Effects of nitrogen and biochar amendment on soil methane concentration profiles and diffusion in a rice-wheat annual rotation system

Could soil microplastic pollution exacerbate climate change? A meta-analysis of greenhouse gas emissions and global warming potential

Microplastics pollution and climate change are primarily investigated in isolation, despite their joint threat to the environment. Greenhouse gases (GHGs) are emitted during: the production of plastic and rubber, the use and degradation of plastic, and after contamination of environment. This is the first meta-analysis to assess underlying causal relationships and the influence of likely mediators. We included 60 peer-reviewed empirical studies; estimating GHGs emissions effect size and global warming potential (GWP), according to key microplastics properties and soil conditions. We investigated interrelationships with microbe functional gene expression. Overall, microplastics contamination was associated with increased GHGs emissions, with the strongest effect (60%) on CH4 emissions. Polylactic-acid caused 32% higher CO2 emissions, but only 1% of total GWP. Phenol-formaldehyde had the greatest (175%) GWP via 182% increased N2O emissions. Only polystyrene resulted in reduced GWP by 50%, due to N2O mitigation. Polyethylene caused the maximum (60%) CH4 emissions. Shapes of microplastics differed in GWP: fiber had the greatest GWP (66%) whereas beads reduced GWP by 53%. Films substantially increased emissions of all GHGs: 14% CO2, 10% N2O and 60% CH4. Larger-sized microplastics had higher GWP (125%) due to their 9% CO2 and 63% N2O emissions. GWP rose sharply if soil microplastics content exceeded 0.5%. Higher CO2 emissions, ranging from 4% to 20%, arose from soil which was either fine, saturated or had high-carbon content. Higher N2O emissions, ranging from 10% to 95%, arose from soils that had either medium texture, saturated water content or low-carbon content. Both CO2 and N2O emissions were 43%-56% higher from soils with neutral pH. We conclude that microplastics contamination can cause raised GHGs emissions, posing a risk of exacerbating climate-change. We show clear links between GHGs emissions, microplastics properties, soil characteristics and soil microbe functional gene expression. Further research is needed regarding underlying mechanisms and processes.

Nitric oxide: A core signaling molecule under elevated GHGs (CO2, CH4, N2O, O3)-mediated abiotic stress in plants

Nitric oxide (NO), an ancient molecule with multiple roles in plants, has gained momentum and continues to govern plant biosciences-related research. NO, known to be involved in diverse physiological and biological processes, is a central molecule mediating cellular redox homeostasis under abiotic and biotic stresses. NO signaling interacts with various signaling networks to govern the adaptive response mechanism towards stress tolerance. Although diverging views question the role of plants in the current greenhouse gases (GHGs) budget, it is widely accepted that plants contribute, in one way or another, to the release of GHGs (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3)) to the atmosphere, with CH4 and N2O being the most abundant, and occur simultaneously. Studies support that elevated concentrations of GHGs trigger similar signaling pathways to that observed in commonly studied abiotic stresses. In the process, NO plays a forefront role, in which the nitrogen metabolism is tightly related. Regardless of their beneficial roles in plants at a certain level of accumulation, high concentrations of CO2, CH4, and N2O-mediating stress in plants exacerbate the production of reactive oxygen (ROS) and nitrogen (RNS) species. This review assesses and discusses the current knowledge of NO signaling and its interaction with other signaling pathways, here focusing on the reported calcium (Ca2+) and hormonal signaling, under elevated GHGs along with the associated mechanisms underlying GHGs-induced stress in plants.

