Soil organic carbon mineralization is controlled by the application dose of exogenous organic matter

Substantial input of exogenous organic matter (EOM) may be required to offset the projected decline in soil organic carbon (SOC) stocks in croplands caused by global warming. However, information on the effectivity of the EOM application dose in preserving SOC stocks is surprisingly limited. Therefore, we set up a 90 d incubation experiment with large soil volumes (sandy loam and silt loam) to compare the mineralization of EOM (13C-labelled ryegrass) and SOC as a function of three EOM application doses (0.5, 1.5, and 5 g dry matter kg-1 soil). The percentage of mineralized EOM was expected to increase linearly with a higher EOM dose in sandy loam soil and to level off in silt loam soil due to the limited O2 supply in order to maintain aerobic microbial activity. In the sandy loam soil, the percentage of mineralized EOM was not affected by EOM dose, while SOC mineralization increased proportionally with an increasing EOM dose (+49.6 mg C g-1 EOM). Likewise, the formation of microbial biomass carbon was proportional to EOM dose, suggesting no reduction in microbial growth efficiency at a higher C concentration. In the silt loam soil, a decreasing tendency in the percentage of mineralized EOM was apparent but could not be confirmed statistically. We therefore conclude that, as in the sandy loam, the proportion of EOM mineralization was not affected with an increasing EOM dose, while SOC mineralization increased at a higher rate than in the sandy loam soil (+117.2 mg C g-1 EOM). Consistently with this lack of response in the proportion of EOM mineralization to EOM dose, soil EH did not decrease with an increasing EOM dose, indicating no O2 limitations. In both soils, an increasing EOM dose possibly supplied energy for microbial growth and enzyme production, which, in turn, stimulated mineralization of native SOC (i.e. co-metabolism). The observed stimulation of soil macroporosity at higher EOM doses in the silt loam soil might have contributed to sustaining the aerobic conditions required for SOC mineralization. In sum, this experiment and our previous research suggest that EOM mineralization is mostly independent of EOM dose, but EOM dose modulates the mineralization of native SOC. Provisional C balances compared to unamended controls indicated that, at low doses, less C remained than when EOM was added at normal or high doses in sandy loam soil, while no effect was found in silt loam soil. These findings tentatively indicate that using larger EOM doses could help preserve more added EOM-C, but longer-term confirmation in the field will firstly be required before we can draw any conclusion for soil C management.