Stoichiometric ratio of dissolved organic carbon to nitrate regulates nitrous oxide emission from the biochar-amended soils

Effects of the three amendments on NH3 volatilization, N2O emissions, and nitrification at four salinity levels: An indoor experiment

The marked salinity and alkaline pH of coastal saline soil profoundly impact the nitrogen conversion process, leading to a significantly reduced nitrogen utilization efficiency and substantial gaseous nitrogen loss. The application of soil amendments (e.g. biochar, manure, and gypsum) was proved to be effective for the remediation of saline soils. However, the effects of the three amendments on soil nitrogen transformation in soils with various salinity levels, especially on NH3 volatilization and N2O emission, remain elusive. Here, we reported the effects of biochar, manure, and gypsum on NH3 volatilization and N2O emission under four natural salinity gradients in the Yellow River Delta. Also, high-throughput sequencing and qPCR analysis were performed to characterize the response of nitrification (amoA) and denitrification (nirS, nirK, and nosZ) functional genes to the three amendments. The results showed that the three amendments had little effect on NH3 volatilization in low- and moderate-salinity soils, while biochar stimulated NH3 volatilization in high-salinity soils and reduced NH3 volatilization in severe-salinity soils. Spearman correlation analysis demonstrated that AOA was significantly and positively correlated with the NO3--N content (r = 0.137, P < 0.05) and N2O emissions (r = 0.174, P < 0.01), which indicated that AOA dominated N2O emissions from nitrification in saline soils. Structural equation modeling indicated that biochar, manure, and gypsum affected N2O emission by influencing soil pH, conductivity, mineral nitrogen content, and functional genes (AOA-amoA and nosZ). Two-way ANOVA further showed that salinity and amendments (biochar, manure, and gypsum) had significant effects on N2O emissions. In summary, this study provides valuable insights to better understand the effects of gaseous N changes in saline soils, thereby improving the accuracy and validity of future GHG emission predictions and modeling.

Exploring low-carbon mulching strategies for maize and wheat on-farm: Spatial responses, factors and mitigation potential

Mulching strategies - including plastic film mulching (FM) and straw mulching (SM) - can enhance crop yields while affecting multiple greenhouse gas (GHG) fluxes. However, most of currently published site-based studies only focus on a certain gas, resulting in an inability to spatially integrated understanding of changes in agricultural global warming potential (GWP) and greenhouse gas intensity (GHGI) caused by mulching across China. Thus, we developed an optimal model considering crop type, meteorology, soil and management variables by four machine learning methods, namely support vector machine, multilayer perceptron, random forest, and gradient boosting machine (GBM). Then we mapped the relative changes in yield and GHG fluxes caused by mulching strategies. The GBM model had the best simulation capability for yield and GHGs in China. Our result showed that FM increased yield in maize (25 %) and wheat (19 %), while SM respectively increased by 14 % and 11 %. Among the relative changes due to mulching strategies, yield and N2O emissions were mainly influenced by soil fertility and soil properties, CH4 uptakes and CO2 emissions were more affected by environmental factors. GWP in maize and wheat average increased by 40 % under FM, while SM decreased GWP by 14 % and 2 %, respectively. Besides, FM average increased GHGI in maize and wheat by 17 % and 9 %, and SM decreased GHGI by 22 % and 12 %, respectively. Spatially, FM reduced maize GWP on 19 % of cropland, while SM reduced maize and wheat GWP on 71 % and 64 % of cropland, respectively. Soil pH was significantly correlated with ΔGHGI in maize and wheat. Our analysis not only estimated for the first time the spatial effects of mulching strategies across China, but also systematically analyzes the agricultural carbon emission mitigation potential of mulching strategies, which promote the development of low-carbon agriculture based on locally appropriate mulching strategies.

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

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

Using isotope pool dilution to understand how organic carbon additions affect N2 O consumption in diverse soils

