Biochar mitigates the N2O emissions from acidic soil by increasing the nosZ and nirK gene abundance and soil pH

Effects of biochar on the transformation and utilization of nitrogen fertilizer in the black soil region of Northeast China

Nitrogen (N) fertilizer is often used in production practice to effectively maintain crop productivity; however, low nitrogen use efficiency (Nue) has always been a problem. Specifically, the transformation and utilization of nitrogen fertilizer by biochar and the driving mechanisms remain unclear. We used four biochar application rates (0, 3750, 7500, and 11,250 kg·ha-1) and analyzed the effects of biochar on nitrogen fertilizer utilization, residue, and loss over three years using 15N isotope tracer technology. The results showed that (1) biochar improved the nitrogen use efficiency of maize plants, reduced total nitrogen loss, and increased the maize yield. Compared to the control treatment in the same year, the application of 7500 kg·ha-1 biochar increased the nitrogen use efficiency by 24.27 %, 27.77 %, and 35.82 %, and the yield increased by 21.1 %, 26.7 %, and 24.5 %, respectively. (2) Biochar increased the proportion of mineral nitrogen supplied by fertilizer in the mineral nitrogen pool. The application of 7500 kg·ha-1 biochar increased mineral nitrogen by 3.05 %, 3.22 %, and 3.8 %, respectively, compared to the control treatments in the same year. Biochar promoted the transformation of nitrogen in the 0-40 cm soil layer to three different soil nitrogen pools, especially the organic nitrogen pool. (3) Biochar significantly improved the soil bacterial community and increased the abundances of N transformation functional genes. The redundancy analysis (RDA) showed that the gdhA mineralization gene was the driving factor of nitrogen fertilizer transformation, contributing 43.6 % of the variance. In summary, the application of 7500 kg·ha-1 of biochar for two consecutive years was conducive to maintaining farmland soil fertility, while its use would not be recommended for more than three consecutive years.

Biochar mitigates the stimulatory effects of straw incorporation on N2O emission and N2O/(N2O + N2) ratio in upland soil

Straw incorporation, a common agricultural strategy designed to enhance soil organic carbon (SOC), often leads to increased nitrous oxide (N2O) emission, potentially offsetting benefits of SOC sequestration. However, the mechanism and mitigation options for the enhanced N2O emission following straw incorporation remain unclear. Here, N2 and N2O emission rate, as well as N2O/(N2O + N2) ratio under four different fertilization treatments [i.e., non-fertilization (Control), conventional chemical fertilization (CF), conventional chemical fertilization plus straw incorporation (SWCF), and conventional chemical fertilization plus straw and biochar incorporation (SWBCF)] were investigated by a robotized sampling and analysis system. High-throughput sequencing was also employed to assess the variation of bacterial community across different treatments. The results showed CF, SWCF, and SWBCF fertilization treatments significantly increased N2O emission rate by 1.04, 2.01, and 1.29 folds, respectively, relative to Control treatment. Albeit no significant enhancements in N2 emission rate, the N2O/(N2O + N2) ratio significantly increased by 65.53%, 1.10 folds, and 69.49% in CF, SWCF, and SWBCF treatments, respectively. The partial least squares path modeling analysis further revealed that fertilization treatments slightly increased N2 emission rate by increasing DOC content and keystone OTUs abundance. While the enhanced N2O emission rate and N2O/(N2O + N2) ratio in the fertilization treatments was primarily determined by reducing DOC/NO3- ratio and specific bacteria module abundance dominated by Gaiellales, Solirubrobacterales, and Micrococcales. Furthermore, SWBCF treatment alleviated the increase in net global warming potential due to straw incorporation, as indicated by the higher SOC sequestration and lower N2O/(N2O + N2) ratio therein. Collectively, these findings suggest that simultaneous application of straw and biochar has the potential to mitigate the risk of increased N2O emission from straw incorporation. This study provides valuable insights for developing targeted strategies in C sequestration and greenhouse gas mitigation, tackling the challenge presented by global climate change.

Microplastics and biochar interactively affect nitrous oxide emissions from tobacco planting soil

Biochar application to amend acidified tobacco-soils can enhance tobacco quality and reduce nitrous oxide (N2O) emissions. Microplastics from agricultural mulch are commonly found in cash-crop farmland soils and, together with biochar, affect soil N2O emissions. In this study, we applied three types of microplastics (polyethylene, PE; polylactic acid, PLA; polybutylene adipate terephthalate, PBAT) and rice biochar alone or in combination to acidified tobacco planting soil in central China to investigate their effects on soil N2O emissions, soil chemical properties, nitrogen-cycle-related functional genes, and microbial functional diversity during a 35-day laboratory incubation period. Significant increases in N₂O emissions were observed with PE and PLA, which raised emissions by 15.96 % and 21.52 %, respectively. Additionally, different microplastics affected soil N₂O emissions through distinct regulatory pathways. Co-application of microplastics and biochar suppressed N2O emissions compared to microplastics alone. Biochar mitigates N2O emissions mainly by increasing the abundance of the nosZ gene. It can remediate soil contaminated by microplastics and reduce their negative impacts on the soil environment. This study provides deeper insight into the effects of microplastics on soil nitrogen cycling and biochar-mitigated remediation of microplastic-contaminated soil.

