Can plants stimulate soil microbes and their own nutrient supply? Evidencefrom a grazing tolerant grass

The primary source of mineral nutrients for plants is the decomposition of organic matter by soil microbes. Plants are traditionally viewed as largely passive participants in the decomposition process, incapable of directly a ffecting rates of decomposition significantly and primarily assimilating nu trients unused by the microbial pool. We performed formed a C-13 pulse-chas e experiment on a common grazing tolerant grass, Poa pratensis L., of Yello wstone National Park, to follow carbon flow into the soil rhizosphere and m icrobial biomass and the associated effects on soil N availability and plan t N dynamics. Grazing promoted root exudation of carbon, which was quickly assimilated into a burgeoning microbial population in the rhizosphere of cl ipped plants. Moreover, these facilitating effects of defoliation on rhizos pheric processes positively fed back on soil inorganic N pools, plant N upt ake, leaf N content, and photosynthesis. Such findings are the first eviden ce, to our knowledge, that suggest (1) plants are capable of promoting rhiz ospheric microbial populations to facilitate uptake of a limiting soil reso urce and (2) that there is a general positive feedback mechanism by which h erbivory promotes plant regrowth as well as energy and nutrient flows in gr azed landscapes.