Ecological Research (LTER)/Long-Term Agroecosystem Research (LTAR) site to better understand the controls of N2O emissions in four corn-soybean-winter wheat rotations employing conventional, no-till, reduced input, and biologically based/organic inputs. We used a random forest machine learning model to predict daily N2O fluxes, trained separately for each system with 70% of observations, using variables such as crop species, daily air temperature, cumulative 2-day precipitation, water-filled pore space, and soil nitrate and ammonium concentrations. The model explained 29%-42% of daily N2O flux variability in the test data, with greater predictability for the corn phase in each system. The long-term rotations showed different controlling factors and threshold conditions influencing N2O emissions. In the conventional system, the model identified ammonium (>15 kg N ha-1) and daily air temperature (>23°C) as the most influential variables; in the no-till system, climate variables such as precipitation and air temperature were important variables. In low-input and organic systems, where red clover (Trifolium repens L.; before corn) and cereal rye (Secale cereale L.; before soybean) cover crops were integrated, nitrate was the predominant predictor of N2O emissions, followed by precipitation and air temperature. In low-input and biologically based systems, red clover residues increased soil nitrogen availability to influence N2O emissions. Long-term data facilitated machine learning for predicting N2O emissions in response to differential controls and threshold responses to management, environmental, and biogeochemical drivers.

Crop-Specific Emission Projection Suggests Peaking of Agricultural N2O by the Middle Century

Agriculture is the largest anthropogenic source of N2O emissions and plays a crucial role in global greenhouse gas mitigation. In an increasingly populated world with growing food demands, a precise and high-resolution spatial prediction of agricultural N2O emissions becomes essential in reducing global emissions. In this study, an integrated assessment model coupled with the land cover downscaling module is employed to predict crop-specific N2O emissions at a 0.05° resolution under various SSP-RCP scenarios from 2025 to 2100. Our findings show that global agricultural N2O emissions will peak around 2065, ranging from 5.2 to 6.6 Tg N a-1. Corn cultivation is the primary N2O contributor, while rice production will produce fewer emissions and peak before 2025. The emission hotspots are concentrated in western Europe, India, eastern China, and the west coast and east-central region of the USA. By 2100, the crop-specific N2O emissions in China are predicted to decrease below the levels observed in 2015, while the emissions in the USA and India may double in some socio-economic pathways. Our projection of N2O emission patterns is supportive of implementing targeted policies and strategies to achieve global emission reduction targets.

Effect of tillage state of paddy soils with heavy metal pollution on the nosZ gene of N2O reductase

Paddy soils are an important source of atmospheric nitrous oxide (N2O). However, numerous studies have focused on N2O production during the soil tillage period, neglecting the N2O production during the dry fallow period. In this study, we conducted an incubation experiment using the acetylene inhibition technique to investigate N2O emission and reduction rates of paddy soil profiles (0-1 m) from Guangdong Province and Jinlin Province in China, with different heavy-metal pollution levels. The abundance and community structures of denitrifying bacteria were determined via quantitative-PCR and Illumina MiSeq sequencing of nosZ, nirK, and nirS genes. Our results showed that the potential N2O emission rate, N2O production rate, and denitrification rate have decreased with increasing soil vertical depth and heavy-metal pollution. More importantly, we found that the functional gene type of N2O reductase switched with the tillage state of paddy soils, which clade Ⅱ nosZ genes were the dominant gene during the tillage period, while clade Ⅰ nosZ genes were the dominant gene during the dry fallow period. The heavy-metal pollution has less effect on the niche differentiation of the nosZ gene. The N2O emission rate was significantly regulated by the genus Bradyhizobium, which contains both N2O reductase and nitrite reductase genes. Our findings suggests that the nosZ gene of N2O reductase can significantly impact the N2O emission from paddy soils.

Soil properties and fungal community jointly explain N2O emissions following N and P enrichment in an alpine meadow

Nutrient enrichment, such as nitrogen (N) and phosphorus (P), typically affects nitrous oxide (N2O) emissions in terrestrial ecosystems, predominantly via microbial nitrification and denitrification processes in the soil. However, the specific impact of soil property and microbial community alterations under N and P enrichment on grassland N2O emissions remains unclear. To address this, a field experiment was conducted in an alpine meadow of the northeastern Qinghai-Tibetan Plateau. This study aimed to unravel the mechanisms underlying N and P enrichment effects on N2O emissions by monitoring N2O fluxes, along with analyzing associated microbial communities and soil physicochemical properties. We observed that N enrichment individually or in combination with P enrichment, escalated N2O emissions. P enrichment dampened the stimulatory effect of N enrichment on N2O emissions, indicative of an antagonistic effect. Structural equation modeling (SEM) revealed that N enrichment enhanced N2O emissions through alterations in fungal community composition and key soil physicochemical properties such as pH, ammonium nitrogen (NH4+-N), available phosphorus (AP), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN)). Notably, our findings demonstrated that N2O emissions were significantly more influenced by fungal activities, particularly genera like Fusarium, rather than bacterial processes in response to N enrichment. Overall, the study highlights that N enrichment intensifies the role of fungal attributes and soil properties in driving N2O emissions. In contrast, P enrichment exhibited a non-significant effect on N2O emissions, which highlights the critical role of the fungal community in N2O emissions responses to nutrient enrichments in alpine grassland ecosystems.

Current investigations on global N2O emissions and reductions: Prospect and outlook

Global nitrous oxide (N2O) emissions merit scrutiny, because N2O is the third most important greenhouse gas for global warming and the predominant ozone-depleting substance in this century. Here we recapitulate global natural and anthropogenic N2O sources, comprehensively depict global sectoral human-induced N2O emissions by country, thoroughly survey all existing approaches for mitigating human-induced N2O emissions, preview the economic costs and social benefits from abating N2O emissions, and summarize roadblocks for achieving its emission reductions. From 1970 to 2018, the annual global anthropogenic N2O emissions increased by 64%-about 3.6 teragrams (Tg); agricultural sources primarily accounted for 78% of this increment. We find the social benefits from reducing N2O emissions override the economic costs for abatements, only except precision farming for agricultural sources and replacement by Xe for anesthetic, thus justifying the motivation for crafting policies to limit its emissions. Net zero N2O emissions cannot be achieved via applying current technologies and breeding N2O-reducing microbes is a potential method to accrue N2O sinks.

Carbon and nitrogen fractions control soil N2O emissions and related functional genes under land-use change in the tropics

Converting natural forests to managed ecosystems generally increases soil nitrous oxide (N2O) emission. However, the pattern and underlying mechanisms of N2O emissions after converting tropical forests to managed plantations remain elusive. Hence, a laboratory incubation study was investigated to determine soil N2O emissions of four land uses including forest, eucalyptus, rubber, and paddy field plantations in a tropical region of China. The effect of soil carbon (C) and nitrogen (N) fractions on soil N2O emissions and related functional genes was also estimated. We found that the conversion of natural forests to managed forests significantly decreased soil N2O emissions, but the conversion to paddy field had no effect. Soil N2O emissions were controlled by both nitrifying and denitrifying genes in tropical natural forest, but only by nitrifying genes in managed forests and by denitrifying genes in paddy field. Soil total N, extractable nitrate, particulate organic C (POC), and hydrolyzable ammonium N showed positive relationship with soil N2O emission. The easily oxidizable organic C (EOC), POC, and light fraction organic C (LFOC) had positive linear correlation with the abundance of AOA-amoA, AOB-amoA, nirK, and nirS genes. The ratios of dissolved organic C, EOC, POC, and LFOC to total N rather than soil C/N ratio control soil N2O emissions with a quadratic function relationship, and the local maximum values were 0.16, 0.22, 1.5, and 0.55, respectively. Our results provided a new evidence of the role of soil C and N fractions and their ratios in controlling soil N2O emissions and nitrifying and denitrifying genes in tropical soils.

