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nirK -Denitrifying bacteria are closely related to N 2 O emissions and they have been studied widely in agro-ecosystems. However, the mechanisms associated with maintenance of the diversity of nirK bacterial communities in agro-ecosystems are unclear. In particular, the ecological roles of "generalists" and "specialists" in nirK bacterial communities under different soil organic carbon (SOC) levels have not been characterized. In this study, we divided 102 soil samples (0–20 and 20–40 cm) from 62 apple orchards in China's main apple producing areas (Shaanxi and Shandong provinces) into four groups according to the SOC content and soil depth, i.e., high organic carbon soils in the 0–20 cm depth, low organic carbon soils in the 0–20 cm depth, high organic carbon soils in the 20–40 cm depth, and low organic carbon soils in the 20–40 cm depth. In the nirK bacterial community, 4187 operational taxonomic units (OTUs) (∼51.32 %) were classified as specialists and 1781 OTUs (∼21.83 %) as generalists. The α-diversity of generalists was higher in the high SOC soils than the low SOC soils, which was consistent with the α-diversity of the whole nirK bacterial community. However, the observed number of specialist species was lower in high SOC soils than low SOC soils. Analysis based on the Spearman's correlation coefficients also showed that the α-diversity and relative abundances of generalists and specialists responded differently to environmental factors. Both deterministic and stochastic processes contributed to the assembly of generalists and specialists. Among the deterministic processes, variable selection was important for the assembly of the generalist community, whereas homogeneous selection was important for the assembly of the specialist community. The niche breadth of generalists was higher than that of specialists, whereas the niche overlap of specialists was higher than that of generalists. The niche breadth of generalists was higher in high SOC soils than low SOC soils. In both networks, generalists had higher degrees than specialists, although the number of generalists was much lower than that of specialists. Our findings demonstrate the contributions of generalists and specialists to the changes in the diversity of the nirK bacterial community at different SOC levels as well as providing new insights into the mechanisms responsible for maintaining the diversity of the nirK bacterial community. [ABSTRACT FROM AUTHOR]

The effect of dairy manure amendments to agricultural soil on the yield-scaled nitrous oxide (N 2 O) emissions remains unclear. We hypothesize that an optimum ratio of dairy manure to synthetic fertilizers leads to large nitrogen use efficiency (NUE) and small yield-scaled N 2 O emissions. The aims of this study were to (1) quantify the variations in the crop yields and N 2 O emissions from winter wheat-summer maize cropping systems in Northwest China, (2) determine the responses of the NUE and yield-scaled N 2 O emission to the ratio of organic materials to synthetic fertilizers, and (3) evaluate the relationship between the NUE and yield-scaled N 2 O emissions. Field measurements were conducted within long- and short-term fertilization experiments between the years of 2014 and 2016. Treatments included synthetic fertilizers, synthetic fertilizers plus crop residues, and synthetic fertilizers plus dairy manure at both sites. The annual grain yields and N 2 O emissions varied from 13.3 to 18.0 Mg ha -1 and from 1.3 to 3.6 kg N ha -1 , respectively, across the treatments. The yield-scaled N 2 O emissions related negatively to the NUE, suggesting that agronomic aims of improving NUE are an effective approach to mitigate N 2 O emissions. The ratio of organic materials to synthetic fertilizers was not a significant limit on the NUE and yield-scaled N 2 O emissions. We conclude that organic amendments appeared to play a minor influence on the promotion of the NUE and N 2 O mitigation.

Application of organic manure combined with synthetic fertilizer can maintain crop yield and improve soil fertility, but the long-term effects of substituting different proportions of synthetic fertilizers with organic manure on N 2 O emission remain unclear. In this study, field experiments and DNDC model simulations were used to study the long-term effects of substituting synthetic fertilizers with organic manure on crop yield and N 2 O emission. The field experiment was conducted at Guanzhong Plain, northern China, under a wheat-maize cropping system. Six treatments were included: no fertilization (CK); synthetic nitrogen (N), phosphorus (P) and potassium (K) fertilizers (NPK); and 25%, 50%, 75% and 100% of the synthetic N substituted by dairy manure (25%M, 50%M, 75%M, and 100%M), respectively. The DNDC model was calibrated using the field data from the NPK treatment from 2014 to 2017 and was validated for the other treatments. The results showed that the DNDC model can successfully simulate the crop yield (e.g. nRMSE < 5%) and annual N 2 O emission (nRMSE < 20%). In addition, a 30-year simulation found that organic manure substitution treatments could maintain wheat yield well, and the yield variation between different years was small. However, relative to the NPK treatment, the maize yields for the first 6 and 7 years were lower under 50%M and 75%M, and under 100%M maize yields were reduced for the first 15 years. The long-term simulation showed that N 2 O emission of fertilized treatment had an increasing trend over time, especially the 75%M treatment where the N 2 O emission was higher than that of NPK treatment after 25 years of fertilization. The annual mean N 2 O emission under different treatments was, in decreasing order, NPK > 25%M > 50%M > 75%M > 100%M > CK. The yield-scale N 2 O emission and emission factor were highest for the NPK treatment. Considering crop yield, yield stability and N 2 O emission, substitution of 25% synthetic fertilizer by organic manure can simultaneously ensure crop productivity and environmental protection under the tested environment., 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.)