Root respiration represents a major, largely unknown fraction of the C budget of grasslands. Little is known about how the proportion of the community-assimilated C that is respired by roots varies seasonally and across spatially heterogeneous grassland landscapes, and how grazing ungulates may affect rates of root respiration. Limited information about root respiration additionally has hampered an understanding of the contribution of roots to the frequently measured flux of soil respiration rates that includes both decomposer and plant components. The objective of this study was to measure the seasonal influence of grazing herbivores (simulated by clipping) on root respiration and its relative contribution to soil respiration in three distinct grassland communities that varied in elevation, topographic position, edaphic properties, and plant productivity in Yellowstone National Park.
Clipping reduced mass-specific (µmol CO2 g-1 s-1) and ecosystem-level (µmol CO2 m-2 s-1) root respiration over the first 15 days after plants were defoliated. However, long-term (seasonal) average mass-specific root respiration rates were unaffected by clipping. Total seasonal CO2 evolution from plant root communities decreased by an average of 23% on grazed plots at all sites. After accounting for soil pore space CO2 concentrations, root respiration contributed proportionately the greatest percentage (75-100%) to soil respiration early in the growing season and thereafter dropped to 15-38%. Seasonal averages showed that root respiration accounted for 50% and 60% of grazed and ungrazed total soil respiration, respectively. Data suggest how seasonal variation, topographic position and the influence of herbivory play important roles in determining root respiration and its relative contribution to soil respiration.