Different indicative roles of stable nitrogen isotope in soil N dynamics of tropical leguminous and non-leguminous forests following nutrient addition

Aims Plant and soil nitrogen (N) isotope ratios (δ 15 N) have been used as indicators of N cycling processes of ecosystems. However, the effects of N and Phosphorus (P) addition on δ 15 N values and their indicative roles in N-cycling in legume-dominated tropical forests remain poorly understood. Methods We compared leguminous ( Acacia auriculiformis ) and non-leguminous ( Eucalyptus urophylla ) forests in terms of N concentration and δ 15 N in the leaf–litter–soil continuum under long-term N and P addition. Typical soil N-cycling processes, including rates of mineralization, nitrification, and N loss, were also examined to determine the differences between the corresponding 15 N signatures observed in the leguminous and non-leguminous forests. Results The plants in both tropical forests were found to be 15 N-depleted. N addition significantly increased the foliage δ 15 N in understorey species in the non-leguminous but not the leguminous forest. In both forests, the addition of N resulted in an increase in nitrate (NO 3 − ) leaching and nitrous oxide (N 2 O) emission. Conversely, P addition led to a decrease in NO 3 − leaching and N 2 O emission. Additionally, foliage δ 15 N was positively correlated with soil N loss rates (N 2 O emission and NO 3 − leaching) in the non-leguminous forest but showed no correlation in the leguminous forest. Conclusions In contrast to the traditional convention derived from non-leguminous forests, our results suggest that foliar 15 N isotope signals are not applicable for indicating the soil N dynamic status of leguminous forests. This study highlights the importance of distinguishing legumes from other plant species in terrestrial N-cycling models when incorporating 15 N isotope signals.