Biochar amendments and climate warming affected nitrification associated N2O and NO production in a vegetable field.

Soil nitrification driven by ammonia-oxidizing microorganisms is the most important source of nitrous oxide (N2O) and nitric oxide (NO). Biochar amendment has been proposed as the most promising measure for combating climate warming; both have the potential to regulate the soil nitrification process. However, the comprehensive impacts of different aged biochars and warming combinations on soil nitrification-related N2O and NO production are not well understood. Here, 1-octyne and acetylene were used to investigate the relative contributions of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential nitrification-mediated N2O and NO production from the fertilized vegetable soil with different aged biochar amendments and soil temperatures in microcosm incubations. Results demonstrated that AOB dominated nitrification-related N2O and NO production across biochar additions and climate warming. Biochar amendment did not significantly influence the relative contribution of AOB and AOA to N2O and NO production. Field-aged biochar markedly reduced N2O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N2O yield while fresh- and lab-aged biochar produced negligible effects on AOB-dependent N2O yield. Climate warming significantly increased N2O production and AOB-dependent N2O yield but less so on NO production. Notably, the relative contribution of AOB to N2O production was enhanced by climate warming, whereas AOB-derived NO showed the opposite tendency. Overall, the results revealed that field-aged biochar contributed to mitigating warming-induced increases in N2O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N2O yield. Our findings provided guidance for mitigating nitrogen oxide emissions in intensively managed vegetable production under the context of biochar amendments and climate warming.

Proper organic substitution attenuated both N2O and NO emissions derived from AOB in vegetable soils by enhancing the proportion of Nitrosomonas.

The ammonia oxidation process driven by microorganisms is an essential source of nitrous oxide (N2O) and nitric oxide (NO) emissions. However, few evaluations have been performed on the changes in the community structure and abundance of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) under substituting portion of chemical fertilizers with organic manure (organic substitution) and their relative contribution to the ammonia oxidation process. Here, five long-term fertilization strategies were applied in field (SN: synthetic fertilizer application; OM: organic manure; M1N1: substituting 50 % of chemical N fertilizer with organic manure; M1N4: substituting 20 % of chemical N fertilizer with organic manure; and CK: no fertilizer). We investigated the response characteristics of AOB and AOA community structures by selective inhibitor shaking assays and high-throughput sequencing and further explained their relative contribution to the ammonia oxidation process during three consecutive years of vegetable production. Compared to SN and M1N4, the potential of ammonia oxidation (PAO) was significantly reduced by 26.4 % and 22.3 % in OM and 9.5 % and 4.4 % in M1N1, resulting in N2O reductions of 38.9 % and 30.8 % (OM) and 31.2 % and 21.1 % (M1N1), respectively, and NO reductions of 45.0 % and 34.1 % (OM) and 40.1 % and 28.3 % (M1N1). RDA and correlation analyses showed that the soil organic carbon and ammonium nitrogen content increased while AOB gene abundance and diversity significantly decreased with increasing organic replacement ratio; however, the relative abundance of Nitrosomonas in AOB increased in OM and M1N1, which further demonstrates that AOB are the main driver in vegetable soils. Therefore, the appropriate proportion of organic substitution (OM and M1N1) could decrease the N2O and NO emissions contributed by AOB by affecting the soil physicochemical properties and AOB community structure.

Contrasting effects of different field-aged biochars on potential methane oxidation between acidic and saline paddy soils.

There is recognition that biochar addition is an appropriate measure to mitigate methane (CH4) emissions by promoting potential methane oxidation (PMO) in the field. However, the mechanism for different field-aged biochars and effective duration after field application are not well documented. Based on a long-term field experiment, biochar was field aged and separated from two contrasting acidic (Ba) and saline (Bs) paddy fields. Then, the effects of different aged biochars on PMO in acidic and saline paddy soils were explored by incubation experiment. There were five treatments for each soil group: soil without biochar (CK), biochar-enriched paddy soil (2 or 6 years) (NB), fresh biochar amendment (Bf), aged biochar separated from acidic paddy soil amendment (Ba), and aged biochar separated from saline paddy soil amendment (Bs). Results showed that saline paddy soils had a significantly higher PMO than acidic paddy soils under treatment without biochar, and that PMO in acidic paddy soil was enhanced by various biochar amendments, whereas those biochar amendments had no significant effects on PMO in saline paddy soil. PMO was positively correlated with pmoA abundance, N consumption rate and pH of soil-biochar mixture. Aged biochar separated from different fields had conflicting influences on soil pH, N consumption rate and PMO. Ba lost its initial effect on changing PMO as compared to Bf treatment when added back into acidic paddy soil. To the contrary, the acidic paddy soil NB treatment containing biochar added six years before possessed the highest value of PMO among all ten treatments. This study suggested that acidic paddy soil with biochar amendment could mitigate CH4 emissions by promoting PMO for a prolonged period, though aged biochar separated from the same field had a limited impact on reducing CH4 emissions.

