Role of added carbon in the transformation of surplus soil nitrate-nitrogen to organic forms in an intensively managed calcareous soil

Excessive amounts of nitrate have accumulated in many soils on the North China Plain due to the large amounts of chemical N fertilizer used combined with low carbon inputs. The present study investigated the promotion of soil nitrate transformation to soil organic N by different carbon amendments. A laboratory incubation experiment using 15N tracer (K15NO3) was employed to elucidate the proportion of soil organic N derived from accumulated soil nitrate following amendment with glucose (G) or maize straw (S) at controlled soil temperature and moisture content. During the 56-d incubation period we determined the dynamics and isotopic abundance of mineral N (NO3- and NH4+) and soil organic N and greenhouse gas (N2O and CO2) emissions. Carbon amendment markedly stimulated transformation of nitrate to soil organic N. Mineralization of native soil organic N was also enhanced because soil organic N at the end of the incubation period was not significantly different from that in the control without added C. Glucose had a greater effect on nitrate-N immobilization (25.0 mg kg-1) than maize straw (9.4 mg kg-1) and glucose addition also led to highest greenhouse gas emissions. With glucose and straw amendment total N2O-N emissions were 1.01 and 0.03 mg kg-1, respectively. Similarly, glucose amendment lowered the concentration and abundance of NO3--N and increased that of NH4+-N more than maize straw. In agricultural practice straw amendment may be an effective strategy to deplete the excess NO3--N pool and restrict leaching, to promote nitrate-N immobilization and to decrease greenhouse gas emissions.