The 15N-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 15N-depleted through isotopic fractionation. However, because of a lack of an appropriate analytical methodology to confirm that supposition, the δ15N 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 δ15N 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 δ15N values of TDN were −0.7‰ in precipitation and +1.2‰ in stream water. The difference in δ15N between soil (+3.9‰) and TDN in the stream water was 2.7‰. In contrast, soil-emitted N2O was strongly 15N-depleted (−14.3‰): 18‰ lower than that of the soil. Our results demonstrate that the discharged N loss is 15N-depleted only slightly compared with soil N, and gaseous N losses can be a strong driver for raising the terrestrial ecosystem δ15N. Our findings suggest that the relation between ecosystem δ15N and the open N cycle can be interpreted better by considering the net discrimination against 15N determined by the balance between gaseous and discharge N losses. Steady state 15N 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 N2 emission) are quite important for better estimation of gaseous N losses from the ecosystem.