Methane Emission Reduction and Biological Characteristics of Landfill Cover Soil Amended With Hydrophobic Biochar

Biochar-amended landfill cover soil (BLCS) can promote CH₄ and O₂ diffusion, but it increases rainwater entry in the rainy season, which is not conducive to CH₄ emission reduction. Hydrophobic biochar–amended landfill cover soil (HLCS) was prepared to investigate the changes in CH₄ emission reduction and biological characteristics, and BLCS was prepared as control. Results showed that rainwater retention time in HLCS was reduced by half. HLCS had a higher CH₄ reduction potential, achieving 100% CH₄ removal at 25% CH₄ content of landfill gas, and its main contributors to CH₄ reduction were found to be at depths of 10–30 cm (upper layer) and 50–60 cm (lower layer). The relative abundances of methane-oxidizing bacteria (MOB) in the upper and lower layers of HLCS were 55.93% and 46.93%, respectively, higher than those of BLCS (50.80% and 31.40%, respectively). Hydrophobic biochar amended to the landfill cover soil can realize waterproofing, ventilation, MOB growth promotion, and efficient CH₄ reduction.

Metabolic potential of anaerobic methane oxidizing archaea for a broad spectrum of electron acceptors

Methane (CH₄) is a potent greenhouse gas significantly contributing to the climate warming we are currently facing. Microorganisms play an important role in the global CH₄ cycle that is controlled by the balance between anaerobic production via methanogenesis and CH₄ removal via methanotrophic oxidation. Research in recent decades advanced our understanding of CH₄ oxidation, which until 1976 was believed to be a strictly aerobic process. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction is now known to be an important sink of CH₄ in marine ecosystems. Furthermore, in 2006 it was discovered that anaerobic CH₄ oxidation can also be coupled to nitrate reduction (N-DAMO), demonstrating that AOM may be much more versatile than previously thought and linked to other electron acceptors. In consequence, an increasing number of studies in recent years showed or suggested that alternative electron acceptors can be used in the AOM process including Fe^III, Mn^IV, As^V, Cr^VI, Se^VI, Sb^V, V^V, and Br^V. In addition, humic substances as well as biochar and perchlorate (ClO₄^-) were suggested to mediate AOM. Anaerobic methanotrophic archaea, the so-called ANME archaea, are key players in the AOM process, yet we are still lacking deeper understanding of their metabolism, electron acceptor preferences and their interaction with other microbial community members. It is still not clear whether ANME archaea can oxidize CH₄ and reduce metallic electron acceptors independently or via electron transfer to syntrophic partners, interspecies electron transfer, nanowires or conductive pili. Therefore, the aim of this review is to summarize and discuss the current state of knowledge about ANME archaea, focusing on their physiology, metabolic flexibility and potential to use various electron acceptors.

Does biochar accelerate the mitigation of greenhouse gaseous emissions from agricultural soil? - A global meta-analysis

Greenhouse gaseous (GHGs) emissions from cropland soils are one of the major contributors to global warming. However, the extent and pattern of these climatic breakdowns are usally determined by the management practices in-place. The use of biochar on cropland soils holds a great promise for increasing the overall crop productivity. Nevertheless, biochar application to agricultural soils has grown in popularity as a strategy to off-set the negative feedback associated with agriculture GHGs emissions, i.e., CO₂ (carbon dioxide), CH₄ (methane), and N₂O (nitrous oxide). Despite increasing efforts to uncover the potential of biochar to mitigate the farmland GHGs effects, there has been little synthesis of how different types of biochar affect GHGs fluxes from cropland soils under varied experimental conditions. Here, we presented a meta-analysis of the interactions between biochar and GHGs emissions across global cropland soils, with field experiments showing the strongest GHG mitigation potential, i.e. CO₂ (RR = -0.108) and CH₄ (RR = -0.399). The biochar pyrolysis temperature, feedstock, C: N ratio, and pH were also found to be important factors influencing GHGs emissions. A prominent reduction in N₂O (RR = -0.13) and CH₄ (RR = -1.035) emissions was observed in neutral soils (pH = 6.6-7.3), whereas acidic soils (pH ≤ 6.5) accounted for the strongest mitigation effect on CO₂ compared to N₂O and CH₄ emissions. We also found that a biochar application rate of 30 t ha^-1 was best for mitigating GHGs emissions while achieving optimal crop yield. According to our meta-analysis, maize crop receiving biochar amendment showed a significant mitigation potential for CO₂, N₂O, and CH₄ emissions. On the other hand, the use of biochar had shown significant impact on the global warming potential (GWP) of total GHGs emissions. The current data synthesis takes the lead in analyzing emissions status and mitigation potential for three of the most common GHGs from cropland soils and demonstrates that biochar application can significantly reduce the emissions budget from agriculture.

