Long-term nitrogen addition promotes soil acidity in two tropical montane rainforests

Abstract

Tropical forest soils are susceptible to acidification owing to high weathering rates and low buffering capacity. Nutrient additions, particularly nitrogen (N) and phosphorus (P) inputs, can alter soil acidity; however, their long-term effects on the dynamics and underlying mechanisms of soil pH in tropical rainforests are not well understood. Here, we conducted two 13-year N and P fertilization experiments in primary and secondary tropical montane rainforests in Hainan, China. Results showed that long-term high-N addition reduced soil pH, and the effects increased with the rate and duration of N addition in both rainforests. The P-limited primary rainforest was more susceptible to N-induced soil acidification than the N-limited secondary rainforest with higher stand density during the experimental periods. Moreover, the depletion of base cations (primarily Ca²⁺) and the generation of exchangeable H⁺ were the main drivers of N-induced soil acidification. However, low- and medium-N additions, single P addition, and combined N and P addition did not significantly change soil pH or cation concentrations in both forests. These findings suggest that elevated soil N availability induced by long-term fertilization may alter soil cation composition, thus leading to soil acidification and impacting ecosystem functions in tropical forests.

Graphical abstract

Keywords

nutrient additions / tropical rainforest / soil acidity / exchangeable cations

Highlight

	● High-N addition accelerated soil acidification in tropical montane rainforests.
	● Primary rainforest was more sensitive to soil acidification than secondary rainforest.
	● Changes in exchangeable Ca2+ and H+ induced by N input regulated soil acidification.
	● P50 or N50P50 addition did not change soil pH or cations in both rainforests.

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