Soil organic carbon decomposition responding to warming under nitrogen addition across Chinese vegetable soils.

Chemical fertilization in excess and warming disrupt the soil microbes and alter resource stoichiometry, particularly in intensive vegetable soils, while the effects of these variables on the temperature sensitivity of soil organic carbon (SOC) decomposition (Q10) and SOC stability remain elusive. Thus, we collected six long-term vegetable soils along a climatic gradient to examine the microbial mechanisms and resource stoichiometry effects on fluctuations in Q10 and SOC stability induced by warming and fertilization from vegetable soils. Our results showed that the SOC decomposition was dominated by microbes and regulated by stoichiometry. Compared to cold sites, higher Q10 of SOC decomposition was observed in warm sites, accompanied by lower enzyme activities, microbial CUE, and C:N ratio. In this context, warming reduced SOC stability as evidenced by up to 31.8% greater Q10 (1.45) at warm sites than at cold sites (1.10) owing to less richness of microbial communities and lower microbial CUE. The relatively lower pH and labile organic C value restricted the development of microbial richness, and decreased C- and N-related enzyme activities and a lower C:N ratio resulted in microbial CUE reduction. Additionally, N fertilization altered the C:N imbalance and enhanced SOC stability in vegetable soils, exhibiting an increase of Q10 values, particularly of great importance in warm sites. Collectively, our findings emphasize the importance of the microbial mechanism and resource stoichiometry in predicting variations in Q10 and fluctuations in SOC stability, and provide theoretical advice on improving management policies in the context of warming and fertilization from vegetable soils.

N2O and NO production and functional microbes responding to biochar aging process in an intensified vegetable soil.

Vegetable soils with high nitrogen input are hotspots of nitrous oxide (N2O) and nitric oxide (NO), and biochar amended to soil has been documented to effectively decrease N2O and NO emissions. However, the aging effects of biochar on soil N2O and NO production and the relevant mechanisms are not thoroughly understood. A15N tracing microcosm study was conducted to clarify the responses of N2O and NO production pathways to the biochar aging process in vegetable soil. The results showed that autotrophic nitrification was the predominant source of N2O production. Biochar aging increased the O-containing functional groups while lowering the aromaticity and pore size. Fresh biochar enhanced the AOB-amoA gene abundance and obviously stimulated N2O production by 15.5% via autotrophic nitrification and denitrification. In contrast, field-aged biochar markedly weakened autotrophic nitrification and denitrification and thus decreased N2O production by 17.0%, as evidenced by the change in AOB-amoA and nosZI gene abundances. However, the amendment with artificially lab-aged biochar had no effect on N2O production. With the extension of aging time, biochar application reduced the soil NO production dominated by nitrification. Changes in the N2O and NO fluxes were closely associated with soil NH4+-N and NO2--N contents, indicating that autotrophic nitrification played a critical role in NO production. Overall, our study demonstrated that field-aged biochar suppressed N2O production via autotrophic nitrification and denitrification by regulating associated functional genes, but not for lab-aged biochar or fresh biochar. These findings improved our insights regarding the implications of biochar aging on N2O and NO mitigation in vegetable soils.

Biochar addition stabilized soil carbon sequestration by reducing temperature sensitivity of mineralization and altering the microbial community in a greenhouse vegetable field.

