The influence of soil fertility on rhizosphere effects in northern hardwood forest soils

Enhanced levels of microbial biomass and activity in the rhizosphere arise from labile C released from roots but the factors that mediate such "rhizosphere effects" are poorly understood. We hypothesized that the magnitude of rhizosphere effects would be reduced by increases in soil fertility, consistent with plant C allocation theory, which predicts decreased C flux to roots in fertile soils. Replicate plots of sugar maple (Acer saccharum Marshall) and northern red oak (Quercus rubra L.) at the Turkey Hill Plantations, NewYork, and yellow birch (Betula allenghaniensis Britton) at the Hubbard Brook Experimental Forest, New Hampshire, were fertilized from 2001 to 2003. In fall of 2003, rhizosphere and bulk soils were sampled from the organic horizon of control and fertilized plots and analyzed for microbial biomass and nutrient transformation rates. In general, fertilization reduced microbial biomass and activity in plots of all three species, and the magnitude of such effects was generally greater in rhizosphere than in bulk soil. In red oak soils, fertilization reduced rhizosphere effects on microbial biomass, net N mineralization rates, and phosphatase enzyme activity (P=0.026, 0.091, and 0.061, respectively). In contrast, fertilization only reduced rhizosphere effects on microbial biomass in sugar maple soils (P=0.019). In yellow birch soils, fertilization decreased rhizosphere effects on microbial biomass (P=0.015). These results suggest that soil fertility may mediate the degree to which roots affect microbial activity in forest soils, possibly due to fertilizer-induced shifts in belowground C supply.