Nitrous oxide (N2 O) is a formidable greenhouse gas with a warming potential ~300× greater than CO2 . However, its emissions to the atmosphere have gone largely unchecked because the microbial and environmental controls governing N2 O emissions have proven difficult to manage. The microbial process N2 O consumption is the only know biotic pathway to remove N2 O from soil pores and therefore reduce N2 O emissions. Consequently, manipulating soils to increase N2 O consumption by organic carbon (OC) additions has steadily gained interest. However, the response of N2 O emissions to different OC additions are inconsistent, and it is unclear if lower N2 O emissions are due to increased consumption, decreased production, or both. Simplified and systematic studies are needed to evaluate the efficacy of different OC additions on N2 O consumption. We aimed to manipulate N2 O consumption by amending soils with OC compounds (succinate, acetate, propionate) more directly available to denitrifiers. We hypothesized that N2 O consumption is OC-limited and predicted these denitrifier-targeted additions would lead to enhanced N2 O consumption and increased nosZ gene abundance. We incubated diverse soils in the laboratory and performed a 15 N2 O isotope pool dilution assay to disentangle microbial N2 O emissions from consumption using laser-based spectroscopy. We found that amending soils with OC increased gross N2 O consumption in six of eight soils tested. Furthermore, three of eight soils showed Increased N2 O Consumption and Decreased N2 O Emissions (ICDE), a phenomenon we introduce in this study as an N2 O management ideal. All three ICDE soils had low soil OC content, suggesting ICDE is a response to relaxed C-limitation wherein C additions promote soil anoxia, consequently stimulating the reduction of N2 O via denitrification. We suggest, generally, OC additions to low OC soils will reduce N2 O emissions via ICDE. Future studies should prioritize methodical assessment of different, specific, OC-additions to determine which additions show ICDE in different soils.

Polyethylene microplastics alter the microbial functional gene abundances and increase nitrous oxide emissions from paddy soils

The accumulation of microplastics (MPs) in terrestrial ecosystems can affect greenhouse gases (GHGs) production by changing soil structure and microbial functions. In this study, microcosm experiments were conducted to investigate the impact of polyethylene (PE) MP addition on soil carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions from paddy soils and their associated microbial functional genes. Methane was not considered due to the negligible emissions throughout the incubation. The amendment of both virgin and aged PE MPs did not significantly (p > 0.05) affect soil CO₂ emissions, but significantly (p < 0.05) increased the abundances of microbial functional genes encoding enzymes involved in hemicellulose (abfA) and lignin (mnp) decomposition, indicating plastic particle has potential to stimulate soil organic carbon decomposition. The presence of PE MP significantly increased N₂O emissions by 3.7-fold, which was probably due to PE MP increased the abundances of nirS gene involved in nitrite reductase. In addition, compared with virgin PE MP treatment, artificially aged PE MP did not significantly (p > 0.05) influence soil CO₂ and N₂O emissions. Our results provide evidence that PE MP likely cause a high risk of N₂O emission from paddy soils, this factor should be considered in future estimates of GHGs emissions from rice fields.

Biochar derived from spent mushroom substrate reduced N2O emissions with lower water content but increased CH4 emissions under flooded condition from fertilized soils in Camellia oleifera plantations

Agricultural soils are major sources of greenhouse gases (GHGs) that related with intensive fertilizer input. Biochar is widely used to mitigate GHGs, which may interact with soil water content impacting GHG emissions. Camellia oleifera fruit shell (FS) and spent mushroom substrate (MS) are ideal biochar feedstocks. However, the impact of water content and biochar on soil GHG emissions has not been thoroughly understood. Here, we examined CH₄ and N₂O emissions from C. oleifera plantation soils as affected by biochar (derived from MS or FS, 1 g 25 g^-1 soil), water content (60%, 120%, 240% or 360% water holding capacity, WHC), and fertilization (control or chicken manure, CM 2.5 g 25 g^-1 soil). We determined the abundance of related microbial functional genes to obtain the underlining mechanisms. The results showed that higher N₂O emissions occurred in soils with 120%WHC, due to increased abundance of AOA, AOB and nirS. MS or FS biochar differed in their effects on soil GHG emissions with different WHC. MS biochar was higher in pH, C/N and specific surface area, and mitigated more N₂O emissions from soils with CM and 120%WHC relative to FS biochar (by 92.9% and 34.6%, respectively). MS biochar significantly decreased abundance of nitrification related functional genes (AOA, AOB) in soils with 120%WHC and CM, which explained the decrease in N₂O emissions. However, MS biochar increased cumulative CH₄ emissions from flooded soils via increase in mcrA abundance. Thereby, biochar feedstocks should be considered in CH₄ and N₂O mitigations from soils with different water contents.

Quantifying soil N2O emissions from soil and anaerobically digested swine manure, nitrification and denitrification using 15N isotope labeling method

Increasing use of anaerobically digested swine manure in the farmland makes it necessary to understand its impact on N2O emissions, regarding the source of N2O and the corresponding mechanism of action. We used a 15N-labeled sulfate modifying the soil in order to identify the sources of N2O and the pathways of nitrification and denitrification. Three soil moisture contents (50% WHC, 75% WHC, and 100% WHC) along with three levels of anaerobically digested swine manure (0 g·kg-1, 10 g·kg-1, and 25 g·kg-1) were tested using randomized block design. Although the combined effect of contents of anaerobically digested swine manure and the soil moisture contents added to the system stimulated the utilization of soil N and promoted denitrification, the process of nitrification dominated. In anaerobically digested swine manure-treated soils, the rate of contribution of anaerobically digested swine manure to N2O accounted for 68.6-99.8%. In the 25 g·kg-1 treatment, the maximum of N2O produced by denitrification and nitrification were 14.1% and 93.1%.

Contrasting effects of maize residue, coal gas residue and their biochars on nutrient mineralization, enzyme activities and CO2 emissions in sandy loess soil

Mismanagement of crop straw and coal gas residue threatens the atmosphere and the economy. Nevertheless, thermal-pyrolysis is an option for management that turns bio-waste into biochar; its viability and adoption by the public as soil amendments is dependent on the agronomic and environmental values compared between biochar and the raw materials. We undertook a 60-day short-term analysis to assess the impact of various wastes and biochars, as well as inorganic nutrients (N), on carbon dioxide (CO₂) fluxes, soil enzyme activities, soil fertility status, and microbial activities. There were eight treatments of soil amendments: without an amendment (CK), Nutrients (N), straw + nutrients (S+N), straw biochar + nutrients (SB+N), coal gas residue + nutrients (C+N), coal gas residue biochar + nutrients (CB+N), straw + straw biochar + nutrients (S+SB+N) and coal gas residue waste + coal gas residue biochar + nutrients (C+ CB +N). The results indicated that soil EC, pH, nitrate N (NO₃^–- N), SOC, TN and available K were significantly (p < 0.05) increased coal gas residue biochar and combined with coal fly ash as compared to maize straw biochar and combined with maize straw and N treatments. The higher concentrations of soil MBC and MBN activities were increased in the maize straw application, while higher soil enzyme activity such as, invertase, urease and catalase were enhanced in the coal fly ash derived biochar treatments. The higher cumulative CO₂ emissions were recorded in the combined applications of maize straw and its biochar as well as coal gas residue and its biochar treatment. Our study concludes, that maize straw and coal fly ash wastes were converted into biochar product could be a feasible substitute way of discarding, since land amendment and decreased CO₂ fluxes and positive changes in soil microbial, and chemical properties, and can be confirmed under long-term conditions for reduction of economical and environment issues.

Effects of phosphorus modified nZVI-biochar composite on emission of greenhouse gases and changes of microbial community in soil

The effect of modified biochar on the greenhouse gas emission in soil is not clear until now. In this study, biochar (BC) was modified by phosphoric acid (P) and further combined with nano-zero-valent iron (nZVI) to form nZVI-P-BC composite. The P modified biochar could significantly increase the available phosphorus in soil. The release of CO₂ and N₂O in soil was inhibited during the initial stage of the experiment, with inhibition becoming more obvious over time. On the contrary, CH₄ and N₂O emission in soil was enhanced by nZVI-P-BC composite. The proportion of Sphingomonas and Gemmatimonas were the most abundant bacterial species, which were related to the metabolism and transformation of nitrogen. The community structure of the fungus was also affected by nZVI-P-BC composite with Fusarium as the main species. PCoA analysis result suggested that bacterial community was more affected by the incubation time while fungal community was more related to the addition of different biochar and modified biochars.

The Effect of Untreated and Acidified Biochar on NH3-N Emissions from Slurry Digestate

The development of new options to reduce ammonia (NH3) emissions during slurry manure storage is still required due to the shortcomings of the current technologies. This study aimed to identify to what extent untreated and acid-treated biochar (BC) and pure acids could reduce ammonia nitrogen (NH3-N) volatilization and increase nitrogen retention in slurry digestate. The NH3-N emissions were effectively reduced by H2SO4 and H3PO4 acids, untreated BC when applied mixed into the digestate and acidified BC treatments applied on the surface of the digestate. Acidification increased the specific surface area and number of O-containing surface functional groups of the BC and decreased the pH, alkalinity and the hydrophobic property. Compared to untreated BC, the ability of BC to reduce NH3-N emissions was greater when it was acidified with H2SO4 and applied to the digestate surface. The effect on digestate pH of acidified BC when applied mixed into the digestate was not different, except for H2O2, from that of the addition of the respective pure acid to digestate. The total N concentration in digestate was not significantly correlated with NH3-N emissions. These findings indicate that acidified BC could be an effective conditioner to reduce NH3-N emissions from slurry digestate storage.