Biochar is widely used for soil carbon sequestration and fertility improvement. However, the effects of biochar interacted with nitrogen (N) on the mineralization of soil organic carbon (SOC) and microbial community have not been thoroughly understood, particularly no reports have been published on the long term effects of biochar in vegetable field. Here, we examined soil properties, SOC mineralization and microbial community affecting by biochar (0, 20 and 40 t ha-1; C0, C1 and C2, respectively), N (0 or 240 t ha-1; N0 or N1, respectively) and their interaction in a greenhouse vegetable field. Results indicated that biochar addition increased soil pH, SOC, recalcitrant C pool, especially for the 40 t ha-1 treatment. Biochar addition generally decreased soil C-cycling enzyme activity while increasing N and P-cycling enzyme and oxidase activities. Biochar combined with N addition reduced SOC mineralization rate and metabolic quotient (qCO2) by 10.2-22.0% and 6.85-30.4%, respectively, across 15-35 °C and the temperature sensitivity (Q10) by 0.96-4.70%, except for the N1C2 at 25-35 °C. Apparent changes in bacterial alpha diversity and community structures were observed among treatments. Besides, biochar mixed with N application significantly enhanced the relative abundance of Proteobacteria and decreased Acidobacteria, while did not result in significant differences in fungal diversity and community composition. Redundancy analysis indicated that the microbial community composition shifts induced by the interaction between N and biochar were attributed to the changes in soil chemical properties, such as pH and SOC. Overall, the combination of biochar and N fertilizer is recommended to improve SOC sequestration potential and regulate bacterial community diversity and composition in vegetable field for sustainable intensification.

Linkages of nitrogen-cycling microbial resistance and resilience to soil nutrient stoichiometry under dry-rewetting cycles with different fertilizations and temperatures in a vegetable field.

Multiple dry-rewetting (DRW) cycles occur in intensively managed vegetable fields due to frequent tillage and irrigation. Soil nitrogen (N) cycling depends on the resistance and resilience of related microbial populations to DRW cycles, which could be closely related to soil nutrient status. However, the linkage of N-cycling microbial resistance and resilience and soil nutrient stoichiometry remains unknown in vegetable field. Here, we established four fertilization treatments in a four-year greenhouse vegetable field: no N fertilization, synthesized N fertilization, substituting 50% of chemical N with organic fertilizer or biofertilizer. Then, we set up an 85-day DRW-cycling incubation at 15, 25 and 35 °C including a 55-day fluctuating moisture for microbial resistance and then a 30-day constant moisture for microbial resilience. The results showed that microbial resistance was high (resistance index = 0.87- 0.99) in response to DRW cycles, but microbial resilience was generally low (resilience index = -0.36- 0.76), especially in 50% organic substitution or 15 °C. N-cycling microbes showed an important trade-off between their resistance and resilience to DRW cycles. Furthermore, most treatments showed microbial carbon limitation and N abundance during DRW cycles and recovered gradually to the undisturbed state. Microbial resistance was significantly related to the soil nutrient stoichiometry of carbon, N and phosphorus, while microbial resilience was mainly correlated with carbon-related indicators. In conclusion, N-cycling microbes presented good stability with oligotrophic strategy to frequent DRW cycles, which was linked to not only the historical legacy effect of DRW cycles but also soil nutrient stoichiometry in the vegetable field.

Biochar amendment mitigated N2O emissions from paddy field during the wheat growing season.

Biochar may variably impact nitrogen (N) transformation and N-cycle-related microbial activities. Yet the mechanism of biochar amendment on nitrous oxide (N2O) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (15N-18O) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to N2O production. Then the field experiment provided quantitative data on dynamic N2O emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using 15N-18O isotope, biochar decreased N2O emission derived from ND (by 45-94%), HD (by 35-46%) and NCD (by 30-64%) compared to the values under N application. Biochar increased the relative contribution of NN to total N2O production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived N2O. The field experiment found that the majority of the N2O emissions peaked following fertilization, in parallel with soil NH4+ and nitrite dynamics. Soil N2O emissions during the wheat growing stage were effectively decreased (by 38-48%) by biochar amendment. Based on the correlation analyses and random forest analysis in both microcosm and field experiments, the decrease in nitrite concentration (by 62-65%) and increase in N2O consumption were mainly responsible for net N2O mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS, nirK and fungal nirK) and N2O reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated N2O emissions by weakening the ND processes, since NCD and HD contributed little during the N2O emission "peaks" following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during N2O emission peaks and extrapolated to all agroecosystems.

