Carbon Cycling and Priming of Soil Organic Matter Decomposition in a Forest Soil Following Glucose Additions.

Many field and lab studies dating back several decades have demonstrated enhanced soil organic matter (SOM) decomposition following additions of simple sugars through natural rhizodeposition or experimental application. This characteristic of SOM turnover is commonly known as the priming effect, but little is known about the mechanisms governing SOM priming. To determine the presence and characteristics of priming in forest soils in the Oregon Cascade Range, we utilized paired one-time injections in the field of ¹³C-labeled glucose and water across experimental plots that have had roots and/or litter inputs selectively excluded (Detrital Input and Removal Treatments). Soil samples from days 0, 1, and 10 were analyzed for carbon content and δ¹³C of microbial biomass, dissolved organic carbon, and bulk soil. Enzyme assays were performed for β-glucosidase, phenol oxidase, and peroxidase potential activities. Soil gas CO₂ flux rates and δ¹³C of soil efflux was measured on days 0-4, 7, 9, and 22. SOM turnover was determined by partitioning soil CO₂ efflux into glucose-derived and SOM-derived CO₂. Priming responses were strongest 2-3 days following glucose additions and then decreased over time. Positive priming occurred in Control (+97% on day 2) and No Litter (+23% on day 3) plots while negative priming occurred in No Roots (-17% on day 2) and No Inputs (-27% on day 2) plots. No consistent trends or treatment effects were found in any of the enzyme assays. These results indicate the priming responses in these soils were not related to potential enzyme activity and do not appear to correspond to de novo enzyme production following glucose additions. Observations of positive priming in soil with roots and negative priming in root-free soil are consistent with earlier results and suggest that variations among microbial communities associated with these soils may drive observed differences in SOM priming.  A novel relationship showing more negative SOM priming for plots with higher background respiration rates was also documented in No Roots, No Litter, and No Inputs plots but was not found in Control plots.