Meta-analysis shows global water availability thresholds for the contents of soil organic carbon and its fractions are only partly affected by nitrogen addition

Soils are the largest terrestrial reservoir of organic carbon, with water availability creating a global threshold for soil organic carbon (SOC) sequestration. Nitrogen (N) deposition is known to affect SOC, but its potential to shift global water availability thresholds for the contents of SOC and its particulate, mineral-associated and microbial fractions remain unclear. Here, we synthesized 2529 paired observations from 339 global N addition studies conducted in surface soil (0–20 cm) to investigate these effects under different water availability conditions, defined by the ratio of precipitation to potential evapotranspiration. Our results show that although nitrogen addition does not alter the water availability threshold for SOC, it significantly increases SOC content, particularly below the threshold, with an increase of 4.62 % below and 2.66 % above the threshold. Similarly, nitrogen does not affect the water availability thresholds for mineral-associated organic carbon and microbial biomass and necromass carbon. Nitrogen addition also increases microbial necromass carbon by 18.2 %, particularly bacterial necromass carbon, while having no significant effect on fungal necromass carbon. In contrast, nitrogen addition lowers the threshold for particulate organic carbon and increases its content by a total of 19.2 %. This differential response likely reflects a closer coupling of particulate organic carbon with plant litter inputs whereas mineral-associated organic carbon is more closely linked to microbial processing and transformation of organic matter. Overall, most SOC fractions showed a water availability threshold around 0.5, indicating that abrupt changes in SOC content may occur when precipitation drops below 50 % of an ecosystem’s evapotranspiration demand. Our results highlight how water availability and N addition interact to shape SOC fractions, offering key insights into SOC responses to global environmental change. Understanding these mechanisms is essential for predicting future SOC stocks and climate feedbacks under increasing N deposition and shifting precipitation patterns.