Organic substitutions aggravated microbial nitrogen limitation and decreased nitrogen-cycling gene abundances in a three-year greenhouse vegetable field.

Partially substituting chemical fertilizer with organic fertilizer has substantially changed the stoichiometric imbalances of carbon (C), nitrogen (N) and phosphorus (P) between microbial communities and their available resources in agroecosystems. However, how organic substitution alters microbial nutrient limitation and then affects soil N cycle in intensive greenhouse vegetable ecosystem, remain unknown. Thus, we performed a three-year greenhouse vegetable field experiment in China with different fertilization strategies: no N fertilization, chemical N fertilization, and substituting 20% (1M4N) or 50% (1M1N) of chemical N with organic fertilizer (organic substitutions). Our results demonstrated that the microbial communities presented N limitation, accompanying with a strong N:P but a weak C:N (or P) microbial homeostasis in response to high N:P imbalance among all treatments. Organic substitutions at 1M1N and 1M4N significantly aggravated microbial N limitation and decreased the gene abundances of nitrification and denitrification by 4.7%-27.3% than that of chemical N fertilization. Microbial N limitation was strongly influenced by N:P stoichiometric imbalance illustrated from regression analysis. The N-cycling gene abundances were not only dependent on the inorganic N pool and soil physicochemical properties (i.e. pH and electrical conductivity), but also affected by microbial nutrient limitation inferred from random forest analysis. Furthermore, the 1M1N treatment performed better than the 1M4N in terms of improved crop yield and less microbial N limitation. Overall, these results highlight the importance of ecological stoichiometry in regulating soil N cycle under different fertilization strategies for intensive greenhouse vegetable ecosystem.

The effect of long-term biochar amendment on N2O emissions: Experiments with N15-O18 isotopes combined with specific inhibition approaches.

Numerous studies reporting a transient decrease in soil nitrous oxide (N2O) emissions after biochar amendment have mainly used short-term experiments. Thus, long-term field trials are needed to clarify the actual impact of biochar on N2O emissions and the underlying mechanisms. To address this, both a 15N18O labeling technique and gene analyses were applied to investigate how N2O production pathways and microbial mediation were affected by long term biochar amendment in field. Then, 1-octyne and 2-phenyl l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) were used in combination with potassium chlorate to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and the associated N2O production. Acidic and alkaline greenhouse vegetable soils that had each received two separate treatments were collected (control, no biochar amendment; biochar, biochar amended in the field after 2 or 7 years). The results showed that biochar decreased N2O emissions by 48% in acidic soils and by 22% in alkaline soils compared to those in control. These results were explained by decreases in nitrifier denitrification- (by 74%) and heterotrophic denitrification-derived N2O production (by 58%), as further evidenced by a decrease in NO2- (by 87%) and the (nirK+nirS+fungal nirK):(nosZ-I + nosZ-II) ratio (by 5%) in both greenhouse vegetable soils. However, biochar increased nitrifier nitrification-derived N2O in both soils because of increases in pH and PAO, which were attributed to an increased abundance of AOB rather than AOA. The contribution of AOB to PAO (or N2O) exceeded 69% (or 68%) of the total in acidic soil and 88% (or 85%) of the total in alkaline soil after biochar amendment. Our findings demonstrated that the mitigation of N2O by biochar is linked to specific N2O production pathways.

Temperature decouples ammonia and nitrite oxidation in greenhouse vegetable soils.

