Functional microbial inoculants affect the balance of DNRA and denitrification pathways in compost amended soil

Dissimilatory Nitrate Reduction to Ammonium (DNRA) and denitrification pathways utilize nitrates as substrates and both participate in the nitrogen cycle. Through isotope labeling, the results show that DNRA and denitrification compete by regulating arginine metabolism to decrease the abundance of citrulline. In the organic composting for soil fertilization, three experimental groups were set up: E1 (added denitrifying functional bacteria agent), E2 (added DNRA functional bacteria agent), and E3 (added both functional bacteria agents). The results showed that the addition of the E2 inoculant significantly promoted the utilization of nitrate nitrogen (NO₃^--N) and the generation of ammonium nitrogen (NH₄⁺-N). Compared to E2 and E3, the ammonium accumulation content was maximally increased by 13.54 %. The relative abundance of nrfA in E2 was approximately eightfold higher than that in E3, while suppressing nosZ and nirK gene expression. E2 also increased the number of DNRA microbial genera and their relative abundance. Thermopetrobacter, Thermomonas, and Pseudomonas were significantly correlated with the nrfA gene. E2 strengthened the positive correlation between NO₃^--N and Bauldia, which has potential DNRA functionality, and improved the correlation between microbes and NH₄⁺-N. Overall, the addition of the E2 inoculant altered the balance between DNRA and denitrification, making the DNRA pathway dominant.