Effects of long-term nitrogen and phosphorus additions on soil acidification in an N-rich tropical forest

Tropical forests with highly-weathered soils are considered to be high sensitive to nitrogen (N) deposition and subsequent soil acidification. Phosphorus (P) shortage in tropical forest ecosystems is suggested to be one important factor that makes the ecosystems more vulnerable to N-derived acidification. However, it remains poorly understood on how changes in soil P availability affect soil acidification processes in humid tropical zones with elevated atmospheric N deposition. To address this question, we conducted a long-term N and P addition experiment in an N-rich tropical forest, consisting of four treatments: control, N-addition, P-addition, and NP-addition, respectively (both N and P are 150 kg ha(-1) yr(-1)). We hypothesized that relieving P-limitation by exogenous P addition will mitigate N-derived soil acidification. Results showed that six-year N addition significantly decreased soil pH (by 0.23,p = 0.013), soil base saturation (BS) (by 32.8%, p = 0.0017), and increased the ratio of aluminum to calcium (Al/Ca) (p = 0.015), suggesting that soil acidification was accelerated by N input. As we expected, P addition increased soil pH in the first few years, probably because of the increased biotic uptakes of nitrate and phosphate. However, after the first few years, continuous N addition promoted acidification and made the buffering effects by P addition invisible in our soils at the Al-buffering stage. Our results suggest that P inputs cannot alleviate excess N-derived soil acidification in N-rich tropical ecosystems, and that it is urgent to reduce reactive N emission for sustainable development of the affected ecosystems in the future. (C) 2016 Elsevier B.V. All rights reserved.