Long-term manuring enhances soil gross nitrogen mineralization and ammonium immobilization in subtropical area

Manure amendments have been generally considered to enhance soil nutrient supply, thus improving soil fertility and crop productivity, especially in subtropical area. However, the underlying mechanisms of manure application effects on soil nitrogen (N) supply capacity and N transformation processes remain uncertain. Here, 15N isotope pool dilution techniques were used to determine the gross rates of N mineralization, ammonium (NH4+) immobilization and nitrification in red soil subjected to three decades of long-term fertilization regimes. Results showed that compared to treatments without manure, manure application alone or combined with chemical fertilizers significantly increased gross N mineralization rates by 4.55−8.31 folds, resulting from increased N-acquiring enzyme activity and decreased microbial N use efficiency (NUE). The decreased microbial NUE in the manure application treatments was due to alleviated imbalance of C:N between soil and microbial biomass. Gross NH4+ immobilization rates were 7.35−11.34 folds higher in the treatments amended with manure (1.51−2.23 mg N kg−1 d−1) relative to without manure, attributed to increased microbial biomass and bacterial abundance. Compared to chemical fertilizers application, manure application decreased gross nitrification rates by 38−55%, due to the decline in NH4+ availability via enhanced NH4+ immobilization. The significantly lower ratio of gross nitrification to NH4+ immobilization in the treatments with manure suggested that manure amendments could potentially mitigate the risk of soil N losses. Overall, long-term manuring has the potential to improve soil N supply and retention capacities by enhancing gross N mineralization and NH4+ immobilization accompanying with decreased gross nitrification.