Insights into CO2 and N2O emissions driven by applying biochar and nitrogen fertilizers in upland soil

Biochar and soil carbon sequestration hold promise in mitigating global warming by storing carbon in the soil. However, the interaction between biochar properties, soil carbon-nitrogen cycling, and nitrogen fertilizer application's impact on soil carbon-nitrogen balance remained unclear. Herein, we conducted batch experiments to study the effects and mechanisms of rice straw biochar application (produced at 300, 500, and 700 °C) on net greenhouse gas emissions (CO2, N2O, CH4) in upland soils under different forms of nitrogen fertilizers. The findings revealed that (NH4)2SO4 and urea significantly elevated soil carbon dioxide equivalent emissions, ranging from 28 to 61.7 kg CO2e/ha and 8.2 to 37.7 kg CO2e/ha, respectively. Conversely, KNO3 reduced soil CO2e emissions, ranging from 2.2 to 13.6 kg CO2e/ha. However, none of these three nitrogen forms exhibited a significant effect on CH4 emissions. The pyrolysis temperature of biochar was found negatively correlated with soil CO2 and N2O emissions. The alkaline substances presented in biochar pyrolyzed at 500-700 °C raised soil pH, increased the ratio of Gram-negative to Gram-positive bacteria, and enhanced the relative abundance of Sphingomonadaceae. Moreover, the co-application of KNO3 based nitrogen fertilizer and biochar increased the total carbon/inorganic nitrogen ratio and reduces the relative abundance of Nitrospirae. This series of reactions led to a significant increase in soil DOC content, meanwhile reduced soil CO2 emissions, and inhibited the nitrification process and decreased the emission of soil N2O. This study provided a scientific basis for the rational application of biochar in soil.

Biochar's dual role in greenhouse gas emissions: Nitrogen fertilization dependency and mitigation potential

Biochar was popularly used for reducing greenhouse gas (GHG) emissions in vegetable production, but using biochar does not necessarily guarantee a reduction in GHG emissions. Herein, it's meaningful to elucidate the intricate interplay among biochar properties, soil characteristics, and GHG emissions in vegetable production to provide valuable insights for informed and effective mitigation strategies. Therefore, in current research, a meta-analysis of 43 publications was employed to address these issues. The boost-regression analysis results indicated that the performance of biochar in inhibiting N2O emissions was most affected by the N application rate both in high and low N application conditions. Besides, biochar had dual roles and showed well performance in reducing GHG emissions under low N input (≤300 kg N ha-1), while having the opposite effect during high N input (>300 kg N ha-1). Specifically, applying biochar under low N fertilization input could obviously reduce soil N2O emissions, CO2 emissions, and CH4 emissions by 18.7 %, 17.9 %, and 16.9 %, respectively. However, the biochar application under high N fertilization input significantly (P < 0.05) increased soil N2O emissions, CO2 emissions, and CH4 emissions by 39.7 %, 43.0 %, and 27.7 %, respectively. Except for the N application rate, the soil pH, SOC, biochar C/N ratio, biochar pH, and biochar pyrolysis temperature are also the key factors affecting the control of GHG emissions in biochar-amended soils. The findings of this study will contribute to deeper insights into the potential application of biochar in regulating GHG under consideration of N input, offering scientific evidence and guidance for sustainable agriculture management.

Comprehensive impacts of different integrated rice-animal co-culture systems on rice yield, nitrogen fertilizer partial factor productivity and nitrogen losses: A global meta-analysis

Integrated rice-animal co-culture (IRAC) is an ecological agricultural system combining rice cultivation with animal farming, which holds significant implications for food security and agriculture sustainable development. However, the comprehensive impacts of the co-culture on rice yield, nitrogen (N) losses, and N fertilizer partial factor productivity (NPFP) remain elusive and may vary under different environmental conditions and N management. Here, we conducted a meta-analysis of data from various IRAC systems on a global scale, including 371, 298, and 115 sets of data for rice yield, NPFP, and N losses, respectively. The results showed that IRAC could significantly increase rice yield (by 3.47 %) and NPFP (by 4.26 %), and reduce N2O emissions (by 16.69 %), NH3 volatilization (by 11.03 %), N runoff (by 17.72 %), and N leaching (by 19.10 %). Furthermore, there were significant differences in rice yield, NPFP, and N loss among different IRAC systems, which may be ascribed to variations in regional climate, soil variables, and N fertilizer management practices. The effect sizes of rice yield and NPFP were notably correlated with the rate and frequency of N application and the soil clay content. Moreover, a higher amount of precipitation corresponded to a larger effect size on rice NPFP. N2O emissions were closely associated with mean annual air temperature, annual precipitation, N application frequency, soil pH level, soil organic matter content, soil clay content, and soil bulk density. However, NH3 volatilization, N runoff, and N leaching exhibited no correlation with either the environmental conditions or the N management. Multivariate regression analysis further demonstrated that the soil clay content and N application rate are pivotal in predicting the effect sizes of rice yield, NPFP, and N2O emissions under IRAC. Specifically, IRAC with a low N application rate in soils with a high clay content could augment the effect size to increase rice NPFP and yield and reduce N2O emissions. In conclusion, IRAC offers a potent strategy to optimize rice yield and NPFP as well as mitigate N losses.

