Partitioning Soil Respiration during Pasture Regrowth

Abstract

ABSTRACTUnderstanding the regulation of net ecosystem exchange and soil C sequestration within pasture systems requires knowledge of its component fluxes, including photosynthetic uptake and respiratory loss. However, little information is available on the partitioning of soil respiration (RS) between its heterotrophic respiration (RH) and rhizosphere respiration (RR) components for perennial cool‐season pasture species. The purpose of this study was to use stable C isotopes to examine the effects of regrowth after defoliation on the components of soil respiration for grassland plots planted as an orchardgrass (Dactylis glomerata L.)–white clover (Trifolium repens L.) mixture but dominated by orchardgrass (90 to 100% of harvested biomass). Respiration components were quantified during the first 3 wk following defoliation at the beginning of July in 2010 and 2011. In 2010, RS, RH, and RR were all lowest 1 d after cutting (DAC). Both RS and RH significantly increased by 8 DAC but no further increase occurred over the next 2 wk. In contrast, RR continued to increase throughout the regrowth period. In 2011, RS did not differ among harvests and averaged across dates was less than in 2010. During the 2011 regrowth period RH increased slightly whereas RR did not change significantly. The rhizosphere contribution (RC) to RS averaged 0.49, increasing from 0.35 to 0.56 during regrowth in 2010 but decreasing from 0.62 to 0.43 in 2011. Differences among years in respiration responses during regrowth were possibly due to a combination of factors including resource limitations, soil moisture stress, and differences in soil temperature.