化合态氮过多，是会抑制根瘤菌的结瘤固氮的。5 mM的硝酸盐即可抑制结瘤固氮。但FEMS Microbiology Ecology杂志上最新报道了3株慢生大豆根瘤菌，它们可以在高氮条件下(20 mM 硝酸盐)与大豆结瘤固氮，而且比5 mM硝酸盐浓度条件下结瘤固氮还好。这一研究虽然打破了之前人们认为的传统观点，但未来的应用前景并不一定好。我们总不能一边施更多的化学氮肥，一边又希望根瘤菌结更多的根瘤，固更多的氮素吧。如果所结的根瘤，在大豆花期和鼓粒期需要更多的氮时，仍能发挥共生固氮作用，那么，倒是不错的结果。论文全文还看不到，不知道根瘤固氮的维持期能有多长。

Novel rhizobia exhibit superior nodulation and biological nitrogen fixation even under high nitrate concentrations

Hien P Nguyen, Hiroki Miwa, Jennifer Obirih-Opareh, Takuya Suzaki, Michiko Yasuda,Shin Okazaki

FEMS Microbiology Ecology, fiz184, https://doi.org/10.1093/femsec/fiz184

Published: 20 December 2019

https://academic.oup.com/femsec/advance-article/doi/10.1093/femsec/fiz184/5682489

Legume-rhizobium symbiosis leads to the formation of nitrogen-fixing root nodules. However, externally applied chemical nitrogen fertilizers (nitrate and ammonia) strongly inhibit nodule formation and nitrogen fixation. Here, we isolated several rhizobial strains exhibiting a superior nodulation and nitrogen fixation with soybean at high nitrate concentrations. The nodulation of soybean symbiont Bradyrhizobium diazoefficiens USDA110 was significantly inhibited at 12.5 mM nitrate; however, three isolates (NKS4, NKM2, and NKTG2) were capable of forming nitrogen-fixing nodules, even at 20 mM nitrate. These isolates exhibited higher nodulation competitiveness and induced larger nodules with higher nitrogen-fixation activity than USDA110 at 5 mM nitrate. Furthermore, these isolates induced more nodules than USDA110 even in nitrate-free conditions. These isolates had a distant lineage within the Bradyrhizobium genus; though they were relatively phylogenetically close to B. japonicum, their morphological and growth characteristics were significantly different. Notably, in the presence of nitrate, expression of the soybean symbiosis-related genes (GmENOD40 and GmNIN) was significantly higher and expression of GmNIC1 which is involved in nitrate-dependent nodulation inhibition was lower in the roots inoculated with these isolates in contrast with inoculation of USDA110. These novel rhizobia serve as promising inoculants for soybeans cultivated in diverse agroecosystems, particularly on nitrate-applied soils.