Comparison analysis of microbial agent and different compost material on microbial community and nitrogen transformation genes dynamic changes during pig manure compost

This study aimed to assess the impact of microbial agent and different compost material, on physicochemical parameters dynamic change, nitrogen-transfer gene/bacterial community interaction network during the pig manure composting. Incorporating a microbial agent into rice straw-mushroom compost reduced the NH3 and total ammonia emissions by 25.52 % and 14.41 %, respectively. Notably, rice straw-mushroom with a microbial agent reduced the total ammonia emissions by 37.67 %. NH4+-N and pH emerged as primary factors of phylum-level and genus-level microorganisms. Microbial agent increased the expression of narG, nirK, and nosZ genes. Rice straw-mushroom elevated the content of amoA, nirK, nirS, and nosZ genes. Alcanivorax, Luteimonas, Pusillimonas, Lactobacillus, Aequorivita, Clostridium, Moheibacter and Truepera were identified as eight core microbial genera during the nitrogen conversion process. This study provides a strategy for reducing ammonia emissions and analyzes the potential mechanisms underlying compost processes.

Soil N2 O emissions from specialty crop systems: A global estimation and meta-analysis

Nitrous oxide (N2 O) exacerbates the greenhouse effect and thus global warming. Agricultural management practices, especially the use of nitrogen (N) fertilizers and irrigation, increase soil N2 O emissions. As a vital sector of global agriculture, specialty crop systems usually require intensive input and management. However, soil N2 O emissions from global specialty crop systems have not been comprehensively evaluated. Here, we synthesized 1137 observations from 114 published studies, conducted a meta-analysis to evaluate the effects of agricultural management and environmental factors on soil N2 O emissions, and estimated global soil N2 O emissions from specialty crop systems. The estimated global N2 O emission from specialty crop soils was 1.5 Tg N2 O-N year-1 , ranging from 0.5 to 4.5 Tg N2 O-N year-1 . Globally, soil N2 O emissions exponentially increased with N fertilizer rates. The effect size of N fertilizer on soil N2 O emissions generally increased with mean annual temperature, mean annual precipitation, and soil organic carbon concentration but decreased with soil pH. Global climate change will further intensify the effect of N fertilizer on soil N2 O emissions. Drip irrigation, fertigation, and reduced tillage can be used as essential strategies to reduce soil N2 O emissions and increase crop yields. Deficit irrigation and non-legume cover crop can reduce soil N2 O emissions but may also lower crop yields. Biochar may have a relatively limited effect on reducing soil N2 O emissions but be effective in increasing crop yields. Our study points toward effective management strategies that have substantial potential for reducing N2 O emissions from global agricultural soils.

Biochar modulating soil biological health: A review

Biochar can be used for multifunctional applications including the improvement of soil health and carbon storage, remediation of contaminated soil and water resources, mitigation of greenhouse gas emissions and odorous compounds, and feed supplementation to improve animal health. A healthy soil preserves microbial biodiversity that is effective in supressing plant pathogens and pests, recycling nutrients for plant growth, promoting positive symbiotic associations with plant roots, improving soil structure to supply water and nutrients, and ultimately enhancing soil productivity and plant growth. As a soil amendment, biochar assures soil biological health through different processes. First, biochar supports habitats for microorganisms due to its porous nature and by promoting the formation of stable soil micro-aggregates. Biochar also serves as a carbon and nutrient source. Biochar alters soil physical and chemical properties, creating optimum soil conditions for microbial diversity. Biochar can also immobilize soil pollutants and reduce their bioavailability that would otherwise inhibit microbial growth. However, depending on the pyrolysis settings and feedstock resources, biochar can be comprised of contaminants including polycyclic aromatic hydrocarbons and potentially toxic elements that can inhibit microbial activity, thereby impacting soil health.

Response of soil N2O production pathways to biochar amendment and its isotope discrimination methods

Reducing nitrous oxide (N2O) emission from farmland is crucial for alleviating global warming since agriculture is an important contributor of atmospheric N2O. Returning biochar to agricultural fields is an important measure to mitigate soil N2O emissions. Accurately quantifying the effect of biochar on the process of N2O production and its driving factors is critical for achieving N2O emission mitigation. Recently, stable isotope techniques such as isotope labeling, natural abundance, and site preference (SP) value, have been widely used to distinguish N2O production pathways. However, the different isotope methods have certain limitations in distinguishing N2O production in biochar-amended soils where it is difficult to identify the relative contribution of individual pathways for N2O production. This paper systematically reviews the pathways of soil N2O production (nitrification, nitrifier denitrification, bacterial denitrification, fungal denitrification, coupled nitrification-denitrification, dissimilatory nitrate reduction to ammonium and abiotic processes) and their response mechanism to the addition of biochar, as well as the development history and advantages of isotopes in differentiating N2O production pathways in biochar-amended soils. Moreover, the limitations of current research methods and future research directions are proposed. These results will help resolve how biochar affects different processes that lead to soil N2O generation and provide a scientific basis for sustainable agricultural carbon sequestration and the fulfilment of carbon neutrality goals.