N2O emission factors for cattle urine: effect of patch characteristics and environmental drivers

Urine patches from grazing cattle are hotspots of nitrous oxide (N2O) emissions. The default IPCC emission factor for urine patches (EFurine) is 0.77% for wet climates and 0.32% for dry climates. However, literature reports a considerable range of cattle urine EF values and urine characteristics used in experimental studies, revealing contrary results on the effects of urine patch characteristics and seasonal pattern. Therefore, we examined N2O emissions and corresponding EFurine values in relation to urine patch characteristics (urine N concentration, urine volume, patch area, urine composition) and environmental drivers (precipitation, water filled pore space, soil temperature). Ten artificial urine application experiments were performed from July 2020 to June 2022 on a pasture located in Eastern Switzerland. Urine N concentration, patch area, volume and urine N composition showed no significant effects on the EFurine value (p > 0.05). EFurine varied, however, strongly over time (0.17-2.05%). A large part of the variation could be predicted either by cumulative precipitation 20 days after urine application using a second order polynomial model (Adj. R2 = 0.60) or average WFPS 30 days after urine application using a linear model (Adj. R2 = 0.45). The derived precipitation model was used to simulate EFurine weekly over the last 20 years showing no significant differences between the seasons of a year. The resulting overall average EFurine was 0.67%. More field studies are needed across sites/regions differing in climate and soil properties to implement a country-specific EF3 for Switzerland and to improve the quantification of N2O emissions at the national scales.

Soil glomalin-related protein affects aggregate N2O fluxes by modulating denitrifier communities in a fertilized soil

Agricultural ecosystems contribute significantly to atmospheric emissions of soil nitrous oxide (N₂O), which exacerbate environmental pollution and contribute to global warming. Glomalin-related soil protein (GRSP) stabilizes soil aggregates and enhances soil carbon and nitrogen storage in agricultural ecosystems. However, the underlying mechanisms and relative importance of GRSP on N₂O fluxes within soil aggregate fraction remain largely unclear. We examined the GRSP content, denitrifying bacterial community composition, and potential N₂O fluxes across three aggregate-size fractions (2000-250 μm, 250-53 μm, and <53 μm) under a long-term fertilization agricultural ecosystem, subjected to mineral fertilizer or manure and their combination. Our findings indicated that various fertilization treatments have no discernible impact on the size distribution of soil aggregates, paving the way to further research into the impact of soil aggregates on GRSP content, the denitrifying bacterial community composition, and potential N₂O fluxes. GRSP content increased with the increase in soil aggregate size. Potential N₂O fluxes (including gross N₂O production and N₂O reduction and net N₂O production) among aggregates were highest in microaggregates (250-53 μm), followed by macroaggregates (2000-250 μm) and lowest in silt + clay (<53 μm) fractions. Potential N₂O fluxes had a positive response to soil aggregate GRSP fractions. The non-metric multidimensional scaling analysis revealed that soil aggregate size could drive the denitrifying functional microbial community composition, and deterministic processes play more critical roles than stochasticity processes in driving denitrifying functional composition under soil aggregate fractions. Procrustes analysis revealed a significant correlation between denitrifying microbial community, soil aggregate GRSP fractions, and potential N₂O fluxes. Our study suggests that soil aggregate GRSP fractions influence potential nitrous oxide fluxes by affecting denitrifying microbial functional composition within soil aggregate.

Denitrification contributes to N2O emission in paddy soils

Denitrification is vital to nitrogen removal and N₂O release in ecosystems; in this regard, paddy soils exhibit strong denitrifying ability. However, the underlying mechanism of N₂O emission from denitrification in paddy soils is yet to be elucidated. In this study, the potential N₂O emission rate, enzymatic activity for N₂O production and reduction, gene abundance, and community composition during denitrification were investigated using the ¹⁵N isotope tracer technique combined with slurry incubation, enzymatic activity detection, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. Results of incubation experiments showed that the average potential N₂O emission rates were 0.51 ± 0.20 μmol⋅N⋅kg^-1⋅h^-1, which constituted 2.16 ± 0.85% of the denitrification end-products. The enzymatic activity for N₂O production was 2.77-8.94 times than that for N₂O reduction, indicating an imbalance between N₂O production and reduction. The gene abundance ratio of nir to nosZ from qPCR results further supported the imbalance. Results of metagenomic analysis showed that, although Proteobacteria was the common phylum for denitrification genes, other dominant community compositions varied for different denitrification genes. Gammaproteobacteria and other phyla containing the norB gene without nosZ genes, including Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus, may contribute to N₂O emission from paddy soils. Our results suggest that denitrification is highly modular, with different microbial communities collaborating to complete the denitrification process, thus resulting in an emission estimation of 13.67 ± 5.44 g N₂O⋅m^-2⋅yr^-1 in surface paddy soils.

Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N₂ O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. -30%) influenced soil N₂ O and carbon dioxide (CO₂ ) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N₂ O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂ O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N₂ O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N₂ O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.

Chromosome-level genome and high nitrogen stress response of the widespread and ecologically important wetland plant Typha angustifolia

Typha angustifolia L., known as narrowleaf cattail, is widely distributed in Eurasia but has been introduced to North America. Typha angustifolia is a semi-aquatic, wetland obligate plant that is widely distributed in Eurasia and North America. It is ecologically important for nutrient cycling in wetlands where it occurs and is used in phytoremediation and traditional medicine. In order to construct a high-quality genome for Typha angustifolia and investigate genes in response to high nitrogen stress, we carried out complete genome sequencing and high-nitrogen-stress experiments. We generated a chromosomal-level genome of T. angustifolia, which had 15 pseudochromosomes, a size of 207 Mb, and a contig N50 length of 13.57 Mb. Genome duplication analyses detected no recent whole-genome duplication (WGD) event for T. angustifolia. An analysis of gene family expansion and contraction showed that T. angustifolia gained 1,310 genes and lost 1,426 genes. High-nitrogen-stress experiments showed that a high nitrogen level had a significant inhibitory effect on root growth and differential gene expression analyses using 24 samples found 128 differentially expressed genes (DEGs) between the nitrogen-treated and control groups. DEGs in the roots and leaves were enriched in alanines, aspartate, and glutamate metabolism, nitrogen metabolism, photosynthesis, phenylpropanoid biosynthesis, plant-pathogen interaction, and mitogen-activated protein kinase pathways, among others. This study provides genomic data for a medicinal and ecologically important herb and lays a theoretical foundation for plant-assisted water pollution remediation.

Carbon-sink potential of continuous alfalfa agriculture lowered by short-term nitrous oxide emission events

Alfalfa is the most widely grown forage crop worldwide and is thought to be a significant carbon sink due to high productivity, extensive root systems, and nitrogen-fixation. However, these conditions may increase nitrous oxide (N₂O) emissions thus lowering the climate change mitigation potential. We used a suite of long-term automated instrumentation and satellite imagery to quantify patterns and drivers of greenhouse gas fluxes in a continuous alfalfa agroecosystem in California. We show that this continuous alfalfa system was a large N₂O source (624 ± 28 mg N₂O m² y^-1), offsetting the ecosystem carbon (carbon dioxide (CO₂) and methane (CH₄)) sink by up to 14% annually. Short-term N₂O emissions events (i.e., hot moments) accounted for ≤1% of measurements but up to 57% of annual emissions. Seasonal and daily trends in rainfall and irrigation were the primary drivers of hot moments of N₂O emissions. Significant coherence between satellite-derived photosynthetic activity and N₂O fluxes suggested plant activity was an important driver of background emissions. Combined data show annual N₂O emissions can significantly lower the carbon-sink potential of continuous alfalfa agriculture.

