Lithologic control of microbial-derived carbon in forest soils

Microbial necromass carbon (MNC) is an important contributor to soil organic carbon. The influence of lithology on MNC remains unclear. MNC is often regarded as a stable, uniform entity, but little consideration has been given to its unprotected and protected fractions. We measured MNC contents in particulate organic matter, which represents the unprotected fraction, and in mineral-associated organic matter, which represents the protected fraction, in forest soils over limestone and clastic rock across a climatic gradient in southwest China. Additionally, nearby croplands with long-term soil tillage were selected for comparison. On average, the contents of protected and unprotected MNC were 52–56% greater in forest soils over limestone compared to clastic rock. Both MNC fractions over clastic rock decreased with increasing the mean annual temperature (MAT), whereas only the unprotected fraction decreased with increasing the MAT over limestone. MNC was regulated by iron oxides and microbial biomass in clastic rock and by exchangeable calcium, iron oxides, and microbial biomass in limestone. However, long-term soil tillage minimized the influences of lithology on MNC. By extrapolating the data to the three provinces of southwest China based on the land-use conversion scenario, we estimated that the existing forestation (recovery and afforestation) has the potential to increase microbial-derived carbon by 17 Tg in topsoil (0–15 cm) over limestone and 11 Tg over clastic rock. The lithology-dependent drivers of microbial-derived carbon accumulation and stability should be considered to predict soil carbon dynamics and minimize carbon emissions under changing environments.