Nitrous Oxide: Emission from Soils During Nitrification of Fertilizer Nitrogen

Impact of particle-attached microbial denitrification on N2O production in an agricultural-urban watershed

Anthropogenically influenced rivers are key hotspots for nitrous oxide (N2O) emissions. However, the seasonal and spatial heterogeneity of N2O emissions in subtropical riverine systems, particularly the role of particle-attached microbes (PAM) in regulating N2O production, remains poorly understood, contributing to uncertainties in global N2O estimates. This study investigates the potential impacts of PAM-driven nitrogen transformations on N2O production in the Dongjiang River under agricultural and urban influences. Water samples collected during the wet and dry seasons were analyzed for N2O concentrations, dual nitrogen-oxygen isotopes (δ15N-NO3-, δ18O-NO3-), and metagenomic sequencing of PAM. All samples exhibited N2O supersaturation, with emissions significantly higher in the dry season than in the wet season. A linearly positive δ15N-δ18O correlation, accompanied by lower NO3- in the bottom layers than the surface layers in the dry season indicates active denitrification, leading to elevated N2O concentrations. PAM-driven denitrification was identified as the dominant nitrogen transformation process, supported by higher abundances of denitrification genes (nirKS, norBC, nosZ) relative to nitrification genes (amoABC). Despite aerobic water column conditions, low-oxygen microhabitats around suspended particles facilitated N2O production. A significantly positive correlation (p < 0.05, R2 = 0.42) between N2O concentrations and the nirK/nosZ gene ratio suggests that gene expression imbalances contributed to net N2O accumulation. Additionally, the downstream urban area exhibited lower DO and higher DOC levels, enhancing denitrification and increasing N2O production by 4.7 % compared to the upstream agricultural region. Seasonal differences further influenced N2O dynamics: higher DOC/NO3- ratios in the dry season promoted heterotrophic denitrification, while elevated temperatures in the wet season favored complete denitrification, reducing N2O emissions. These findings provide critical insights into PAM-driven nitrogen cycling, informing strategies for mitigating N2O emissions and managing nitrogen pollution in subtropical riverine systems.

Variability in N2O emission controls among different ponds within a hilly watershed

While it is well established that small water bodies like ponds play a disproportionately large role in contributing to N2O emissions, few studies have focused on lowland ponds in hilly watersheds. Here, we explored the characteristics of N2O concentrations and emissions from various typical ponds (village, tea, forested, and aquaculture ponds) in a hilly watershed and examined the specific controls influencing N2O production. Our findings revealed that tea ponds exhibited the highest N2O flux (8.42 ± 8.23 μmol m-2 d-1), which was 2.8 to 3.3 times greater than other types of ponds. Remarkable seasonal variations were observed in tea and forested ponds due to the seasonality of nutrient-enriched runoff, whereas such variations were less pronounced in village and aquaculture ponds. Key factors such as nitrogen levels, temperature, and dissolved oxygen (DO) emerged as the primary controls of N2O concentrations in ponds, heavily influenced by land use and human activities in their drainage areas. Specifically, N2O production in tea and aquaculture ponds was driven by N inputs from fertilization and feed, respectively, while DO levels governed the process in village and forested ponds, influenced by abundant algae and forest vegetation. This study emphasizes that environmental factors predominantly drive N2O production in ponds within hilly watersheds, but land use in the pond drainages acts as an indirect yet crucial influence. This highlights the need for future research to develop targeted emission reduction strategies based on land use to effectively mitigate N2O emissions, promising a path toward more sustainable and climate-friendly watershed management.

Green manuring combined with zeolite reduced nitrous oxide emissions in maize field by targeting microbial nitrogen transformations

Green manure is a crucial strategy for increasing cereal yield and mitigating environmental burden while reducing chemical N fertilizer. To effectively tackle climate change, finding ways to reduce nitrous oxide (N2O) emissions from green manuring systems is vital. Herein, field and 15N labeled microcosm experiments were arranged to investigate the effect and mechanisms of green manuring and zeolite application on N2O emission. Both experiments comprised four treatments: conventional chemical N (N100), 70 % chemical N (N70), N70 with green manure (N70 + CV), and N70 + CV combined with zeolite (N70 + CV + Z). Compared with N100, both N70 + CV and N70 + CV + Z maintained maize yield, cumulative N2O emissions decreased by 37.7 % and 34.9 % in N70 + CV + Z in 2022-yr and 2023-yr, and by 12.8 % in N70 + CV in 2022-yr. Moreover, the reduction of N2O emission primarily occurred after incorporating green manure. The N100 and N70 + CV demonstrated a similar transformed proportion of chemical N to N2O (i.e., 4.9 % and 4.7 %) while reducing it to 2.7 % in N70 + CV + Z. Additionally, a mere 0.7 % of green manure N was transformed to N2O in both N70 + CV and N70 + CV + Z treatments. Compared with N100, both N70 + CV and N70 + CV + Z decreased the relative abundances of ammonia oxidation microbes, increased denitrifier and the ratios of (nirK + nirS)/nosZ and norBC/nosZ. Furthermore, compared with N70 + CV, N70 + CV + Z decreased the relative abundances of N2O-producer and the ratios of (nirK + nirS)/nosZ and norBC/nosZ in denitrification. These findings revealed that the reduction of N2O emissions resulting from green manure replaced chemical N was mainly due to weakened nitrification, while zeolite reduced N2O emissions attributed to enhanced conversion of N2O to N2. Moreover, certain key N-cycling functional bacteria, such as Phycisphaerae, Rubrobacteria, and Thermoflexia, were positively correlated with N2O emission. In contrast, Dehalococcoidia, Gammaproteobacteria, and Betaproteobacteria were negatively correlated with N2O emission. This investigation uncovered the underlying mechanisms for effectively reducing N2O emissions through green manuring combined with zeolite.

