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Carbon (C) allocation plays an important role in plant adaptation to water and nutrient stresses. However, the effects of drought and nutrient deficiencies on the allocation of recently fixed C in the plant–soil–microbe system remain largely unknown. Herein, we studied the response of C allocation of Sophora moorcroftiana (an indigenous pioneer shrub in Tibet) to drought, nitrogen (N) deficiency and phosphorus (P) deficiency using a microcosm experiment. The 13CO2 continuous labeling was used to trace C allocation in the plant–soil–microbe system. We found that drought significantly reduced plant 13C, but it increased 13C accumulation in soil. The decreased plant 13C under drought was attributed to the decrease of 13C in stem and root rather than that in leaf. The excess 13C fraction in the microbial biomass (MB13C) was reduced by N deficiency, but it was not affected by the combination of drought and N deficiency, indicating that drought weakened the effects of N deficiency on MB13C. By contrast, MB13C increased under the combination of drought and P deficiency, suggesting that drought enhanced the effects of P deficiency on MB13C. Drought and nutrient deficiencies regulated the belowground 13C allocation. Specifically, drought and P deficiency increased the allocation of 13C to root and N deficiency regulated the allocation of 13C to microbial biomass C and dissolved organic C in soil. Notably, soil 13C decreased with increasing plant 13C, while MB13C first decreased and then increased with increasing plant 13C. Overall, our study demonstrated that drought and nutrient deficiencies interactively affected C allocation in a plant–soil–microbe system and provided insights into C allocation strategies in response to multiple resource (water and nutrient) stresses under environmental changes. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in a subtropical tea (Camellia sinensis (L.) O. Kuntze) plantation in Jiangsu Province, China, including the following treatments: no nitrogen (N) fertilizer (control), conventional mineral N fertilizer (urea) (CN), soybean cake fertilizer (SF), pig manure (PM), cattle manure (CaM), chicken manure (CM), and CM + biochar (CMB). Cumulative nitrous oxide (N2O) and nitric oxide (NO) emissions were 4.8 ± 0.1 and 3.7 ± 0.3 kg N ha−1 year−1 under CN, respectively, and increased to 5.4 ± 0.2 and 4.6 ± 0.3 kg N ha−1 year−1 under SF (P < 0.05), respectively. Treatments with livestock manures (PM, CaM, and CM) reduced N2O (41.4–49.6%) and NO (46.5–59.8%) emission in comparison to CN. Combined amendment of CM and biochar more effectively reduced N2O emissions than CM treatment alone. Based on a meta-analysis of 26 global paired measurements in acid soils, the threshold of C/N ratios of organic fertilizers between the positive and negative responses of N2O emissions to organic fertilizers was 8.6 with a range of 4.5–22.3 (95% confidence interval), indicating that reduced N2O emission under PM, CaM and CM was potentially due to their C/N ratios compared to the threshold. Organic fertilizer application did not influence tea yield, while combined application of CM and biochar increased tea yield and resulted in the least yield-scaled N2O emission. N2O and NO emission factors for N fertilizers applied under CN were 1.9 ± 0.1% and 1.5 ± 0.2%, respectively, and reduced to 0.08 ± 0.04% and 0.12 ± 0.04% under CMB, respectively. The results suggest that tea plantations in the subtropical region are hotspots for N2O and NO emissions. Combined application of chicken manure and biochar could mitigate N gas emissions and increase yield in the tea plantation systems. [ABSTRACT FROM AUTHOR]

A field experiment was conducted to evaluate the influence of the continuous application of organic and mineral N fertilizer on N2O and NO emissions under maize and wheat rotation on the North China Plain. This study included eight treatments: no fertilizer (control); mineral N fertilizer (Nmin) at a rate of 200 kg N ha−1 per season; 50% mineral fertilizer N plus 50% cattle manure N (50% CM), 50% chicken manure N (50% FC) or 50% pig manure N (50% FP); 75% mineral fertilizer N plus 25% cattle manure N (25% CM), 25% chicken manure N (25% FC) or 25% pig manure N (25% FP). The annual N2O and NO emissions were 2.71 and 0.39 kg N ha−1, respectively, under the