Soil N transformation rates are not linked to fertilizer N losses in vegetable soils with high N input

Vegetable soils under plastic shed are characterized by multiple cropping and extremely high rates of nitrogen (N) application, but the N use efficiency (NUE) is generally low and the rate of N loss is often high. The underlying mechanisms behind these N losses following long-term N fertilizer applications remain elusive. Here, a 15N tracing incubation study in conjunction with in-situ monitoring was performed to investigate the effects of different N fertilization regimes on gross N transformations, microbial abundance and N losses via NO3− leaching and N2O emissions in a vegetable soil under plastic shed subjected to 14 years of repeated N fertilization. The field experiment comprised of six treatments: no N fertilization (control, CK), and applications of chicken manure (M), chemical N plus M (CM), CM plus straw (CMS), 42 % of chemical N plus chicken manure (RCM) and RCM plus straw (RCMS). The results showed that the total gross N mineralization rates were significantly higher in CMS, RCMS and M treatments than in CK, CM and RCM treatments. The total gross NH4+ immobilization rates followed the order of CK > CM > CMS > M > RCM ≈ RCMS, indicating a decreasing NH4+ immobilization rate after N application. Gross autotrophic nitrification rate was significantly enhanced by fertilizer application in the order: CK < CM < CMS ≈ RCM < M < RCMS, mainly due to both increased mineralization rates and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB). A significant negative relationship between gross rates of NH4+ immobilization and autotrophic nitrification suggests that higher autotrophic nitrification decreased NH4+ immobilization. Reduced chemical N input alleviated soil acidification, NO3− leaching and N2O emissions. There were significant positive relationships between rates of chemical N input and N loss but no relationships between gross N transformation rates and tomato yields, NO3− leaching losses and N2O emissions. Thus, soil N transformation rates do not reflect rate of fertilizer N losses in vegetable soil managed under plastic shed. We thus suggest that chemical N application rate could be reduced to decrease N losses while sustaining vegetable production.