Phylogenetic and Environmental Distribution of NirB-Ammonifiers

Nitrate ammonification is important for soil nitrogen retention. Dissimilatory nitrate reduction to ammonia (DNRA, also known as nitrate/nitrite ammonification) can compete with denitrification for substrates, retaining nitrogen in the soil as ammonium. There are multiple enzymes in two families that play a crucial role in this processes. The gene distribution of NrfA and Onr enzymes has been intensively studied, but NirB,called Nit-6in fungi, has not been systematically studied. Studies show that NirB-ammonifiers could contribute to the global ammonium pools. Research about the diversity and environmental diversity of NirB-ammonifiers can provide a more precise understanding of their genetic potential, further helping assess the abundance of soil NirB-ammonifiers across terrestrial biomes and evaluate their role in global nitrogen retention. In this study, we used previously published NirB and Nit-6 sequences as seed to search for nirB and nit-6 genes in 105,500 bacterial and fungal genomes and 1,583 metagenomes. The results showed that genes were widely distributed in fungi andbacteria. The abundance of nirB gene was similar to that of nrfA and much higher than that of nit-6. Random forest models found that pH, moisture, soil C/N, and aluminum and iron concentrations had important effects on nirB gene abundance. This study demonstrated the widespread presence of NirB and Nit-6 as well as identified factors that control the fate of soil nitrate.