Effects of biodegradable microplastics and straw addition on soil greenhouse gas emissions

Large pieces of plastic are transformed into microplastic particles through weathering, abrasion, and ultraviolet radiation, significantly impacting the soil ecosystem. However, studies on biodegradable microplastics replacing traditional microplastics as agricultural mulching films to drive the biogeochemical processes influenced by GHG are still in their initial stages, with limited relevant reports available. This study sought to investigate the effects of microplastic and straw addition on CO2 and N2O emissions in different soils. Herein, yellow-brown soil (S1) and fluvo-aquic soil (S2) were utilized, each treated with three different concentrations of PLA (polylactic acid) microplastics (0.25%, 2%, and 7% w/w) at 25 °C for 35 days, with and without straw addition. The results showed that straw (1% w/w) significantly increased soil CO2 by 4.1-fold and 3.2-fold, respectively, and N2O by 1.8-fold and 1.8-fold, respectively, in cumulative emissions in S1 and S2 compared with the control. PLA microplastics significantly increased CO2 emissions by 71.5% and 99.0% and decreased N2O emissions by 30.1% and 24.7% at a high concentration (7% w/w, PLA3) in S1 and S2 compared with the control, respectively. The same trend was observed with the addition of straw and microplastics together. Structural equation modeling and redundancy analysis confirmed that soil physiochemical parameters, enzyme and microbial activities are key factors regulating CO2 and N2O emissions. The addition of microplastics is equivalent to the addition of carbon sources, which can significantly affect DOC, MBC, SOC and the abundance of carbon-associated bacteria (CbbL), thereby increasing soil CO2 emissions. The addition of microplastics alone inhibited the activity of nitrogen cycling enzymes (urease activity), increasing the abundance of denitrifying microbes. However, adding a high amount of microplastics and straw together released plastic additives, inhibiting microbial abundance and reducing the nitrogen cycle. These effects decreased NH4+-N and increased NO3--N, resulting in decreased N2O emissions. This study indicates that biodegradable microplastics could reduce soil plastic residue pollution through degradation. However, their use could also increase CO2 emissions and decrease N2O emissions. Consequently, this research lays the groundwork for further investigation into the implications of utilizing biodegradable microplastics as agricultural mulch, particularly concerning soil geochemistry and GHG emissions.

pH selects for distinct N2O-reducing microbiomes in tropical soil microcosms

Nitrous oxide (N2O), a greenhouse gas with ozone destruction potential, is mitigated by the microbial reduction to dinitrogen catalyzed by N2O reductase (NosZ). Bacteria with NosZ activity have been studied at circumneutral pH but the microbiology of low pH N2O reduction has remained elusive. Acidic (pH < 5) tropical forest soils were collected in the Luquillo Experimental Forest in Puerto Rico, and microcosms maintained with low (0.02 mM) and high (2 mM) N2O assessed N2O reduction at pH 4.5 and 7.3. All microcosms consumed N2O, with lag times of up to 7 months observed in microcosms with 2 mM N2O. Comparative metagenome analysis revealed that Rhodocyclaceae dominated in circumneutral microcosms under both N2O feeding regimes. At pH 4.5, Peptococcaceae dominated in high-N2O, and Hyphomicrobiaceae in low-N2O microcosms. Seventeen high-quality metagenome-assembled genomes (MAGs) recovered from the N2O-reducing microcosms harbored nos operons, with all eight MAGs derived from acidic microcosms carrying the Clade II type nosZ and lacking nitrite reductase genes (nirS/K). Five of the eight MAGs recovered from pH 4.5 microcosms represent novel taxa indicating an unexplored N2O-reducing diversity exists in acidic tropical soils. A survey of pH 3.5-5.7 soil metagenome datasets revealed that nosZ genes commonly occur, suggesting broad distribution of N2O reduction potential in acidic soils.

Inland Waters Increasingly Produce and Emit Nitrous Oxide

Nitrous oxide (N2O) is a long-lived greenhouse gas and currently contributes ∼10% to global greenhouse warming. Studies have suggested that inland waters are a large and growing global N2O source, but whether, how, where, when, and why inland-water N2O emissions changed in the Anthropocene remains unclear. Here, we quantify global N2O formation, transport, and emission along the aquatic continuum and their changes using a spatially explicit, mechanistic, coupled biogeochemistry-hydrology model. The global inland-water N2O emission increased from 0.4 to 1.3 Tg N yr-1 during 1900-2010 due to (1) growing N2O inputs mainly from groundwater and (2) increased inland-water N2O production, largely in reservoirs. Inland waters currently contribute 7 (5-10)% to global total N2O emissions. The highest inland-water N2O emissions are typically in and downstream of reservoirs and areas with high population density and intensive agricultural activities in eastern and southern Asia, southeastern North America, and Europe. The expected continuing excessive use of nutrients, dam construction, and development of suboxic conditions in aging reservoirs imply persisting high inland-water N2O emissions.

Resistance and Resilience of Soil Nitrogen Cycling to Drought and Heat Stress in Rehabilitated Urban Soils

In the context of climate change and biodiversity loss, rehabilitation of degraded urban soils is a means of limiting artificialization of terrestrial ecosystems and preventing further degradation of soils. Ecological rehabilitation approaches are available to reinitiate soil functions and enhance plant development. However, little is known about the long-term stability of rehabilitated soils in terms of soil functions when further natural or anthropogenic perturbations occur. Based on rehabilitated urban soils, the present study sought to evaluate the resistance and resilience of soil functions linked to carbon cycling and phosphate dynamics in addition to nitrogen cycling and related microbial communities after a heat and drought stress. A laboratory experiment was conducted in microcosms under controlled temperature conditions, with four contrasted soils collected from a rehabilitated urban brownfield; an initial, non-rehabilitated soil (IS), a technosol with a high organic matter level (HO), and two technosols with less organic matter (LO1 and LO2), together with their respective controls (no stress). Changes in potential denitrification (PDR), nitrification (PNR) rates, and their interactive relationships with soil microbial activities and soil physicochemical properties were determined following a combined heat (40°C) and drought stress period of 21 days. Measurements were carried out immediately after the stress (resistance), and then also 5, 30, and 92 days after soil rewetting at 60% water holding capacity (resilience). Microbial activities involved in soil functions such as carbon cycling and phosphate dynamics proved to be of low resistance in all soils except for IS; however, they were resilient and recovered rapidly after rewetting. On the other hand, the microbial activities and gene abundances that were measured in relation to nitrogen cycling processes showed that for denitrification, activities were more rapidly resilient than gene abundances whereas for nitrification the activities and gene abundances were resilient in the same way. Results suggest that, unless the soils contain high amounts of organic matter, microbial communities in imported soils can be more vulnerable to environmental pressures such as drought and heat than communities already present. This should be considered when rehabilitating degraded soils.

Enlisting wild grass genes to combat nitrification in wheat farming: A nature-based solution

Globally, wheat farming is a major source of nitrogen pollution. Rapid generation of soil nitrates cause nitrogen leakage and damage ecosystems and human health. Here, we show the 3Ns^bS chromosome arm in wild grass (Leymus racemosus) that controls root nitrification inhibitor production can be transferred into elite wheat cultivars, without disrupting the elite agronomic features. Biological nitrification inhibition (BNI)–enabled wheats can improve soil ammonium levels by slowing down its oxidation and generate significant synergistic benefits from assimilating dual nitrogen forms and improving adaptation to low N systems. Deploying BNI-enabled wheat on a significant proportion of current global wheat area (ca. 225 M ha) could be a powerful nature-based solution for reducing N fertilizer use and nitrogen losses while maintaining productivity.

