The 15N natural abundance of the N lost from an N-saturated subtropical forest in southern China

The N-15-enrichment of plants and soils is believed to indicate characteristics of the open nitrogen (N) cycle in terrestrial ecosystems because N lost from an ecosystem is presumably N-15-depleted through isotopic fractionation. However, because of a lack of an appropriate analytical methodology to confirm that supposition, the delta N-15 value for total dissolved nitrogen (TDN, the sum of ammonium, nitrate, and dissolved organic N) in stream water from forests has been measured only rarely. This report describes the delta N-15 values for TDN, ammonium, and nitrate in precipitation and stream water, together with those for soil-emitted nitrous oxide (N2O; measured once) in an N-saturated subtropical forest in southern China. Concentration-weighted delta N-15 values of TDN were -0.7% in precipitation and +1.2% in stream water. The difference in delta N-15 between soil (+3.9%) and TDN in the stream water was 2.7%. In contrast, soil-emitted N2O was strongly N-15-depleted (-14.3%): 18% lower than that of the soil. Our results demonstrate that the discharged N loss is N-15-depleted only slightly compared with soil N, and gaseous N losses can be a strong driver for raising the terrestrial ecosystem delta N-15. Our findings suggest that the relation between ecosystem delta N-15 and the open N cycle can be interpreted better by considering the net discrimination against N-15 determined by the balance between gaseous and discharge N losses. Steady state N-15 budget calculations proposed by Houlton and Bai (2009) can provide important information about the gaseous N fluxes, which are difficult to measure directly. The steady state calculation for the relationships among gaseous N loss, apparent isotopic fractionation during gaseous N loss, and isotopic signature of N inputs suggests that precise measurements of unmeasured components (e.g., dry deposition, NO and N-2 emission) are quite important for better estimation of gaseous N losses from the ecosystem.