Production of biochar from squeezed liquid of fruit and vegetable waste: Impacts on soil N2O emission and microbial community

The squeezed liquid from fruit and vegetable waste (LW) presents a unique wastewater challenge, marked by recalcitrance in treatment and amplified design risks with the application of conventional processes. Following coagulation of the squeezed liquid, the majority of particulate matter precipitates. The resulting precipitated floc (LWF) is reclaimed and subsequently utilized for the synthesis of biochar. The present study primarily explores the viability of repurposing LWF as biochar to enhance soil quality and mitigate N2O emissions. Findings indicate that the introduction of a 2% proportion of LWFB led to a remarkable 99.5% reduction in total N2O emissions in contrast to LWF. Concurrently, LWFB substantially enhanced nutrients content by elevating soil organic carbon (SOC) and nitrogen levels. Utilizing high-throughput sequencing in conjunction with qPCR, the investigation unveiled that the porous structure and substantial specific surface area of LWFB potentially fostered microbial adhesion and heightened diversity within the soil microbial community. Furthermore, LWFB notably diminished the relative abundance of AOB (Nitrosospira, Nitrosomonas), and NOB (Candidatus_Nitrotoga), thereby curbing the conversion of NH4+ into NO3-. The pronounced elevation in nosZ abundance implies that LWFB holds the potential to mitigate N2O emissions through a conversion to N2.

Nitrogen addition stimulates N2O emissions via changes in denitrification community composition in a subtropical nitrogen-rich forest

Microbially driven nitrification and denitrification play important roles in regulating soil N availability and N2O emissions. However, how the composition of nitrifying and denitrifying prokaryotic communities respond to long-term N additions and regulate soil N2O emissions in subtropical forests remains unclear. Seven years of field experiment which included three N treatments (+0, +50, +150 kg N ha-1 yr-1; CK, LN, HN) was conducted in a subtropical forest. Soil available nutrients, N2O emissions, net N mineralization, denitrification potential and enzyme activities, and the composition and diversity of nitrifying and denitrifying communities were measured. Soil N2O emissions from the LN and HN treatments increased by 42.37% and 243.32%, respectively, as compared to the CK. Nitrogen addition significantly inhibited nitrification (N mineralization) and significantly increased denitrification potentials and enzymes. Nitrification and denitrification abundances (except nirK) were significantly lower in the HN, than CK treatment and were not significantly correlated with N2O emissions. Nitrogen addition significantly increased nirK abundance while maintaining the positive effects of denitrification and N2O emissions to N deposition, challenging the conventional wisdom that long-term N addition reduces N2O emissions by inhibiting microbial growth. Structural equation modeling showed that the composition, diversity, and abundance of nirS- and nirK-type denitrifying prokaryotic communities had direct effects on N2O emissions. Mechanistic investigations have revealed that denitrifier keystone taxa transitioned from N2O-reducing (complete denitrification) to N2O-producing (incomplete denitrification) with increasing N addition, increasing structural complexity and diversity of the denitrifier co-occurrence network. These results significantly advance current understanding of the relationship between denitrifying community composition and N2O emissions, and highlight the importance of incorporating denitrifying community dynamics and soil environmental factors together in models to accurately predict key ecosystem processes under global change.

Using adaptive and aggressive N2O-reducing bacteria to augment digestate fertilizer for mitigating N2O emissions from agricultural soils

Nitrous oxide (N2O) emitted from agricultural soils destroys stratospheric ozone and contributes to global warming. A promising approach to reduce emissions is fertilizing the soil using organic wastes augmented by non-denitrifying N2O-reducing bacteria (NNRB). To realize this potential, we need a suite of NNRB strains that fulfill several criteria: efficient reduction of N2O, ability to grow in organic waste, and ability to survive in farmland soil. In this study, we enriched such organisms by sequential anaerobic batch incubations with N2O and reciprocating inoculation between the sterilized substrates of anaerobic manure digestate and soils. 16S rDNA amplicon sequencing and metagenomics analysis showed that a cluster of bacteria containing nosZ genes encoding N2O-reductase, was enriched during the incubation process. Strains of several dominant members were then isolated and characterized, and three of them were found to harbor the nosZ gene but none of the other denitrifying genes, thus qualifying as NNRB. The selected isolates were tested for their capacities to reduce N2O emissions from three different typical Chinese farmland soils. The results indicated the significant mitigation effect of these isolates, even in very acidic red soil. In conclusion, this study demonstrated a strategy to engineer the soil microbiome with promising NNRB with high adaptability to livestock manure digestate as well as different agricultural soils, which would be suitable for developing novel fertilizer for farmland application to efficiently mitigate the N2O emissions from agricultural soils.

Mitigation of N2O emission from granular organic fertilizer with alkali- and salt-resistant plant growth-promoting rhizobacteria

AIM

Organic fertilizer application significantly stimulates nitrous oxide (N2O) emissions from agricultural soils. Plant growth-promoting rhizobacteria (PGPR) strains are the core of bio-fertilizer or bio-organic fertilizer, while their beneficial effects are inhibited by environmental conditions, such as alkali and salt stress observed in organic manure or soil. This study aims to screen alkali- and salt-resistant PGPR that could mitigate N2O emission after applying strain-inoculated organic fertilizer.

METHODS AND RESULTS

Among the 29 candidate strains, 11 (7 Bacillus spp., 2 Achromobacter spp., 1 Paenibacillus sp., and 1 Pseudomonas sp.) significantly mitigated N2O emissions from the organic fertilizer after inoculation. Seven strains were alkali tolerant (pH 10) and five were salt tolerant (4% salinity) in pure culture. Seven strains were selected for further evaluation in two agricultural soils. Five of these seven strains could significantly decrease the cumulative N2O emissions from Anthrosol, while six could significantly decrease the cumulative N2O emissions from Cambisol after the inoculation into the granular organic fertilizer compared with the non-inoculated control.

