Investigating drivers of active nitrification in organic horizons of tropical forest soils

• Some O horizons showed higher nitrification rate than mineral horizons. • Both total N and pH were positively correlated with nitrification rate in O horizon. • Nutrient richness in litters supported active nitrification in O horizon. • Nitrification rate in O horizon increased along with a pH threshold of 5.5–6.0. High nitrate leaching has been observed from the O horizons of some tropical forests; however, the drivers of high nitrate production (active nitrification) in these O horizons have not yet been identified. This study investigated the drivers of active nitrification in the O horizon of tropical forest soils by focusing on two of the most widely recognized controlling factors of nitrification, total N, and pH. We collected mineral and O horizons from eight tropical forests in Cameroon, Indonesia, and Malaysia and measured gross nitrification rates. Some O horizons showed significantly higher gross nitrification rates than mineral horizons, indicating that these O horizons have a high potential for nitrification. Gross nitrification rates in the O horizons were positively correlated with both total N and pH, and the chemical properties (e.g., total content of N, P, and base cations) were intercorrelated. These correlations suggested that the underlying driver of nitrification in the O horizon was nutrient richness in the litter. Results also indicated a threshold of gross nitrification rates around pH values of 5.5–6.0. We elucidate that active nitrification and subsequent high nitrate leaching from the O horizon could be driven by nutrient-rich litter, possibly derived from soil fertility and tree species.