Variation in the relationship between nitrification and acidification of subtropical soils as affected by the addition of urea or ammonium sulfate

The effects on nitrification and acidification in three subtropical soils to which (NH4)2SO4 or urea had been added at rate of 250 mg N kg−1 was studied using laboratory-based incubations. The results indicated that NH4+ input did not stimulate nitrification in a red forest soil, nor was there any soil acidification. Unlike red forest soil, (NH4)2SO4 enhanced nitrification of an upland soil, whilst urea was more effective in stimulating nitrification, and here the soil was slightly acidified. For another upland soil, NH4+ input greatly enhanced nitrification and as a result, this soil was significantly acidified. We conclude that the effects of NH4+ addition on nitrification and acidification in cultivated soils would be quite different from in forest soils. During the incubation, N isotope fractionation was closely related to the nitrifying capacity of the soils.