Effects of solid oxygen fertilizers and biochars on nitrous oxide production from agricultural soils in Florida

Elevated levels of nitrous oxide (N₂O) emissions are a matter of concern in agricultural soils especially when flooding (hypoxic conditions) results from over irrigation or frequent rains. This study is the first to report the use of two solid oxygen fertilizers (SOFs, calcium peroxide and magnesium peroxide) to reduce N₂O production in mineral and organic soils amended with N fertilizer in a short-term laboratory incubation besides two biochars. In general, organic soil had greater N₂O production than mineral soil. Soils amended with nitrogen fertilizer exhibited increased N₂O production, by 74 times in mineral soil and 2 times in organic soil. Both solid oxygen fertilizers in mineral soil (98–99%) and calcium peroxide in organic soil (25%) successfully reduced N₂O production than corresponding N fertilized treatments. Additionally, a greater level of available nitrate–N (52–57 and 225 mg kg⁻¹ in mineral and organic soil, respectively) was recorded with the solid oxygen fertilizers. Corn residue biochar with N fertilizer increased N₂O production in mineral soil but decreased in organic soil, while pine bark biochar with N did not affect the N₂O production in either soil. Depending on soil, appropriate SOFs applied were able to reduce N₂O production and maintain greater nitrate–N levels in flooded soil. Thus, solid oxygen fertilizers can potentially be used as an effective way to reduce N₂O emission from hypoxic soil in agricultural production systems.

Nitrous oxide emissions in response to straw incorporation is regulated by historical fertilization

The incorporation of crop straw with fertilization is beneficial for soil carbon sequestration and cropland fertility improvement. Yet, relatively little is known about how fertilization regulates the emissions of the greenhouse gas nitrous oxide (N₂O) in response to straw incorporation, particularly in soils subjected to long-term fertilization regimes. Herein, the arable soil subjected to a 31-year history of five inorganic or organic fertilizer regimes (unfertilized; chemical fertilizer application, NPK; 200% NPK application, 2 × NPK; manure application, M; NPK plus manure application, NPKM) was incubated with and without rice straw to evaluate how historical fertilization influences the impact of straw addition on N₂O emissions. The results showed that compared to the unfertilized treatment, historical fertilization strongly increased N₂O emissions by 0.48- to 34-fold, resulting from increased contents of hot water-extracted organic carbon (HWEOC), NO₃^-, and available phosphorus (Olsen-P). Straw addition had little impact on N₂O emission from the unfertilized and NPK treatments, primarily due to Olsen-P limitation. In contrast, straw addition increased N₂O emissions by 102-316% from the 2 × NPK, M, and NPKM treatments as compared to the corresponding straw-unamended treatments. These results indicated that N₂O emissions in response to straw addition were largely regulated by historical fertilization. The N₂O emissions were closely associated with the depletion of NO₃^- and decoupled from change in NH₄⁺ content, suggesting that NO₃^- was the main substrate for N₂O production upon straw addition. The stoichiometric ratios of HWEOC to mineral N and mineral N to Olsen-P were key factors affecting N₂O emissions, underscoring the importance of resource stoichiometry in regulating N₂O emissions. In conclusion, historical fertilization largely regulated the impacts of crop straw incorporation on N₂O emissions via shifts in NO₃^- depletion and the stoichiometry of HWEOC, mineral N, and Olsen-P.

Global nitrogen input on wetland ecosystem: The driving mechanism of soil labile carbon and nitrogen on greenhouse gas emissions

Greenhouse gas emissions from wetlands are significantly promoted by global nitrogen input for changing the rate of soil carbon and nitrogen cycling, and are substantially affected by soil labile carbon and nitrogen conversely. However, the driving mechanism by which soil labile carbon and nitrogen affect greenhouse gas emissions from wetland ecosystems under global nitrogen input is not well understood. Working out the driving factor of nitrogen input on greenhouse gas emissions from wetlands is critical to reducing global warming from nitrogen input. Thus, we synthesized 72 published studies (2144 paired observations) of greenhouse gas fluxes and soil labile compounds of carbon and nitrogen (ammonium, nitrate, dissolved organic carbon, soil microbial biomass nitrogen and carbon), to understand the effects of labile carbon and nitrogen on greenhouse gas emissions under global nitrogen input. Across the data set, nitrogen input significantly promoted carbon dioxide, methane and nitrous oxide emissions from wetlands. In particular, at lower nitrogen rates (<100 kg ha^-1·yr^-1) and with added ammonium compounds, freshwater wetland significantly promoted carbon dioxide and methane emissions. Peatland was the largest nitrous oxide source under these conditions. This meta-analysis also revealed that nitrogen input stimulated dissolved organic carbon, ammonium, nitrate, microbial biomass carbon and microbial biomass nitrogen accumulation in the wetland ecosystem. The variation-partitioning analysis and structural equation model were used to analyze the relationship between the greenhouse gas and labile carbon and nitrogen further. These results revealed that dissolved organic carbon (DOC) is the primary factor driving greenhouse gas emission from wetlands under global nitrogen input, whereas microbial biomass carbon (MBC) more directly affects greenhouse gas emission than other labile carbon and nitrogen.

