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

Plant and soil nitrogen (N) isotope ratios (δ¹⁵N) have been used as indicators of N cycling processes of ecosystems. However, the effects of N and Phosphorus (P) addition on δ¹⁵N values and their indicative roles in N-cycling in legume-dominated tropical forests remain poorly understood.

We compared leguminous (Acacia auriculiformis) and non-leguminous (Eucalyptus urophylla) forests in terms of N concentration and δ¹⁵N 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 ¹⁵N signatures observed in the leguminous and non-leguminous forests.

The plants in both tropical forests were found to be ¹⁵N-depleted. N addition significantly increased the foliage δ¹⁵N 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 (NO₃⁻) leaching and nitrous oxide (N₂O) emission. Conversely, P addition led to a decrease in NO₃⁻ leaching and N₂O emission. Additionally, foliage δ¹⁵N was positively correlated with soil N loss rates (N₂O emission and NO₃⁻ leaching) in the non-leguminous forest but showed no correlation in the leguminous forest.

In contrast to the traditional convention derived from non-leguminous forests, our results suggest that foliar ¹⁵N 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 ¹⁵N isotope signals.