Responses of soil gross nitrogen transformations to three vegetation restoration strategies in a subtropical karst region

Investigation of soil internal nitrogen cycles is useful for unravelling the mechanisms responsible for soil nitrogen availability change. Here, gross nitrogen transformations were investigated in a subtropical karst area after 16 years of vegetation restoration. The experiment included four treatments with one control and three restoration strategies, that is: (i) maize‐soybean rotation field (CR, control); (ii) restoration with Toona sinensis (TS); (iii) restoration with Guimu‐1 hybrid elephant grass (GG, Pennisetum americanum (L.) Leeke × Pennisetum purpureum Schumach); and (iv) restoration with Guimu‐1 hybrid elephant grass and Zenia insignis as a mixture (ZG). Soil NH₄⁺ and NO₃⁻ responded differentially to three restoration strategies, resulting in no significant change of total inorganic N content following vegetation restoration. Gross nitrogen mineralization (GNM) rate increased by 124.0%, 96.8%, and 60.3% in TS, GG, and ZG, respectively, relative to CR (2.88 ± 0.05 mg N kg⁻¹ d⁻¹). The strongest explanatory variable was microbial biomass carbon for both GNM (R² = 0.82) and gross nitrification (R² = 0.66). Dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA) rate in TS or ZG, which was significantly greater than that in CR, was significantly correlated with SOC: NO₃⁻ ratio (R² = 0.61), implying that DNRA was limited by SOC availability. Gross NH₄⁺ immobilization rate, which was highest in GG, and lowest in CR, was best predicted by carbon to nitrogen ratio (R² = 0.65). However, gross NO₃⁻ immobilization rate could not be well predicted by the measured soil properties. Our results suggest that vegetation restoration especially TS significantly enhanced soil N availability as reflected by gross N mineralization rate.