Nmin treatment, with an emission factor of 0.50% for N2O and 0.07% for NO. Compared with the Nmin treatment, N2O emissions did not differ when 50% of the mineral N was replaced with manure N (50% CM, 50% FC and 50% FP), while annual NO emissions were significantly reduced by 49.0% and 27.8% under 50% FC and 50% FP, respectively. In contrast, annual N2O emissions decreased by 21–38% compared to the Nmin treatment when 25% of the mineral N was replaced with manure N (25% CM, 25% FC and 25% FP). Most of the reduction occurred during the maize season. The 25% CM, 25% FC and 25% FP treatments had no effect on NO emissions compared to the Nmin treatment. There was no obvious difference in annual N2O and NO emissions among the organic manures at the same application rate, probably due to their similar C/N ratio. Replacing a portion of the mineral fertilizer N with organic fertilizer N did not significantly affect crop grain yield, except for the 50% FC treatment in the wheat season. Overall, the results suggest that the combined application of 25% organic manure N plus 75% mineral fertilizer N had the most potential to mitigate N2O emissions while not affecting crop yield in the maize and wheat rotation system in this area of China. [ABSTRACT FROM AUTHOR]

15N tracing was carried out on sandy loam soil amended with (i) mineral nitrogen-phosphorus-potassium fertilizer (NPK) alone, (ii) half mineral N and half N from chicken manure (HFC), or (iii) half mineral N and half N from cattle manure (HCM), for 8 years. Cumulative N2O emissions during incubation were 30.2 µg N kg− 1 in the NPK treatment, which increased to 37.8 and 51.3 µg N kg− 1 in the HFC and HCM treatments, respectively. The majority of N2O emissions in all the treatments were attributed to nitrification (81.0% in the NPK treatment, 83.0% in the HFC treatment, and 85.1% in the HCM treatment). Compared with NPK, HCM treatment caused a significant increase in the gross rate of nitrification, while HFC treatment slightly enhanced the rate of dissimilatory NO3− reduction to NH4+. Additionally, HFC treatment achieved higher gross rates of organic N mineralization, and both HFC and HCM treatments had higher NH4+ mineralization-immobilization turnover (MIAT) rates than NPK treatment. The results suggest that application of cattle or chicken manure increased soil NH4+ availability. The gross rate of NO3− adsorption in the HCM treatment was greater than that in the NPK treatment, while the release of adsorbed NO3− in the HFC treatment was slower than that in the NPK treatment, indicating that application of cattle or chicken manure lowered the potential for NO3− leaching in soil. Overall, combining cattle or chicken manure with mineral fertilizer decreased NO3− availability but increased NH4+ availability, leading to higher N2O emissions through nitrification. Our results suggest that organic manures should be applied with nitrification inhibitors in sandy loam soil containing low organic carbon to increase soil fertility and mitigate N2O emissions. [ABSTRACT FROM AUTHOR]

Biochar amendment of upland soil has been generally accepted to mitigate nitrous oxide (N 2 O) emissions. However, this is not always the case in rice paddy soil, and the underlying mechanisms are not well understood. To evaluate how biochar amendment affects N 2 O production and emissions in paddy soil, an incubation experiment was designed including six treatments: wheat straw-derived biochar (slow pyrolyzed at 400 °C) amendment at rates of 0% (Control), 1% and 4% soil mass (w/w), inorganic nitrogen (N) fertilizer amendment (with urea), and N fertilizer plus 1% biochar and 4% biochar. The application of 4% biochar significantly increased N 2 O emissions from N-unfertilized and fertilized soils during the 45-day incubation, by 291% and 256%, respectively, while 1% biochar amendment significantly increased soil N 2 O emissions when accompanied by N fertilizer addition. On day 14, when the N 2 O emission peaks occurred, N 2 O flux was significantly correlated with soil pH in all treatments. Biochar addition also enhanced the abundance of ammonia-oxidizing bacteria (AOB) amoA genes, which was significantly related to soil pH. Among all detected N 2 O-forming and reducing microbial genes, the abundance of AOB amoA genes was most closely related to N 2 O flux. On biochar addition, the AOB community structure shifted from Nitrosospira -dominated toward Nitrosomonas , and the diversity of AOB was significantly increased. Compared with the control, biochar amendment decreased, albeit not significantly, the abundance of the nitrous oxide reductase encoding gene nosZ , but did not alter the abundance of nitrite reductase encoding genes nirK and nirS . Our study suggests that wheat straw-derived biochar amendment of paddy soils increased soil pH, which in turn increased the abundance and diversity of AOB and N 2 O emissions. [ABSTRACT FROM AUTHOR]

