Topsoil Microbial Biomass Carbon Pool and the Microbial Quotient under Distinct Land-Use Types Across China: A Data Synthesis

The status of soil microbiome has become global concern for earth sustainability. The pool size of microbial biomass carbon (MBC) provides a basic tool to generally assess the soil microbes related to soil health in global terrestrial ecosystems. Albeit, the variation of topsoil MBC pool remains largely uncertain with land use patterns across China. In this study, data of topsoil (0−20 cm) MBC measurements were retrieved from 468 observations published in Chinese journals from 2000−2022. With linking to soil organic carbon, microbial quotient (MQ) was calculated as a portion of MBC content as a percentage of SOC content. Meta-analysis showed that, on average, topsoil MBC pool was the greatest (470.8 mg·kg−1) for forest land but the lowest (179.9 mg·kg−1) for dry croplands. MQ was higher (2.7%) for rice paddies with waterlogged conditions than for other land use types (2.0% on average). Integrating the values from all land use patterns, mean MBC pool of China's topsoil was quantified as 323.2−384.3 mg·kg−1 while MQ as 1.7%−2.3%, at 95% confidence. Upscaling with the area data of the land use types allowed a prediction of an overall MBC pool of China's topsoil of 635.8 Tg C (in a range of 614.4−657.1 Tg C). MQ was on average of 2.1% for whole China's topsoil, being prominently higher than the reported global mean of 1.2%. Furthermore, topsoil MBC contents exerted a wide variation (CV of 40%) while MQ showed a much narrow variation (CV of 19%) with land use types across mainland China. The MBC pool was affected significantly by edaphic factors related to organic matter status such as SOC, total N, C/N ratio and bulk density. For environmental drivers, however, MBC was significantly, but less strongly impacted with mean annual precipitation while MQ significantly but slightly influenced with mean annual temperature. The former could be generally ascribed to soil-resource dependent ecosystem productivity though the latter could be related to temperature-sensitive of organic carbon decomposition and microbial carbon assimilation through SOC turnover in soil. To predict a topsoil MBC pool, a statistical linear model was developed with three key edaphic attributes of bulk density, organic carbon and total nitrogen plus one climate attribute of mean annual precipitation, which had an explanatory of 48% of the total variance for soils with varying land conditions. Our study highlights the importance of integrating soil microbial biomass carbon pool into global biogeochemical models of carbon cycles with strong reference to microbial community structure and activity across hierarchical scales.