Awakening soil microbial utilization of nitrate by carbon regulation to lower nitrogen pollution

Soil microbial immobilization of nitrate (NO3--N, I-NO3) and its role in mitigating NO3--N pollution in agricultural ecosystems globally is often neglected because of the tenet that microbes preferentially use ammonium over NO3--N. Soil I-NO3 driven by heterotrophic microorganisms is often carbon (C)-limited, and may therefore be stimulated by C sources with high C/nitrogen (N) ratios. Here, using (NO3-)-N-15-N labelling coupled with acetylene inhibition, quantitative PCR and Illumina high-throughput sequencing approaches, we demonstrated that I-NO3 was stimulated from zero to a substantial level by crop residue amendment without any environmental risk; the degree of stimulation depending on residue quality and soil type. The stimulation was predicted well by C mineralization of the added residue. Soil with a lower initial nutrient status had reduced I-NO3 efficiency due to stronger C constraint and lower return on energy investment. Fast-growing bacterial r-strategists were identified as the key driver of I-NO3. We estimate that, globally, residue amendment could cause a total NO3--N loss reduction of about 33.6 Tg N yr(-1) through I-NO3 stimulation. We suggest that soil I-NO3 driven by addition of exogenous C is an overlooked process that can help curb NO3--N accumulation in soil and its subsequent release to the environment, the "nitrate time bomb".