The effects of biochar derived from feedstock with different Si and Al concentration on soil N2O and CO2 emissions

Desilicification and allitization is important characteristic of acidic soil. While decrease in soil silicon (Si) may generate Si limitation, the increase of aluminum (Al) will aggravate soil acidification. Biochar has been used in acid soil improvement, which could mitigate nitrous oxide (N₂O) emissions and alter soil Si and Al concentration. However, the effect of biochar with different Si and Al concentration on greenhouse gas emissions remains unclear. We evaluated the effects of biochar derived from feedstock with different Si (moso bamboo leaves, BL; rice straw, RS) and Al (Camellia oleifera fruit shell, CFS; C. oleifera leaves, CL) concentration on greenhouse gas emissions and soil acidification. Microbial functional gene abundance associated with N₂O emissions were measured to further explore the response of microbiological community. The results showed that BL, RS, CFS and CL significantly increased soil pH (by 19.2%, 16.7%, 18.7% and 24.9%, respectively), decreased soil exchangeable acid and exchangeable Al content, and reduced N₂O emission rate of soil with nitrogen (N) (by 14.2%, 27.3%, 25.6% and 38.7%, respectively), which correlated with increase in soil nosZ abundance. BL, RS, CFS and CL increased soil nirK (by 325.6%, 66.7%, 155.8%, and 253.2%, respectively) and nosZ (by 198.6%, 174.1%, 72.2%, and 152.0%, respectively) abundance with N. Structural equation model showed that Si input via biochar application directly reduced N₂O emissions, and soil acid-extractable Si is inversely proportional to N₂O emission rate. In addition, Si input reduced carbon dioxide (CO₂) emissions via indirect effects. Al input via biochar addition indirectly affected N₂O and CO₂ emissions through mainly indirect effects on other soil factors. In intensive management and production activities, Si-rich biochar can be considered instead of sole addition as fertilizer, which will be beneficial to the sustainable development of agricultural and forestry production in acid soil areas, and mitigation of global change.

The Contribution of Nitrate Dissimilation to Nitrate Consumption in narG- and napA-Containing Nitrate Reducers with Various Oxygen and Nitrate Supplies

Nitrate reducers containing narG or napA play an important role in the nitrogen cycle, but little is known about their functional differentiations in relation to environmental changes. In this study, three types of nitrate reducers in the genus Pseudomonas, including strains containing narG (G type), napA (A type) and both narG and napA (GA type), were selected to explore their functional performances under varied nitrate and oxygen concentrations. Their growth characteristics, nitrate consumption, and dissimilatory nitrate-reducing activity were investigated. Growth and nitrate consumption of all three types of strains were generally promoted with increasing oxygen and nitrate concentrations. However, their dissimilatory nitrate-reducing activities were restricted by oxygen supply. When supplied with 0.25 mM KNO₃, A-type strains showed a higher growth rate but lower activity of dissimilatory nitrate reduction (DNR) than G-type strains, regardless of oxygen concentration. However, when nitrate concentration increased to 0.75 mM or 5 mM, G-type strains displayed stronger capability of nitrate consumption and DNR than A-type strains under anaerobic conditions, whereas under oxygenated conditions, A-type strains exhibited higher growth and nitrate consumption but weaker DNR than G-type strains. The GA-type strains appeared similar to G type under anaerobic conditions but performed more similarly to A type in aerobic environments. In summary, the nitrate consumption of narG-containing nitrate reducers is mainly caused by DNR in both anaerobic and aerobic environments, while the large proportion of nitrate consumption in A-type nitrate reducers under the aerobic condition is attributed to the assimilation by cell growth. IMPORTANCE Nitrate reducers containing narG or napA are ubiquitous, but little is known about their functional performance in various environments. Our study provides an important clue that the nitrate consumption of narG-containing strains is mainly caused by dissimilatory reduction in the environments, while that of napA-containing nitrate reducers under anaerobic conditions is ascribed to nitrate dissimilation but under the aerobic condition is attributed to the assimilation by cell growth. This finding broadens the understanding of aerobic nitrate reduction in the nitrogen cycle and highlights the important role of narG-containing bacteria in nitrate reduction under aerobic conditions.

Microbial-Mediated Emissions of Greenhouse Gas from Farmland Soils: A Review

The greenhouse effect is one of the concerning environmental problems. Farmland soil is an important source of greenhouse gases (GHG), which is characterized by the wide range of ways to produce GHG, multiple influencing factors and complex regulatory measures. Therefore, reducing GHG emissions from farmland soil is a hot topic for relevant researchers. This review systematically expounds on the main pathways of soil CO2, CH4 and N2O; analyzes the effects of soil temperature, moisture, organic matter and pH on various GHG emissions from soil; and focuses on the microbial mechanisms of soil GHG emissions under soil remediation modes, such as biochar addition, organic fertilizer addition, straw return and microalgal biofertilizer application. Finally, the problems and environmental benefits of various soil remediation modes are discussed. This paper points out the important role of microalgae biofertilizer in the GHG emissions reduction in farmland soil, which provides theoretical support for realizing the goal of “carbon peaking and carbon neutrality” in agriculture.

Enhancing N uptake and reducing N pollution via green, sustainable N fixation-release model

The overuse of N fertilizers has caused serious environmental problems (e.g., soil acidification, excessive N₂O in the air, and groundwater contamination) and poses a serious threat to human health. Improving N fertilizer utilization efficiency and plant uptake is an alternative for N fertilizers overuses. Enterobacter cloacae is an opportunistic pathogen, also used as plant growth-promoting rhizobacteria (PGPR), has been widely presented in the fields of bioremediation and bioprotection. Here we developed a new N fixation-release model by combining biochar with E. cloacae. The efficiency of the model was evaluated using a greenhouse pot experiment with maize (Zea mays L.) as the test crop. The results showed that biochar combined with E. cloacae significantly increased the N content. The application of biochar combined with E. cloacae increased total N in soil by 33% compared with that of N fertilizers application. The N-uptake and utilization efficiency (NUE) in plant was increased 17.03% and 14.18%, respectively. The activities of urease, dehydrogenase and fluorescein diacetate hydrolase (FDA) was improved, the catalase (CAT) activity decreased. Analysis of the microbial community diversity revealed the abundance of Proteobacteria, Actinobacteria, Firmicutes, and Gemmatimonadetes were significantly improved. The mechanism under the model is that E. cloacae acted as N-fixation by capturing N₂ from air. Biochar served as carrier, supporting better living environment for E. cloacae, also as adsorbent adsorbing N from fertilizer and from fixed N by E. cloacae, the adsorption in turn slower the N release. Altogether, the model promotes N utilization by plants, improves the soil environment, and reduces N pollution.

