The rhizosphere priming effect is the change in rate of soil organic matter (SOM) decomposition in the presence of roots, which is a crucial aspect of soil carbon cycling and the global carbon cycle. This experiment measured the rhizosphere priming effect at the field scale and lasted for two growing seasons, which is significantly different from previous studies that are often conducted in greenhouses over the span of a few weeks. The natural encroachment of tree species obtained by reduced fire occurrence in the area allows for the study microbial influence on SOM decomposition in the presence of native species in native soil without artificial labeling. The rhizosphere priming effect of encroaching wood species in C4 grassland was studied in situ at the Konza Prairie Biological Research Station. A natural abundance 13C method was used to study SOM decomposition and the effects of live roots on the rate of SOM decomposition. Individual isolated second order roots from two native trees species, juniper virginiana and gleditsia tricanthos, were placed in root chambers and respiration was sampled seven times over the course of two growing seasons. At the end of the first growing season a representative sample of root chambers was destructively harvested. Priming, root biomass, carbon and nitrogen content of roots and soil were measured.
In both the first and second growing season roots inside the chamber appeared to have grown normally. Available carbon in the root chambers decreased during the first growing season. Respiration was lower in the second season meaning that all treatments (unrooted, juniper virginiana, and gleditsia tricanthos) saw a decrease in microbial activity. Negative rhizosphere priming was observed during the first growing season, and no statistically significant rhizosphere priming occurred during the second growing season. This may be related to the initial high nutrient content of the prairie soil because the priming effect has been shown to depend on the nutrient and carbon availability.