CONCLUSIONS

Inoculating alkali- and salt-resistant PGPR into organic fertilizer can reduce N2O emissions from soils under microcosm conditions. Further studies are needed to investigate whether these strains will work under field conditions, under higher salinity, or at different soil pH.

Understanding the relative contributions of fungi and bacteria led nitrous oxide emissions in an acidic soil amended with industrial waste

Amendment of fertilized arable soil with alkaline industrial waste has the potential to ameliorate soil acidification whilst also improving crop yield. Another co-benefit is nitrous oxide (N₂O) emission abatement but the contribution of fungi and bacteria involved in this process remains unclear. Two incubation experiments were conducted to: 1) examine how amendment of acidic soils with a mixture of phosphorus tailings mixture and insoluble potassium-containing rocks (PT) affect N₂O emissions and 2) understand the microbial mechanisms and relative contributions of fungi and bacteria responsible for N₂O emissions. In the first incubation experiment, the four treatments consisted of: i) the study control, ii) urea, iii) PT amendment and iv) PT amendment plus urea. Results showed that the PT amendment significantly increased soil pH from 4.8 to above 6.0, and reduced N₂O emissions by 65.7%. PT-amended soils had higher N₂ emissions and faster O₂ consumption. The PT amendment significantly increased extracellular enzyme activities of leucine aminopeptidase and N-Acetyl-β-glucosaminidase, while it significantly decreased activities of β-1, 4-glucosidase and β-cellobiosidase. Two antibiotics (cycloheximide and streptomycin) combined with substrate-induced respiration method were used in the second incubation experiment. Compared to soil with urea, urea with PT amendment raised soil bacteria-related N₂O from 9.2% to 18.8% while decreasing fungi-related N₂O from 50.5% to 43.2%. These findings suggest that the N₂O emissions from acidic soils can be considerably mitigated by the application of alkaline industrial wastes. The contribution of fungi should be considered when designing and applying N₂O mitigation strategies in acidic soils. DATA AVAILABILITY: Data will be made available on request.

The inhibitory effects and underlying mechanism of high ammonia stress on sulfide-driven denitrification process

Sulfide-driven denitrification (SD) process has been widely studied for treating wastewater containing sulfate and ammonia in recent years. But influence of high ammonia stress on the SD process and microbial community remained unclear. In this work, a series of tests were conducted to investigate effects of different ammonia stress (200-3000 mg-total ammonia nitrogen (TAN)/L) on denitrification efficiency, byproduct accumulation and microbial community of the SD process. According to our results, the SD process was severely inhibited, and 32.67 ± 5.15 mg/L NO₂^--N was accumulated when ammonia stress reached 3000 mg TAN/L. But the inhibited SD process could recover in about 40 days when ammonia stress was decreased to 200 mg TAN/L. After analyzing the microbial community, Thiobacillus sp. (Thiobacillus sp. 65-29, Thiobacillus sp. SCN 64-317, Thiobacillus sp. 63-78 and Thiobacillus denitrificans) was confirmed as dominant bacteria responsible for the SD process. Further, expression of narG, napA, nirK and nirS were inhibited under high ammonia stress, thus making the SD process stuck in NO₃^- and NO₂^- reduction step. This study reveals the inhibitory effects of high ammonia stress on the SD process and its possible underlying mechanism with discussion in gene level.

Banana, pineapple, cassava and sugarcane residue biochars cannot mitigate ammonia volatilization from latosols in tropical farmland

Ammonia (NH₃) volatilization is a major pathway of soil nitrogen loss in tropical farmland, causing many environmental issues. Biochar can improve soil quality and affect soil NH₃ volatilization. However, little is known about the effects of tropical crop residue biochar on soil NH₃ volatilization in tropical farmland. Therefore, a laboratory incubation study was conducted using four kinds of tropical crop residue biochar (pineapple straw (stem and leaves), banana straw, cassava straw and sugarcane bagasse pyrolyzed at 500 °C) with five addition rates (0.5%, 1%, 2%, 4%, and 6%) to evaluate their impact on NH₃ volatilization from tropical latosols. The results showed that NH₃ volatilization peaked twice under biochar application, once at 1-5 days and again at 12-16 days. The cumulative NH₃ volatilization (0.14-0.47 mg kg^-1) of the 20 biochar treatments was higher than that of the control (0.12 mg kg^-1). With the increase in the biochar addition rate, the soil pH, soil organic matter (SOM), urease activity, nitrate nitrogen content (NO₃^--N), nitrification rate and cumulative NH₃ volatilization increased gradually, and the 6% biochar treatment resulted in the highest NH₃ volatilization loss (0.19-0.47 mg kg^-1). The type of biochar is also a main factor affecting soil NH₃ volatilization. The cumulative NH₃ volatilization was the highest under pineapple straw biochar, as it was 19-43% higher than when the other three biochars were applied. However, sugarcane bagasse biochar had the lowest cumulative NH₃ volatilization due to its low quartz, sylvite and calcite contents, lack of -OH hydroxyl groups and high adsorbability. NH₃ volatilization was positively correlated with the soil pH, SOM, urease activity, NO₃^--N and nitrification rate. In conclusion, four tropical crop residue biochars can increase NH₃ volatilization in tropical latosols, so reducing NH₃ volatilization needs to be further considered in tropical crop residue biochar applications.