Active nitrifiers and rapid nitrification are major contributing factors to nitrogen losses in global wheat production. Suppressing nitrifier activity is an effective strategy to limit N losses from agriculture. Production and release of nitrification inhibitors from plant roots is termed “biological nitrification inhibition” (BNI). Here, we report the discovery of a chromosome region that controls BNI production in “wheat grass” Leymus racemosus (Lam.) Tzvelev, located on the short arm of the “Lr#3Ns^b” (Lr#n), which can be transferred to wheat as T3BL.3Ns^bS (denoted Lr#n-SA), where 3BS arm of chromosome 3B of wheat was replaced by 3Ns^bS of L. racemosus. We successfully introduced T3BL.3Ns^bS into the wheat cultivar “Chinese Spring” (CS-Lr#n-SA, referred to as “BNI-CS”), which resulted in the doubling of its BNI capacity. T3BL.3Ns^bS from BNI-CS was then transferred to several elite high-yielding hexaploid wheat cultivars, leading to near doubling of BNI production in “BNI-MUNAL” and “BNI-ROELFS.” Laboratory incubation studies with root-zone soil from field-grown BNI-MUNAL confirmed BNI trait expression, evident from suppression of soil nitrifier activity, reduced nitrification potential, and N₂O emissions. Changes in N metabolism included reductions in both leaf nitrate, nitrate reductase activity, and enhanced glutamine synthetase activity, indicating a shift toward ammonium nutrition. Nitrogen uptake from soil organic matter mineralization improved under low N conditions. Biomass production, grain yields, and N uptake were significantly higher in BNI-MUNAL across N treatments. Grain protein levels and breadmaking attributes were not negatively impacted. Wide use of BNI functions in wheat breeding may combat nitrification in high N input–intensive farming but also can improve adaptation to low N input marginal areas.

Competition and community succession link N transformation and greenhouse gas emissions in urine patches

Nitrous oxide (N₂O) is a strong greenhouse gas produced by biotic/abiotic processes directly linked to both fungal and prokaryotic communities that produce, consume or create conditions leading to its emission. In soils exposed to nitrogen (N) in the form of urea, an ecological succession is triggered resulting in a dynamic turnover of microbial populations. However, knowledge of the mechanisms controlling this succession and the repercussions for N₂O emissions remain incomplete. Here, we monitored N₂O production and fungal/prokaryotic community changes (via 16S and 18S amplicon sequencing) in soil microcosms exposed to urea. Contributions of microbes to emissions were determined using biological inhibitors. Results confirmed that urea leads to shifts in microbial community assemblages by selecting for certain microbial groups (fast growers) as dictated through life history strategies. Urea reduced overall community diversity by conferring dominance to specific groups at different stages in the succession. The diversity lost under urea was recovered with inhibitor addition through the removal of groups that were actively growing under urea indicating that species replacement is mediated in part by competition. Results also identified fungi as significant contributors to N₂O emissions, and demonstrate that dominant fungal populations are consistently replaced at different stages of the succession. These successions were affected by addition of inhibitors which resulted in strong decreases in N₂O emissions, suggesting that fungal contributions to N₂O emissions are larger than that of prokaryotes.

Urea-functionalized amorphous calcium phosphate nanofertilizers: optimizing the synthetic strategy towards environmental sustainability and manufacturing costs

Nanosized fertilizers are the new frontier of nanotechnology towards a sustainable agriculture. Here, an efficient N-nanofertilizer is obtained by post-synthetic modification (PSM) of nitrate-doped amorphous calcium phosphate (ACP) nanoparticles (NPs) with urea. The unwasteful PSM protocol leads to N-payloads as large as 8.1 w/w%, is well replicated by using inexpensive technical-grade reagents for cost-effective up-scaling and moderately favours urea release slowdown. Using the PSM approach, the N amount is ca. 3 times larger than that obtained in an equivalent one-pot synthesis where urea and nitrate are jointly added during the NPs preparation. In vivo tests on cucumber plants in hydroponic conditions show that N-doped ACP NPs, with half absolute N-content than in conventional urea treatment, promote the formation of an equivalent amount of root and shoot biomass, without nitrogen depletion. The high nitrogen use efficiency (up to 69%) and a cost-effective preparation method support the sustainable real usage of N-doped ACP as a nanofertilizer.

Tracking denitrification in green stormwater infrastructure with dual nitrate stable isotopes

Strategies to mitigate watershed nitrogen export are critical in managing water resources. Green infrastructure (GI) has shown the ability to remove nitrogen from stormwater, but the removal mechanism is unclear. Denitrification removes nitrate from water permanently, making it the most desirable removal mechanism. The year-round field performance of a roadside infiltration GI practice (bioretention) in Northern Virginia was monitored to investigate the transport of nitrogen and the occurrence and contribution of denitrification. Stormwater runoff volumes, nitrogen concentrations, and nitrate isotope ratios (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were measured at the inlet and outlet of the bioretention during 24 storm events over 14 months. Nitrate concentration reductions (inlet vs. outlet) displayed seasonal trends, with higher reductions happening during warmer events and lower reductions or increases occurring during colder events. Cumulative bioretention nitrate and total dissolved nitrogen load reductions were 73% and 70%, respectively. Two out of 24 monitored events displayed denitrification isotope trends, indicating that although bioretention has denitrification potential, it is infrequent and other nitrogen removal mechanisms (i.e. infiltration and plant uptake) are primarily responsible for nitrogen surface effluent reductions. Only approximately 1.4% of the total reduced nitrate surface effluent load over the monitoring period was attributable to denitrification. Denitrification occurred during two of the largest monitored events, suggesting increased hydraulic retention time (HRT) promotes denitrification. Future GI designs should consider increasing HRT to encourage the important ecosystem service denitrification provides.

