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) transformation processes of three Malaysian acid soils (pHH2O 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°C separately under aerobic and anaerobic conditions. The N mineralization followed zero order kinetics during 15 days of the incubation with a large accumulation of NH4 +-N under both moisture levels. Addition of sugar beet leaves, having a lower C/N ratio, influenced N mineralization in terms of NH4 + accumulation to a greater extent. Wheat straw, having a wider C/N ratio, limited 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 with NH4 +-fertilizer indicated a priming effect might occur. Under anaerobic conditions, disappearance of NO3 − followed first order kinetics. The disappearance was very rapid supplied with both NO3 −-fertilizer and sugar beet leaves. The acidity did not limit intense NO3 − reduction rather showed a predominant accumulation of NH4 +-N under anaerobic conditions. Accumulation of NO2 − was low because of the small nitrification or slow NO3 − reduction rate under aerobic conditions. The NO3 − fertilizer and sugar beet leaves increased NO2 − accumulation to several-folds under anaerobic conditions followed by its consumption. The N transformations varied with N sources and moisture regime.