Distinct fates of atmogenic NH4 + and NO3 − in subtropical, N-saturated forest soils

Abstract

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH4 +) and nitrate (NO3 −). Previous long-term studies indicate efficient turnover of atmogenic NH4 + to NO3 − in weathered, acidic soils of the subtropics, leading to excessive NO3 − leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ 15N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. 15NH4NO3, NH 4 15 NO3 and 15N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N2O) were monitored for nine days. Incorporation of 15NO3 − into soil organic N was negligible and 80% of the applied label was lost from the top soil (0–15 cm) primarily by leaching within 9 days. In contrast, 15NH4 + was largely retained in soil organic N. However, instant production of 15NO3 − in the 15NH4 + treatment suggested active nitrification. In both the 15NH4 + and 15N-glutamic acid treatments, the 15N enrichment in the NO3 − pool exceeded that in the NH4 + pool one day after 15N application, suggesting preferential nitrification of added 15NH4 + with subsequent dilution of the NH4 + pool and/or immobilization of 15NH4 + followed by heterotrophic nitrification. The cumulative recovery of 15N in N2O after 9 days ranged from 2.5 to 6.0% in the 15NO3 − treatment, confirming the previously reported significant response of N2O emission to N deposition. Source partitioning of 15N2O demonstrated a measurable contribution of nitrification to N2O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH4 + in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO3 − and NH4 +) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO3 − in the soil and (ii) rapid immobilization-mineralization of NH4 + followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N2O emission.