Aggregate size and their disruption affect 14C-labeled glucose mineralization and priming effect

Abstract

Soil organic matter (SOM) pools, allocated within various aggregates, are characterized by different degradability and turnover rates that depend on the spatial accessibility of organics and their recalcitrance. Hence, to understand the processes and mechanisms of SOM cycling and stability, the contribution of individual aggregate size classes to the total CO2 efflux including extra mineralization via priming effect (PE) should be considered. In this study, we determined whether aggregate size classes and their disruption affected the mineralization of SOM and induced PE depending on the primer amount. Soil samples were separated into three aggregate size classes (>2mm, 2–0.25mm macroaggregates and <0.25mm microaggregates). Half of the samples within each class were left intact, whereas half were crushed. After the addition of two levels of 14C-labeled glucose, the amount of 14C in CO2 efflux and microbial biomass were measured several times during the 49-day incubation.Cumulative SOM-derived CO2 production from the macroaggregates was 16–21% greater than the CO2 production from the microaggregates after 49 days. The percentage of glucose mineralized to CO2 increased with the level of glucose addition, but 14C incorporation into microbial biomass decreased, indicating lower carbon (C) use efficiency at high substrate availability. Aggregate disruption had no effect on the cumulative total and SOM-derived CO2 production, but it increased glucose mineralization up to 11.2% while the percentage of added glucose incorporated into microbial biomass in macroaggregates decreased. The PE increased with an increased glucose level for the intact aggregates. Aggregate disruption increased the PE in all aggregates sizes under low glucose level. In summary, our findings demonstrate that the aggregate size class has clear effects on C mineralization while their disruption affects the added labile C decomposition and transformation, indicating the relevance of soil structure for SOM cycling in terms of priming and C sequestration.