δ15N of soil N and plants in a N‐saturated, subtropical forest of southern China

We investigated the δ 15 N profile of N (extractable NH 4 + , NO 3 - , and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of δ 15 N in the soil was EON > NH 4 + > NO 3 - . Although the δ 15 N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5‰. The difference in δ 15 N between bulk soil N and EON (Δ 15 N bulk-EON ) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high δ 15 N of EON, as in the case of soil microbial biomass δ 15 N. A thorough δ 15 N survey of available N (NH 4 + , NO 3 , and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the δ 15 N of the available N forms did not fully overlap with the δ 15 N of plants. This mismatch in δ 15 N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest.