Nitrification can regulate N loss from soil because nitrates easily can be leached out and denitrified. In many tropical forest ecosystems, however, the open N cycle and fast nitrification have been observed. This study aimed to elucidate the cause of high nitrification rates in tropical forest soils by focusing on litterfall-N input (N-availability in soil) and functions of the organic (O) horizon. Soil samples from mineral and O horizons were collected from 3 sites in Cameroon (High-N input) and 3 sites in Indonesia (low-N input) to measure gross nitrification rates. In the mineral horizon, faster nitrification was observed in N-rich soil from Cameroon than in N-poor soil from Indonesia. Total N content in soil also was positively correlated with gross nitrification rates (r=0.81, p=0.001, n=12). This implied litterfall-N input may regulate nitrification rates. In Cameroonian soils, gross nitrification rates were higher in the O horizon than in the mineral horizon because the O horizon can serve as a N-rich pool and has a higher pH, whereas the O horizon in Indonesian soils did not exhibit fast nitrification rates, probably owing to lower N content. Therefore, litterfall-N input may regulate nitrification in both mineral and O horizons of tropical forest soils.