Multiple Facets of Nitrogen: From Atmospheric Gas to Indispensable Agricultural Input

Nitrogen (N) is a gas and the fifth most abundant element naturally found in the atmosphere. N's role in agriculture and plant metabolism has been widely investigated for decades, and extensive information regarding this subject is available. However, the advent of sequencing technology and the advances in plant biotechnology, coupled with the growing interest in functional genomics-related studies and the various environmental challenges, have paved novel paths to rediscovering the fundamentals of N and its dynamics in physiological and biological processes, as well as biochemical reactions under both normal and stress conditions. This work provides a comprehensive review on multiple facets of N and N-containing compounds in plants disseminated in the literature to better appreciate N in its multiple dimensions. Here, some of the ancient but fundamental aspects of N are revived and the advances in our understanding of N in the metabolism of plants is portrayed. It is established that N is indispensable for achieving high plant productivity and fitness. However, the use of N-rich fertilizers in relatively higher amounts negatively affects the environment. Therefore, a paradigm shift is important to shape to the future use of N-rich fertilizers in crop production and their contribution to the current global greenhouse gases (GHGs) budget would help tackle current global environmental challenges toward a sustainable agriculture.

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

Permafrost-affected tundra soils are large carbon (C) and nitrogen (N) reservoirs. However, N is largely bound in soil organic matter (SOM), and ecosystems generally have low N availability. Therefore, microbial induced N-cycling processes and N losses were considered negligible. Recent studies show that microbial N processing rates, inorganic N availability, and lateral N losses from thawing permafrost increase when vegetation cover is disturbed, resulting in reduced N uptake or increased N input from thawing permafrost. In this review, we describe currently known N hotspots, particularly bare patches in permafrost peatland or permafrost soils affected by thermokarst, and their microbiogeochemical characteristics, and present evidence for previously unrecorded N hotspots in the tundra. We summarize the current understanding of microbial N cycling processes that promote the release of the potent greenhouse gas (GHG) nitrous oxide (N2O) and the translocation of inorganic N from terrestrial into aquatic ecosystems. We suggest that certain soil characteristics and microbial traits can be used as indicators of N availability and N losses. Identifying N hotspots in permafrost soils is key to assessing the potential for N release from permafrost-affected soils under global warming, as well as the impact of increased N availability on emissions of carbon-containing GHGs.

Suburban agriculture increased N levels but decreased indirect N2O emissions in an agricultural-urban gradient river

The effects of land use on riverine N₂O emissions are not well understood, especially in suburban zones between urban and rural with distinct anthropogenic perturbations. Here, we investigated in situ riverine N₂O emissions among suburban, urban, and rural sections of a typical agricultural-urban gradient river, the Qinhuai River of Southeastern China from June 2010 to September 2012. Our results showed that suburban agriculture greatly increased riverine N concentration compared to traditional agricultural rivers (TAR). The mean total dissolved nitrogen (TDN) concentration was 8.18 mg N L^-1 in the suburban agricultural rivers (SUAR), which was almost the same as that in the urban rivers (UR, of 8.50 mg N L^-1), compared to that in TAR (0.92 mg N L^-1). However, the annual average indirect N₂O flux from the SUAR was only 27.15 μg N₂O-N m^-2 h^-1, which was slightly higher than that from the TAR (13.14 μg N₂O-N m^-2 h^-1) but much lower than that from the UR (131.10 μg N₂O-N m^-2 h^-1). Moreover, the average N₂O emission factor (EF_5r, N₂O-N/DIN-N) in the SUAR (0.0002) was significantly lower than those in the TAR (0.0028) and UR (0.0004). The limited indirect N₂O fluxes from the SUAR are best explained by the high riverine dissolved organic carbon (DOC) and low dissolved oxygen, which probably reduced the denitrification source N₂O by favoring complete denitrification to produce N₂ and inhibited the nitrification source N₂O, respectively. An exponential decrease model incorporating dissolved inorganic nitrogen and DOC could greatly improve our EF_5r predictions in the agricultural-urban gradient river. Given the unprecedented suburban agriculture in the world, more studies in suburban agricultural rivers are needed to further refine the EF_5r and better reveal the mechanisms behind indirect N₂O emissions as influenced by suburban agriculture.

Anthropogenic nitrate attenuation versus nitrous oxide release from a woodchip bioreactor

Nitrogen loss via overland flow from agricultural land use is a global threat to waterways. On-farm denitrifying woodchip bioreactors can mitigate NO₃^- exports by increasing denitrification capacity. However, denitrification in sub-optimal conditions releases the greenhouse gas nitrous oxide (N₂O), swapping the pollution from aquatic to atmospheric reservoirs. Here, we assess NO₃^--N removal and N₂O emissions from a new edge-of-field surface-flow bioreactor during ten rain events on intensive farming land. Nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO₃^--N m^-3 wetted woodchip d^-1 with a mean of 30.3 ± 7.3 g NO₃^--N m^-3. The nitrate removal efficiency (NRE) was ∼73% in ideal hydrological conditions and ∼18% in non-ideal conditions. The fraction of NO₃^--N converted to N₂O (rN₂O) in the bioreactor was ∼3.3 fold lower than the expected 0.75% IPCC emission factor. We update the global bioreactor estimated Q₁₀ (NRR increase every 10 °C) from a recent meta-analysis with previously unavailable data to >20 °C, yielding a new global Q₁₀ factor of 3.1. Mean N₂O CO₂-eq emissions (431.9 ± 125.4 g CO₂-eq emissions day^-1) indicate that the bioreactor was not significantly swapping aquatic NO₃^- for N₂O pollution. Our estimated NO₃^--N removal from the bioreactor (9.9 kg NO₃^--N ha^-1 yr^-1) costs US$13.14 per kg NO₃^--N removed and represents ∼30% NO₃^--N removal when incorporating all flow and overflow events. Overall, edge-of-field surface-flow bioreactors seem to be a cost-effective solution to reduce NO₃^--N runoff with minor pollution swapping to N₂O.

A Decreasing Trend of Nitrous Oxide Emissions From California Cropland From 2000 to 2015

Mitigation of greenhouse gas emissions from agriculture requires an understanding of spatial-temporal dynamics of nitrous oxide (N₂O) emissions. Process-based models can quantify N₂O emissions from agricultural soils but have rarely been applied to regions with highly diverse agriculture. In this study, a process-based biogeochemical model, DeNitrification-DeComposition (DNDC), was applied to quantify spatial-temporal dynamics of direct N₂O emissions from California cropland employing a wide range of cropping systems. DNDC simulated direct N₂O emissions from nitrogen (N) inputs through applications of synthetic fertilizers and crop residues during 2000-2015 by linking the model with a spatial-temporal differentiated database containing data on weather, crop areas, soil properties, and management. Simulated direct N₂O emissions ranged from 3,830 to 7,875 tonnes N₂O-N yr^-1, representing 0.73%-1.21% of the N inputs. N₂O emission rates were higher for hay and field crops and lower for orchard and vineyard. State cropland total N₂O emissions showed a decreasing trend primarily driven by reductions of cropland area and N inputs, the trend toward growing more orchard, and changes in irrigation. Annual direct N₂O emissions declined by 47% from 2000 to 2015. Simulations showed N₂O emission variations could be explained not only by cropland area and N fertilizer inputs but also climate, soil properties, and management besides N fertilization. The detailed spatial-temporal emission dynamics and driving factors provide knowledge toward effective N₂O mitigation and highlight the importance of coupling process-based models with high-resolution data for characterizing the spatial-temporal variability of N₂O emissions in regions with diverse croplands.

