Nitrogen mineralization as influenced by climate, soil and vegetation in a subtropical humid forest in northeast India

Nitrogen mineralization was studied in 7-, 13- and 16-year old regrowing forest stands by determining ammonium-N, nitrate-N, ammonification and nitrification rates during different seasons for two annual cycles. Mean concentrations of ammonium and nitrate-N were highest in the 16-year old stand, and lowest in the 7-year old stand. Inorganic-N (ammonium+nitrate) was highest during winter and lowest during rainy season in the three stands, whereas net ammonification, nitrification and N mineralization rates showed reverse seasonal trend. The net ammonification and mineralization rates were significantly higher (P<0.01) in the 16-year old stand than in the 7- and 13-year old stands, but nitrification rate followed a reverse trend. Inorganic-N and net N mineralization rates were significantly higher (P<0.01) in the surface soil layer (0-10 cm) than the subsurface layer (10-20 cm). Ammonium was the dominant form of inorganic-N. Nitrogen mineralization rate was positively correlated with the density and basal area of the woody vegetation, whereas nitrification rate was negatively correlated. Further, net N mineralization was heavily governed by rainfall, soil moisture, temperature and microbial population. (C) 1998 Elsevier Science B.V.