delta N-15 of soil N and plants in a N-saturated, subtropical forest of southern China

We investigated the delta N-15 profile of N (extractable NH4+, NO3-, and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of delta N-15 in the soil was EON > NH4+ > NO3-. Although the delta N-15 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 parts per thousand. The difference in delta N-15 between bulk soil N and EON (Delta N-15(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 delta N-15 of EON, as in the case of soil microbial biomass delta N-15. A thorough delta N-15 survey of available N (NH4+, NO3-, and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the delta N-15 of the available N forms did not fully overlap with the delta N-15 of plants. This mismatch in delta N-15 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. Copyright (C) 2010 John Wiley & Sons, Ltd.