Century-long changes and drivers of soil nitrous oxide (N2 O) emissions across the contiguous United States

The atmospheric concentration of nitrous oxide (N₂ O) has increased by 23% since the pre-industrial era, which substantially destructed the stratospheric ozone layer and changed the global climate. However, it remains uncertain about the reasons behind the increase and the spatiotemporal patterns of soil N₂ O emissions, a primary biogenic source. Here, we used an integrative land ecosystem model, Dynamic Land Ecosystem Model (DLEM), to quantify direct (i.e., emitted from local soil) and indirect (i.e., emissions related to local practices but occurring elsewhere) N₂ O emissions in the contiguous United States during 1900-2019. Newly developed geospatial data of land-use history and crop-specific agricultural management practices were used to force DLEM at a spatial resolution of 5 arc-min by 5 arc-min. The model simulation indicates that the U.S. soil N₂ O emissions totaled 0.97 ± 0.06 Tg N year^-1 during the 2010s, with 94% and 6% from direct and indirect emissions, respectively. Hot spots of soil N₂ O emission are found in the US Corn Belt and Rice Belt. We find a threefold increase in total soil N₂ O emission in the United States since 1900, 74% of which is from agricultural soil emissions, increasing by 12 times from 0.04 Tg N year^-1 in the 1900s to 0.51 Tg N year^-1 in the 2010s. More than 90% of soil N₂ O emission increase in agricultural soils is attributed to human land-use change and agricultural management practices, while increases in N deposition and climate warming are the dominant drivers for N₂ O emission increase from natural soils. Across the cropped acres, corn production stands out with a large amount of fertilizer consumption and high-emission factors, responsible for nearly two-thirds of direct agricultural soil N₂ O emission increase since 1900. Our study suggests a large N₂ O mitigation potential in cropland and the importance of exploring crop-specific mitigation strategies and prioritizing management alternatives for targeted crop types.

Management Strategies to Mitigate N2O Emissions in Agriculture

The concentration of greenhouse gases (GHGs) in the atmosphere has been increasing since the beginning of the industrial revolution. Nitrous oxide (N2O) is one of the mightiest GHGs, and agriculture is one of the main sources of N2O emissions. In this paper, we reviewed the mechanisms triggering N2O emissions and the role of agricultural practices in their mitigation. The amount of N2O produced from the soil through the combined processes of nitrification and denitrification is profoundly influenced by temperature, moisture, carbon, nitrogen and oxygen contents. These factors can be manipulated to a significant extent through field management practices, influencing N2O emission. The relationships between N2O occurrence and factors regulating it are an important premise for devising mitigation strategies. Here, we evaluated various options in the literature and found that N2O emissions can be effectively reduced by intervening on time and through the method of N supply (30–40%, with peaks up to 80%), tillage and irrigation practices (both in non-univocal way), use of amendments, such as biochar and lime (up to 80%), use of slow-release fertilizers and/or nitrification inhibitors (up to 50%), plant treatment with arbuscular mycorrhizal fungi (up to 75%), appropriate crop rotations and schemes (up to 50%), and integrated nutrient management (in a non-univocal way). In conclusion, acting on N supply (fertilizer type, dose, time, method, etc.) is the most straightforward way to achieve significant N2O reductions without compromising crop yields. However, tuning the rest of crop management (tillage, irrigation, rotation, etc.) to principles of good agricultural practices is also advisable, as it can fetch significant N2O abatement vs. the risk of unexpected rise, which can be incurred by unwary management.

Microorganism community composition analysis coupling with 15N tracer experiments reveals the nitrification rate and N2O emissions in low pH soils in Southern China

Nitrification in agricultural soil is an important process for food production. In acidic soil, nitrification is however also considered to be a major source of N₂O production. The nitrification rate largely depends on the community composition of ammonia-oxidizing organisms. To obtain a view of the nitrification rates and N₂O emission situations in low pH soils in Southern China and understand their relations with the microbial community composition, here we conducted ¹⁵N tracer experiments and microorganism community composition analysis using four acidic agricultural soil samples collected in Southern China. A single dominant community (relative abundance >68%) of the ammonia-oxidizing bacteria and ammonia-oxidizing archaea was observed in the soils with pH = 4.81–6.02. A low amount of NO3–\documentclass[10pt]{article}\usepackage{wasysym}\usepackage[substack]{amsmath}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage[mathscr]{eucal}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}{\text{NO}}_{3}^{\mbox{--}}]\end{document} was produced from the nitrification in the strongly acidic soil (pH = 4.03), and the calculated nitrification rate in this soil was significantly lower than those of other soils with pH = 4.81–6.02. High N₂O emissions but low ¹⁵N–N₂O emissions were observed in the soil with pH = 4.03. Our results suggest that, under aerobic conditions, soil pH is an important factor affecting nitrification through modifying the microorganism composition.

Suppressive effect of the deep placement of lime nitrogen on N2O emissions in a soybean field

Deep placement of slow-release nitrogen (N) fertilizers improves the growth and yield of soybean with a high N use efficiency. This study examined the effectiveness of deep placement of lime nitrogen (LN) in reducing N2O emissions in a soybean field and compared it with conventional fertilization. Before sowing soybeans, the starter N fertilizer (16 kg-N ha-1 ammonium sulfate) was mixed in the surface soil and the following four treatments were installed: the control with only the starter N (CT), conventional top-dressing of 60 kg-N ha-1 coated urea (CV), deep placement (20 cm depth) of 100 kg-N ha-1 urea (DU), and deep placement (20 cm depth) of 100 kg-N ha-1 LN (DL). The seasonal patterns of N2O emission rates measured using the closed chamber method differed among the treatments: in CT, N2O emissions were relatively low; in CV, N2O emissions derived from the top-dressed coated urea were observed from 91 days after sowing; in DU and DL, deeply-placed N was converted to N2O in the early growth stages. The cumulative N2O emissions in DL (1.8 kg-N ha-1) during the soybean cultivation period were significantly lower than those in DU (3.1 kg-N ha-1) and CV (2.8 kg-N ha-1), and slightly higher than CT (1.2 kg-N ha-1). The magnitude of N2O emissions was significantly lower in DL than DU, indicating that the choice of N fertilizer is important to reduce N2O emissions. Focusing on N2O emissions per unit coarse grain yield of soybeans, the value in DL was 0.45 g-N kg-1, which was significantly lower than 0.74 g-N kg-1 in CV. In conclusion, the deep placement of LN has the potential to be a sustainable farming method that can promote yields and reduce N2O emissions in soybean cultivation for high yield with N fertilization.

Ammonia-oxidizing bacterial communities are affected by nitrogen fertilization and grass species in native C4 grassland soils

Background

Fertilizer addition can contribute to nitrogen (N) losses from soil by affecting microbial populations responsible for nitrification. However, the effects of N fertilization on ammonia oxidizing bacteria under C₄ perennial grasses in nutrient-poor grasslands are not well studied.

Methods

In this study, a field experiment was used to assess the effects of N fertilization rate (0, 67, and 202 kg N ha⁻¹) and grass species (switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)) on ammonia-oxidizing bacterial (AOB) communities in C₄ grassland soils using quantitative PCR, quantitative reverse transcription-PCR, and high-throughput amplicon sequencing of amoA genes.

Results

Nitrosospira were dominant AOB in the C₄ grassland soil throughout the growing season. N fertilization rate had a stronger influence on AOB community composition than C₄ grass species. Elevated N fertilizer application increased the abundance, activity, and alpha-diversity of AOB communities as well as nitrification potential, nitrous oxide (N₂O) emission and soil acidity. The abundance and species richness of AOB were higher under switchgrass compared to big bluestem. Soil pH, nitrate, nitrification potential, and N₂O emission were significantly related to the variability in AOB community structures (p < 0.05).

