Rhizosphere Priming Regulates Soil Carbon Cycle.

The impact of plant roots and their rhizospheres on soil carbon dynamics remains a major issue in soil science. Results from laboratory studies suggest that the presence of living roots and their associated rhizospheric activities has the potential to enhance the rate of soil organic matter decomposition by fourfold in some instances or to suppress the rate of soil organic matter decomposition by half under other conditions. This wide range of the rhizosphere priming effect indicates that plants can impose a major control on soil organic matter dynamics via roots and their rhizosphere microbial activities to a similar degree as temperature and moisture do. However, our understanding on quantitative relationships between rhizosphere priming effect and soil organic carbon dynamics are lacking. Data from a greenhouse experiment involving two plant species and three levels of fertilization indicated that rhizosphere-primed soil C loss scales non-isometrically with root N content, with a scaling equation exponent much less than one (0.523). Nevertheless, the relationship is highly significant (P<0.0001). Whereas data from another greenhouse experiment using four tree species and three herbaceous species suggested that level of the rhizosphere priming effect on soil organic carbon decomposition scales isometrically (i.e., the scaling equation exponents are not significantly different from one) with live root biomass. Results from a 400-day isotope labeling experiment indicated that the quantitative relationship between levels of the rhizosphere priming effect and total rhizosphere input may roughly follows a modified Michaelis–Menten function. Evidence so far suggests that roots and their rhizospheres impose significant controls on soil processes across various temporal and spatial scales.