Microbial regulation of nitrous oxide emissions from chloropicrin-fumigated soil amended with biochar

The microbial mechanism underpinning biochar's ability to reduce emissions of the potent greenhouse gas nitrous oxide (N₂O) is little understood. We combined high-throughput gene sequencing with a dual-label ¹⁵N-¹⁸O isotope to examine microbial mechanisms operative in biochar made from Crofton Weed (BC1) or pine wood pellets (BC2) and the N₂O emissions from those biochar materials when present in chloropicrin (CP)-fumigated soil. Both BC1 and BC2 reduced N₂O total emissions by 62.9-71.9% and 48.8-52.0% in CP-fumigated soil, respectively. During the 7-day fumigation phase, however, both BC1 and BC2 increased N₂O production by significantly promoting nirKS and norBC gene abundance, which indicated that the N₂O emission pathway had switched from heterotrophic denitrification to nitrifier denitrification. During the post-fumigation phase, BC1 and BC2 significantly decreased N₂O production as insufficient nitrogen was available to support rapid population increases of nitrifying or denitrifying bacteria. BC1 and BC2 significantly reduced CP's inhibition of nitrifying archaeal bacteria (AOA, AOB) and the denitrifying bacterial genes (nirS, nirK, nosZ), which promoted those bacterial populations in fumigated soil to similar levels observed in unfumigated soil. Our study provided insight on the impact of biochar and microbes on N₂O emissions.

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.

Characterization of halophyte biochar and its effects on water and salt contents in saline soil

Biochar is a beneficial soil amendment; however, biochar-based properties are mainly determined by the feedstocks and the pyrolysis temperature. Nevertheless, considering the vast biomass of halophyte, little is known about how the halophyte-derived biochar improves saline soils. In this study, we firstly produced biochars by using three different halophytes, including Tamarix chinensis (recretohalophyte), Suaeda salsa (euhalophyte), and Phragmites australis (pseudo-halophyte) at 300, 500, and 700 °C, and compared their chemical and physical properties. We applied halophyte (Tamarix chinensis and Phragmites australis) biochars (pyrolysis at 500 °C) into 0-20 cm saline soil at 2% and 4% (w/w) rates to investigate the saline soil water, salt, and pH dynamics in a 12-month column experiment. The results showed that as the pyrolytic temperature increase, biochar yield and pore diameter decreased by 37.5-44.0% and 34.6-89.7%, respectively; in contrast, biochar pH, specific surface area, and total volume increased by 24.8-47.8%, 3-37 times and 1-9 times, respectively. The halophyte types significantly controlled biochar carbon and dissolved salt content and electrical conductivity. Halophyte biochar application can increase soil water and salt content, and application of 4% of Tamarix chinensis-derived biochar can increase more soil moisture than the soil salinity, and it can maintain soil pH at a stable level, which would be a potential way to improve saline soil properties. The results are valuable for choosing halophyte types and optimizing pyrolytic temperatures for halophyte biochar production through specific environmental usage.

Biochar application for the remediation of trace metals in contaminated soils: Implications for stress tolerance and crop production

In modern agriculture and globalization, the release of trace metals from manufacturing effluents hinders crop productivity by polluting the atmosphere and degrading food quality. Sustaining food safety in polluted soils is critical to ensure global food demands. This review describes the negative effects of trace metals stress on plant growth, physiology, and yield. Furthermore, also explains the potential of biochar in the remediation of trace metal's contaminations in plants by adoption of various mechanisms such as reduction, ion exchange, electrostatic forces of attraction, precipitation, and complexation. Biochar application enhances the overall productivity, accumulation of biomass, and photosynthetic activity of plants through the regulation of various biochemical and physiological mechanisms of plants cultivated under trace metals contaminated soil. Moreover, biochar scavenges the formation of reactive oxygen species, by activating antioxidant enzyme production i.e., ascorbate peroxidase, catalase, superoxide dismutase, peroxidase, etc. The application of biochar also improves the synthesis of stressed proteins and proline contents in plants thus maintaining the osmoprotectant and osmotic potential of the plant under contaminates stress. Integrated application of biochar with other amendments i.e., microorganisms and plant nutrients to improve trace metal remediation potential of biochar and improving crop production was also highlighted in this review. Moreover, future research needs regarding the application of biochar have also been addressed.

Biochar-induced reduction of N2O emission from East Asian soils under aerobic conditions: Review and data analysis

Global meta-analyses showed that biochar application can reduce N₂O emission. However, no relevant review study is available for East Asian countries which are responsible for 70% of gaseous N losses from croplands globally. This review analyzed data of the biochar-induced N₂O mitigation affected by experimental conditions, including experimental types, biochar types and application rates, soil properties, and chemical forms and application rates of N fertilizer for East Asian countries. The magnitude of biochar-induced N₂O mitigation was evaluated by calculating N₂O reduction index (R_index, percentage reduction of N₂O by biochar relative to control). The R_index was further standardized against biochar application rate by calculating R_index per unit of biochar application rate (ton ha^-1) (Unit R_index). The R_index averaged across different experimental types (n = 196) was -21.1 ± 2.4%. Incubation and pot experiments showed greater R_index than column and field experiments due to higher biochar application rate and shorter experiment duration. Feedstock type and pyrolysis temperature also affected R_index; either bamboo feedstock or pyrolysis at > 400 °C resulted in a greater R_index. The magnitude of R_index also increased with increasing biochar rate. Soil properties did not affect R_index when evaluated across all experimental types, but there was an indication that biochar decreased N₂O emission more at a lower soil moisture level in field experiments. The magnitude of R_index increased with increasing N fertilizer rate up to 500-600 kg N ha^-1, but it decreased thereafter. The Unit R_index averaged across experimental types was -1.2 ± 0.9%, and it was rarely affected by experimental type and conditions but diminished with increasing biochar rate. Our results highlight that since N₂O mitigation by biochar is affected by biochar application rate, R_index needs to be carefully evaluated by standardizing against biochar application rate to suggest the best conditions for biochar usage in East Asia.

