Importance of lithology control over microbial-derived carbon in forest soils

Microbial necromass carbon (MNC), also known as microbial-derived carbon, is an important contributor to soil organic carbon (SOC). Understanding of factors influencing MNC has been mainly confined to the effects of soil and climate; whether and how lithology influences MNC remains unclear. Furthermore, MNC is often regarded as a stable, uniform entity, but little consideration has been given to its labile and recalcitrant fractions. We measured MNC contents in particulate and mineral-associated organic matter, which represent the labile and recalcitrant pools, respectively, in forest soils over limestone and clasolite across a climatic gradient in southwest China. Nearby croplands with long-term soil tillage were selected as references for comparison. We found that, on average, the contents of labile and recalcitrant MNC were 56% and 52% greater, respectively, in forest soils over limestone than over clasolite. We also found that the responses of the MNC fractions to climatic and edaphic factors differed between lithologies. Lithology regulates the effect of mean annual temperature on soil minerals (calcium and iron oxides) and microbial biomass carbon, leading to recalcitrant MNC being more resistant to temperature changes in forest soils over limestone than over clasolite. However, long-term soil tillage could distort the influence 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 carbon by 418 and 241 Tg in topsoil (0–15 cm) over limestone and clasolite, respectively, of which 20% and 13% are attributed to MNC for limestone and clasolite, respectively. Our findings highlight that lithology controls the climatic pattern of microbial-derived carbon fractions in undisturbed soils and emphasize the necessity of considering lithology-based efforts to prevent carbon emissions.