Nitrogen transformations and emission of greenhouse gases from three acid soils of humid tropics amended with N sources and moisture regime. I. nitrogen transformations

Laboratory studies were carried out to investigate the nitrogen (N) transfo rmation processes of three Malaysian acid soils (pH(H2)O ranged from 3.74-4 .44) belonging to Aeric Tropic Fluvaquent and Xanthic Hapludox. The soils, amended with inorganic and organic N sources, were incubated at 25 degreesC separately under aerobic and anaerobic conditions. The N mineralization fo llowed zero order kinetics during 15 days of the incubation with a large ac cumulation of NH4+-N under both moisture levels. Addition of sugar beet lea ves, having a lower C/N ratio, influenced N mineralization in terms of NH4 accumulation to a greater extent. Wheat straw, having a wider C/Nratio, li mited N mineralization i.e. might induce immobilization of N. Under aerobic conditions, nitrification was either small with NH4+ fertilizer or caused some NO3- reduction with organic residues and the increased mineralized N w ith NH4+-fertilizer indicated a priming effect might occur. Under anaerobic conditions, disappearance of NO3- followed first order kinetics. The disap pearance was very rapid supplied with both NO3--fertilizer and sugar beet l eaves. The acidity did not limit intense NO3- reduction rather showed a pre dominant accumulation of NH4+-N under anaerobic conditions. Accumulation of NO2- was low because of the small nitrification or slow NO3- reduction rat e under aerobic conditions. The NO3- fertilizer and sugar beet leaves incre ased NO2- accumulation to several-folds under anaerobic conditions followed by its consumption. The N transformations varied with N sources and moistu re regime.