Microbial necromass as the source of soil organic carbon in global ecosystems

Despite the recognized importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, at a global scale, there has been no quantification for cropland, grassland, and forest ecosystems. To address this knowledge gap, the contents of fungal and bacterial necromass were estimated based on glucosamine and muramic acid contents in cropland (986 samples), grassland (278 samples), and forest (452 samples) soils. On an average, microbial necromass C contributed 51%, 47%, and 35% to the SOC in cropland, grassland, and forest soils, respectively, in the first 20 cm of topsoil. The contribution of microbial necromass to SOC increased with soil depth in grasslands (from 47% to 54%) and forests (from 34% to 44%), while it decreased in croplands (from 51% to 24%). The microbial necromass accumulation coefficient (the ratio between necromass and living microbial biomass C) was higher in soil from croplands (41) and grasslands (33) than in forest (20) soils. These results suggest that the turnover of living microbial biomass is faster in grassland and cropland soils than in forest soils, where the latter contains more partially decomposed plant residues. Fungal necromass C (>65% of total necromass) had consistently higher contributions to SOC than bacterial necromass C (32–36%) in all soils due to i) a larger living fungal biomass than bacterial biomass, and ii) fungal cell compounds being decomposed slowly and, thus able to persist longer in soil. The ratio of fungal:bacterial necromass C increased from 2.4 to 2.9 in the order of croplands < grasslands < forests, because fungi are the principal decomposers of complex substrates dominant in grassland and, especially, in forest soils. The ratios of bacterial:microbial necromass and bacterial:fungal necromass in cropland soils are larger than those in grassland and forest soils. This result indicates that the relative contribution of fungal necromass to total microbial necromass is lowest in cropland among the three land uses. Moreover, fungal and bacterial necromass increased with the total living microbial C and N contents. Lower temperatures and soil pH (e.g., in temperate and boreal ecosystems) stimulate fungal and bacterial necromass accumulation. These findings highlight the fact that shifts in the bacterial:fungal necromass ratio and the microbial necromass contribution to SOC are ecosystem-specific and depend on climate. In conclusion, microbial necromass contributes to approximately half of the SOC in cropland and grassland soils, and only 35% in forest soils; whereas, two-thirds of microbial necromass are of fungal origin.