Nitrogen-induced soil acidification mitigates the negative effects of nitrogen addition on SOC stability

Abstract

While nitrogen (N) deposition is a well-established driver of soil organic carbon (SOC) stability, quantitative syntheses assessing its global-scale impacts remain surprisingly limited. This study conducted a meta-analysis of 4418 observations from 131 independent studies to investigate how SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) response to N addition. Our findings showed that N addition increased SOC (6.53 %), POC (12.59 %) and MAOC (3.45 %) pools. Nevertheless, the POC:SOC ratio increased by 5.46 %, whereas the MAOC:SOC ratio decreased by 2.2 %, indicating that N addition reduced SOC stability. Under N addition, soil acidification was categorized into three levels based on the magnitude of soil pH decline: non-acidified [≥ 0], mildly acidified [-0.5–0] and severely acidified [≤ -0.5]. Soil acidification inhibited the positive responses of SOC and POC to N addition. With increasing acidification, both SOC and POC declined progressively. In contrast, Soil acidification generally promoted the accumulation effect of N addition on MAOC, as the 4.21 % increase in MAOC under mild acidification exceeded the 0.25 % loss under severe acidification. Furthermore, the MAOC:SOC ratio steadily increased under N-addition treatments, indicating that N-induced soil acidification contributed to SOC stability. Overall, our study demonstrated that N-induced soil acidification mitigated the negative effects of N addition on SOC stability by regulating the POC and MAOC response to N addition. These findings underscore the crucial role of soil acidification in regulating SOC dynamics under N addition and offer new insights into the interactions between soil C and N.