Biochar application is thought to improve crop yield and reduce N leaching and gas emissions; however, little is known about how field-aged biochar affects fertilizer N retention and N 2 O emissions. Here, a field microplot experiment is established in the North China Plain at maize season by applying 15 N-labeled urea to the sandy loam soil both with (Biochar) and without (Control) application of 3-year field-aged biochar at 12 t ha -1 . Overall, 25.6-26.2% of the urea N was taken up by maize aboveground biomass, field-aged biochar did not affect yield or fertilizer N recovery efficiency. After maize harvest, the residual ratio of applied N in the soil profile (0-40 cm) was 21.6 and 20.3% under Control and Biochar treatment, respectively, with an increase of 10.2% in the topsoil (0-20 cm) and decrease of 37.2% in the subsoil (20-40 cm) following biochar amendment, probably due to reduced NO 3 - leaching. Cumulative N 2 O emissions and urea N-induced N 2 O emissions under Control treatment were 2.06 and 0.78 kg N ha -1 , and significantly decreased to 1.89 and 0.74 kg N ha -1 after Biochar treatment, respectively. N 2 O emissions derived from the applied N accounted for 38.0 and 39.4% of the total emissions under Control and Biochar treatment, respectively. N 2 O emissions from decomposition of soil organic N induced by the priming effect of the applied N was 0.69 and 0.56 kg N ha -1 under Control and Biochar treatment, respectively, contributing 33.7 and 29.7% of the total emissions. Overall, our results suggest that field-aged biochar increased the retention of fertilizer N in the topsoil by reducing NO 3 - leaching, while effectively reduced N 2 O emissions from fertilizer N and mineralization of organic N in the sandy loam soil., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Biochar and inhibitors applications have been proposed for mitigating soil greenhouse gas emissions. However, how biochar, inhibitors and the combination of biochar and inhibitors affect CH 4 emissions remains unclear in paddy soils. The objective of this study was to explore the effects of biochar application alone, and in combination with urease (hydroquinone) and nitrification inhibitors (dicyandiamide) on CH 4 emissions and yield-scaled CH 4 emissions during three rice growing seasons in the Taihu Lake region (Suzhou and Jurong), China. In Suzhou, N fertilization rates of 120-280 kg N ha -1 increased CH 4 emissions compared to no N fertilization (Control) (P < 0.05), and the highest emission was observed at 240 kg N ha -1 , possibly due to the increase in rice-derived organic carbon (C) substrates for methanogens. Biochar amendment combined with N fertilization reduced CH 4 emissions by 13.2-27.1% compared with optimal N (ON, Suzhou) and conventional N application (CN-J, Jurong) (P < 0.05). This was related to the reduction in soil dissolved organic C and the increase in soil redox potential. Addition of urease and nitrification inhibitor (ONI) decreased CH 4 emissions by 15.7% compared with ON treatment. Combined application of biochar plus urease, nitrification and double inhibitors further decreased CH 4 emissions by 22.2-51.0% compared with ON and CN-J treatment. ON resulted in the highest yield-scaled CH 4 emissions, while combined application of biochar alone and in combination with the inhibitors decreased yield-scaled CH 4 emissions by 12.7-54.9% compared with ON and CN-J treatment (P < 0.05). The lowest yield-scaled CH 4 emissions were observed under combined application of 7.5 t ha -1 biochar with both urease and nitrification inhibitors. These findings suggest that combined application of biochar and inhibitors could mitigate total CH 4 and yield-scaled CH 4 emissions in paddy fields in this region., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)