Fates of residual nitrate under reductive soil disinfestation with different schemes

Intensive vegetable cultivation has caused severe nitrate (NO 3 − -N) accumulation and associated environmental issues. Reductive soil disinfestation (RSD), originally developed for suppressing soil-borne diseases, has proven effective in removing residual NO 3 − -N of topsoil at the cost of substantial nitrogen (N) loss. Therefore, optimizing RSD schemes by screening the appropriate carbon (C) source and treatment duration is of utmost importance. However, how different C sources and durations affect the fates of residual NO 3 − -N during RSD remains unknown. Here, using a 15 N tracing approach with qPCR technique, we investigated the dynamics of residual NO 3 − -N in three nitrate-rich soils (previously cultivated with tomato, celery, and zucchini) treated with RSD. Sugarcane bagasse and alfalfa meal, differing in C/N ratios, were incorporated into RSD (RSD_SB and RSD_AM, respectively) over a 5-week incubation with no C incorporation as control (RSD_CK). Across all soils, RSD_CK removed less than 11% of NO 3 − -N during the entire incubation. In contrast, RSD_AM removed approximately 85–100% of NO 3 − -N at week 1, and RSD_SB removed 37–67% of NO 3 − -N at weeks 4–5. Across all soils and C sources, insoluble organic N (ION) and undetermined N (mainly complete denitrification loss) accounted for 2–14% and 32–97% of residual NO 3 − -N, respectively. Conversion of NO 3 − -N to ION was stronger in RSD_SB than in RSD_AM across soils, whereas the opposite trend was observed for its conversion to undetermined N. Mineralization of added C was identified as the main factor regulating these fates by altering the soil ammonium concentration or nosZ gene abundance. Overall, optimizing RSD protocols by selecting the C source and duration could improve the removal capacity and efficiency of residual NO 3 − -N but simultaneously enhance the risk of N loss to the environment.