A global pattern of soil acidification caused by nitrogen deposition

Abstract:

Nitrogen (N) deposition-induced soil acidification has become a global problem. However, the response patterns of soil acidification to N addition and the underlying mechanisms remain far from unclear. Here, we conducted a meta-analysis of 106 studies to reveal global patterns of soil acidification in responses to N addition. We found that N addition significantly reduced soil pH by 0.23 on average globally. However, the response ratio of soil pH varied with ecosystem types, N addition rate, N fertilization forms, and experimental durations. Soil pH decreased most in grassland, whereas boreal forest was insensitive to N addition in soil acidification. Soil pH decreased linearly with N addition rates. Addition of urea and NH₄NO₃ contributed more to soil acidification than NH₄-form fertilizer. When experimental duration was longer than 20 years, N addition effects on soil acidification diminished. Environmental factors such as initial soil pH, soil carbon and nitrogen content, precipitation, and temperature all influenced the response ratio of soil pH. Base cations of Ca²⁺, Mg²⁺ and K⁺ were critical important in buffering against N-induced soil acidification at the early stage. However, N addition has shifted global soils into the Al³⁺ buffering phase. Overall, this study indicates that acidification in global soils is very sensitive to N deposition, which is greatly modified by biotic and abiotic factors. Global soils are now at a buffering transition from base cations (Ca²⁺, Mg²⁺ and K⁺) to non-base cations (Mn²⁺ and Al³⁺). This calls our attention to care about the limitation of base cations and the toxic impact of non-base cations for terrestrial ecosystems with N deposition.