Intermittent oxygenation is essential for modulating nitrite ammonifiers in an agricultural upland soil to minimize nitrogen loss

Abstract

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA), two divergent nitrogen metabolism pathways, share nitrite as a common intermediate during anaerobic reduction. Unlike denitrification, which involves nitrogen loss through gaseous nitrogen emission and nitrate leaching, DNRA is beneficial for the conservation of nitrogen. Previous studies have reported factors such as carbon and oxygen can independently affect DNRA. Nevertheless, the key mechanism underlying the joint regulation of aerobic carbon metabolism to nitrite ammonifiers in soil is still unclear. Here, microcosm experiments with agricultural upland soil were conducted under different aeration conditions supplemented with labile carbon to analyze the process of denitrification and DNRA. The results indicated that denitrification exclusively dominated nitrite reduction when the soil was directly placed in an anaerobic environment. Nonetheless, a significant increase in DNRA activity and the attenuation of denitrification were detected when the soil was incubated aerobically with the addition of glucose prior to anaerobic incubation. Specifically, up to 55.8 % of nitrite reduction switched to nitrogen conservation mainly via DNRA under high‑carbon conditions. Quantitative assays of the nrfA gene showed that aerobic incubation with 1000 mg·kg⁻¹ carbon addition increased DNRA by 3.74 ± 0.24 fold. Sequence analysis of nrfA gene indicated a marked shift in nitrite ammonifiers, with Firmicutes being the most altered phylum. These results intriguingly indicate that nitrate/nitrite metabolic flux in the soil could be regulated to enhance DNRA by stimulating facultative anaerobic nitrite ammonifiers such as Sedimentibacter under alternating aerobic and anaerobic environments with carbon metabolism.