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

Nitrous oxide (N₂O) is a pervasive greenhouse gas, and soil management practices greatly affect its release into the atmosphere. Soil pH management (particularly increasing the pH) using biochar can seriously affect soil N₂O emissions. The current incubation experiment was conducted to explore the response of N₂O emissions from acidic soils using various doses of biochar. Soil with a pH of 5.48 was treated with rice straw biochar at different doses (0%, 1% and 2%) and incubated with 60% water-filled pore spaces (WFPS). The experiment was conducted in a completely randomized design (CRD) with three replications. The soil N₂O emissions, pH, NH₄⁺-N, NO₃^--N, microbial biomass carbon (MBC), and nosZ and nirK gene abundance were determined at various intervals throughout the study. The biochar application (2%) increased the soil pH (from 5.48 to 6.11), triggered the transformation of nitrogen, and augmented the abundance of nosZ and nirK genes. Higher magnitudes of cumulative soil N₂O emissions (48.60 μg kg^-1) were noted in the control (no biochar) compared to 1% (28.10 μg kg^-1) and 2% (14.50 μg kg^-1) biochar application. The 2% biochar application more effectively decreased the soil N₂O emissions, mainly because of the increased nosZ and nirK gene abundance at higher soil pH levels. The findings suggest that the amelioration of acidic soil with rice straw biochar can considerably control soil N₂O emissions by elevating the soil pH and the abundance of nosZ and nirK genes.

Nitrogen turnover and N2O/N2 ratio of three contrasting tropical soils amended with biochar

Biochar has been reported to reduce emission of nitrous oxide (N₂O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N₂O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The ¹⁵N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N₂O emissions measurements. Direct measurements of N₂ emissions were conducted using the helium gas flow soil core method. N₂O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N₂O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH₄ ⁺ immobilization and NO₃ ^-consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N₂O emissions suggested that the inhibiting effect of biochar on soil N₂O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N₂ emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N₂O and N₂ flux data thus support an explanation of direct promotion of gross N₂O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya.

Effects of Different Biochars on Wheat Growth Parameters, Yield and Soil Fertility Status in a Silty Clay Loam Soil

The conversion of organic wastes into biochar via the pyrolysis technique could be used to produce soil amendments useful as a source of plant nutrients. In this study, we investigated the effects of fruit peels and milk tea waste-derived biochars on wheat growth, yield, root traits, soil enzyme activities and nutrient status. Eight amendment treatments were tested: no amendment (CK), chemical fertilizer (CF), banana peel biochar 1% (BB1 + CF), banana peel biochar 2% (BB2 + CF), orange peel biochar 1% (OB1 + CF), orange peel biochar 2% (OB2 + CF), milk tea waste biochar 1% (TB1 + CF) and milk tea waste biochar 2% (TB2 + CF). The results indicated that chlorophyll values, plant height, grain yield, dry weight of shoot and root were significantly (p < 0.05) increased for the TB2 + CF treatment as compared to other treatments. Similarly, higher contents of nutrients in grains, shoots and roots were observed for TB2 + CF: N (61.3, 23.3 and 7.6 g kg⁻¹), P (9.2, 10.4 and 8.3 g kg⁻¹) and K (9.1, 34.8 and 4.4 g kg⁻¹). Compared to CK, the total root length (41.1%), surface area (56.5%), root volume (54.2%) and diameter (78.4%) were the greatest for TB2 + CF, followed by BB2 + CF, OB2 + CF, TB1 + CF, BB1 + CF, OB1 + CF and CF, respectively. However, BB + CF and OB + CF treatments increased β-glucosidase and dehydrogenase, but not urease activity, as compared to the TB + CF amendment, while all enzyme activity decreased with the increased biochar levels. We concluded that the conversion of fruit peels and milk tea waste into biochar products contribute the benefits of environmental and economic issues, and should be tested as soil amendments combined with chemical fertilizers for the improvement of wheat growth and grain yield as well as soil fertility status under field conditions.

Aged biochar alters nitrogen pathways in bauxite-processing residue sand: Environmental impact and biogeochemical mechanisms

Low nitrogen (N) content and retention in bauxite-processing residue sand (BRS) disposal areas pose a great challenge to the establishment of sustainable vegetation cover in this highly alkaline environment. The budget and fate of applied N in BRS and its potential environmental impacts are largely unknown. We investigated the effect of combined application of biochars [aged acidic (AC) vs alkaline pine (PC)] and di-ammonium phosphate (DAP) fertiliser on ammonia (NH₃) volatilisation, nitrous oxide (N₂O) emission and N retention in a 116-day glasshouse study. The application of AC to BRS decreased pH (≈0.5 units) in BRS, while PC biochar increased pH (≈0.3 units). The application of AC reduced NH₃ volatilisation by ca. 80%, while PC by ca. 25%. On the other hand, the AC treatment increased N₂O emission by 5 folds. However, the N loss via N₂O emission in the AC treatment only accounted for ca. 0.4% of applied N. The reduction in BRS pH and increased retention of mineral N due to the presence of oxygen-containing (phenolic and carboxylic) functional groups in AC may be responsible for reduced NH₃ volatilisation and increased N₂O emission. This study has highlighted the potential of biochar (particularly aged biochar) in improving N retention and minimising environmental impacts in highly alkaline environments.