Assessment of Indirect N2O Emission Factors from Agricultural River Networks Based on Long-Term Study at High Temporal Resolution

Assessment of indirect emission factors (EF_5r) of nitrous oxide (N₂O) from agricultural river networks remains challenging, and results are uncertain due to limited data availability. This study compared two methods of assessing EF_5r using data from long-term observations at high temporal resolution in a typical agricultural catchment in subtropical central China. The concentration method (method 1) and the Intergovernmental Panel on Climate Change (IPCC) 2006 method (method 2) were employed to evaluate the emission factor. EF_5r estimated using method 1 (i.e., EF_5r1) was 0.00077 ± 0.00025 (0.00038-0.00097). EF_5r calculated using method 2 (i.e., EF_5r2) was lower than EF_5r1, with a mean value of 0.00004 (0.000015-0.00012). Both EF_5r1 and EF_5r2 were significantly lower than the IPCC 2006 default value of 0.0025, suggesting that N₂O emissions from China and world river networks may be grossly overestimated. A complex N₂O production pathway and diffusion mechanism were responsible for the transfer of N₂O from the sediment to river water and then to the atmosphere. These findings provide essential data for refining national greenhouse gas inventories and contribute evidence for downward revision of indirect emission factors adopted by the IPCC.

Cobalt-dependent inhibition of nitrite oxidation in Nitrobacter winogradskyi

Nitrobacter winogradskyi is an abundant, intensively studied autotrophic nitrite-oxidizing bacterium, which is frequently used as a model strain in the two-step nitrification of ammonia (NH₃) to nitrate (NO₃^-) via nitrite (NO₂^-), either in activated sludge, agricultural field studies or more recently in artificial microbial consortia for organic hydroponics. We observed a hitherto unknown cobalt ion-dependent inhibition of cell growth and NO₂^- oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca²⁺ and Mg²⁺ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. l-Histidine effectively restored cell growth and NO₂^- oxidation activity of N. winogradskyi in mineral media containing Co²⁺ with >90% recovery. Our results suggest that Co²⁺ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation.

Tillage and Nitrogen Source Impacts on Relationships between Nitrous Oxide Emission and Nitrogen Recovery Efficiency in Corn

Quantitative understanding of relationships between NO emission and plant N uptake are needed to select environmentally optimal management systems for corn ( L.) production. Studies were conducted from 2014 to 2016 in Indiana to assess long-term tillage and N source effects on NO emission, and in 2015 and 2016 on relationships between NO losses and N recovery efficiency (NRE) and N use efficiency (NUE), in a continuous corn system. Tillage treatments (mostly in place since 1975) consisted of no till (NT), strip till (ST), chisel plow (CP), and moldboard plow (MP), whereas the N source comparison involved sidedress urea ammonium nitrate applied at 220 kg N ha with and without nitrapyrin. Grain yield averaged 6.5% greater for MP than for CP and NT in the 3-yr period. Nitrapyrin never increased grain yield or NRE but reduced cumulative seasonal NO emission in 1 yr. Tillage affected NO emission in 2 of 3 yr, when emissions decreased in the order MP > CP > ST > NT. Significant negative linear relationships existed between NO emission and NRE under NT and ST, and between NO and NUE under ST, but not for CP and MP. Overall, NO losses under ST and NT decreased by 17 and 13 g N ha, respectively, per unit increase of NRE, and by 63 g N ha per unit increase of NUE under ST. Our results confirmed that selected management systems such as NT or ST that improved NRE and/or NUE can potentially reduce NO emissions during continuous corn production.

Changes in bacterial community structure and antibiotic resistance genes in soil in the vicinity of a pharmaceutical factory

China is the largest global producer of antibiotics. With the demand for antibiotics increasing every year, it is necessary to assess potential environmental risks and the spread of antibiotic resistance genes (ARGs) associated with antibiotic production. Here, we investigated the occurrence and distribution of ARGs in soil in the vicinity of a pharmaceutical factory. The results showed that antibiotic concentrations were under the detection limit; however, ARGs were present in soil and tended to be enriched near the factory. A significant correlation between the relative abundance of intI-1 and tetracycline ARGs implied that horizontal gene transfer might play an important role in the spread of ARGs. The occurrence of these ARGs could be the results of previous antibiotic contamination. However, the soil bacterial community structure seemed to be more affected by nutrients or other factors than by antibiotics. Overall, this study supports the viewpoint that long-term pharmaceutical activity might have a negative effect on environmental health, thus, underscoring the need to regulate antibiotic production and management.

Influence of the Potential Carbon Sources for Field Denitrification Beds on Their Microbial Diversity and the Fate of Carbon and Nitrate

Nitrogen based eutrophication of ecosystems is a global problem that gains momentum through a growing global population. The water quality of nitrate or ammonium contaminated rivers and streams cannot always be amended in centralized waste water treatment plants. Field denitrification plants were suggested as a solution for a decentralized reduction of nitrate to dinitrogen. Here, stable and cheap organic carbon sources serve as carbon and electron source for a microbial community. Still, our knowledge on the impact of these organic carbon sources on the development and diversity of these cultures is sparse. Moreover, the stability of these denitrification plants at different nitrate loading rates especially in the higher concentration regime were not tested so far. In this study, we compare the fate of carbon and nitrogen as well as the microbial community of wood pellet (WP) (pressed sawdust), wheat straw, and wood chips (WC) based laboratory denitrification reactors. Our study reveals that the diversity and composition of the community is strongly dependent on the carbon source. The diversity decreased in the order WC, wheat straw, and WPs. The three reactor types were characterized by different nitrate reduction kinetics and were affected differently by high nitrate loading rates. While the nitrate reduction kinetics were negatively influenced by higher nitrate doses in the wheat straw reactors, WPs as carbon source sustained the opposite trend and WC lead to an overall slower but concentration independent nitrate reduction rate. Counterintuitively, the concentration of soluble organic carbon was highest in the WP reactors but methane emission was not detectable. This is corroborated by the microbial diversity data in which methanogenic species were highly underrepresented compared to the other two reactor types. In contrary, the methane emissions in the wheat straw and WC reactors were comparable to each other.

Impacts of sugarcane agriculture expansion over low-intensity cattle ranch pasture in Brazil on greenhouse gases

Sugarcane is a widespread bioenergy crop in tropical regions, and the growing global demand for renewable energy in recent years has led to a dramatic expansion and intensification of sugarcane agriculture in Brazil. Currently, extensive areas of low-intensity pasture are being converted to sugarcane, while management in the remaining pasture is becoming more intensive, i.e., includes tilling and fertilizer use. In this study, we assessed how such changes in land use and management practices alter emissions of greenhouse gases (GHG) such as CO₂, N₂O and CH₄ by measuring in situ fluxes for one year after conversion from low-intensity pasture to conventional sugarcane agriculture and management-intensive pasture. Results show that CO₂ and N₂O fluxes increased significantly in pasture and sugarcane with tillage, fertilizer use, or both combined. Emissions were highly variable for all GHGs, yet, cumulatively, it was clear that annual emissions in CO₂-equivalent (CO₂-eq) were higher in management-intense pasture and sugarcane than in unmanaged pasture. Surprisingly, tilled pasture with fertilizer (management-intensive pasture) resulted in higher CO₂-eq emissions than conventional sugarcane. We concluded that intensification of pasture management and the conversion of pasture to sugarcane can increase the emission factor (EF) estimated for sugarcane produced in Brazil. The role of management practices and environmental conditions and the potential for reducing emissions are discussed.

