Mechanisms behind the stimulation of nitrification by N input in subtropical acid forest soil

Input of N as NH4 (+) is known to stimulate nitrification and to enhance the risk of N losses through NO3 (-) leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed. In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg(-1) was pre-incubated at 25 A degrees C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a N-15 tracing methodology. Gross rates of mineralization and nitrification of NH4 (+)-N increased (P < 0.05), while gross rate of NO3 (-) immobilization significantly decreased with increasing N input rates (P < 0.001). A significant relationship was established between the gross nitrification rate of NH4 (+) and the gross mineralization rate (R (2) = 0.991, P < 0.01), so was between net nitrification rate of NH4 (+) and the net mineralization rate (R (2) = 0.973, P < 0.05). Stimulation effect of N input on the gross rate of nitrification of NH4 (+)-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH4 (+) in acid soils (De Boer et al., Soil Biol Biochem 20:845-850, 1988; Prosser, Advan Microb Physiol 30:125-181, 1989). The stimulated gross nitrification rate with the decreased gross NO3 (-) immobilization rate under the elevated N inputs could lead to accumulation of NO3 (-) and to enhance the risk of NO3 (-) loss from humid forest soils.