Biochar, soil and land-use interactions that reduce nitrate leaching and N2O emissions: A meta-analysis

Biochar can reduce both nitrous oxide (N₂O) emissions and nitrate (NO₃^-) leaching, but refining biochar's use for estimating these types of losses remains elusive. For example, biochar properties such as ash content and labile organic compounds may induce transient effects that alter N-based losses. Thus, the aim of this meta-analysis was to assess interactions between biochar-induced effects on N₂O emissions and NO₃^- retention, regarding the duration of experiments as well as soil and land use properties. Data were compiled from 88 peer-reviewed publications resulting in 608 observations up to May 2016 and corresponding response ratios were used to perform a random effects meta-analysis, testing biochar's impact on cumulative N₂O emissions, soil NO₃^- concentrations and leaching in temperate, semi-arid, sub-tropical, and tropical climate. The overall N₂O emissions reduction was 38%, but N₂O emission reductions tended to be negligible after one year. Overall, soil NO₃^- concentrations remained unaffected while NO₃^- leaching was reduced by 13% with biochar; greater leaching reductions (>26%) occurred over longer experimental times (i.e. >30 days). Biochar had the strongest N₂O-emission reducing effect in paddy soils (Anthrosols) and sandy soils (Arenosols). The use of biochar reduced both N₂O emissions and NO₃^- leaching in arable farming and horticulture, but it did not affect these losses in grasslands and perennial crops. In conclusion, the time-dependent impact on N₂O emissions and NO₃^- leaching is a crucial factor that needs to be considered in order to develop and test resilient and sustainable biochar-based N loss mitigation strategies. Our results provide a valuable starting point for future biochar-based N loss mitigation studies.

Biochar amendment suppresses N2 O emissions but has no impact on 15 N site preference in an anaerobic soil

RATIONALE

Biochar amendments often decrease N₂ O gas production from soil, but the mechanisms and magnitudes are still not well characterized since N₂ O can be produced via several different microbial pathways. We evaluated the influence of biochar amendment on N₂ O emissions and N₂ O isotopic composition, including ¹⁵ N site preference (SP) under anaerobic conditions.

METHODS

An agricultural soil was incubated with differing levels of biochar. Incubations were conducted under anaerobic conditions for 10 days with and without acetylene, which inhibits N₂ O reduction to N₂ . The N₂ O concentrations were measured every 2 days, the SPs were determined after 5 days of incubation, and the inorganic nitrogen concentrations were measured after the incubation.

RESULTS

The SP values with acetylene were consistent with N₂ O production by bacterial denitrification and those without acetylene were consistent with bacterial denitrification that included N₂ O reduction to N₂ . There was no effect of biochar on N₂ O production in the presence of acetylene between day 3 and day 10. However, in the absence of acetylene, soils incubated with 4% biochar produced less N₂ O than soils with no biochar addition. Different amounts of biochar amendment did not change the SP values.

CONCLUSIONS

Our study used N₂ O emission rates and SP values to understand biochar amendment mechanisms and demonstrated that biochar amendment reduces N₂ O emissions by stimulating the last step of denitrification. It also suggested a possible shift in N₂ O-reducing microbial taxa in 4% biochar samples.

Effects of dairy manure biochar on adsorption of sulfate onto light sierozem and its mechanisms

Amendment of dairy manure biochar exhibits negative effect on sulfate adsorption onto light sierozem.

