In situ 15 N labeling experiment reveals different long‐term responses to ammonium and nitrate inputs in N‐saturated subtropical forest

Chronically elevated deposition of reactive nitrogen (N), as ammonium (NH 4 + ) and nitrate (NO 3 − ), in subtropical forests with monsoonal climate has caused widespread N leaching in southern China. So far, little is known about the effect of further increases in N input and changes in the relative proportion of NH 4 + and NO 3 − on turnover rate and fate of atmogenic N. Here we report a 15 N tracer experiment in Tieshanping (TSP) forest, SW China, conducted as part of a long‐term N fertilization experiment, using NH 4 NO 3 and NaNO 3 , where effects of a doubling of monthly N inputs were compared. In June 2012, the regular N fertilizers were replaced by their 15 N‐labeled forms, viz., 15 NH 4 NO 3 and Na 15 NO 3 , as a single‐dose addition. Mass balances of N for the initial 1.5 years following label addition showed that for both treatments, 70% to 80% of the annual N input was leached as NO 3 − , both at ambient and at double N input rates. This confirms the earlier reported extreme case of N saturation at TSP. The 15 N, added as Na 15 NO 3 , showed recoveries of about 74% in soil leachates, indicating that NO 3 − input at TSP is subject to a rapid and nearly quantitative loss through direct leaching as a mobile anion. By contrast, recoveries of 15 N in soil leachates of only 33% were found if added as 15 NH 4 NO 3 . Much of the 15 N was immobilized in the soil and to a lesser extent in the vegetation. Thus, immobilization of fresh N input is significantly greater if added as NH 4 + , than as NO 3 − .