Subtropical forest catchments in South China provide an efficient biological N sink alleviating regional N pollution: evidence from dual nitrate isotopes

In warm-humid subtropics of South China, enhanced N deposition has led to extreme N saturation in a wide range of forests, with significant nitrate (NO3-) leaching from well-drained soils. Yet, the imbalanced N budget, with limited N export by stream runoff, reveals strong N retention in these forest catchments. Here, to strengthen our understanding of catchment-scale N turnover, we combine a dual NO3- isotope study with weekly inorganic N flux monitoring for two years, across five southern forest catchments in China. In each catchment, data were collected along a hydrological continuum consisting of a hill slope (HS), a groundwater discharge zone (GDZ) and a stream outlet. Combined for all catchments, the data show that the atmogenic ammonium (NH4+) is efficiently removed in the soil pore water of HS, while NO3- is produced. Depleted δ15N and δ18O signals in NO3- on the HS suggested the occurrence of efficient nitrification. In the GDZ, the strong decline in NO3- concentration associated with significant increase in δ15N and δ18O of NO3- indicated denitrification as the dominant N sink. Such uniform N turnover pattern may be attributed to similar climatic and topographic conditions, which facilitate efficient N transformation and transport along the hydrological continuum. In-out N budgets across these sites revealed that elevated N deposition stimulated catchment N retention, which was matched by increasing difference in 15N enrichment of NO3- between HS and GDZ. Thus, N attenuation by denitrification in South Chinese forest catchments appears to be driven by N deposition load in a range of 10 to 50 kg ha-1 yr-1. Our study demonstrates widely overlooked N sink function of subtropical forest catchments, which may be of great importance for alleviating regional N pollution.