A tradeoff between denitrification and nitrate leaching into the subsoil in nitrate-rich vegetable soils treated by reductive soil disinfestation

Intensive vegetable cultivation often leads to substantial accumulation of nitrate (NO3--N) in the topsoil, thus posing serious threat to groundwater safety by NO3- -N leaching and food safety by excessive uptake of NO3--N by vegetables. Reductive soil disinfestation (RSD) may remove over-accumulated NO3--N in the topsoil due to its water-saturated, strongly reductive, and carbon-rich characteristics that favor the removal of NO3--N via denirification, immobilization, and leaching. However, how RSD affects the fates of NO3--N of the topsoil in intensive vegetable lands remains largely unknown. Here, a soil column experiment using N-15 tracing technique was conducted to quantify the fates of NO3- -N in the topsoil under RSD in six representative intensive vegetable soils in China. The results showed that an average of 99.7% of the added (NO3-)-N-15-N in the topsoil was removed by RSD in all intensive vegetable lands. Denitrification in the entire soil profile and leaching of NO3--N into the subsoil were responsible for 21.8-78.3% and 14.9-68.4% of the removed (NO3-)-N-15 -N, respectively, whereas microbial NO3- -N immobilization and dissimilatory nitrate reduction to ammonium in the topsoil played a negligible role in removing (NO3-)-N-15 -N. A tradeoff was found between denitrification and leaching of NO3--N into the subsoil among the six soils under RSD. The structural equation modeling demonstrated that the positive effects of RSD on topsoil pH, dissolved organic carbon, and bacterial abundance decreased the leaching of NO3--N into the subsoil by enhancing denitrification. Overall, the results provided the first direct evidence for complete removal of NO3--N in the topsoil by RSD. Such an evidence has implications for controlling NO3--N accumulation through implementing RSD in intensive vegetable soils.