Nitrification catalyzed by ammonia-oxidizing prokaryotes
is affected not only by the concentration of substrate, but
also by the soil properties, types of N fertilizers, and activity of ammonia-oxidizing prokaryotes. In this study,there was nitrification at very low pH in both Ultisols
with no N fertilizer added. Addition of urea accelerated the nitrification and thus soil acidification in both the Ultisols from subtropical regions. However, nitrification was inhibited by addition of (NH4)2SO4, and higher input of (NH4)2SO4 led to greater inhibition of nitrification.AOB played an important role in nitrification of the cropland soil under acidic conditions. Addition of urea stimulated the increase in AOB population and thus accelerated
nitrification and soil acidification; while addition of