Effects of nitrogen-enriched biochar on rice growth and yield, iron dynamics, and soil carbon storage and emissions: A tool to improve sustainable rice cultivation

Biochar is often applied to paddy soils as a soil improver, as it retains nutrients and increases C sequestration; as such, it is a tool in the move towards C-neutral agriculture. Nitrogen (N) fertilizers have been excessively applied to rice paddies, particularly in small farms in China, because N is the major limiting factor for rice production. In paddy soils, dynamic changes in iron (Fe) continuously affect soil emissions of methane (CH₄) and carbon dioxide (CO₂); however, the links between Fe dynamics and greenhouse gas emissions, dissolved organic carbon (DOC), and rice yields following application of biochar remain unclear. The aims of this study were to examine the effects of two rates of nitrogen (N)-enriched biochar (4 and 8 t ha^-1 y^-1) on paddy soil C emissions and storage, rice yields, and Fe dynamics in subtropical early and late rice growing seasons. Field application of N-enriched biochar at 4 and 8 t ha^-1 increased C emissions in early and late rice, whereas application at 4 t ha^-1 significantly increased rice yields. The results of a culture experiment and a field experiment showed that the application of N-enriched biochar increased soil Fe²⁺concentration. There were positive correlations between Fe²⁺concentrations and soil CO₂, CH₄, and total C emissions, and with soil DOC concentrations. On the other way around, these correlations were negative for soil Fe³⁺concentrations. In the soil culture experiment, under the exclusion of plant growth, N-enriched biochar reduced cumulative soil emissions of CH₄ and CO₂. We conclude that moderate inputs of N-rich biochar (4 t ha^-1) increase rice crop yield and biomass, and soil DOC concentrations, while moderating soil cumulative C emissions, in part, by the impacts of biochar on soil Fe dynamics. We suggest that water management strategies, such as dry-wet cycles, should be employed in rice cultivation to increase Fe²⁺ oxidation for the inhibition of soil CH₄ and CO₂ production. Overall, we showed that application of 4 t ha^-1 of N-enriched biochar may represent a potential tool to improve sustainable food production and security, while minimizing negative environmental impacts.

Coconut husk biochar amendment enhances nutrient retention by suppressing nitrification in agricultural soil following anaerobic digestate application

Anaerobic digestate and biochar are by-products of the biogasification and pyrolysis of agricultural wastes. This study tested the hypothesis that combined application of anaerobic pig/cattle manure digestate and coconut husk (CH) biochar can improve soil nutrient conditions, whilst minimizing atmospheric and groundwater pollution risks. Microcosms simulated digestate application to agricultural soil with and without CH biochar. Ammonia volatilization and nutrient leaching were quantified after simulated heavy rainfalls. Archaeal and bacterial community and abundance changes in soils were quantified via next generation sequencing and qPCR of 16S rRNA genes. Nitrifying bacteria were additionally quantified by qPCR of functional genes. It was found that CH biochar retarded nitrate leaching via slower nitrification in digestate-amended soil. CH biochar reduced both nitrifying archaea and bacteria abundance in soil by 71-83 percent in the top 4 cm soil layer and 66-80 percent in the deeper soil layer one month after the digestate application. Methanotroph abundances were similarly reduced in the CH biochar amended soils. These findings demonstrate combined benefits of anaerobic digestate and CH biochar application which are relevant for the development of a more circular rural economy with waste minimization, renewable energy production, nutrient recycling and reduced water pollution from agricultural land.

