Disturbance and soil amendment effects on C allocation in grazed temperate pastures.
Julie Doll, Richard Cates, and Randall Jackson. UW-Madison, Agronomy Dept., 1575 Linden Drive, Madison, WI 53706
Atmospheric CO2 concentrations are increasing at an alarming rate, therefore, the ability of grasslands to sequester C is of great interest. Plant biomass is both a potential reservoir and the C source for rhizodeposition-the input of C to soil from root systems. Grasslands allocate a large portion of C belowground, and because roots are the primary source of organic C to grassland soils, it is important to understand how management affects not only the total plant biomass production, but also the allocation of C to roots and shoots. In a temperate cool-season pasture, we measured above- and below-ground production under two disturbance regimes (fire and grazing) and three nutrient levels (ambient, C+, and N+) in a split-split-split plot experiment. Burning resulted in higher root:shoot ratios than grazing while N fertilization decreased the root:shoot ratio under both disturbance regimes. To better understand the effects of grazing on C allocation patterns, we conducted a factorial glasshouse experiment manipulating the defoliation frequency (weekly or monthly) and intensity (5 or 10 cm residual) of cool-season (Bromus inermis) and warm-season (Andropogon gerardii) grasses. Warm-season grasses generally have higher root:shoot ratios and greater belowground C than cool-season grasses, but there is a paucity of research comparing C allocation between them under identical defoliation regimes. In our experiment, both the frequency and intensity of defoliation significantly affected the cool-season grass root:shoot ratio, but these treatments had no effect on the warm-season grass. With an increasing amount of land under managed grazing and interest rising in the use of warm-season grasses in the Upper Midwest, it is important to understand how management will affect C allocation of both warm- and cool-season grasses in temperate pastures.