Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

Changes in soil N-cycling and retention processes in subtropical/tropical acidic forest ecosystems under anthropogenic N inputs are not well understood. We conducted a laboratory 15N tracing study on an acid soil (pH values: 4.6 to 5.0) from a subtropical forest fertilized for more than 2.5 years at a rate of 0, 40, and 120 kg NH4ClN ha−1 yr−1, respectively. To get a better resolution of mechanistic changes in soil N cycling and retention processes under NH4+ additions, we used a conceptual 15N tracing model to quantify process-specific and pool-specific N transformation rates in soils. Gross N mineralization rates decreased at high NH4+ additions, which were paralleled by a reduction in fungal biomass and mineralization of recalcitrant organic N. Gross NH4+ immobilization rates did not show a change with increasing NH4+ additions. Interestingly, soil NO3− production (heterotrophic, autotrophic, and gross nitrification) and retention (NO3− immobilization and dissimilatory nitrate reduction to ammonium) showed insensitivity to increasing additions of NH4+. The mechanisms behind the lack of response of heterotrophic nitrification were unclear, but possibly related to the absence of significant changes in soil C: N ratio and soil acidity under increased NH4+ additions. Because of the low autotrophic nitrification potential and the lack of NH4+ limitation to autotrophic nitrifiers, autotrophic nitrification was unresponsive to NH4+ additions. NO3− immobilization rates appeared to be controlled by the NO3− produced from heterotrophic nitrification, as indicated by the positive relationship between NO3− immobilization and heterotrophic nitrification (R2 = 0.59, p = 0.015), thus showing a lack of a change under increased NH4+ additions. DNRA seemed to be inherently less responsive to environmental changes such as NH4+ deposition. Our work demonstrates that enhanced NH4+ deposition has a low potential to stimulate soil NO3− production and weaken soil retention of NO3− in this, and perhaps other subtropical/tropical acidic forest ecosystems.