Denitrification Rate and Controlling Factors for Accumulated Nitrate in the Deep Subsoil of Intensive Farmlands: A Case Study in the North China Plain

Denitrification in subsoil (to a depth of 12 m) is an important mechanism to reduce nitrate (NO₃^-) leaching into groundwater. However, regulating mechanisms of subsoil denitrification, especially those in the deep subsoil beneath the crop root zone, have not been well documented. In this study, soil columns of 0-12 m depth were collected from intensively farmed fields in the North China Plain. The fields had received long-term nitrogen (N) fertilizer inputs at 0 (N0), 200 (N200) and 600 (N600) kg N ha^-1 year-1. Main soil properties related to denitrification, i.e., soil water content, NO₃^-, dissolved organic carbon (DOC), soil organic carbon (SOC), pH, denitrifying enzyme activity (DEA), and anaerobic denitrification rate (ADR), were determined. Statistical comparisons among the treatments were performed. The results showed that NO₃^- was more heavily accumulated in the entire soil profile of the N600 treatment, compared to the N0 and N200 treatments. The SOC, DOC, and ADR decreased with increasing soil depth in all treatments, whereas considerable DEA was observed throughout the subsoil. The long-term fertilizer rates affected ADR only in the upper 4 m soil layers. The ADRs in the N200 and N600 treatments were significantly correlated with DOC. Multiple regression analysis indicated that DOC rather than DEA was the key factor regulating denitrification beneath the root zone. Additional research is required to determine if carbon addition into subsoil can be a promising approach to enhance NO₃^- denitrification in the subsoil and consequently to mitigate groundwater NO₃^- contamination in the intensive farmlands.