Carbon cycling and priming of soil organic matter decomposition in a forest soil following glucose additions

Graduation date: 2008Soils are a globally significant carbon (C) pool and have the potential to respond to elevated CO2 and environmental changes through positive feedback cycles that enhance the turnover of soil organic matter (SOM). Understanding the mechanisms governing the turnover of SOM is particularly important for modeling the fate of C in soils under predicted environmental changes. The change in turnover of SOM following additions of labile C through natural root and litter deposition as well as from human activities such as fertilizer application is known as priming, and the mechanisms governing this process are poorly understood but may be important components determining soil C balance. Many studies have utilized experimental additions of labile C and variously interpreted the primed C to derive from the activation and turnover of microbial biomass or from the increased decomposition of non-microbial SOM. The objective of this study was to evaluate changes in the activities of SOM-degrading enzymes as a potential mechanism for observed priming effects and to determine whether the priming response was related to the availability of “primable” C from root and litter inputs to soil.\ud 13C-labeled glucose was injected in the field into surface soils of an old-growth coniferous forest on the western side of the central Oregon Cascade Range. The fate of these small glucose additions (100 µg C g-1 dry soil) was traced into soil C pools and captured as respired CO2 over the course of a 22-day experiment. The forest soils considered in this study have undergone ten years of selective exclusion of root and/or litter inputs as part of a larger experiment known as the Detrital Input and Removal Treatments (DIRT). Plots without roots showed negative priming responses to glucose additions with reduced turnover of SOM due to a preferential switch from SOM to glucose as a C source and through an apparent generalized suppression of microbial activity. Positive priming was observed in soils receiving regular C inputs from roots and litter. Both positive and negative priming were observed in plots with litter inputs excluded depending upon which method of priming quantification was used. In soils with C-input limitations (i.e., No Roots, No Litter, No Inputs), the magnitude and direction of priming was negatively related to background respiration rates with highly active soils in these plots showing the strongest negative priming effects. Control soils showed no relationship between background respiration rates and priming effects. \ud The potential activities of β-glucosidase, phenol oxidase, and peroxidase showed no consistent relationship to glucose additions or priming effects and suggest that the production of enzymes in response to labile C inputs may not be the controlling mechanism for priming in these soils. The amount of primed C mineralized throughout the course of the experiment is consistent with earlier hypotheses that the activation and increased turnover of microbial biomass C is the primary source of primed C. \ud Modeling the turnover of glucose-derived C in soil revealed at least two pools of glucose-C with mean turnover times of 1 d and 30 d. The initial respiratory response of soils to glucose additions was dependent upon the rapid turnover of an easily degradable pool whereas 13CO2 efflux after four days was tied more directly to the turnover of a slowly degrading pool. A significant portion of glucose-C remained in soils throughout the course of the experiment in a non-extractable pool comparable in size to the amount of glucose-C taken up into microbial cells.\ud The results of this study contribute new and challenging problems for mechanistic interpretation of SOM priming. The lack of a discernible connection between enzyme activities and SOM priming suggests that observed priming was not related to accelerated turnover of stable SOM but it also seems unlikely that microbes in this C-limited system would maintain large intracellular reserves of C. The relationship between background respiration rates and negative priming as well as the generalized suppression of microbial activity following glucose additions are novel observations and defy common expectations of glucose effects on microbial activity