Effects of Exogenous Glucose on Soil Organic Carbon Sequestration and Carbon Metabolism in Root of Malus baccata (L.) Bork

Labile carbon (C) inputs to soil affect organic C dynamics in soil and the pathway of energy metabolism in tissue of plants. However, studies focusing on orchard soil are still scarce. In this study, the fate of 13C-glucose in organic C components, and the C metabolism in root of Malus baccata (L.) Borkh. in response to exogenous glucose at two levels, 1.50 g kg−1 (GLC1) and 7.50 g kg−1 (GLC2), were evaluated. Glucose addition increased the δ13C values and contents of soil organic carbon (SOC) and active fractions including microbial biomass carbon (MBC), dissolved organic carbon (DOC) pools, and particulate organic carbon (POC). At the end of the experiment, the MBC, DOC, and POC contents in GLC1 were 3.20%, 14.26%, and 5.16% higher than those in GLC2, respectively. In addition, no marked difference was found for SOC content between these two glucose rates. At the end of incubation, glucose-C was mostly allocated to SOC (56.79–85.05%), followed by DOC (4.72–20.88%) and MBC (4.46–18.1%); the lowest percentage was in POC (3.86–5.77%). Exogenous glucose increased the contents of organic acids and activities of key enzymes related with C metabolism in roots of M. baccata. In addition, adenosine triphosphate content, the energy matter, in root under GLC1 was 22.35% higher than that under GLC2. Correlation analyses suggested that the soil organic C fractions were positively correlated with C metabolism parameters in the roots of M. baccata. Overall, glucose addition improved the C metabolism in roots and the allocation of glucose-derived C in soil.