Soil microbial necromass carbon in forests: A global synthesis of patterns and controlling factors

Soil microbial necromass carbon (MNC) is an important contributor to soil organic carbon (SOC) and plays a vital role in carbon sequestration and climate change mitigation. However, it remains unclear whether the content, contribution to SOC (MNC/SOC), and fungal-to-bacterial necromass carbon ratio (FNC/BNC) of MNC vary across forest biomes and types. By summarizing data from 1 704 points across 93 forest sites, we explored the spatial patterns of MNC, MNC/SOC, and FNC/BNC in the surface layer of 0–20 cm of forest soils, as well as the controlling factors involved. Overall, boreal and temperate forests had higher MNC and FNC/BNC values than tropical, subtropical, and Mediterranean forests, whereas both boreal and Mediterranean forests had low MNC/SOC values. Mixed forests had higher MNC and lower FNC/BNC than broadleaved and coniferous forests, whereas MNC/SOC was higher in broad-leaved forests than that in coniferous forests. Interestingly, the dependence of MNC on forest type also varies among forest biomes. Regression analyses identified soil total N as one of the most important factors affecting MNC and MNC/SOC; whereas MAT, soil pH, and clay content were identified as the important factors affecting FNC/BNC. This synthesis is critical for managing soil MNC to mitigate climate change in forests.