Separated pathways for biochar to affect soil N2O emission under different moisture contents

Dry land is a massive contributor to global nitrous oxide (N₂O) production and biochar is a potential material for soil amendment that can impact soil N₂O emission. Considering that the moisture content of dry land is usually changeable, it is essential to investigate the effect of biochar on soil N₂O emission under different moisture contents. Therefore, column experiments were conducted with two biochars (B300 and B500, biochars pyrolyzed at 300 and 500 °C, respectively) under five moisture contents (18%, 21%, 24%, 27% and 30%, w/w). The results showed that B300 promoted N₂O emission under the moisture contents of 18%, 21% and 24% by increasing the content of dissolved organic carbon and thus enhancing the microbial processes related to N₂O production. However, when the moisture contents were 27% and 30%, the promotion of N₂O production was overwhelmed by the improvement in N₂O reduction due to the B300 induced increase in the abundance ratio of nosZ to nirS, leading to the decrease in N₂O emission. Moreover, B500 did not alter the content of dissolved organic matter significantly and thus caused no significant change in N₂O emission when the moisture contents were 18%, 21% and 24%. But it was able to increase the abundance ratio of nosZ to nirS and thus decrease N₂O emission when the moisture contents were 27% and 30%. The results further clarified the effect of biochar on soil N₂O emission and helped to evaluate the N₂O-suppressing-potential of biochar.

Peanut-Shell Biochar and Biogas Slurry Improve Soil Properties in the North China Plain: A Four-Year Field Study

Biochar and biogas slurry have been proved to improve the quality of some soil types, but the long-term effects on fluvo-aquic soil are not fully understood. This study aimed to compare the continuity effects of peanut-shell biochar and biogas slurry on the physicochemical properties, microbial population size, and enzyme activities of fluvo-aquic soil. We conducted a four-year field experiment of winter wheat-summer maize rotation in the North China Plain. Along with equal nitrogen inputs, three treatments were applied—conventional fertilizers, peanut-shell biochar, and hoggery biogas slurry—after which various soil quality indicators were compared. Compared with those of control, both biochar and biogas slurry increased the soil total nitrogen and organic matter content, and improved soil aggregation, microbial biomass, and actinomycetes. Biogas slurry decreased soil pH and improved urease and protease activities. With biochar and biogas slurry treatments, wheat yield increased by 8.46% and 23.47%, and maize yield by 18% and 15.46%, respectively. Biogas slurry increased the content of crude protein and starch in the grains. Both biogas slurry and peanut-shell biochar improved fluvo-aquic soil nutrient content, water-stable macroaggregates, and microbial population, which might be related to their high nutrient content, large specific surface area, adsorption capacity, and functional groups. Biogas slurry generally exhibited stronger effects than biochar probably because of its richness in nutrients and bioactive substances.

Impact of hydrochar on rice paddy CH4 and N2O emissions: A comparative study with pyrochar

Hydrothermal carbonization (HTC) thermally converts wet biomasses to carbon materials, dramatically reducing energy use for drying and improving solid product yield compared to pyrolysis process. However, researches regarding agricultural usage of hydrochar (HC) are limited. In the present study, the influence of HC amendment on CH₄ and N₂O emissions, as well as global warming potential (GWP) and greenhouse gas intensity (GHGI) were investigated. Additionally, pyrochar (PC) treatments as well as two char-free control treatments with (CKU) or without (CK) N fertilizer were also included for comparison. Chars were produced from wheat straw (WC) and saw dust (SC) and applied at different rates (0.5% and 3%, w/w). Both hydrochar and pyrochar decreased paddy CH₄ emissions when amended at a lower rate (0.5%) compared to CKU treatment, which was more obvious for pyrochar when applied at the rate of 3%. Contrarily, 3%-HC significantly stimulated CH₄ emissions, which were around 5 and 3 times higher than that of CKU for WC and SC, respectively. Furthermore, hydrochar showed the potential to decrease paddy N₂O emissions (6.06-32.32%) at both application rates. However, N₂O emissions with PC treatments varied depending on application rate (20.20-75.76%). GWP and GHGI values of 0.5%-HC and PC treatments were similar, 6.67-25.00% and 3.85-25.00% lower than those of CKU treatment, respectively. However, 3%-HC amendments led to significantly increased GWP and GHGI. This study suggested that application rate of hydrochar used in rice fields should be taken into serious consideration to fulfill its potential in GHGs mitigation and minimize environmental side effects.

Offsetting global warming-induced elevated greenhouse gas emissions from an arable soil by biochar application

Global warming will likely enhance greenhouse gas (GHG) emissions from soils. Due to its slow decomposability, biochar is widely recognized as effective in long-term soil carbon (C) sequestration and in mitigation of soil GHG emissions. In a long-term soil warming experiment (+2.5 °C, since July 2008) we studied the effect of applying high-temperature Miscanthus biochar (0, 30 t/ha, since August 2013) on GHG emissions and their global warming potential (GWP) during 2 years in a temperate agroecosystem. Crop growth, physical and chemical soil properties, temperature sensitivity of soil respiration (R_s ), and metabolic quotient (qCO₂ ) were investigated to yield further information about single effects of soil warming and biochar as well as on their interactions. Soil warming increased total CO₂ emissions by 28% over 2 years. The effect of warming on soil respiration did not level off as has often been observed in less intensively managed ecosystems. However, the temperature sensitivity of soil respiration was not affected by warming. Overall, biochar had no effect on most of the measured parameters, suggesting its high degradation stability and its low influence on microbial C cycling even under elevated soil temperatures. In contrast, biochar × warming interactions led to higher total N₂ O emissions, possibly due to accelerated N-cycling at elevated soil temperature and to biochar-induced changes in soil properties and environmental conditions. Methane uptake was not affected by soil warming or biochar. The incorporation of biochar-C into soil was estimated to offset warming-induced elevated GHG emissions for 25 years. Our results highlight the suitability of biochar for C sequestration in cultivated temperate agricultural soil under a future elevated temperature. However, the increased N₂ O emissions under warming limit the GHG mitigation potential of biochar.