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.

Biochar decreases the efficacy of the nitrification inhibitor nitrapyrin in mitigating nitrous oxide emissions at different soil moisture levels

Unprecedented increases in agricultural nitrous oxide (N₂O) emissions in recent years have caused substantial environmental pollution that leads to ozone depletion and global warming. Application of biochar and/or nitrification inhibitors (NIs) has the potential to reduce N₂O emissions; however, it is not clear how biochar application may affect the efficacy of NI in reducing nitrification rates, soil enzyme activities, and N₂O emissions under different soil moisture regimes. We conducted a 60-day laboratory incubation experiment to study the effects of manure biochar and nitrapyrin (as a NI) on N₂O emissions from a urea fertilized soil with either 60 (low) or 80% (high) water-filled pore space (WFPS). Nitrification rates were significantly affected by biochar × NI × WFPS and biochar × WFPS interactions. Biochar initially increased and then decreased the rates, resulting in 45.2 and 26.6% (P < 0.001 for both) overall reductions in low and high WFPS, respectively while NI reduced the rates only in the first 10 days at 60% WFPS. Biochar decreased (P < 0.001) and NI increased (P = 0.007) β-1,4-N-acetyl glucosaminidase activities while urease activities were increased (P < 0.001) by biochar across WFPS. Biochar had significant interaction with NI in cumulative N₂O emissions with the efficacy of NI being reduced when co-applied with biochar. Cumulative N₂O emissions were greater at high than at low WFPS; the emissions were decreased by biochar at 60% WFPS and NI at both 60 and 80% WFPS. We conclude that biochar reduces efficacy of nitrapyrin in mitigating N₂O emissions and their effects on net nitrification rates, enzyme activities and N₂O emissions are dependent on soil moisture level.

Clay-hydrochar composites mitigated CH4 and N2O emissions from paddy soil: A whole rice growth period investigation

With the favorable microporous structure and excellent adsorption capacity, clay-hydrochar composites (CHCs) serve as promising materials to mitigate greenhouse gas emissions (GHG) from the paddy fields. Three clays were co-pyrolyzed with hydrochar derived from poplar sawdust to obtain CHCs, which were applied to the paddy fields to investigate the effects on methane (CH₄) and nitrous oxide (N₂O) emissions. Three CHCs were labeled as bentonite-hydrochar composite (BTHC), montmorillonite-hydrochar composite (MTHC), and kaolinite-hydrochar composite (KTHC), respectively. The effects of these three CHCs on GHG emissions were determined by monitoring the dynamic CH₄ and N₂O emissions in the paddy soil column ecosystem during the rice-growing season. The results showed that compared with the control group, three CHCs significantly mitigated CH₄ and N₂O emissions by 21.4%-47.5% and 5.2%-36.8%, respectively. Furthermore, the fluorescent components result displayed CHCs increased humic-like content by 29.62%-59.72%. A structural equation model was used to assess the hypothesis mitigation mechanism, which exemplified that GHG emissions negatively correlated with pmoA and nosZ genes, possibly resulting in the CH₄ and N₂O mitigation. Among the three CHCs, the KTHC amendment mitigated the CH₄ and N₂O emissions by 47.5% and 36.8%, respectively, which was superior to BTHC and MTHC. Hence, it was recommended for application to the field. Overall, this study demonstrates the mitigating effects of CHCs on GHG emissions for the first time, and the reduced CH₄ and N₂O emissions could contribute to increased soil C and N retention for better agricultural nutrients management.

Graphitic carbon nitride/biochar composite synthesized by a facile ball-milling method for the adsorption and photocatalytic degradation of enrofloxacin

In order to enhance the removal performance of graphitic carbon nitride (g-C₃N₄) on organic pollutant, a simultaneous process of adsorption and photocatalysis was achieved via the compounding of biochar and g-C₃N₄. In this study, g-C₃N₄ was obtained by a condensation reaction of melamine at 550°C. Then the g-C₃N₄/biochar composites were synthesized by ball milling biochar and g-C₃N₄ together, which was considered as a simple, economical, and green strategy. The characterization of resulting g-C₃N₄/biochar suggested that biochar and g-C₃N₄ achieved effective linkage. The adsorption and photocatalytic performance of the composites were evaluated with enrofloxacin (EFA) as a model pollutant. The result showed that all the g-C₃N₄/biochar composites displayed higher adsorption and photocatalytic performance to EFA than that of pure g-C₃N₄. The 50% g-C₃N₄/biochar performed best and removed 45.2% and 81.1% of EFA (10 mg/L) under darkness and light with a dosage of 1 mg/mL, while g-C₃N₄ were 19.0% and 27.3%, respectively. Besides, 50% g-C₃N₄/biochar showed the highest total organic carbon (TOC) removal efficiency (65.9%). Radical trapping experiments suggested that superoxide radical (•O₂^-) and hole (h⁺) were the main active species in the photocatalytic process. After 4 cycles, the composite still exhibited activity for catalytic removal of EFA.

