COS 296-5 - The effects of livestock grazing duration and timing on soil health and organic carbon sequestration in a semi-arid rangeland of the western U.S.

Abstract: In the western United States, publicly-owned rangelands are used for livestock grazing, water supply, wildlife habitat, and recreation. As such, they provide an array of ecosystem services of economic, environmental, and societal importance. Balancing these services to satisfy rangeland stakeholders is challenging but critical to developing sustainable agroecosystems. We examined how improved livestock grazing – via managing the presence, duration, and timing of grazing – can meet this challenge while promoting good rangeland stewardship and supporting producer livelihoods. To do this, we synergized with stakeholders on a semi-arid rangeland in northeastern Utah that have historically grazed livestock in pastures comprising gradients of duration (0 months [exclosure], 0.5 months [short], 2 months [medium], and 4 months [long]) and timing (variable, early-season, and late-season). In particular, we explored how these factors affect soil health and soil organic carbon (SOC) sequestration by measuring water infiltration, nitrogen (N) availability and cycling, microbial biomass and respiration, bulk density, and SOC stocks at a landscape-scale that includes riparian and upland areas. We found that riparian areas – though small in extent – were biological hotspots for soil microbial activity across the rangeland. For example, infiltration, net N cycling, and microbial respiration rates were fastest, and N availability and microbial biomass were largest on the riparian benches directly along the streams relative to the riparian upland and adjacent upland areas. More, these metrics of soil health on the riparian benches were greatest in exclosures and pastures that experienced short-duration, variable-timing grazing relative to pastures that experienced medium- and long-duration grazing. These metrics of soil health on the riparian benches, and in the exclosures and short-duration, variable-timing pastures were related to the largest stocks of SOC. Though the upland areas had lower SOC stocks than the riparian areas, we found that once accounting for the spatial heterogeneity of the rangeland, stocks were also highest in the short-duration, variable-timing pastures relative to the medium- and long-duration pastures. Such results suggest that reducing the duration of grazing and shifting its timing from year-to-year are effective strategies to improve soil health and increase SOC sequestration. Therefore, focusing on the management of grazing duration and timing has the potential to provide a model of agroecosystem sustainability benefitting all stakeholders of public lands.