Un-fertilized rice paddies have shown maintained soil nitrogen (N) status, stable N supply to the rice plant and sustained rice yields at moderate levels for hundreds of years. Microbial N 2 fixation is known to contribute N to un-fertilized paddies, but it cannot fully explain the maintained N nutrition, where favourable conditions exist for N loss by denitrification. We used 15 N tracer, 15 N 2 uptake, acetylene reduction assay and qPCR to simultaneously investigate N 2 fixation, dissimilatory nitrate reduction to ammonium (DNRA), denitrification and related microbial gene abundances in long-term low (or no) and high N input rice paddies of Myanmar. We also determined how varying soil organic carbon-to-nitrate (SOC/NO 3 − ) ratios affect nitrate partitioning between DNRA and denitrification by manipulating these ratios through labile organic carbon addition. We observed more than 2.5 times higher N 2 fixation (1.49–2.08 μg N g −1 soil day −1 ) and significantly higher N 2 fixing gene ( nifH ) abundance in low compared with high N input paddies. Up to 60% of the soil nitrate (1.51–2.67 μg NO 3 − -N g −1 soil day −1 ) was ammonified through DNRA, and only 15% was lost as N 2 through denitrification in low N input paddies, whereas denitrification exceeded DNRA in high N input paddies. The microbial gene related to DNRA activity ( nrfA ) was also higher in low input than in high input rice paddies. We found that nitrate retention can be improved in high N input rice paddies by maintaining a higher soil organic carbon-to-nitrate ratio. Our findings highlight the unique microbial N-cycling strategies in resource-limited paddies which support maintained N nutrition of the paddy system.

中文翻译：

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异化硝酸盐氨化和固氮有助于维持资源有限稻田的氮营养

数百年来，未施肥的稻田一直保持土壤氮 (N) 状态，水稻植株的氮供应稳定，水稻产量保持在中等水平。已知微生物 N 2 固定为未施肥的稻田贡献 N，但它不能完全解释维持的 N 营养，其中存在有利于反硝化 N 损失的条件。我们使用 15 N 示踪剂、15 N 2 吸收、乙炔还原测定和 qPCR 来同时研究 N 2 固定、异化硝酸盐还原成铵 (DNRA)、反硝化作用以及长期低氮（或无氮）和高氮条件下的相关微生物基因丰度输入缅甸的稻田。我们还确定了不同的土壤有机碳与硝酸盐 (SOC/NO 3 - ) 比率如何通过添加不稳定的有机碳来操纵这些比率来影响 DNRA 和反硝化作用之间的硝酸盐分配。我们观察到，与高 N 输入稻田相比，低 N 2 固定（1.49–2.08 μg N g -1 土壤日 -1 ）高出 2.5 倍以上，且 N 2 固定基因 ( nifH ) 丰度显着更高。高达 60% 的土壤硝酸盐（1.51–2.67 μg NO 3 - -N g -1 土壤日 -1 ）通过 DNRA 氨化，只有 15% 通过低氮输入稻田中的反硝化作用以 N 2 形式损失，而反硝化作用在高氮输入稻田中超过 DNRA。与 DNRA 活性相关的微生物基因 (nrfA) 在低投入稻田中也高于高投入稻田。我们发现通过保持较高的土壤有机碳与硝酸盐的比率，可以提高高氮输入稻田的硝酸盐截留率。我们的研究结果突出了资源有限稻田中独特的微生物 N 循环策略，该策略支持稻田系统的 N 营养维持。