Long-term fertilization and tillage regimes have limited effects on structuring bacterial and denitrifier communities in a sandy loam UK soil

Denitrification causes loss of available nitrogen from soil systems, thereby reducing crop productivity and increasing reliance on agrochemicals. The dynamics of denitrification and denitrifying communities are thought to be altered by land management practices, which affect the physicochemical properties of the soil. In this study, we look at the effects of long-term tillage and fertilization regimes on arable soils following 16 years of treatment in a factorial field trial. By studying the bacterial community composition based on 16S rRNA amplicons, absolute bacterial abundance and diversity of denitrification functional genes (nirK, nirS and nosZ), under conditions of minimum/conventional tillage and organic/synthetic mineral fertilizer, we tested how specific land management histories affect the diversity and distribution of both bacteria and denitrification genes. Bacterial and denitrifier communities were largely unaffected by land management history and clustered predominantly by spatial location, indicating that the variability in bacterial community composition in these arable soils is governed by innate environmental differences and Euclidean distance rather than agricultural management intervention.

Comparing GHG Emissions from Drained Oil Palm and Recovering Tropical Peatland Forests in Malaysia

For agricultural purposes, the drainage and deforestation of Southeast Asian peatland resulted in high greenhouse gases’ (GHGs, e.g., CO2, N2O and CH4) emission. A peatland regenerating initiative, by rewetting and vegetation restoration, reflects evidence of subsequent forest recovery. In this study, we compared GHG emissions from three Malaysian tropical peatland systems under the following different land-use conditions: (i) drained oil palm plantation (OP), (ii) rewetting-restored forest (RF) and (iii) undrained natural forest (NF). Biweekly temporal measurements of CO2, CH4 and N2O fluxes were conducted using a closed-chamber method from July 2017 to December 2018, along with the continuous measurement of environmental variables and a one-time measurement of the soil physicochemical properties. The biweekly emission data were integrated to provide cumulative fluxes using the trapezoidal rule. Our results indicated that the changes in environmental conditions resulting from draining (OP) or rewetting historically drained peatland (RF) affected CH4 and N2O emissions more than CO2 emissions. The cumulative CH4 emission was significantly higher in the forested sites (RF and NF), which was linked to their significantly higher water table (WT) level (p < 0.05). Similarly, the high cumulative CO2 emission trends at the RF and OP sites indicated that the RF rewetting-restored peatland system continued to have high decomposition rates despite having a significantly higher WT than the OP (p < 0.05). The highest cumulative N2O emission at the drained-fertilized OP and rewetting-restored RF sites was linked to the available substrates for high decomposition (low C/N ratio) together with soil organic matter mineralization that provided inorganic nitrogen (N), enabling ideal conditions for microbial mediated N2O emissions. Overall, the measured peat properties did not vary significantly among the different land uses. However, the lower C/N ratio at the OP and the RF sites indicated higher decomposition rates in the drained and historically drained peat than the undrained natural peat (NF), which was associated with high cumulative CO2 and N2O emissions in our study.

In-depth analysis of N2O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis

Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (N₂O) or elemental nitrogen (N₂). The release of N₂O is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of N₂O; however, despite being the world's second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating N₂O fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual N₂O fluxes in the Congo Basin's lowland forests (0.97 ± 0.53 kg N ha^-1 year^-1) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha^-1 year^-1). Measurements of soil pore air N₂O isotope data at multiple depths suggests that a microbial reduction of N₂O to N₂ within the soil may account for the observed low surface N₂O fluxes and low soil pore N₂O concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (N₂O, N₂) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N₂ via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.

Global greenhouse gas emissions from animal-based foods are twice those of plant-based foods

Agriculture and land use are major sources of greenhouse gas (GHG) emissions but previous estimates were either highly aggregate or provided spatial details for subsectors obtained via different methodologies. Using a model-data integration approach that ensures full consistency between subsectors, we provide spatially explicit estimates of production- and consumption-based GHG emissions worldwide from plant- and animal-based human food in circa 2010. Global GHG emissions from the production of food were found to be 17,318 ± 1,675 TgCO₂eq yr^-1, of which 57% corresponds to the production of animal-based food (including livestock feed), 29% to plant-based foods and 14% to other utilizations. Farmland management and land-use change represented major shares of total emissions (38% and 29%, respectively), whereas rice and beef were the largest contributing plant- and animal-based commodities (12% and 25%, respectively), and South and Southeast Asia and South America were the largest emitters of production-based GHGs.

Improvement of the spectroscopic parameters of the air- and self-broadened N2O and CO lines for the HITRAN2020 database applications

This paper outlines the major updates of the line-shape parameters that were performed for the nitrous oxide (N2O) and carbon monoxide (CO) molecules listed in the HITRAN2020 database. We reviewed the collected measurements for the air- and self-broadened N2O and CO spectra to determine proper values for the spectroscopic parameters. Careful comparisons of broadening parameters using the Voigt and speed-dependent Voigt line-shape profiles were performed among various published results for both N2O and CO. Selected data allowed for developing semi-empirical models, which were used to extrapolate/interpolate existing data to update broadening parameters of all the lines of these molecules in the HITRAN database. In addition to the line broadening parameters (and their temperature dependences), the pressure shift values were revised for N2O and CO broadened by air and self for all the bands. The air and self speed-dependence of the broadening parameter for these two molecules were added for every transition as well. Furthermore, we determined the first-order line-mixing parameters using the Exponential Power Gap (EPG) scaling law. These new parameters are now available at HITRAN online.

Spatial patterns of net greenhouse gas balance and intensity in Chinese orchard system

Fruit production has been expanding due to the pursuit of healthier lifestyles in China. Determining the greenhouse gas (GHG) emissions status of the orchard system could contribute to adopting appropriate measures to alleviate climate change pressure from the growing fruit production. In this study, the net GHG balance and GHG intensity (GHGI) in the Chinese fruit production were estimated at the regional level using a meta-analysis based on databases compiled from relevant publications during 2000-2019, including soil nitrous oxide (N₂O) and methane (CH₄) emissions or uptake, upstream carbon dioxide (CO₂) emissions related to farm practices, and the change of soil organic carbon (SOC) storage from the life cycle perspective. Results showed that the net GHG balance and GHGI varied among six regions, with ranges of 6.4 ± 0.3 to 10.0 ± 0.6 Mg CO₂e ha^-1 yr^-1, and 2.2 ± 0.2 to 3.0 ± 0.2 kg CO₂e kg^-1, respectively. Synthetic nitrogen (N) fertilization was the largest source of overall GHG emissions from fruit production throughout China, accounting for 46% and ranging from 43% to 55% in the six fruit production regions. Fertilizer-induced N₂O emissions were responsible for 22-31% of the total GHG emissions, and the N₂O-N emission factor was identified as 0.7%. Also, power use for irrigation contributed a non-negligible 17% to the emissions on a national level, yet with large regional variations. In addition, fruit production in North, Northeast, Central, and East, and South China have relatively lower GHGIs than in Northwest and Southwest China. The estimated total GHG emissions from the Chinese fruit production were 102 Tg CO₂e, with the contribution of SOC change to a decrease by 11% for the year 2018. Our results highlight an urgency to lower fruit production-related carbon emissions by extending optimized N fertilization and irrigation modes in China's orchard system.

