Purpose

Roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification under straw return conditions are rarely discriminated, limiting our understanding on nitrogen biogeochemistry in soils amended with straw. The objective of this study was to explore the microbial mechanisms accounting for the altered rhizosphere and bulk soil denitrification with straw amendment.

Methods

In this study, straw was added into two representative paddy soils (0.5% w/w) from Yixing (YX) and Taizhou (TZ) in Jiangsu city, China, for rice growth over a 37-day period. Denitrification potentials (net N₂O emission, total denitrification, and N₂O reduction rates) of rhizosphere and bulk soils at the end of rice growth were measured using an acetylene inhibition technique. In addition, denitrifying bacterial community compositions and functional gene abundances were analyzed using Illumina sequencing and quantitative real-time PCR.

Results and discussion

For control treatment without straw addition, total denitrification and N₂O reduction rates were significantly higher in the rhizosphere than that in bulk soils; however, net N₂O emission potentials were similar between rhizosphere and bulk for TZ (22.6–53.3 vs. 9.44–46.6 mg N kg⁻¹) and YX (946 ± 126 vs. 699 ± 350 mg N kg⁻¹). Under straw return condition, N₂O emission potentials from the bulk of both soils were significantly elevated, corresponding to the increase in the relative abundance of some taxa of denitrifying bacteria (unnamed environmental samples and unclassified proteobacteria) and increase in nirK, nirS, and nosZ genes. This indicates a positive response of bulk soil denitrifying bacteria to straw addition. In contrast, straw addition produced inhibitive effects on the growth of denitrifying bacteria with decreased nirK, nirS, and nosZ gene abundances in the rhizosphere. This led to decreased N₂O emission potentials at the end of incubation for rhizosphere soils, which were 98.2% and 25.2% lower for TZ and YX compared to those of bulk soils.

Conclusions

These results suggested that the roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification shifted with straw addition, which might inspire further studies to target the denitrification hotspots to effectively mitigate greenhouse emissions at the early period of rice growth.

中文翻译：

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秸秆还田条件下散装和根际反硝化细菌在稻田反硝化中的作用

目的

在秸秆还田条件下，散装和根际反硝化细菌在稻田土壤反硝化中的作用很少被区分，这限制了我们对秸秆改良土壤中氮生物地球化学的理解。这项研究的目的是探索解决秸秆改良剂改变根际和土壤松土反硝化作用的微生物机制。

方法

在这项研究中，将秸秆添加到来自江苏省宜兴市（YX）和台州（TZ）的两种代表性稻田（0.5％w / w）中，以在37天的时间内水稻生长。使用乙炔抑制技术测量了水稻生长结束时根际和散装土壤的反硝化潜能（净N ₂ O排放，总反硝化和N ₂ O还原速率）。此外，使用Illumina测序和定量实时PCR分析了反硝化细菌群落组成和功能基因丰度。

结果与讨论

对于不添加秸秆的对照处理，根际土壤的总反硝化和N ₂ O还原率显着高于散土。但是，TZ的根际和体积之间的净N ₂ O排放潜力相似（分别为22.6-53.3和9.44-46.6 mg N kg ^-1）和YX（946±126 vs. 699±350 mg N kg ^-1）。在秸秆还田条件下，两种土壤中大部分土壤的N ₂ O排放势均显着升高，这与反硝化细菌（未命名的环境样品和未分类的变形细菌）某些分类单元的相对丰度增加以及nirK和nirS的增加有关。和nosZ基因。这表明散装土壤反硝化细菌对秸秆添加有积极反应。相反，添加秸秆对根际中的nirK，nirS和nosZ基因丰度降低，反硝化细菌的生长产生抑制作用。这导致根际土壤在培养结束时N ₂ O排放潜能降低，与散装土壤相比，TZ和YX分别降低了98.2％和25.2％。

结论

这些结果表明，秸秆的添加改变了土壤和根际反硝化细菌在稻田土壤反硝化中的作用，这可能激发了针对反硝化热点的有效研究，以有效减轻水稻生长初期的温室气体排放。