Leachate treatment in landfills is a significant N2O source

The importance of methane (CH₄) emissions from landfills has been extensively documented, while the nitrous oxide (N₂O) emissions from landfills are considered negligible. In this study, three landfills were selected to measure CH₄ and N₂O emissions using the static chamber method. Dongbu (DB) and Dongfu (DF) landfills, both located in Xiamen city, Fujian Province, were classified as sanitary. The former started to receive solid waste from Xiamen city in 2009, and the latter was closed in 2009. Nanjing (NJ) landfill, located in Nanjing county, Fujian Province, was classified as managed. Results showed that for the landfill reservoirs, CH₄ emissions were significant, while N₂O emissions occurred mainly in operating areas (on average, 16.3 and 19.0mgN₂Om^-2h^-1 for DB and NJ landfills, respectively) and made a negligible contribution to the total greenhouse gas emissions in term of CO₂ equivalent. However, significant N₂O emissions were observed in the leachate treatment systems of sanitary landfills and contributed 72.8% and 45.6% of total emissions in term of CO₂ equivalent in DB and DF landfills, respectively. The N₂O emission factor (EF) of the leachate treatment systems was in the range of 8.9-11.9% of the removed nitrogen. The total N₂O emissions from the leachate treatment systems of landfills in Xiamen city were estimated to be as high as 8.55gN₂O-Ncapita^-1yr^-1. These results indicated that N₂O emissions from leachate treatment systems of sanitary landfills were not negligible and should be included in national and/or local inventories of greenhouse gas emissions.

Microbial volatile compounds alter the soil microbial community

Volatile organic compounds (VOCs) from soil bacteria are likely to have an important role in the interactions among soil microorganisms. However, their effects on the soil microbial community have not been extensively studied. In this study, the effect of bacterial VOCs generated by growing Bacillus amyloliquefaciens NJN-6 on modified MS medium on soil microbial community was evaluated. B. amyloliquefaciens NJN-6 was able to produce 48 volatile compounds as determined by solid-phase microextraction-GC/MS. MiSeq sequencing data showed that bacterial VOCs could alter the composition of both soil bacterial and soil fungal communities and could decrease the alpha-diversity of the soil microbial community. Taxonomic analysis revealed that bacterial VOCs significantly increased the relative abundance of Proteobacteria, Bacteroidetes, and Firmicutes. Moreover, bacterial VOCs significantly increased the relative abundance of Ascomycota. The qPCR data showed that bacterial VOCs of strain NJN-6 decreased the soil fungal biomass and increased the soil bacterial biomass. Further evaluation of the effect of bacterial VOCs on functional genes revealed that VOCs could reduce the copies of nifH, nirS, and a gene encoding nonribosomal peptide synthase, while increasing the copy number of the ammonium-oxidizing bacteria gene. The effect on gene encoding polyketide synthase was insignificant. Results from this study indicated that bacterial VOCs could influence the soil microbial community as well as functional gene abundance.

Hydrogeological Controls on Regional-Scale Indirect Nitrous Oxide Emission Factors for Rivers

Indirect nitrous oxide (N₂O) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF_5r) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF_5r estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, N₂O and nitrate (NO₃^-) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median N₂O-N concentration (3.0 μg L^-1), EF_5r (0.00036), and N₂O-N flux (10.8 kg ha^-1 a^-1). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median N₂O-N concentration (0.8 μg L^-1), EF_5r (0.00016), and N₂O-N flux (2.6 kg ha^-1 a^-1), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high N₂O yield from high permeability chalk contrasting with significantly lower median N₂O-N concentration (0.7 μg L^-1), EF_5r (0.00020), and N₂O-N flux (2.0 kg ha^-1 a^-1) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF_5r can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale N₂O emission estimates.

Effects of Positively Charged Dicyandiamide and Nitrogen Fertilizer Sources on Nitrous Oxide Emissions in Irrigated Corn

Synthetic nitrogen (N) fertilizer formulations vary in their effects as substrates on nitrous oxide (NO) emissions. Mitigation of NO emissions can potentially be achieved through appropriate choice of N fertilizer sources combined with stabilizers. The effects of three N fertilizers and urease and nitrification inhibitors on NO emissions, crop N uptake, and yields were determined in a furrow-irrigated corn ( L.) system in Reiff loam soil in the Sacramento Valley of California for one growing season. Aqua ammonia (Aq. NH), urea ammonium nitrate (UAN), and calcium nitrate were sidedressed at the rate of 202 kg N ha. The control treatment received only starter fertilizer (20 kg N ha). Total seasonal emissions were in the order Aq. NH > UAN > calcium nitrate = control with 1.38, 0.97, 0.35, and 0.27 kg NO-N ha, respectively. A novel, positively charged form of dicyandiamide, KAS-771G77 (G77), was combined with Aq. NH and UAN to test the effectiveness of this nitrification inhibitor in reducing NO emissions. When combined with Aq. NH, G77 did not reduce the emissions, but G77 significantly lowered them in the UAN treatment. A similar reduction of NO emissions in the UAN treatment was achieved with the urease and nitrification inhibitor AgrotainPlus. Yields and N use efficiency did not differ among the fertilized treatments. Ammoniacal fertilizers had higher NO emissions than nitrate-based fertilizers, which could imply nitrification pathways as a source of NO emissions. The use of G77 or AgrotainPlus, when applied with UAN, was an effective NO mitigation practice.

Coordinated nitrogen and carbon remobilization for nitrate assimilation in leaf, sheath and root and associated cytokinin signals during early regrowth of Lolium perenne

Background and Aims

The efficiency of N assimilation in response to defoliation is a critical component of plant regrowth and forage production. The aim of this research was to test the effect of the internal C/N balance on NO3- assimilation and to estimate the associated cytokinin signals following defoliation of perennial ryegrass ( Lolium perenne L. 'Grasslands Nui') plants.

