Responses of root and soil phosphatase activity to nutrient addition differ between primary and secondary tropical montane forests

Phosphatases play a vital role in phosphorus (P) acquisition for both plant roots and soil microbes. However, how this P-acquisition strategy responds to exogenous nitrogen (N) and P input in tropical forests of different successional stages remains unclear. Using the 7-yr N and P addition experiment in two tropical montane forests under different successional stages (primary and secondary), we investigated the responses of root and soil acid phosphatase (AcP) activity to nutrient addition. Results showed that in both the primary and secondary forest, there is a general pattern that root and soil AcP were both enhanced by N addition while suppressed by P addition and NP addition, but the effect varies between two sources and between two forest types. Specifically, P addition significantly reduced root AcP activity by 74.8% in the secondary forest, while reduced soil AcP activity by 42.8% in the primary forest; only root AcP activity in the primary forest was significantly enhanced with increasing N addition rates. Both root and soil AcP activities were negatively correlated with soil total P when data from two forest types were combined, whereas no significant relationship was detected between soil available P and root or soil AcP activity in the primary forest or in the secondary forest. Moreover, root AcP activity was significantly higher in the slow-growth primary forest (12.3 μmol g−1 h−1) compared to the fast-growth secondary forest (6.3 μmol g−1 h−1). Overall, we verified the widespread pattern that N addition increases while P addition suppresses soil AcP activity and found a similar pattern in root AcP activity in two tropical montane forests, indicating that plants and soil microbes have consistent responses to nutrient addition. We also emphasized the potential role of the successional stage of tropical forests in regulating root AcP activity.