Does grazing matter for soil organic carbon sequestration in the western North American Great Plains?

Authors: Derner, Justin; Augustine, David; FRANK, DOUGLAS - Syracuse University

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/2018
Publication Date: 12/12/2018
Citation: Derner, J.D., Augustine, D.J., Frank, D. 2018. Does grazing matter for soil organic carbon sequestration in the western North American Great Plains? Ecosystems. 22(5):1088-1094. https://doi.org/10.1007/s10021-018-0324-3.
DOI: https://doi.org/10.1007/s10021-018-0324-3

Interpretive Summary: Influences of livestock grazing on sequestration of soil carbon in semiarid grasslands remain unclear despite considerable prior efforts on shorter-term studies with carbon flux measurements and some directly measured soil carbon values from grazing experiments. Here we evaluate soil organic carbon sequestration rates from 12 pairs of 74-year old grazing exclosures and moderately grazed sites, in the western North American shortgrass steppe in northeastern Colorado. Soil radiocarbon (‰14C) measurements allowed us to determine turnover of the soil organic pool over a seven-decade period in the presence versus absence of grazing. Although removal of grazing for > 7 decades substantially altered plant community composition, this did not influence total soil carbon, soil organic carbon, soil radiocarbon (‰14C), values or turnover rate of soil organic carbon. Long-term removal of grazing from semiarid grassland ecosystems in the western North American Great Plains does not improve soil organic carbon sequestration despite substantial changes in the plant community.

Technical Abstract: Considerable uncertainty remains regarding grazing-induced influences on soil organic carbon (SOC) sequestration in semiarid grassland ecosystems due to three important complications associated with studying such effects: (1) ecologically meaningful shifts in SOC pools attributable to grazing are difficult to detect relative to the inherently large SOC pools in these grassland ecosystems, (2) a lack of baseline (pre-treatment) data, and (3) frequent lack of or limited replication of long-term grazing manipulations. Here is described a study that examined SOC sequestration in grazed grassland that overcame those challenges. SOC sequestration rates were determined in 74-year old grazing exclosures and paired moderately grazed sites, established across a soil texture gradient, in the western North American shortgrass steppe in northeastern Colorado. We sampled soils (0-20 cm) from 12 exclosures and paired grazed sites to measure SOC concentration and soil radiocarbon (‰14C); the latter allowed us to determine turnover of the SOC pool over a seven-decade period in the presence versus absence of grazing. Removal of grazing for > 7 decades substantially altered plant community composition but did not affect total soil C, SOC, soil delta 14C, SOC turnover rate, or total soil N. Grazing effect also did not interact with soil texture to influence any of those soil properties. Soil texture (silt + clay content) did influence total soil C and SOC, and total soil N, but not delta 14C or SOC turnover. Results provide evidence that long-term removal of grazing from semiarid grassland ecosystems in the western North American Great Plains does not enhance long-term SOC sequestration, in spite of changes in the relative dominance of C3 versus C4 grasses.