Total and denitrifying bacterial communities associated with the interception of nitrate leaching by carbon amendment in the subsoil

Nitrate leaching is severe in greenhouse where excessive nitrogen is often applied to maintain high crop productivities. In this study, we investigated the effects of carbon amendment in the subsoil on nitrate leaching and the emission of greenhouse gases (CH₄ and N₂O) using a soil column experiment. Carbon amendment resulted in over 39% reduction in nitrate leaching and 25.3% to 60.6% increase of total N content in the subsoil zone as compared to non-amended control. Strikingly, the abundance of nirS, nosZ, and 16S rRNA were higher in the treatment than the corresponding controls while no significant effect was detected for nirK. Carbon amendment explained 14%, 10%, and 4% of the variation in the community of nosZ, nirS, and nirK, respectively. It also considerably (more than 7 times) enriched genera such as Anaerovorax, Pseudobacteroides, Magnetospirillum, Prolixibacter, Sporobacter, Ignavibacterium, Syntrophobacter, Oxobacter, Hydrogenispora, Desulfosporomusa, Mangrovibacterium, and Sporomusa, as revealed by the analysis of 16S rRNA amplicon. Network analysis further uncovered that carbon amendment enriched three microbial hubs which mainly consists of positively correlated nirS, nosZ, and anaerobic bacterial populations. In summary, carbon amendment in the subsoil mitigated nitrate leaching and increased the nitrogen pool by possible activation of denitrifying and anaerobic bacterial populations. KEY POINTS: • Carbon amendment in subsoil reduced NO₃^- leaching by over 39% under high N input. • Carbon amendment increased the total N in subsoil from 25.3% to 60.6%. • Carbon amendment enriched nirS- and nosZ-type denitrifying bacteria in subsoil.

Feedstock particle size and pyrolysis temperature regulate effects of biochar on soil nitrous oxide and carbon dioxide emissions

Atmospheric greenhouse gas (GHG) concentration increases are a serious problem impacting global climate. Mitigation of agricultural GHG production is crucial as fertilized soils contribute substantially to changes in GHG atmospheric composition. Biochar derived from agricultural or forestry biowaste has been widely used in agriculture and may help mitigate GHG emissions. While different kinds of biochar and their effects on GHG emissions have been studied, feedstock particle size may interact with pyrolysis temperature to impact biochar effects on GHG emissions, but this has not been investigated. Here, feedstock particle size effects on biochar characteristics and soil nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions were studied using Camellia oleifera fruit shell feedstock with three particle size fractions (0.5-2, 2-5, and 5-10 mm) each pyrolyzed at 300, 450, and 600 °C. Results showed that dissolved organic carbon in biochar increased with particle size when pyrolyzed at 300 °C, but decreased with pyrolysis temperature. The 0.5-2 mm shell-derived biochar was associated with the lowest N₂O and CO₂ emission rates but the highest net nitrogen mineralization rates compared to 2-5 mm and 5-10 mm shell-derived biochar when pyrolyzed at 300 °C. Overall, shell particle size was more important for soil processes at lower pyrolysis temperatures with less variation among particle sizes at higher pyrolysis temperatures. The results indicated that feedstock particle size may interact with pyrolysis temperature and impact mitigation of soil N₂O and CO₂ emissions.

Editorial: New Research on Soil Degradation and Restoration

The Virtual Special Issue (VSI) "New Research on Soil Degradation and Restoration" was proposed by the Guest-Editors (the authors of this editorial piece) to Journal of Environmental Management taking into account the following aspects: (a) Firstly, soil degradation is a main issue all over the world; (b) Secondly, physical, chemical and biological degradation of soil environments need detailed research, also going deeper in some new aspects poorly covered up to now; and (c) Similarly, new quality research on restoration of degraded soils, dumping sites, different areas affected by mining activities, and so on, would be clearly useful in order to prevent and/or solve critical environmental hazards. As a result, 110 manuscripts were submitted to the VSI by authors from around the world, and near 50 high quality works were finally published. The Guest-Editors of the VSI consider that the papers published will be of great interest for researchers working in this field, as well as for the overall community, as they include aspects clearly relevant at a global level.

Applying microwave vacuum pyrolysis to design moisture retention and pH neutralizing palm kernel shell biochar for mushroom production

Microwave vacuum pyrolysis of palm kernel shell was examined to produce engineered biochar for application as additive in agriculture application. The pyrolysis approach, performed at 750 W of microwave power, produced higher yield of porous biochar (28 wt%) with high surface area (270 cm²/g) compared to the yield obtained by conventional approach (<23 wt%). Addition of the porous biochar in mushroom substrate showed increased moisture content (99%) compared to the substrate without biochar (96%). The mushroom substrate added with biochar (150 g) was optimal in shortening formation, growth, and full colonization of the mycelium within one month. Using 2.5% of the biochar in mushroom substrate desirably maintained the optimum pH level (6.8-7) during the mycelium colonization period, leading to high mycelium growth (up to 91%) and mushroom yield (up to 280 g). The engineered biochar shows great potential as moisture retention and neutralizing agent in mushroom cultivation.