Biogeochemical transformation of greenhouse gas emissions from terrestrial to atmospheric environment and potential feedback to climate forcing

Carbon dioxide (CO₂) is mainly universal greenhouse gas associated with climate change. However, beyond CO₂, some other greenhouse gases (GHGs) like methane (CH₄) and nitrous oxide (N₂O), being two notable gases, contribute to global warming. Since 1900, the concentrations of CO₂ and non-CO₂ GHG emissions have been elevating, and due to the effects of the previous industrial revolution which is responsible for climate forcing. Globally, emissions of CO₂, CH₄, and N₂O from agricultural sectors are increasing as around 1% annually. Moreover, deforestation also contributes 12-17% of total global GHGs. Perhaps, the average temperature is likely to increase globally, at least 2 °C by 2100-by mid-century. These circumstances are responsible for climate forcing, which is the source of various human health diseases and environmental risks. From agricultural soils, rhizospheric microbial communities have a significant role in the emissions of greenhouse gases. Every year, microbial communities release approximately 1.5-3 billion tons of carbon into the atmospheric environment. Microbial nitrification, denitrification, and respiration are the essential processes that affect the nitrogen cycle in the terrestrial environment. In the twenty-first century, climate change is the major threat faced by human beings. Climate change adversely influences human health to cause numerous diseases due to their direct association with climate change. This review highlights the different anthropogenic GHG emission sources, the response of microbial communities to climate change, climate forcing potential, and mitigation strategies through different agricultural management approaches and microbial communities.

Biochar can mitigate methane emissions by improving methanotrophs for prolonged period in fertilized paddy soils

Biochar application to fertilized paddy soils has been recommended as an effective countermeasure to mitigate methane (CH₄) emissions, but its mechanism and effective duration has not yet been adequately elucidated. A laboratory incubation experiment was performed to gain insight into the combined effects of fresh and six-year aged biochar on potential methane oxidation (PMO) in paddy soils with ammonium or nitrate-amendment. Results showed that both ammonium and nitrate were essential for CH₄ oxidation though high ammonium (4 mM) inhibited PMO as compared to low ammonium (1 mM and 2 mM), and that nitrate was better in promoting PMO than ammonium. Moreover, ammonium-amendment promoted type I pmoA, and nitrate-amendment enhanced type II pmoA abundance. Both fresh and aged biochar increased PMO as well as nitrification by enhancing the total, type I and type II methanotrophs as compared to the control. Increased soil PMO with mineral N input in both six-year aged biochar and fresh biochar amendment, indicating that biochar mitigated CH₄ by promoting PMO for prolonged period in fertilized paddy soils.

Biochar-Mediated Anaerobic Oxidation of Methane

Biochar was recently identified as an effective soil amendment for CH₄ capture. Corresponding mechanisms are currently recognized to be from physical properties of biochar, providing a favorable growth environment for aerobic methanotrophs which perform aerobic methane (CH₄) oxidation. However, our study shows that the chemical reactivity of biochar can also stimulate anaerobic oxidation of CH₄ (AOM) by anaerobic methanotrophic archaea (ANME) of ANME-2d, which proposes another plausible mechanism for CH₄ mitigation by biochar amendment in anaerobic environments. It was found that, by adding biochar as the sole electron acceptor in an anaerobic environment, CH₄ was biologically oxidized, with CO₂ production of 106.3 ± 5.1 μmol g^-1 biochar. In contrast, limited CO₂ production was observed with chemically reduced biochar amendment. This biological nature of the process was confirmed by mcr gene transcript abundance as well as sustained dominance of ANME-2d in the microbial community during microbial incubations with active biochar amendment. Combined FTIR and XPS analyses demonstrated that the redox activity of biochar is related to its oxygen-based functional groups. On the basis of microbial community evolution as well as intermediate production during incubation, different pathways in terms of direct or indirect interactions between ANME-2d and biochar were proposed for biochar-mediated AOM.

