Contrasting effects of different acid rain types on soil nitrogen pools

Acid rain represents a critical global change driver with profound consequences for soil nitrogen (N) pools. While atmospheric deposition has transitioned from predominantly sulfuric acid rain (SAR) to mixed acid rain (MAR) and nitric acid rain (NAR), the effects of these acid rain types on soil N pools remain poorly understood, particularly due to the lack of large-scale, comprehensive analyses. Here, we conducted a meta-analysis of 1195 paired observations from 74 studies to systematically assess the effects of different acid rain types on soil N pools across diverse ecosystems. We found that SAR significantly increased soil total N (TN; +5.5 %) and ammonia N (NH₄⁺; +46.0 %) pools while suppressing nitrate (NO₃^-; -33.9 %) and microbial biomass N (MBN; -17.4 %), reflecting sulfate (SO₄^2-)-induced inhibition of nitrification coupled with reduced microbial immobilization. Conversely, NAR reduced soil TN (-6.8 %), NH₄⁺ (-10.9 %), and MBN (-8.2 %) while enhancing alkaline hydrolyzable N (AN; +11.0 %), indicative of NO₃^--driven N losses through leaching and denitrification. MAR exhibited intermediate effects, elevating AN by approximately 11 %. The responses of soil N pools to different acid rain types were ecosystem-dependent, with significantly stronger negative effects observed in forests and grasslands compared to croplands and potted systems. The responses of soil N pools to different acid rain types were primarily driven by pH-related variables (including changes in soil pH, initial soil pH, and acid rain pH), rather than by climatic factors or soil texture. These findings demonstrate that acid rain composition fundamentally alters soil N dynamics, where pH is the primary controlling factor. These results highlight the particular vulnerability of NAR-affected ecosystems and emphasize the necessity of incorporating these differential effects into Earth system models to accurately project soil N cycling under future atmospheric deposition scenarios.