Graduate School of Agricultural and Life Sciences, The University of Tokyo.
Niigata Agricultural Research Institute.
Microbes Environ. 2020;35(4). doi: 10.1264/jsme2.ME20069.
Nitrification-denitrification processes in the nitrogen cycle have been extensively examined in rice paddy soils. Nitrate is generally depleted in the reduced soil layer below the thin oxidized layer at the surface, and this may be attributed to high denitrification activity. In the present study, we investigated dissimilatory nitrate reduction to ammonium (DNRA), which competes with denitrification for nitrate, in order to challenge the conventional view of nitrogen cycling in paddy soils. We performed paddy soil microcosm experiments using N tracer analyses to assess DNRA and denitrification rates and conducted clone library analyses of transcripts of nitrite reductase genes (nrfA, nirS, and nirK) in order to identify the microbial populations carrying out these processes. The results obtained showed that DNRA occurred to a similar extent to denitrification and appeared to be enhanced by a nitrate limitation relative to organic carbon. We also demonstrated that different microbial taxa were responsible for these distinct processes. Based on these results and previous field observations, nitrate produced by nitrification within the surface oxidized layer may be reduced not only to gaseous N via denitrification, but also to NH via DNRA, within the reduced layer. The present results also indicate that DNRA reduces N loss through denitrification and nitrate leaching and provides ammonium to rice roots in rice paddy fields.
氮循环中的硝化-反硝化过程在稻田土壤中得到了广泛研究。硝酸盐通常在地表薄氧化层以下的还原土壤层中耗尽，这可能归因于高反硝化活性。在本研究中，我们研究了异化硝酸盐还原为铵（DNRA），它与反硝化作用竞争硝酸盐，以挑战稻田土壤中氮循环的传统观点。我们使用 N 示踪剂分析进行稻田土壤微宇宙实验，以评估 DNRA 和反硝化作用速率，并进行亚硝酸盐还原酶基因（nrfA、nirS 和 nirK）的转录本克隆文库分析，以确定进行这些过程的微生物种群。所得结果表明，DNRA 的发生程度与反硝化作用相当，并且似乎受到相对于有机碳的硝酸盐限制的增强。我们还证明了不同的微生物类群负责这些不同的过程。基于这些结果和以前的田间观察，在地表氧化层内硝化作用产生的硝酸盐可能不仅通过反硝化作用还原为气态 N，而且还通过 DNRA 还原为 NH 在还原层内。本研究结果还表明，DNRA 通过反硝化作用和硝酸盐淋失减少氮损失，并为稻田中的水稻根系提供铵。