Pasture enclosures increase soil carbon dioxide flux rate in Semiarid Rangeland, Kenya

BACKGROUND

Pasture enclosures play an important role in rehabilitating the degraded soils and vegetation, and may also influence the emission of key greenhouse gasses (GHGs) from the soil. However, no study in East Africa and in Kenya has conducted direct measurements of GHG fluxes following the restoration of degraded communal grazing lands through the establishment of pasture enclosures. A field experiment was conducted in northwestern Kenya to measure the emission of CO₂, CH₄ and N₂O from soil under two pasture restoration systems; grazing dominated enclosure (GDE) and contractual grazing enclosure (CGE), and in the adjacent open grazing rangeland (OGR) as control. Herbaceous vegetation cover, biomass production, and surface (0-10 cm) soil organic carbon (SOC) were also assessed to determine their relationship with the GHG flux rate.

RESULTS

Vegetation cover was higher enclosure systems and ranged from 20.7% in OGR to 40.2% in GDE while aboveground biomass increased from 72.0 kg DM ha^-1 in OGR to 483.1 and 560.4 kg DM ha^-1 in CGE and GDE respectively. The SOC concentration in GDE and CGE increased by an average of 27% relative to OGR and ranged between 4.4 g kg^-1 and 6.6 g kg^-1. The mean emission rates across the grazing systems were 18.6 μg N m^-2 h^-1, 50.1 μg C m^-2 h^-1 and 199.7 mg C m^-2 h^-1 for N₂O, CH₄, and CO₂, respectively. Soil CO₂ emission was considerably higher in GDE and CGE systems than in OGR (P < 0.001). However, non-significantly higher CH₄ and N₂O emissions were observed in GDE and CGE compared to OGR (P = 0.33 and 0.53 for CH₄ and N₂O, respectively). Soil moisture exhibited a significant positive relationship with CO₂, CH₄, and N₂O, implying that it is the key factor influencing the flux rate of GHGs in the area.

CONCLUSIONS

The results demonstrated that the establishment of enclosures in tropical rangelands is a valuable intervention for improving pasture production and restoration of surface soil properties. However, a long-term study is required to evaluate the patterns in annual CO₂, N₂O, CH₄ fluxes from soils and determine the ecosystem carbon balance across the pastoral landscape.

Field-aged biochar stimulated N2O production from greenhouse vegetable production soils by nitrification and denitrification

Evidence suggests that biochar is among ideal strategies for climate change mitigation and sustainable agriculture. However, the effects of soil aging on the physicochemical characteristics of biochar and nitrous oxide (N₂O) production remain elusive. We set up a microcosm experiment with two greenhouse vegetable production (GVP) (alkaline and acid) soils by using the ¹⁵N tracing technique and quantitative polymerase chain reaction (qPCR) to investigate the mechanisms of N₂O production as affected by fresh (FB) and aged biochar (AB) amendment. The results showed that AB increased the specific surface area, organic C, ammonium sorption capacity and cation exchange capacity, whereas decreased the pore size and pH relative to the FB. Results also demonstrated that FB effectively decreased N₂O emissions from both soils while it enhanced the abundance of nirK and nosZI genes in alkaline soil and reduced the abundance of ammonia-oxidizing bacteria (AOB) amoA and increased nirK and nosZII genes in acid soil. In contrast, AB significantly stimulated nitrification and denitrification in both soils and thus significantly increased the N₂O emissions by 43-78%. Furthermore, AB induced increases in ammonia-oxidizing archaeal (AOA) amoA and nirK gene abundances in alkaline soil and fungal nirK gene abundances in acid soil. These results suggest that AB may not be suitable for the mitigation of soil N₂O emissions in GVP soils thus improving our understanding of the potential mechanism of biochar in N₂O emissions.

