The highest acidification risk is not at the acidest region: Results from soil’s and leave’s responses across a 3,300 km gradient of wet deposition

Sulfur (S) and nitrogen (N) deposition-induced-ecosystem’s acidification has been broadly explored in hotspot regions. In contrast, ecosystems' sensitivity or risk were untested both at the organ level and across the natural deposited gradient. In this study, we examined the foliar pH of dominant woody plants and soil pH from 27 sites across a 3,300 km precipitation and deposition gradient in China. Significant relationships were observed in the humid and sub-humid regions between the response ratio of foliar pH (lnRRL) or soil pH (lnRRS) and acidic deposition. Acidic deposition’s negative effects on lnRRL disappeared in the semi-arid and arid regions but still occurred on lnRRS in the semi-arid region. Scaling exponents relating lnRRL to S&N deposition and lnRRS to S&N deposition were much higher in the humid region (-0.738 ~ -0.356) than in the sub-humid region (-2.774 ~ -1.935). The response ratio (lnRR) exhibited a sigmoid-shaped curve along the acidic deposition gradient, but the R2 of 0.936 for lnRRS was much higher than the R2 of 0.356 for lnRRL. We conclude that: the sub-humid region has higher ecosystem susceptibility or risk to acidic stress, while the humid region with the highest deposition maintains a relative acidification equilibrium; soils are more responsive to S and N inputs, while leaves pose a lagged and weaker response to acidic deposition. Our findings provide a predictive understanding of how plant leaves respond to acidic deposition across natural depositional gradients, which can help policy-makers structure a sustainable management approach that aims to mitigate future acidic risk.