Soil respiration analysis using a mid-infrared quantum cascade laser and calibration-free WMS-based dual-gas sensor

A high response and sensitive dual-gas sensor based on calibration-free wavelength modulation spectroscopy (CF-WMS) has been developed for the simultaneous detection of carbon monoxide (CO) and nitrous oxide (N2O) to eliminate the detection errors caused by light intensity variations. A multi-pass cell (MPC) was employed to lengthen the optical path to improve the precision of the sensing system by combining with a 4.56 μm mid-infrared quantum cascade laser (MIR-QCL). Meanwhile, a LabVIEW-based bi-molecular iterative fitting algorithm was used to infer the respective abundances of each species. The performance of the completed system was accurately evaluated with precisions of 3.4 ppb for CO and 3.8 ppb for N2O at a 1 s averaging time, which could be improved to 0.48 ppb for CO and 0.53 ppb for N2O at an averaging time of 154 s and 278 s, respectively. The grassland soil respiration analysis of CO and N2O was performed under different moisture conditions, which indicated that dried soil samples appeared to be a significant source of CO, while the sinks of CO and the sources of N2O occurred in the moist soil samples. The maximum exchange rates of the two gases were exhibited in moderate moisture soil samples rather than in the over-wet or arid soil samples. Moreover, a possible positive relationship between the sinks of CO and sources of N2O was established to illustrate the correlation of the two species in soil respiration.

Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters

Inland waters are significant sources of nitrous oxide (N₂O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N₂O efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of N₂O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018-2020. N₂O concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the N₂O emission factor (N₂O-N/DIN, median 0.0005) was lower than global medians and the N₂O yield (ΔN₂O/(ΔN₂O + ΔN₂), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N₂O emissions and denitrifying bacteria as well as the Michaelis-Menten relationship between N₂O emissions and NO₃^--N concentrations suggested that bacteria control the net production of N₂O in waters of Beijing with N saturation, leading to a low N₂O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N₂O accumulation during denitrification, resulting in high N₂O yields. This study demonstrates the significant N₂O emissions and their distinctive patterns and controls in urban inland waters and suggests that N₂O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.

Nitrous oxide emission and sweet potato yield in upland soil: Effects of different type and application rate of composted animal manures

The aims of this study were to determine type and application rate of composted animal manure to optimize sweet potato yield relative to N₂O emissions from upland soils. To this end, the study was conducted on upland soils amended with different types and rates of composted animal manure and located at two geographically different regions of South Korea. Field trials were established at Miryang and Yesan in South Korea during the sweet potato (Ipomoea batatas) growing season over 2 years: 2017 (Year 1) and 2018 (Year 2). Three composted animal manures (chicken, cow, and pig) were applied at the rates of 0, 10, and 20 Mg ha^-1 to upland soils in both locations. In both Years and locations, manure type did not affected significantly cumulative N₂O emissions from soil during the sweet potato growing season or the belowground biomass of sweet potato. However, application rate of animal manures affected significantly the cumulative N₂O emission, nitrogen (N) in soil, and belowground biomass of sweet potato. An increase in cumulative N₂O emission with application rates of animal manures was related to total N and inorganic N concentration in soil. The belowground biomass yield of sweet potato but also the cumulative N₂O emission increased with increasing application rate of composted animal manures up to 7.6 and 16.0 Mg ha^-1 in Miryang and Yesan, respectively. To reduce N₂O emission from arable soil while increasing crop yield, composted animal manures should be applied at less than application rate that produce the maximum belowground biomass of sweet potato.

Diversity of nitrogen cycling genes at a Midwest long-term ecological research site with different management practices

Nitrogen fertilizer results in the release of nitrous oxide (N₂O), a concern because N₂O is an ozone-depleting substance and a greenhouse gas. Although the reduction of N₂O to nitrogen gas can control emissions, the factors impacting the enzymes involved have not been fully explored. The current study investigated the abundance and diversity of genes involved in nitrogen cycling (primarily denitrification) under four agricultural management practices (no tillage [NT], conventional tillage [CT], reduced input, biologically-based). The work involved examining soil shotgun sequencing data for nine genes (napA, narG, nirK, nirS, norB, nosZ, nirA, nirB, nifH). For each gene, relative abundance values, diversity and richness indices, and taxonomic classification were determined. Additionally, the genes associated with nitrogen metabolism (defined by the KEGG hierarchy) were examined. The data generated were statistically compared between the four management practices. The relative abundance of four genes (nifH, nirK, nirS, and norB) were significantly lower in the NT treatment compared to one or more of the other soils. The abundance values of napA, narG, nifH, nirA, and nirB were not significantly different between NT and CT. The relative abundance of nirS was significantly higher in the CT treatment compared to the others. Diversity and richness values were higher for four of the nine genes (napA, narG, nirA, nirB). Based on nirS/nirK ratios, CT represents the highest N₂O consumption potential in four soils. In conclusion, the microbial communities involved in nitrogen metabolism were sensitive to different agricultural practices, which in turn, likely has implications for N₂O emissions. KEY POINTS: • Four genes were less abundant in NT compared to one or more of the others soils (nifH, nirK, nirS, norB). • The most abundant sequences for many of the genes classified within the Proteobacteria. • Higher diversity and richness indices were observed for four genes (napA, narG, nirA, nirB). • Based on nirS/nirK ratios, CT represents the highest N₂O consumption potential.

Nitrous oxide emission from agricultural soils: Application of animal manure or biochar? A global meta-analysis

Organic amendments (animal manure and biochar) to agricultural soils may enhance soil organic carbon (SOC) contents, improve soil fertility and crop productivity but also contribute to global warming through nitrous oxide (N₂O) emission. However, the effects of organic amendments on N₂O emissions from agricultural soils seem variable among numerous research studies and remains uncertain. Here, eighty-five publications (peer-reviewed) were selected to perform a meta-analysis study. The results of this meta-analysis study show that the application of animal manure enhanced N₂O emissions by 17.7%, whereas, biochar amendment significantly mitigated N₂O emissions by 19.7%. Moreover, coarse textured soils increased [lnRR‾ = 182.6%, 95% confidence interval (CI) = 151.4%, 217.7%] N₂O emission after animal manure, in contrast, N₂O emission mitigated by 7.0% from coarse textured soils after biochar amendment. In addition, this study found that 121-320 kg N ha^-1 and ⩽ 30 T ha^-1 application rates of animal manure and biochar mitigated N₂O emissions by 72.3% and 22.5%, respectively. Soil pH also played a vital role in regulating the N₂O emissions after organic amendments. Furthermore, > 10 soil C: N ratios increased N₂O emissions by 121.4% and 27.6% after animal and biochar amendments, respectively. Overall, animal manure C: N ratios significantly enhanced N₂O emissions, while, biochar C: N ratio had not shown any effect on N₂O emissions. Overall, average N₂O emission factors (EFs) for animal manure and biochar amendments were 0.46% and -0.08%, respectively. Thus, the results of this meta-analysis study provide scientific evidence about how organic amendments such as animal manure and biochar regulating the N₂O emission from agricultural soils.

