Aggregate size and glucose level affect priming sources: A three-source-partitioning study

Decomposition of soil organic matter (SOM) protected within aggregates can be accelerated via priming effect (PE) by the addition of fresh substrates. However, the knowledge of the sources of mineralization and PE in aggregate size classes is absent. We applied the three-source-partitioning isotopic (C-14 + delta C-13) approach to determine how aggregate size classes affect the contribution of three C sources (substrate added, recent and old SOM) to CO2 efflux and PE depending on the amount of added primer. Soil from a field with 3 years of maize cropping (C-4 plants) after long-term C-3 vegetation was used to differentiate between recent C (C-4-C; < 3 years) and old C (C-3 C; >3 years). Soil samples were separated into three aggregate size classes (>2 mm, 2-0.25 mm macroaggregates and <0.25 mm microaggregates) and were incubated for 49 days after being amended with two levels of C-14 labeled glucose. The proportion of glucose mineralized to CO2 increased with decreasing aggregate size, but C-14 incorporation into microbial biomass decreased, indicating higher C use efficiency in macroaggregates compared with microaggregates. The short-time PE was positive and was accompanied by a rapid reduction of dissolved organic C. After 49 days, the PE was higher in macro-versus microaggregates at both glucose levels. Positive PE induced by a low glucose level was observed only in large macroaggregates (>2 mm), but was observed in both macroaggregates (>0.25 mm) and microaggregates (