Assessment of N2O emissions from a fertilised vegetable cropping soil under different plant residue management strategies using 15N tracing techniques

Combined application of plant residues and N fertilisers strongly affect soil mineral N dynamics and N₂O emissions depending on the quality of the plant residues, their application methods and other management strategies. We investigated the effect of combined application of two vegetable plant residues (cauliflower and sweet corn) and ¹⁵N fertiliser on N dynamics and N₂O emission in a glasshouse pot study. The experiment was conducted under two residue management practices (soil incorporation vs surface mulching) over 98days with growing basil (Ocimum basilicum) plants. We also assessed the efficacy of applying the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) to the plant residues, for reducing N loss and mitigating N₂O emissions. Application of plant residues, both on the soil surface or into soil, resulted in net N mineralisation and increased cumulative N₂O emission compared with the application of N fertiliser alone. Soil surface mulching of sweet corn decreased total and residue-induced cumulative N₂O emission compared with the incorporation method, while it showed opposite effect on N₂O emissions from cauliflower residue. The application of DMPP with sweet corn residue reduced total, residue- and fertiliser-induced N₂O emissions; however its application with cauliflower residue did not show any mitigating effect on the N₂O emissions. The residue application methods and the use of DMPP did not significantly affect ¹⁵N recovery by the basil plants. In contrast, soil incorporation of these residues doubled the microbial immobilisation of applied ¹⁵N into soil organic matter. Linear regression analysis of N₂O emission during the experimental period indicated that in the treatments without DMPP application, soil NO₃^--N concentration was the most important factor in controlling the magnitude of N₂O emissions, while the application of DMPP changed the dominant regulating factor from NO₃^--N to NH₄⁺-N concentration.

Sulfate sorption on rape (Brassica campestris L.) straw biochar, loess soil and a biochar-soil mixture

The effects of biochar amendment on sulfur behavior in soils are unknown. In this paper, sulfate (SO₄^2-) sorption on rape (Brassica campestris L.) straw biochar produced at 600 °C (BC), loess soil (soil) and a 1:9 (w/w) biochar-soil mixture (BC-soil) was investigated by batch experiments. The effects of contact time, initial SO₄^2- concentration, temperature and solution pH value on sorption were tested. Kinetics, isotherms and thermodynamics for sorption were investigated. Pre- and post-sorption characterizations of BC and soil were respectively studied using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectroscopy, respectively. It has been shown that SO₄^2- sorption on three sorbents was well described by pseudo-second-order kinetic model. The sorption isotherms could be fitted using Langmuir and Freundlich models. BC amendment did not increase the sorption capacity of soil for SO₄^2-. The values of ΔG⁰, ΔH⁰ and ΔS⁰ indicated that the nature of sorption was spontaneous, endothermic and feasible. Increasing solution pH value led to a slight reduction in the sorption amount of SO₄^2-. Sulfate was mainly sorbed onto BC through electrostatic interaction, whereas onto the soil via electrostatic interaction and formation of poorly soluble CaSO₄.

Application of biochar and nitrogen influences fluxes of CO2, CH4 and N2O in a forest soil

Nitrogen (N) fertilization of forests for increasing carbon sequestration and wood volume is expected to influence soil greenhouse gas (GHG) emissions, especially to increase N₂O emissions. As biochar application is known to affect soil GHG emissions, we investigated the effect of biochar application, with and without N fertilization, to a forest soil on GHG emissions in a controlled laboratory study. We found that biochar application at high (10%) application rates increased CO₂ and N₂O emissions when applied without urea-N fertilizer. At both low (1%) and high biochar (10%) application rates CH₄ consumption was reduced when applied without urea-N fertilizer. Biochar application with urea-N fertilization did not increase CO₂ emissions compared to biochar amended soil without fertilizer. In terms of CO₂-eq, the net change in GHG emissions was mainly controlled by CO₂ emissions, regardless of treatment, with CH₄ and N₂O together accounting for less than 1.5% of the total emissions.