Straw returns to the soil is an effective way to improve soil organic carbon and reduce air pollution by straw burning, but this may increase CH4 and N2O emissions risks in paddy soils. Biochar has been used as a soil amendment to improve soil fertility and mitigate CH4 and N2O emissions. However, little is known about their interactive effect on CH4 and N2O emissions and the underlying microbial mechanisms. In this study, a 2-year pot experiment was conducted on two paddy soil types (an acidic Utisol, TY, and an alkaline Inceptisol, BH) to evaluate the influence of straw and biochar applications on CH4 and N2O emissions, and on related microbial functional genes. Results showed that straw addition markedly increased the cumulative CH4 emissions in both soils by 4.7- to 9.1-fold and 23.8- to 72.4-fold at low (S1) and high (S2) straw input rate, respectively, and significantly increased mcrA gene abundance. Biochar amendment under the high straw input (BS2) significantly decreased CH4 emissions by more than 50% in both soils, and increased both mcrA gene and pmoA gene abundances, with greatly enhanced pmoA gene and a decreased mcrA/pmoA gene ratio. Moreover, methanotrophs community changed distinctly in response to straw and biochar amendment in the alkaline BH soil, but showed slight change in the acidic TY soil. Straw had little effect on N2O emissions at low input rate (S1) but significantly increased N2O emissions at the high input rate (S2). Biochar amendment showed inconsistent effect on N2O emissions, with a decreasing trend in the BH soil but an increasing trend in the TY soil in which high ammonia existed. Correspondingly, increased nirS and nosZ gene abundances and obvious community changes in nosZ gene containing denitrifiers in response to biochar amendment were observed in the BH soil but not in the TY soil. Overall, our results suggested that biochar amendment could markedly mitigate the CH4 and N2O emissions risks under a straw return practice via regulating functional microbes and soil physicochemical properties, while the performance of this practice will vary depending on soil parent material characteristics.
Dissimilarity nitrate reduction to ammonium (DNRA) is of significance in agriculture ecosystems as the process is beneficial to N retention in soils. However, how fertilization regimes influence DNRA rates and functional microbes in agriculture was rarely estimated. In the present study, a 2-year pot experiment was conducted in two contrasting paddy soils to evaluate the effects of straw and nitrogen addition on DNRA process and the related functional microbes, using stable isotope tracer and molecular ecology techniques. The results showed that the abundance and transcription activity of nitrite reductase encoding gene (nrfA) involved in DNRA process and DNRA rates were significantly higher in alkaline soils than in acidic soils. Straw incorporation significantly enhanced nrfA gene abundance and transcription activity, with a greater effect in alkaline soil than in acidic soil. The rates of DNRA, abundance and transcription activity of nrfA gene positively correlated to soil C/N and C/NO 3 induced by straw application. Sequencing analysis based on nrfA gene transcript showed that Deltaproteobacteria was the most dominant group in both soil types (30.9%-67.4%), while Gammaproteobacteria, Chloroflexi, Actinobacteria were selectively enriched by straw incorporation. These results demonstrated that DNRA activity can be improved by straw return practice in paddy soils while the effect will vary among soil types due to differentiated functional microbial communities and edaphic properties.
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