Grazing lands take up almost half of the Earth’s land surface and play a major role in climate change due to their massive stores and fluxes of carbon. In California, rangelands cover more than half of California’s total land area, approximately 34 million acres of which are actively grazed. How do different grazing techniques affect grazing lands’ soil health and climate change mitigation potential?
Recent research in the Journal of Environmental Quality aims to answer that question. The paper, by Byrnes, Eastburn, Tate, and Roche (all associated with UC Rangelands out of UC Davis), is a global meta-analysis of the impacts of different grazing techniques and intensities on soil health.
The study analyzed 64 different grazing research articles. The experimental management strategies were grouped into continuous, rotational, or no grazing, as well as light, moderate, and heavy intensity grazing. Thus, the authors were able to correlate grazing management techniques with changes in soil health indicators, including soil organic carbon, total nitrogen, carbon-to-nitrogen ratio, and bulk density (an indicator of soil compaction and pore space).
Core findings of the analysis include:
- Any grazing intensity or strategy increases soil compaction relative to no grazing.
- Rotational grazing reduces compaction and increases soil carbon relative to continuous grazing.
- Reduced grazing intensity reduces compaction and increases carbon stores.
- Site conditions such as soil texture and total annual precipitation moderate grazing impacts.
- Rotational grazing could create climate change mitigation opportunities over continuous grazing.
“Our findings suggest that rotational grazing can improve soil health over continuous grazing strategies. Decisions about grazing strategy and intensity significantly influence soil health outcomes, and site-specific conditions play important roles in shaping these outcomes,” writes co-author Ken Tate in a UC Rangelands blog summarizing their research.
More specifically, the research found that continuous grazing activities increased soil compaction (i.e., increased bulk density) while decreasing the soil organic carbon (SOC). Additionally, heavy grazing activities had more negative effects on soil health indicators like soil compaction and SOC than moderate and light grazing. Meanwhile, rotational grazing improved both the bulk density and SOC of the soil relative to continuous grazing.
How does grazing affect soil health?
The short answer is: in many complex, inter-related ways. For example, plant defoliation (e.g., cows eating grass) can affect plant photosynthetic rates, carbon allocation, fine root mass, and plant root exudates (substances roots secret into the soil). All of those changes affect grazing lands’ biogeochemical cycles. Meanwhile, excessive trampling of plants and soil from grazing animals’ hooves can compact soils, resulting in decreased soil pore space, reduced soil infiltration, and less water available to plants.
Despite that complexity, the results of this study offer important insight into the potential of rotational and light intensity grazing as tools for improving soil health, water infiltration, and carbon sequestration in the soil.