Influence of the El Niño and La Niña climate events and litter removal on inorganic nitrogen dynamics in pine forest soils on Central Java, Indonesia

To determine to what extent environmental factors and anthropogenic disturbances dictate N dynamics in tropical forest soils, changes of concentrations of inorganic N in soil were investigated during a period of extreme climatic conditions caused by El Niño and La Niña. This allowed the determination of factors driving the N-dynamics in tropical soils more clearly than during normal seasonal cycles. Three N-limited pine forests in Central Java, Indonesia, were studied monthly for over a year. N-NH4+ and N-NO3− were abundant in the organic layers, and decreased with increasing depth. Regular litter removal from the forest floor and branch-cutting at two pine forest sites significantly reduced the N-NH4+ concentration in the organic and mineral layers, but had only a minor influence on N-NO3− concentration. The N-NH4+ and N-NO3− concentrations and pH showed a distinct seasonal variation with extremely large amplitude in all studied soil layers, this corresponded to variation in soil moisture content. Concentrations were low during the extreme dry period of El Niño of 1997, and significantly increased during the subsequent long wet period of La Niña. The largest changes occurred in the organic top layers and changes were more pronounced than during normal seasonal cycles. It is concluded that N-dynamics in litter and fragmentation layers were most influenced by seasonal precipitation and to a lesser degree by litter removal.