Dissimilatory nitrate reduction to ammonium in a soil under greenhouse vegetable cultivation as affected by organic amendments

Abstract

Excessive nitrate (NO3 −) accumulation in greenhouse vegetable soils often results in nitrogen (N) loss through leaching or gaseous emission as a result of denitrification. Dissimilatory NO3 − reduction to ammonium (NH4 +) (DNRA) as affected by organic amendments in a greenhouse vegetable soil was studied. Soil incubations were carried out following the amendment of rice straw (RS), Chinese milk vetch (CMV), and a control (CK) without organic amendment using two 15 N isotope methods in the laboratory. Gross DNRA rates were 15.2–26.2 and 0.21–1.06 mg N kg−1 day−1 when estimated by the recovery of 15NO3 − in NH4 + and organic N (ON) pools (M1) and by mean residence time (MRT) of NH4 + (M2), respectively. Microbial NO3 − assimilation might occur due to high NO3 − content in the tested soil (1.27 g N kg−1), and thus, M1 probably overestimated gross DNRA rates. Gross DNRA rates estimated by M2 were higher in RS and CMV treatments than those in CK and were significantly (p < 0.05) correlated with soil dissolved organic carbon (DOC) content and the ratio of DOC to NO3 −–N. Therefore, the DNRA rate was promoted by organic amendments attributed to the enhancement of soil DOC to NO3 −–N ratio. Further studies on DNRA in more soil types under different conditions are required in order to better understand the potential contribution of DNRA to N transformation and conservation in vegetable soils.