Soil Carbon Stocks and Root Characteristics of Perennial Warm-Season Grasslands Managed for Grazing and Biomass Production.

Although grazinglands in the eastern US are largely dominated by cool-season (C3) forage species, perennial warm-season (C4) grasses may serve as a suitable forage alternative to complement traditional cool-season pastures.  While some benefits of warm-season species have been examined, including high herbage mass and tolerance of drought and low nutrient conditions, relatively few studies have investigated the impacts that grazing or species diversity may have on belowground properties in this system.  We measured C stocks using natural 13C abundances and used minirhizotrons to study root growth characteristics in three year-old fields established with native perennial grasses managed either for grazing or biomass / bioenergy in Virginia.  Compared to C3 pastures, soil C stocks under C4 grasses were 80% lower in the top 10 cm (~30 Mg ha^-1 under C4 and 54 Mg ha^-1 under C3 grasses), but were 27% higher between 10-40 cm depths (44 Mg ha^-1 vs. 32 Mg ha^-1, respectively).  The C concentration in soils under grazing was 10-14% higher than under the ungrazed exclosures.  The amount of new C added to the soil by C4 species, measured by 13C, suggests that increasing C4 species richness may increase surface C additions in grazed and ungrazed areas.  However, at 10-40 cm depths, the C4-C was higher under exclosures than under grazed areas.  Over a 13-month period, grazing reduced total root volume and average root diameter.  Under management for biomass, total root volume was 40% higher, average root diameters were 20% larger, and the root distribution by root length was deeper in soils.  These observed belowground differences may be influenced by shifts in plant species composition caused by grazing, as grazing was found to reduce C4 grass biomass and increase the invasion of annual weedy species.