The influence of temperature on soil ammonia (NH3) and nitrite (NO2-) oxidation and related NO2- accumulation in soils remain unclear. The soil potential NH3 oxidation (PAO) and NO2- oxidation (PNO) rates were evaluated over a temperature gradient of 5-45 °C in six greenhouse vegetable soils using inhibitors. The values of temperature sensitivity traits such as temperature minimum (Tmin), temperature optimum (Topt), and maximum absolute temperature sensitivity (Tm_sens) were also fitted to the square root growth (SQRT) and macromolecular rate theory (MMRT) models. The ammonia-oxidizing archaea (AOA) and bacteria (AOB) were determined by quantifying amoA, and nitrite-oxidizing bacteria (NOB) were determined by quantifying the nxrA and nxrB. Both models identified that Topt for PAO (34.0 °C) was significantly greater than that for PNO (26.0 °C). The Tm_sens (23.4 ± 2.1 °C) and Tmin (1.0 ± 2.0 °C) for PAO were higher than those for PNO (16.8 ± 3.2 °C and - 11.7 ± 6.7 °C). PAO was positively correlated with AOB-amoA at 20-30 °C and with AOA-amoA at 30-35 °C, while PNO was positively correlated with nxrB at 5-30 °C. Additionally, NO2- and N2O were positively correlated with the (AOA + AOB amoA) to (nxrA + nxrB) ratio, and the concentration of N2O was positively correlated with NO2- accumulation. These results highlight that elevated temperatures resulted in the uncoupling of NH3 oxidation and NO2- oxidation, leading to NO2- accumulation, which could stimulate N2O emissions.

Biochar-enriched soil mitigated N2O and NO emissions similarly as fresh biochar for wheat production.

Biochar amendment has been recommended as a potential strategy to mitigate nitrous oxide (N2O) and nitric oxide (NO) emissions for wheat production, but its mechanism and effective duration are not well understood. The 1-octyne and 2-pheny l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in combination with potassium chlorate were used to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and N2O and NO production as affected by biochar. Acidic and alkaline soils were collected during wheat-growing season, and four treatments were installed in each soil type: CK, urea alone; BE, biochar-enriched soil for 2-6 years; FB, fresh biochar added to CK; and AB, aged biochar added to CK. The results showed that octyne and PTIO efficiently assessed AOB and AOA activities in soil incubation. The AOB-driven PAO in acidic soil was largely enhanced by increased soil pH in BE and FB treatments, whereas AOA-driven PAO was not. And the contribution of AOB to PAO exceeded 80% in alkaline soil. The N2O and NO production were positively correlated with PAO in both soils. BE treatment decreased the direct N2O and NO production in alkaline soil, while both BE and FB treatments decreased the N2O and NO yields in acidic soil, indicating that biochar mitigated soil N2O and NO emissions for wheat production. The lack of differences between AB and CK treatments indicated that aged biochar lost its initial effects on PAO, while the biochar-enriched soil amended with biochar years earlier still functioned similarly as fresh biochar.

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

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

Aged biochar stimulated ammonia-oxidizing archaea and bacteria-derived N2O and NO production in an acidic vegetable soil.

Both nitrous oxide (N2O) and nitric oxide (NO) emissions are typically high in greenhouse-based high N input vegetable soils. Biochar amendment has been widely recommended for mitigating soil N2O emissions in agriculture. However, knowledge of the regulatory mechanisms of fresh and aged biochar for both N2O and NO production during ammonia oxidation is lacking. Two vegetable soils with different pH values were used in aerobic incubation experiments with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), 1-octyne and acetylene. The relative importance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) to N2O and NO production was investigated as influenced by fresh and aged biochar amendments. The results showed that AOA dominated N2O production in acidic soil, while AOB dominated N2O production in alkaline soil. Aged biochar stimulated both AOA- and AOB-derived N2O and NO production by 84.8 and 340%, respectively, in acidic soil but only increased AOA-derived N2O and NO production in alkaline soil. Fresh biochar amendment increased AOA- and AOB-derived NO in acidic soil and AOA-derived NO in alkaline soil but had negligible effects on AOA- and AOB-derived N2O in both soils. Fresh biochar decreased AOA-amoA but increased AOB-amoA gene abundances in acidic soil, whereas aged biochar increased AOA- and AOB-amoA gene abundances in both soils. These findings improved our understanding of N2O and NO production mechanisms under different biochar amendments in alkaline and acidic vegetable soils.

Field-aged biochar reduces the greenhouse gas balance in a degraded vegetable field treated by reductive soil disinfestation.

