Published results from laboratory experiments indicate that the rhizosphere priming effect (RPE), stimulation of soil organic carbon (SOC) decomposition by live roots and their rhizospheric activities, is an important mechanism in the global carbon cycle. However, the magnitude and controls of the RPE under realistic environmental conditions remain largely unknown and unstudied. Because of widely recognized limitations in applying laboratory results to field ecosystems, studies of the RPE under field conditions are needed. To meet this need we directly quantified the RPE under field environmental conditions at multiple locations using 13C isotope methods which enable us to definitively distinguish CO2 derived from the RPE of C3 plant roots on SOC decomposition in “C4 soils” (i.e., soils developed under C4-dominated vegetation). For quantifying the RPE of woody species, we first isolated an "intact" root system, and immediately lead the root system into a plastic chamber which contained C4-soil. The RPE of herbaceous species was measured by growing C3 plants in C4-soils in large plastic pots buried in field plots. Un-rooted soil controls were included in all experiments.
Results from the field root chamber experiment at Konza Prairie showed that some woody species invading Tallgrass prairie produced negative RPEs of approximately 20% - 45% during the initial phase of soil organic carbon decomposition right after soil disturbance. Results from the experiment at two locations in Heilongjiang Province in China showed that soybean plants grown in a soil from a 23-year continuous corn field produced RPEs ranging from 0.0% to 135.4% stimulation of SOC decomposition during two growing seasons, and that cottonwood (Populus alba) produced RPEs ranging from 16% suppression to 121% stimulation. Results from the experiment located in Northern Inner Mongolia indicated that a rhizome-forming grass (Leymus chinensis Tzvel.) rendered RPEs of -20% to 254% stimulation of SOC decomposition, while the RPE of alfalfa (Medicago sativa) ranged from -13% to 159%. Peak RPE values were observed at the flowering stage for most herbaceous plants. Total plant biomass was positively correlated with the RPE except for data from the Konza experiment. Overall, these results obtained under realistic environmental conditions provides the needed support for using RPE data from laboratories in a broader context, and further suggest that understanding the RPE at the ecosystem level requires more investigation.