Rhizosphere Priming Effects of Encroaching Woody Species in a Tallgrass Prairie Ecosystem.

The influence of plant roots and the associated rhizosphere microbial activities on decomposition of soil organic carbon (C) and nitrogen (N), the rhizosphere priming effect, has emerged as a crucial mechanism affecting soil organic matter dynamics with potentially important consequences for global cycling of C and N. Our laboratory studies indicate that rhizosphere priming can reduce the rate of soil organic matter decomposition by 50% or accelerate it up to four-fold, depending on plant-soil couplings and experimental conditions. The rhizosphere priming effect in laboratory studies can be as large as those of soil temperature and moisture; however, the magnitude and controls of rhizosphere priming on soil C and N dynamics under realistic field conditions remain largely unknown and rarely studied. We explored rhizosphere priming associated with the encroachment of woody plant species into native tallgrass prairie at the Konza Prairie Biological Station near Manhattan, Kansas, using a natural ¹³C tracer method under field conditions. Our initial results suggest that the rhizosphere effects of honey locust (Gleditisia triacanthose) and eastern red cedar (Juniperus virginiana) are negative, reducing the decomposition rate of soil organic matter during the first growing season. This may reflect a suppression of soil organic matter mineralization as woody plants invade grasslands, or imply that the effect of soil disturbance associated with root chamber installation overpowered the rhizosphere priming effect at this stage of the study. Longer-term measurements are underway to resolve these possibilities.