The Effect of Waterlogging on Soil Organic Carbon Decomposition Is Dependent on Its Biochemistry

The mechanism of soil organic carbon (SOC) mineralization stimulated by anaerobic conditions is unclear. The present study was designed to test whether SOC mineralization stimulated by waterlogging is dependent on abundances of plant and microbial origins in substrate. Two soils were sampled from alpine meadows (silty texture and sandy texture), and the abundance of plant materials was mediated through maize residue addition. The soil aeration conditions were adjusted by three moisture levels: 60% (aerobic), 100% field capacity (suboxic), and saturated (anaerobic). All treatments were then incubated for 121 days in the dark at 25 °C. Dynamics of CO2 flux and dissolved SOC (DOC) content were monitored. The effect of anaerobic conditions on CO2 release in each soil changed with incubation period and the abundance of plant or microbial-processed origins in substrate but DOC contents were always higher under anaerobic relative to aerobic or suboxic conditions throughout the entire 121 days. Moreover, anaerobic conditions reduced proportions of maize-derived carbon in the CO2 and microbial biomass in the first week, and lowered the ratios of hexoses to pentoses in non-cellulosic carbohydrates by the end of incubation compared with non-anaerobic conditions. Anaerobic incubation reduced microbial use of plant-derived carbon for respiration and assimilation compared with non-anaerobic incubation. Waterlogging stimulated SOC mineralization does not necessarily result from the increased SOC solubility and instead changes with SOC biochemistry.