Mineral N suppressed priming effect while increasing microbial C use efficiency and N2O production in sandy soils under long-term conservation management

Restoring soil organic carbon (SOC) with residue incorporation is an important component of soil health management. In the present study, we investigated the interactive impacts of residue amendment and mineral nitrogen (N) additions on preserving the added residues and SOC in a sandy Ultisols. Soil samples were collected from fields under long-term reduced tillage and incubated with either 13 C-labelled crop residues, ammonium nitrate (NH 4 NO 3 ), both, or neither for 55 days. Respiration, microbial biomass C, enzyme activities, and C use efficiency (CUE) were measured along with nitrous oxide (N 2 O) production. Residue amendment increased CO 2 production by 206% at the end of the incubation, inducing positive priming effects (PE). Mineral N reduced the positive impacts of residue amendment on CO 2 production and PE, resulting in higher microbial CUE. However, the N addition had no effects on the measured enzyme activities involved in organic C reactions, except for β-glucosidase activity when residues were present. Additions of mineral N reduced residue decomposition by 89% at the end of the experiment. Total CO 2 (R 2 = − 0.56), residue-derived CO 2 (R 2 = − 0.62), SOC-derived CO 2 (R 2 = − 0.53), and the primed production (R 2 = − 0.52) were all negatively correlated to soil NH 4 + concentrations. Residue amendment instantly stimulated N 2 O production, which was augmented by mineral N addition. Denitrification was seemingly the main N 2 O production pathway. The results reinforced the concept that N can regulate SOC dynamics through direct and indirect impacts on soil microbial activities. Combining N fertilization and residue management is seemingly promising to increase SOC stability and preservation in sandy soils. However, the trade-offs of N 2 O production need to be considered.