Microbial explanations for field-aged biochar mitigating greenhouse gas emissions during a rice-growing season

Knowledge about the impacts of fresh and field-aged biochar amendments on greenhouse gas (CH₄, N₂O) emissions is limited. A field experiment was initiated in 2012 to study the effects of fresh and field-aged biochar additions on CH₄ and N₂O emissions and the associated microbial activity during the entire rice-growing season in typical rice-wheat rotation system in Southeast China. CH₄ and N₂O fluxes were monitored, and the abundance of methanogen (mcrA), methanotrophy (pmoA), ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite reductase (nirS, nirK), N₂O reductase (nosZ), and potential soil enzyme activities related to CH₄ and N₂O were simultaneously measured throughout different rice developmental stages. There were three treatments: control (urea without biochar), fresh BC (urea with fresh biochar added in 2015), and aged BC (urea with 3-year field-aged biochar added in 2012). Results showed that field-aged biochar significantly decreased seasonal CH₄ emissions by 16.8% in relation to the fresh biochar, though no significant differences were detected between biochars and control treatment. The structural equation model indicated that soil pH, microbial biomass carbon (MBC), pmoA, and mcrA were the main factors directly influenced by fresh and aged biochar amendments; aged biochar showed a negative effect while fresh biochar showed positive effects on CH₄ fluxes. Both fresh and field-aged biochar obviously increased AOA and AOB abundances and reduced the (nirS+nirK)/nosZ ratio during the entire rice-growing season, although no significant effects were observed on seasonal N₂O emissions. Therefore, biochar amendment produced long-term effects on total CH₄ and N₂O emissions through observed influences of soil pH and functional gene abundance. The figure shows how fresh and field-aged biochar differentially affected CH4 production and oxidation and N2O production and reduction through related functional gene abundances. Blue arrows indicate suppressing while pink arrows indicate promoting effect.

Nitrogen Fertilizer Management in Dryland Wheat Cropping Systems

Wheat is the most widely cultivated food crop in the world, which provides nutrition to most of the world population and is well adapted to a wide range of environmental conditions. Timely and efficient rates of nitrogen (N) application are vital for increasing wheat grain yield and protein content, and maintaining environmental sustainability. The goal of this study was to investigate the effect of using different rates and split application of N on the performance of spring wheat in dryland cropping systems. The experiment was conducted in three different locations in Montana and Idaho during two consecutive growing seasons. A split-plot experimental design was used with three at planting N fertilization application (0, 90 and 135 kg N ha-1) and two topdressing N fertilization strategies as treatments. A number of variables such as grain yield (GY), protein content (GP) in the grains and N uptake (NUP) were assessed. There was a significant effect of climate, N rate, and time application on the wheat performance. The results showed that at-planting N fertilizer application of 90 kg N ha-1 has significantly increased GY, GP and NUP. On the other hand, for these site-years, increasing at-planting N fertilizer rate to 135 kg N ha-1 did not further enhance wheat GY, GP and NUP values. For all six site-years, topdress N fertilizer applied at flowering did not improve wheat GY, GP and NUP compared to at-planting fertilizer alone. As the risk of yield loss is minimal with split N application, from these results we concluded the best treatment for study is treatments that had received 90 kg N ha-1 split as 45 kg N ha-1 at planting and 45 kg N ha-1 at flowering.

Variations in nutrient and trace element composition of rice in an organic rice-frog coculture system

Introducing frogs into paddy fields can control pests and diseases, and organic farming can improve soil fertility and rice growth. The aim of this 2-year field study was compare the yield and elemental composition of rice between an organic farming system including frogs (ORF) and a conventional rice culture system (CR). The grain yields were almost the same in the ORF system and the CR system. The ORF significantly increased the contents of phosphorus (P), ion (Fe), zinc (Zn), molybdenum (Mo) and selenium (Se) in rice grain at one or both years. However, the ORF system decreased the calcium (Ca) content in grice grains, and increased the concentration of cadmium, which is potentially toxic. A principal components analysis showed the main impacts of ORF agro-ecosystem on the rice grain ionome was to increase the concentration of P and trace metal(loid)s. The results showed that the ORF system is an ecologically, friendly strategy to avoid excessive use of chemical fertilizers, herbicides and pesticides without decreasing yields, and to improve the nutritional status of rice by increasing the micronutrient contents. The potential risks of increasing Cd contents in rice grain should be addressed if this cultivation pattern is used in the long term.