Reductive soil disinfestation (RSD) is proposed as a pre-plant, non-chemical soil disinfestation technique to control several soilborne phytosanitary issues. However, limited information is available on the evaluation of greenhouse gas (GHG) balance and soil quality during the soil remediation process as affected by RSD method. A 44-day field experiment including four different treatments was conducted to investigate the effects of conventional RSD and field-aged biochar-amended RSD on GHG balance and soil quality in a degraded vegetable field. Results showed that the conventional RSD application can significantly decrease the soil nitrate (NO3-) concentrations and electrical conductivity (EC) and oxidation-reduction potential (Eh) by 51.4-67.3%, 5.3-23.6%, and 10.9-15.1%, respectively, while significantly increase soil pH and cation exchange capacity (CEC) by 0.37-0.42 units and 7.8-32.2%, respectively, in relation to the control (CK). Compared with the conventional RSD treatment, aged biochar-amended RSD significantly reduced soil NO3- concentrations, EC and Eh. No significant differences on CH4 emissions were observed among all the treatments during the experimental period. However, the conventional RSD application significantly increased the cumulative nitrous oxide (N2O) and carbon dioxide (CO2) emissions by 66.2-124.7% and 64.3-130.0%, respectively, and thus resulted in a significant GHG balance of 64.1-130.1% in relation to the CK. On the contrary, although resulted in more N2O emissions compared with the conventional RSD treatment, aged biochar-amended RSD significantly reduced the cumulative CO2 emissions and thus had an overall decrease in GHG balance by 20.7-28.7%. Therefore, aged biochar-amended RSD can simultaneously achieve lower GHG balance and better improvement of soil quality in degraded vegetable field, and thus can be utilized as an effective technology for soil remediation in intensive vegetable production.

Overdose fertilization induced ammonia-oxidizing archaea producing nitrous oxide in intensive vegetable fields.

Little is known about the effects of nitrogen (N) fertilization rates on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) and their differential contribution to nitrous oxide (N2O) production, particularly in greenhouse based high N input vegetable soils. Six N treatments (N1, N2, N3, N4, N5 and N6 representing 0, 293, 587, 880, 1173 and 1760 kg N ha-1 yr-1, respectively) were continuously managed for three years in a typically intensified vegetable field in China. The aerobic incubation experiment involving these field-treated soils was designed to evaluate the relative contributions of AOA and AOB to N2O production by using acetylene or 1-octyne as inhibitors. The results showed that the soil pH and net nitrification rate gradually declined with increasing the fertilizer N application rates. The AOA were responsible for 44-71% of the N2O production with negligible N2O from AOB in urea unamended control soils. With urea amendment, the AOA were responsible for 48-53% of the N2O production in the excessively fertilized soils, namely the N5-N6 soils, while the AOB were responsible for 42-55% in the conventionally fertilized soils, namely the N1-N4 soils. Results indicated that overdose fertilization induced higher AOA-dependent N2O production than AOB, whereas urea supply led to higher AOB-dependent N2O production than AOA in conventionally fertilized soils. Additionally, a positive relationship existed between N2O production and NO2- accumulation during the incubation. Further mechanisms for NO2--dependent N2O production in intensive vegetable soils therefore deserve urgent attention.

Effects of different composting strategies on methane, nitrous oxide, and carbon dioxide emissions and nutrient loss during small-scale anaerobic composting.

Composting is considered as one of the main sustainable methods for the treatment of livestock manure. In this study we investigated the effects of additives (urea and rice straw) on methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) emissions using a traditional Chinese pig slurry composting method over an 81-day period, as well as examining total organic carbon and total nitrogen loss. Four common treatment strategies were examined in this study: a control (MC), urea nitrogen addition (MN), composting using rice straw cover (MScover), and compost mixed with rice straw (MSmix). Our results indicate that the addition of urea resulted in the lowest total CH4 emissions and the highest N2O emissions. MScover treatment had the highest and most significant effect on CH4 emissions, while MSmix treatment had the lowest CO2 emissions. Carbon lost through CH4 and CO2 released during the experiment was 0.1-0.9 and 2.4-3.9% of total carbon loss, respectively, and nitrogen lost through N2O release was 11.1-17.9% of total nitrogen. In general, although MSmix, MScover, and MN treatments increased global warming potential by 21.4, 41.6, and 50.9% per kg of pig slurry, respectively, no statistical differences between the four treatments were recorded. By considering carbon and nitrogen conservation, as well as the improvement of the quality of compost and the mitigation of greenhouse gases (GHGs), the small-scale composting method of pig slurry alone is an acceptable environmentally friendly strategy for use in China.

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

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

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

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