Effects of Organic Fertilizers on the Soil Microorganisms Responsible for N2O Emissions: A Review

The use of organic fertilizers constitutes a sustainable strategy to recycle nutrients, increase soil carbon (C) stocks and mitigate climate change. Yet, this depends largely on balance between soil C sequestration and the emissions of the potent greenhouse gas nitrous oxide (N₂O). Organic fertilizers strongly influence the microbial processes leading to the release of N₂O. The magnitude and pattern of N₂O emissions are different from the emissions observed from inorganic fertilizers and difficult to predict, which hinders developing best management practices specific to organic fertilizers. Currently, we lack a comprehensive evaluation of the effects of OFs on the function and structure of the N cycling microbial communities. Focusing on animal manures, here we provide an overview of the effects of these organic fertilizers on the community structure and function of nitrifying and denitrifying microorganisms in upland soils. Unprocessed manure with high moisture, high available nitrogen (N) and C content can shift the structure of the microbial community, increasing the abundance and activity of nitrifying and denitrifying microorganisms. Processed manure, such as digestate, compost, vermicompost and biochar, can also stimulate nitrifying and denitrifying microorganisms, although the effects on the soil microbial community structure are different, and N₂O emissions are comparatively lower than raw manure. We propose a framework of best management practices to minimize the negative environmental impacts of organic fertilizers and maximize their benefits in improving soil health and sustaining food production systems. Long-term application of composted manure and the buildup of soil C stocks may contribute to N retention as microbial or stabilized organic N in the soil while increasing the abundance of denitrifying microorganisms and thus reduce the emissions of N₂O by favoring the completion of denitrification to produce dinitrogen gas. Future research using multi-omics approaches can be used to establish key biochemical pathways and microbial taxa responsible for N₂O production under organic fertilization.

Greenhouse gas fluxes from turfgrass systems: Species, growth rate, clipping management, and environmental effects

Turfgrass systems can be an important source or sink for greenhouse gases (GHG), including carbon dioxide (CO₂ ), nitrous oxide (N₂ O), and methane (CH₄ ). Further research is required in turfgrass systems; therefore, our objectives were to evaluate the effects of turfgrass species, growth rate, clipping management, and environmental conditions on GHG emissions. Greenhouse gas fluxes were measured in two separate field experiments in West Lafayette, IN. Experiment 1 investigated GHG flux in three cool-season (C₃ ) and two warm-season (C₄ ) turfgrass species during two growing seasons. Experiment 2 investigated fluxes in two C₃ cultivars with varying growth rates and under different clipping management regimes. The C₃ turfgrasses had the highest mean CO₂ flux rates ranging from 0.373 to 0.431 g CO₂ -C m^-2 h^-1 compared with 0.273 to 0.361 g CO₂ -C m^-2 h^-1 for C₄ turfgrasses. Mean hourly N₂ O flux rates ranged from 43.3 to 50.9 μg N₂ O-N m^-2 h^-1 for C₃ compared with 11.1 to 14.4 μg N₂ O-N m^-2 h^-1 for C₄ turfgrasses. Methane flux was more variable across time, but overall C₄ turfgrasses were more likely to be a CH₄ source, whereas C₃ turfgrasses were often a CH₄ sink. Growth rate and grass clipping management treatments had negligible impact on measured GHG flux. The differences in management practices specific to C₃ and C₄ turfgrasses had the largest impact on GHG flux. Results indicate the impact and importance of turfgrass species selection on GHG flux and also provide more information on our overall understanding on carbon and nitrogen cycling in urban soils.

Near-infrared cavity ring-down spectroscopy measurements of nitrous oxide in the (4200)←(0000) and (5000)←(0000) bands

Using frequency-agile rapid scanning cavity ring-down spectroscopy, we measured line intensities and line shape parameters of ¹⁴N₂ ¹⁶O in air in the (4200)←(0000) and (5000)←(0000) bands near 1.6 µm. The absorption spectra were modeled with multi-spectrum fits of Voigt and speed-dependent Voigt profiles. The measured line intensities and air-broadening parameters exhibit deviations of several percent relative to values provided in HITRAN 2016. Our measured intensities for these two bands have relative combined standard uncertainties of ∼1% which is approximately five times smaller than literature values. Comparison of the present air-broadening and speed-dependent broadening parameters to experimental literature values for other rotation-vibration bands of N₂O indicates significant differences in magnitude and J-dependence. For applications requiring high spectral fidelity, these results suggest that the assumption of band-independent line shape parameters is not appropriate.

Effects of untreated or insufficiently treated wastewater discharges on the spatial and temporal variability of nitrous oxide (N2O) emissions from different streams in southeastern Brazil

Increasing atmospheric N₂O concentrations is of great environmental concern due to the role of this gas in climate change and stratospheric ozone destruction. Nitrogen-enriched lotic water bodies are significant sources of N₂O. However, N₂O emissions from rivers and streams, particularly those that receive untreated or insufficiently treated wastewater discharge, are poorly understood, especially in Brazil. The present study investigated the effects of the discharge of untreated or insufficiently treated wastewater on the spatial-temporal variability of N₂O emissions from different streams in Ilha Grande, located within the Abraão hydrographic system, in southeastern Brazil. Estimated N₂O fluxes determined in Abraão streams and upstream of the urbanized stretch ranged from 18.4 and 96.5 μg N m^-2 h^-1. Inside the urbanized stretch, estimated N₂O fluxes ranged from 110 to 561 μg N m^-2 h^-1 under non-limited dissolved oxygen (DO) conditions and 133 to 2,229 μg N m^-2 h^-1 under hypoxic conditions (DO < 2 mg O₂ L^-1). High spatial and temporal variability in N₂O emissions were noted, with the highest emissions in Abraão urban areas. Therefore, the differences observed between N₂O fluxes from the studied streams at Abraão seem to be associated with different lotic water body conditions, such as availability of reactive N and DO.

Global greenhouse vegetable production systems are hotspots of soil N2O emissions and nitrogen leaching: A meta-analysis

Vegetable production in greenhouses is often associated with the use of excessive amounts of nitrogen (N) fertilizers, low NUE (15-35%), and high N losses along gaseous and hydrological pathways. In this meta-analysis, we assess the effects of application rate, fertilizer type, irrigation, and soil properties on soil N₂O emissions and nitrogen leaching from greenhouse vegetable systems on the basis of 75 studies. Mean ± standard error (SE) N₂O emissions from unfertilized control plots (N₂O_control) and N leaching (NL_control) of greenhouse vegetable systems were 3.2 ± 0.4 and 91 ± 20 kg N ha^-1 yr^-1, respectively, indicating legacy effects due to fertilization in preceding crop seasons. Soil organic carbon concentrations (SOC) and irrigation were significantly positively correlated with NL_control losses, while other soil properties did not significantly affect N₂O_control or NL_control. The annual mean soil N₂O emission from fertilized greenhouse vegetable systems was 12.0 ± 1.0 kg N₂O-N ha^-1 yr^-1 (global: 0.067 Tg N₂O-N yr^-1), with N₂O emissions increasing exponentially with fertilization. The mean EF_N2O was 0.85%. The mean annual nitrogen leaching (NL) was 297 ± 22 kg N ha^-1 yr^-1 (global: 1.66 Tg N yr^-1), with fertilization, irrigation, and SOC explaining 65% of the observed variation. The mean leaching factor across all fertilizer types was 11.9%, but 18.7% for chemical fertilizer. Crop NUE was highest, while N₂O emissions and N leaching were lowest, at fertilizer rates <500 kg N ha^-1 year^-1. Yield-scaled N₂O emissions (0.05 ± 0.01 kg N₂O-N Mg^-1 yr^-1) and nitrogen leaching (0.79 ± 0.08 kg N Mg^-1 yr^-1) were lowest at fertilizer rates <1000 kg N ha^-1 yr^-1. Vegetables are increasingly produced in greenhouses, often under management schemes of extreme fertilization (>1500 kg N ha^-1 yr^-1) and irrigation (>1200 mm yr^-1). Our study indicates that high environmental N₂O and N leaching losses can be mitigated by reducing fertilization rates to 500-1000 kg N ha^-1 yr^-1 (mean: ∼762 kg N ha^-1 yr^-1) without jeopardizing yields.