NITROGEN MINERALIZATION AS INFLUENCED BY CLIMATE, SOIL AND VEGETATIONIN A SUBTROPICAL HUMID FOREST IN NORTHEAST INDIA

Nitrogen mineralization was studied in 7-, 13- and 16-year old regrowi ng 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 (ammo nium+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 ammonifica tion and mineralization rates were significantly higher (P<0.01) in th e 16-year old stand than in the 7- and 13-year old stands, but nitrifi cation rate followed a reverse trend. Inorganic-N and net N mineraliza tion rates were significantly higher (P<0.01) in the surface soil laye r (0-10 cm) than the subsurface layer (10-20 cm). Ammonium was the dom inant form of inorganic-N. Nitrogen mineralization rate was positively correlated with the density and basal area of the woody vegetation, w hereas nitrification rate was negatively correlated. Further, net N mi neralization was heavily governed by rainfall, soil moisture, temperat ure and microbial population. (C) 1998 Elsevier Science B.V.