Methods

Plants, manipulated to have contrasting internal N content and contrasting availability of water soluble carbohydrates (WSCs), were obtained by exposure to either continuous light or short days (8:16 h light-dark), and watered with modified N-free Hoagland medium containing either high (5 m m ) or low (50 μ m ) NO3- as sole N source. Half of the plants were defoliated and the root, sheath and leaf tissue were harvested at 8, 24 and 168 h after cutting. The spatiotemporal changes in WSCs, synthesis of amino acids and associated cytokinin content were recorded after cutting.

Key Results

Leaf regrowth following defoliation involved changes in the low- and high-molecular weight WSCs. The extent of the changes and the partitioning of the WSC following defoliation were dependant on the initial WSC levels and the C and N availability. Cytokinin levels varied in the sheath and root as early as 8 h following defoliation and preceded an overall increase in amino acids at 24 h. Subsequently, negative feedback brought the amino acid response back towards pre-defoliation levels within 168 h after cutting, a response that was under control of the C/N ratio.

Conclusions

WSC remobilization in the leaf is coordinated with N availability to the root, potentially via a systemic cytokinin signal, leading to efficient N assimilation in the leaf and the sheath tissues and to early leaf regrowth following defoliation.

Ammonia and Nitrous Oxide Emissions from Broiler Houses with Downtime Windrowed Litter

An emerging poultry manure management practice is in-house windrowing to disinfect the litter. However, this practice is likely to increase emissions of ammonia (NH) and nitrous oxide (NO) from the windrowed litter. The objective of this study was to quantitatively compare NH and NO emissions from broiler houses with and without in-house windrowing. Two broiler houses at a commercial farm were used to compare the NH and NO emissions. Gas emission measurements were conducted continuously and simultaneously for both the control house (without windrowing) and the house with windrowing during the same production periods. The house emission rates were calculated by multiplying the hourly mean gas concentrations and the ventilation rates. The windrowed litter temperature was significantly higher than that of the control litter. The impact of downtime (the time lapse between flocks, during which the bird house is empty) windrowing litter on pathogen reduction was inconclusive because of very low or no recovery of both and spp. from control or windrowed litter samples, respectively. The windrowing house NH emissions were 26.2 and 16.6 kg d house, whereas for the control house, they were 14.6 and 12.8 kg d house in 2012 and 2013, respectively. The NO emissions from the windrowing house were also higher than those from the control house. The total NH and NO emissions from broiler houses practicing windrowing litter management were estimated to be 35.0 and 4.43 g bird, respectively, compared with 31.9 and 3.89 g bird for the control house, respectively.

Antibacterial Compounds-Macrolactin Alters the Soil Bacterial Community and Abundance of the Gene Encoding PKS

Macrolactin produced by many soil microbes has been shown to be an efficient antibacterial agent against many bacterial pathogens. However, studies examining the effect of macrolactin on both the soil bacterial community and the intrinsic bacterial species that harbor genes responsible for the production of this antibiotic have not been conducted so far. In this study, a mixture of macrolactin was isolated from the liquid culture of Bacillus amyloliquefaciens NJN-6, and applied to the soil once a week for four weeks. 16S rRNA Illumina MiSeq sequencing showed that continuous application of macrolactin reduced the α-diversity of the soil bacterial community and thereby changed the relative abundance of microbes at both the phylum and genus level. The relative abundance of Proteobacteria and Firmicutes was significantly increased along with a significant decrease in the relative abundance of Acidobacteria. However, the application of macrolactins had an insignificant effect on the total numbers of bacteria. Further, the native gene responsible for the production of macrolactin, the gene encoding polyketide synthase was reduced in copy number after the application of macrolactin. The results of this study suggested that a bactericide from a microbial source could decrease the diversity of the soil bacterial community and change the bacterial community structure. Moreover, the populations of the intrinsic bacterial species which harbor genes responsible for macrolactin production were inhibited when the external source antibiotic was applied.

Physiological and Genotypic Characteristics of Nitrous Oxide (N2O)-Emitting Pseudomonas Species Isolated from Dent Corn Andisol Farmland in Hokkaido, Japan

Dent corn Andisol at the Hokkaido University Shizunai Livestock Experimental Farm actively emits nitrous oxide (N2O). In order to screen for culturable and active N2O emitters with high N2O emission potential, soft gel medium containing excess KNO3 was inoculated with soil suspensions from farm soil samples collected at different land managements. Dominant bacterial colonies were searched for among 20 of the actively N2O-emitting cultures from post-harvest soil and 19 from pre-tilled soil, and all isolates were subjected to the culture-based N2O emission assay. Ten active N2O-emitting bacteria, four from post-harvest soil and six from pre-tilled soil, out of 156 isolates were identified as genus Pseudomonas by 16S rRNA gene sequencing. These N2O emitters showed clear responses to NO3(-) within a neutral pH range (5.5-6.7), and accelerated N2O production with 1.5-15 mM sucrose supplementation, suggesting the production of N2O during the denitrification process. However, the negative responses of 6 active N2O emitters, 3 from post-harvest soil and 3 from pre-tilled soil, out of the 10 isolates in the acetylene-blocking assay suggest that these 6 N2O emitters are incomplete denitrifiers that have lost their N2O reductase (N2OR) activity. Although the PCR assay for the denitrification-associated genes, narG and nirK/S, was positive in all 10 Pseudomonas isolates, those negative in the acetylene-blocking assay were nosZ-negative. Therefore, these results imply that the high N2O emission potential of dent corn Andisol is partly attributed to saprophytic, nosZ gene-missing pseudomonad denitrifiers.

Effects of land use on greenhouse gas fluxes and soil properties of wetland catchments in the Prairie Pothole Region of North America

Wetland restoration has been suggested as policy goal with multiple environmental benefits including enhancement of atmospheric carbon sequestration. However, there are concerns that increased methane (CH4) emissions associated with restoration may outweigh potential benefits. A comprehensive, 4-year study of 119 wetland catchments was conducted in the Prairie Pothole Region of the north-central U.S. to assess the effects of land use on greenhouse gas (GHG) fluxes and soil properties. Results showed that the effects of land use on GHG fluxes and abiotic soil properties differed with respect to catchment zone (upland, wetland), wetland classification, geographic location, and year. Mean CH4 fluxes from the uplands were predictably low (<0.02 g CH4 m(-2) day(-1)), while wetland zone CH4 fluxes were much greater (<0.001-3.9 g CH4 m(-2) day(-1)). Mean cumulative seasonal CH4 fluxes ranged from roughly 0-650 g CH4 m(-2), with an overall mean of approximately 160 g CH4 m(-2). These maximum cumulative CH4 fluxes were nearly 3 times as high as previously reported in North America. The overall magnitude and variability of N2O fluxes from this study (<0.0001-0.0023 g N2O m(-2) day(-1)) were comparable to previously reported values. Results suggest that soil organic carbon is lost when relatively undisturbed catchments are converted for agriculture, and that when non-drained cropland catchments are restored, CH4 fluxes generally are not different than the pre-restoration baseline. Conversely, when drained cropland catchments are restored, CH4 fluxes are noticeably higher. Consequently, it is important to consider the type of wetland restoration (drained, non-drained) when assessing restoration benefits. Results also suggest that elevated N2O fluxes from cropland catchments likely would be reduced through restoration. The overall variability demonstrated by this study was consistent with findings of other wetland investigations and underscores the difficulty in quantifying the GHG balance of wetland systems.

