Defoliation by grazers can initiate compensatory growth, increased root exudation, and increased nutrient flows that enhance aboveground and belowground feedbacks between plants and microbes. This study examined the effect of defoliation on rates of CO2 exchange in a Yellowstone National Park (YNP) grassland during 2006 using a 50 x 50 x 50 cm plexiglass, temperature-controlled chamber. We compared monthly (July–Sept) measurements of net ecosystem productivity (NEP), ecosystem respiration (ER), ecosystem assimilation (EA), soil respiration (SR), and microbial carbon (MC) in plots that were clipped (grazed; G) with those in control, unclipped plots (ungrazed; UG). Grazing did not influence NEP (p = 0.56). Both treatments showed an increase in NEP from approximately 3 umolm-2s-1 to 7 umolm-2s-1 over the first 20 days of the experiment that declined to 1 umolm-2s-1 in September. There was no effect of grazing on EA (p = 0.11), which fluctuated between 16 and 21 umolm-2s-1 for most of the experiment. ER was marginally greater (p = 0.06) in G compared to UG plots. Grazing had no effect on SR (p = 0.09). SR fluctuated between –12 umolm-2s-1 and –7 umolm-2s-1 over the duration of the experiment (52 days). Although there was no overall difference in MC (p = 0.48), MC was higher in G plots (range ca. 620-560 mgkg-1 soil) 6 and 12 days respectively after clipping compared to UG plots (ca. 525 mgkg-1 soil). All parameters had a significant time effect (p < 0.0001), while SR and MC had a grazing x time interaction (p < 0.005). There was no overall effect of grazing on CO2 allocation processes despite a 30% grazing intensity that was approximately 2x higher than recent levels in YNP.
