Effects of long-term annual fire and mowing on root dynamics in tallgrass prairie

The dynamics of fine root production, mortality, and turnover have important implications for ecological patterns and processes at scales ranging from microscopic to global. Root dynamics are especially important in tallgrass prairie because the majority of biomass and primary productivity occurs belowground in this ecosystem. Fire, grazing by large ungulates, and mowing are important disturbances influencing ecosystem processes in tallgrass prairie. This study examined the effects on root dynamics of long-term annual spring burning and summer mowing in a replicated factorial experiment at the Konza Prairie Biological Station in northeastern Kansas, USA. Using a combination of 90 cm deep soil cores and minirhizotron observations over three field seasons, I determined that fire and mowing acted individually and interactively to significantly influence patterns of fine root biomass, root C and N content, root length density (RLD), production, mortality, and turnover of fine roots. All of the observed fine root parameters varied significantly with depth, with a general pattern of decline with increasing depth. Annual burning resulted in significant increases in fine root biomass (+ ∼50%), C storage in roots (+ ∼40%), RLD (+ ∼100%), and total absolute fine root production (+ ∼45%) and mortality (+ ∼35%). Annual burning also resulted in greater proportions of root biomass, RLD, absolute root production and mortality occurring below 20 cm, and a decrease in fine root N content (∼0.18%) and relative fine root turnover (0.76 year-1 compared to 1.01 year-1 in unburned unmowed plots). Mowing did not affect root biomass or RLD in unburned plots, but significantly reduced root biomass and RLD (∼30% and ∼60%, respectively) in burned plots, and also reduced root N content (∼0.16%) and the relative depth distribution of roots (greater proportion of root biomass and RLD above 20 cm). Mowing alone, therefore, did not appear to have the often-predicted effect of reducing root biomass or root C storage as compared to unmowed tallgrass prairie. The observed effects and interactions of annual burning and mowing and soil depth could have important long-term consequences for ecosystem C and N cycling and other processes in tallgrass prairie.