Over 50% of soil organic carbon is stored in subsurface soil horizons (below 30 cm). Although the mineralization rate of subsoil organic carbon is much lower than that in topsoil, the amount of CO₂ released in deeper soil layers can be substantial because of its large stock size. Because of the barrier and buffer effect of topsoil, global change factors do not directly act on subsoil. Instead, they alter the labile carbon and nutrient input to subsoil through deep root system and flow path. 

Dr. TIAN Qiuxiang, under the supervision of Professor LIU Feng from Wuhan Botanical Garden, investigated the effects of labile carbon (¹³C labelled glucose) and nitrogen addition on soil organic carbon mineralization for three soil layers from a subtropical forest (Badagongshan National Nature Reserve). 

Study revealed that glucose addition stimulated soil organic carbon mineralization (positive priming effect) for all soil layers. The magnitude of priming effect in subsoil was about two times higher than that in topsoil with stronger increase in the microbial activity of mining complex components. The effects of nitrogen addition on the magnitude of priming effect varied with soil natural nitrogen availability.  

In normal-nitrogen soil at surface layer, the lower microbial activity in mining nitrogen-containing substrates (soil organic carbon in soil) decreased the positive priming effect, which was supported by “microbial nitrogen mining” theory.  

However, in nitrogen-poor soil at deeper layer, the stronger microbial activity in mining nitrogen-containing substrates after nitrogen addition increased the positive priming effect, which was supported by “stoichiometric decomposition” theory. 

These results indicate that any future changes in environmental conditions that affect the input and distribution of labile carbon and nitrogen in soil profiles could affect carbon dynamics in deep soil, which could have significant implications on terrestrial carbon cycling. 

Results were published in Biogeochemistry entitled “Microbial community mediated response of organic carbon mineralization to labile carbon and nitrogen addition in topsoil and subsoil”. This study was funded by the Natural Science Foundation of China, the Chinese National Key Development Program for Basic Research, and the China Postdoctoral Science Foundation funded project.