Evaluating the impacts of alternative grazing management practices on soil carbon sequestration and soil health indicators

Globally, grazing lands represent a major carbon (C) sink. However, improper use of these ecosystems can degrade their soils and their C storage capacity. Specifically, prolonged high stocking rates in livestock pro-duction can result in significantly less soil organic carbon (SOC) and soil fertility in grazing lands. This underlines the need to assess key soil health indicators to determine the ecological efficacy of grazing management. The objective of this study was to identify the impacts of alternative grazing management practices, including heavy continuous (HC), light continuous (LC), and adaptive multi-paddock (AMP) grazing, on SOC and soil health indicators at the ranch and watershed scales in the Lower Prairie Dog Town Fork Red River Watershed in Northwest Texas. The stocking rate was the same under AMP and HC grazing, and it was reduced by half under LC grazing. The Soil and Water Assessment Tool-Carbon (SWAT-C) model was modified and used to conduct this assessment over a 40 year period (1979-2018), after a 900-year spin-up simulation (800 years under natural condition and 100 years under the HC grazing) to initiate the SOC pool. The simulated SOC was validated using observed data from several sampling locations. The study results indicated that when grazing management at the study ranch was changed from the current AMP grazing to hypothetical HC grazing, simulated average annual SOC decreased from 84 to 81.8 Mg/ha (a 2.6% decline). At the watershed-scale, when the grazing management was changed from the baseline HC grazing to AMP grazing, the simulated average annual SOC increased from 35.6 to 38.3 Mg/ha (a 7.5% increase). The net primary productivity (NPP), evapotranspiration (ET), biomass C, and soil nitrogen (N) increased, while C losses from runoff, and sediment loads decreased under AMP grazing. These results indicate that compared to HC, AMP grazing performed better with respect to SOC increase, and improvement of soil ecosystem and hydrological functions at both the ranch and watershed scales in the study watershed. Our findings suggest the need to shift from continuous to AMP grazing in order to improve soil health at multiple spatial scales.Data availability statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.