Microbial community mediated response of organic carbon mineralization to labile carbon and nitrogen addition in topsoil and subsoil

Over 50 % of soil carbon (C) is stored in subsoil (below 30 cm). Inputs of labile C and nutrients can stimulate soil organic carbon (SOC) mineralization priming effect (PE) and subsequently affect subsoil C dynamics. However, little is known about the magnitude and mechanism of the PE occurring in subsoil, which complicates the prediction of subsoil C dynamics. Using a lab incubation experiment, the effects of glucose and nitrogen (N) addition on SOC mineralization were studied for three soil layers (0-10, 10-30, and 30-60 cm) from a subtropical forest. Five glucose (5.16 atom % C-13) levels were applied according to soil microbial biomass in each soil layer. Meanwhile, community-level physiological profiling was conducted to reflect microbial functional diversity and activity. We found positive PE for all soil layers. The PE magnitude in subsoil was about two times higher than that in topsoil with stronger increase in the microbial activity in mining components of SOC. N addition led to a reduction of 45 % in the PE magnitude in topsoil with relatively lower microbial activity in mining N-containing substrates (amino acids and amines) but caused an increase of 18 % in subsoil PE. Soil C and N availability were associated with microbial functional activity, the shifts of which then mediated the SOC mineralization in the presence of labile C and nutrient. These results suggested that mineralization of subsoil C was more sensitive to labile C and N addition. Any future changes in environmental conditions that affect the input and distribution of labile C and N in soil profiles could affect C dynamics in deep soil.