Influences of Land Use/Cover Types on Nitrous Oxide Emissions during Freeze-Thaw Periods from Waterlogged Soils in Inner Mongolia

Nitrous oxide emissions during freeze/thaw periods contribute significantly to annual soil N₂O emissions budgets in middle- and high-latitude areas; however, the freeze/thaw-related N₂O emissions from waterlogged soils have hardly been studied in the Hulunber Grassland, Inner Mongolia. For this study, the effects of changes in land use/cover types on N₂O emissions during freeze–thaw cycles were investigated to more accurately quantify the annual N₂O emissions from grasslands. Soil cores from six sites were incubated at varying temperature (ranging from −15 to 10°C) to simulate freeze–thaw cycles. N₂O production rates were low in all soil cores during freezing periods, but increased markedly after soil thawed. Mean rates of N₂O production differed by vegetation type, and followed the sequence: Leymus chinensis (LC) and Artemisia tanacetifolia (AT) steppes > LC steppes ≥ Stipa baicalensis (SB) steppes. Land use types (mowing and grazing) had differing effects on freeze/thaw-related N₂O production. Grazing significantly reduced N₂O production by 36.8%, while mowing enhanced production. The production of N₂O was related to the rate at which grassland was mowed, in the order: triennially (M3) > once annually (M1) ≥ unmown (UM). Compared with the UM control plot, the M3 and M1 mowing regimes enhanced N₂O production by 57.9% and 13.0% respectively. The results of in situ year-round measurements showed that large amounts of N₂O were emitted during the freeze–thaw period, and that annual mean fluxes of N₂O were 9.21 μg N₂O-N m^-2 h^-1 (ungrazed steppe) and 6.54 μg N₂O-N m^-2 h^-1 (grazed steppe). Our results further the understanding of freeze/thaw events as enhancing N₂O production, and confirm that different land use/cover types should be differentiated rather than presumed to be equivalent, regarding nitrous oxide emission. Even so, further research involving multi-year and intensive measurements of N₂O emission is still needed.

Influence of edaphic and management factors on the diversity and abundance of ammonia-oxidizing thaumarchaeota and bacteria in soils of bioenergy crop cultivars

Ammonia-oxidizing thaumarcheota (AOA) and ammonia-oxidizing bacteria (AOB) differentially influence soil and atmospheric chemistry, but soil properties that control their distributions are poorly understood. In this study, the ammonia monooxygenase gene (amoA) was used to identify and quantify presumptive AOA and AOB and relate their distributions to soil properties in two experimental fields planted with different varieties of switchgrass (Panicum virgatum), a potential bioenergy feedstock. Differences in ammonia oxidizer diversity were associated primarily with soil properties of the two field sites, with pH displaying significant correlations with both AOA and AOB population structure. Percent nitrogen (%N), carbon to nitrogen ratios (C : N), and pH were also correlated with shifts nitrifier population structure. Nitrosotalea-like and Nitrosospira cluster II populations were more highly represented in acidic soils, whereas populations affiliated with Nitrososphaera and Nitrosospira cluster 3A.1 were relatively more abundant in alkaline soils. AOA were the dominant functional group in all plots based on quantitative polymerase chain reaction and high-throughput sequencing analyses. These data suggest that AOA contribute significantly to nitrification rates in carbon and nitrogen rich soils influenced by perennial grasses.

Suppression of soil nitrification by plants

Nitrification, the biological oxidation of ammonium to nitrate, weakens the soil's ability to retain N and facilitates N-losses from production agriculture through nitrate-leaching and denitrification. This process has a profound influence on what form of mineral-N is absorbed, used by plants, and retained in the soil, or lost to the environment, which in turn affects N-cycling, N-use efficiency (NUE) and ecosystem health and services. As reactive-N is often the most limiting in natural ecosystems, plants have acquired a range of mechanisms that suppress soil-nitrifier activity to limit N-losses via N-leaching and denitrification. Plants' ability to produce and release nitrification inhibitors from roots and suppress soil-nitrifier activity is termed 'biological nitrification inhibition' (BNI). With recent developments in methodology for in-situ measurement of nitrification inhibition, it is now possible to characterize BNI function in plants. This review assesses the current status of our understanding of the production and release of biological nitrification inhibitors (BNIs) and their potential in improving NUE in agriculture. A suite of genetic, soil and environmental factors regulate BNI activity in plants. BNI-function can be genetically exploited to improve the BNI-capacity of major food- and feed-crops to develop next-generation production systems with reduced nitrification and N2O emission rates to benefit both agriculture and the environment. The feasibility of such an approach is discussed based on the progresses made.

Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts

In this review we explore the biotic transformations of nitrogenous compounds that occur during denitrification, and the factors that influence denitrifier populations and enzyme activities, and hence, affect the production of nitrous oxide (N2O) and dinitrogen (N2) in soils. Characteristics of the genes related to denitrification are also presented. Denitrification is discussed with particular emphasis on nitrogen (N) inputs and dynamics within grasslands, and their impacts on the key soil variables and processes regulating denitrification and related gaseous N2O and N2 emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N2O:N2 production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N2O:N2 ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, (15)N tracer and direct N2 quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N2O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N2O:N2 product ratio and N2O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N2O:N2 product ratio during denitrification to mitigate N2O emissions. Future research needs to focus on evaluating the N2O-reducing ability of the denitrifiers to accelerate the conversion of N2O to N2 and the reduction of N2O:N2 ratio during denitrification.

