Soil carbon sequestration in rangelands: a critical review of the impacts of major management strategies

The agronomic benefits of soil organic matter have been studied for centuries, but contemporary focus has expanded to ask how increasing long-term storage of soil organic carbon (SOC) can contribute to mitigation of climate change. Understanding the potential for SOC sequestration in the vast rangelands is crucial for climate change policy, agricultural land management and carbon market opportunities. In this review, we evaluate the evidence from published field trials and modelling studies for sequestration in Australian rangeland soils managed for livestock grazing. We found few long-term studies with high quality SOC stock change data linked to new management, and our analysis was constrained by data limitations, conflicting results between studies, and highly variable climate, soil and landscape conditions across production systems. Rainfall and soil properties are dominant determinants of variation in SOC stocks in rangelands, and it was difficult to detect management impacts in these environments. However, there was consistent evidence that: (1) Sowing more productive grasses or legumes in existing grass pastures generally increases SOC stocks; (2) Prolonged high stocking is associated with net SOC loss; (3) Destocking or exclusion of grazing results in small SOC increases, especially in degraded soils; (4) Conversion from cropping to permanent pasture results in sequestration, influenced by management history; (5) Rotational grazing strategies show negligible impact on SOC stocks relative to continuous grazing; and (6) Waterponding increased SOC stocks initially but persistence has not been demonstrated. We discuss possible opportunities for SOC sequestration in rangelands in the context of uncertainties and associated benefits and trade-offs for livestock production, and make recommendations to improve the evidence-base for major management strategies.