Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/1/2010
Publication Date: 9/13/2011
Citation: Skinner, R.H. 2011. Quantifying rhizosphere respiration for two cool-season perennial forages. Crop Science. DOI: 10.2135/cropsci2011.03.0155. Interpretive Summary: Information on root respiration rate is important for understanding management and environmental effects on pasture carbon sequestration. However, such measurements are difficult to obtain because of the location of root systems within the soil. In May, July, and September of 2008 and 2009, carbon isotope differences between roots and soil organic matter were used to measure both root and soil respiration in undisturbed field plots containing orchardgrass and white clover. Both root and soil respiration rates were sensitive to soil temperature, but the sensitivity to temperature appeared to be somewhat greater for roots than for soil organisms. Root respiration accounted for about 50% of total soil respiration in July and September but only 40% in May. Root respiration under orchardgrass averaged 50% of total soil respiration compared with 43% under white clover. Accurately measuring the components of pasture respiration will aid in the development of realistic models to simulate carbon uptake and loss.
Technical Abstract: Understanding the regulation of ecosystem carbon dioxide flux from forage production systems requires knowledge of component fluxes, including photosynthetic uptake and respiratory loss. Experimental separation of soil respiration into its heterotrophic (free-living soil organisms) and rhizosphere components (roots and associated organisms) has been difficult, complicating efforts to quantify management and environmental effects on grazing land C sequestration. This study takes advantage of differences in the natural abundance of 13C between C3 and C4 plant species to separate microbial respiration of C4 derived soil organic matter from root respiration by C3 forage species. Respiratory flux and carbon isotope data were collected in May, July, and September, 2008 and 2009 from plots containing either orchardgrass (Dactylis glomerata L.) or white clover (Trifolium repens L.). The site had been a big bluestem (Andropogon gerardii, Vitman) field for about 30 years prior to establishing the orchardgrass and white clover. Thus, the soil organic C had a strong C4 signature. At each sampling period, respiration measurements were made at the mid-point of the regrowth cycle, two weeks following defoliation to a stubble height of 7 cm. Rhizosphere respiration accounted for about 50% of total soil respiration in July and September but only 40% in May. Rhizosphere respiration under orchardgrass averaged 50% of total soil respiration compared with 43% under white clover. Accurate quantification of the components of ecosystem respiration will aid in the development of realistic models to simulate ecosystem C flux.