A paradigm shift towards low-nitrifying production systems: the role of biological nitrification inhibition (BNI)

BACKGROUND

Agriculture is the single largest geo-engineering initiative that humans have initiated on planet Earth, largely through the introduction of unprecedented amounts of reactive nitrogen (N) into ecosystems. A major portion of this reactive N applied as fertilizer leaks into the environment in massive amounts, with cascading negative effects on ecosystem health and function. Natural ecosystems utilize many of the multiple pathways in the N cycle to regulate N flow. In contrast, the massive amounts of N currently applied to agricultural systems cycle primarily through the nitrification pathway, a single inefficient route that channels much of this reactive N into the environment. This is largely due to the rapid nitrifying soil environment of present-day agricultural systems.

SCOPE

In this Viewpoint paper, the importance of regulating nitrification as a strategy to minimize N leakage and to improve N-use efficiency (NUE) in agricultural systems is highlighted. The ability to suppress soil nitrification by the release of nitrification inhibitors from plant roots is termed 'biological nitrification inhibition' (BNI), an active plant-mediated natural function that can limit the amount of N cycling via the nitrification pathway. The development of a bioassay using luminescent Nitrosomonas to quantify nitrification inhibitory activity from roots has facilitated the characterization of BNI function. Release of BNIs from roots is a tightly regulated physiological process, with extensive genetic variability found in selected crops and pasture grasses. Here, the current status of understanding of the BNI function is reviewed using Brachiaria forage grasses, wheat and sorghum to illustrate how BNI function can be utilized for achieving low-nitrifying agricultural systems. A fundamental shift towards ammonium (NH4(+))-dominated agricultural systems could be achieved by using crops and pastures with high BNI capacities. When viewed from an agricultural and environmental perspective, the BNI function in plants could potentially have a large influence on biogeochemical cycling and closure of the N loop in crop-livestock systems.

Nitrous oxide emissions from black soils with different pH

N2O fluxes as a function of incubation time from soil with different available N contents and pH were determined. Cumulative carbon dioxide (CO2) emissions were measured to indicate soil respiration. A 144-hr incubation experiment was conducted in a slightly acidic agricultural soil (pH(H2O) 5.33) after the pH was adjusted to four different values (3.65, 5.00, 6.90 and 8.55). The experiments consisted of a control without added N, and with NH(4+)-N and NO(3-)-N fertilization. The results showed that soil pH contributed significantly to N2O flux from the soils. There were higher N2O emissions in the period 0-12 hr in the four pH treatments, especially those enhanced with N-fertilization. The cumulative N2O-N emission reached a maximum at pH 8.55 and was stimulated by NO(3-)-N fertilization (70.4 microg/kg). The minimum emissions appeared at pH 3.65 and were not stimulated by NO(3-)-N or NH(4+)-N fertilization. Soil respiration increased significantly due to N-fertilization. Soil respiration increased positively with soil pH (R2 = 0.98, P < 0.01). The lowest CO2-C emission (30.2 mg/kg) was presented in pH 3.65 soils without N-fertilization. The highest CO2-C emissions appeared in the pH 8.55 soils for NH(4+)-N fertilization (199 mg/kg). These findings suggested that N20 emissions and soil respiration were significantly influenced by low pH, which strongly inhibits soil microbial nitrification and denitrification activities. The content of NO(3-)-N in soil significantly and positively affected the N20 emissions through denitrification.

Microbial-based inoculants impact nitrous oxide emissions from an incubated soil medium containing urea fertilizers

There is currently much interest in developing crop management practices that will decrease NO emissions from agricultural soils. Many different approaches are being investigated, but to date, no studies have been published on how microbial inoculants affect NO emissions. This study was conducted to test the hypothesis that microbial-based inoculants known to promote root growth and nutrient uptake can reduce NO emissions in the presence of N fertilizers under controlled conditions. Carbon dioxide and CH fluxes were also measured to evaluate microbial respiration and determine the aerobic and anaerobic conditions of the incubated soil. The microbial-based treatments investigated were SoilBuilder (SB), a metabolite extract of SoilBuilder (SBF), and a mixture of four strains of plant growth-promoting spp. Experiments included two different N fertilizer treatments, urea and urea-NHNO 32% N (UAN), and an unfertilized control. Emissions of NO and CO were determined from soil incubations and analyzed with gas chromatography. After 29 d of incubation, cumulative NO emissions were reduced 80% by SB and 44% by SBF in soils fertilized with UAN. Treatment with spp. significantly reduced NO production on Days 1 and 2 of the incubation in soils fertilized with UAN. In the unfertilized treatment, cumulative emissions of NO were significantly reduced 92% by SBF. Microbial-based treatments did not reduce NO emissions associated with urea application. Microbial-based treatments increased CO emissions from soils fertilized with UAN, suggesting a possible increase in microbial activity. Overall, the results demonstrated that microbial-based inoculants can reduce NO emissions associated with N fertilizer application, and this response varies with the type of microbial-based inoculant and fertilizer.

Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability

The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 concentrations following urea or ammonium sulfate [(NH4)2SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by (15)N-(18)O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O2 concentrations with urea producing more N2O than (NH4)2SO4 additions. These findings challenge established thought attributing N2O emissions from soils with high water content to HD due to presumably low O2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N2O production in agricultural soils.

Relationships between nitrogen transformation rates and gene abundance in a riparian buffer soil

Denitrification is a critical biogeochemical process that results in the conversion of nitrate to volatile products, and thus is a major route of nitrogen loss from terrestrial environments. Riparian buffers are an important management tool that is widely utilized to protect water from non-point source pollution. However, riparian buffers vary in their nitrate removal effectiveness, and thus there is a need for mechanistic studies to explore nitrate dynamics in buffer soils. The objectives of this study were to examine the influence of specific types of soluble organic matter on nitrate loss and nitrous oxide production rates, and to elucidate the relationships between these rates and the abundances of functional genes in a riparian buffer soil. Continuous-flow soil column experiments were performed to investigate the effect of three types of soluble organic matter (citric acid, alginic acid, and Suwannee River dissolved organic carbon) on rates of nitrate loss and nitrous oxide production. We found that nitrate loss rates increased as citric acid concentrations increased; however, rates of nitrate loss were weakly affected or not affected by the addition of the other types of organic matter. In all experiments, rates of nitrous oxide production mirrored nitrate loss rates. In addition, quantitative polymerase chain reaction (qPCR) was utilized to quantify the number of genes known to encode enzymes that catalyze nitrite reduction (i.e., nirS and nirK) in soil that was collected at the conclusion of column experiments. Nitrate loss and nitrous oxide production rates trended with copy numbers of both nir and 16s rDNA genes. The results suggest that low-molecular mass organic species are more effective at promoting nitrogen transformations than large biopolymers or humic substances, and also help to link genetic potential to chemical reactivity.