Background Soil pH Governs Nitrogen‐Induced Acidification by Mediating Microbial N‐Cycling and Soil Buffering Capacity

Abstract

Soil acidification caused by increased nitrogen (N) inputs has emerged as a global environmental concern that threatens ecosystem functioning and food security worldwide. However, how background soil pH influences the extent and mechanism of N‐induced acidification remains poorly understood. By assembling a global data set of field N addition experiments, we showed that the degree of acidification was most pronounced in neutral soils (pH 5.98–7.95), followed by acidic (pH ≤ 5.98) and alkaline (pH > 7.95) soils. In acidic soils, ammonia‐oxidizing archaea continuously released protons during the nitrification process, and the relatively weak buffering capacity exacerbated acidification. In neutral soils, N inputs increased the abundance of ammonia‐oxidizing bacteria (AOB) as well as nirS‐ and nirK‐harboring denitrifiers. These microbes exhibited strong N transformation capabilities, resulting in severe soil acidification due to the marked decline in buffering cations such as calcium (Ca ²⁺ ) and magnesium (Mg ²⁺ ). In alkaline soils, the high buffer capacity provided by carbonates (CO ₃ ²⁻ ) initially mitigated acidification. However, high background pH promoted the growth of AOB and nirK denitrifiers after N inputs, potentially initiating acid‐generating reactions and gradually depleting the soil's CO ₃ ²⁻ buffer capacity. Our study suggests that N‐induced soil acidification is strongly regulated by background soil pH, with the microbial N transformation process and soil buffer capacity playing crucial mediating roles. These findings have important implications for developing sustainable soil management strategies under continued N enrichment.