Understanding soil carbon storage mechanisms to support sustainable intensification of pasture-based livestock systems in Brazil

Brazilian livestock production is predominantly based on pastures, characterized by extensive management and low forage productivity. To increase forage and livestock productivity while enhancing soil carbon storage, the use of more productive pastures and the diversification of production systems have been proposed. Integrated systems, such as crop-livestock (ICL) and crop-livestock-forest (ICLF) systems, are widely recommended to achieve these goals. In this review, we synthesize studies comparing permanent pastures and integrated systems, with and without a tree, that applied physical fractionation of soil organic matter. Our objective was to assess the capacity of integrated systems to store soil carbon relative to pastures and to provide insights for farmers, policymakers, and future research. The reviewed studies reaffirm the role of pastures as soil carbon sinks, primarily because they stabilize carbon in mineral-associated organic matter (MAOM), the largest and most persistent soil carbon pool. We found that grazing management and soil fertility in the pasture strongly control the capacity of both integrated and non-integrated systems to store soil carbon, due to the role of grasses in promoting MAOM formation. Integrated systems also have the potential to increase particulate organic matter (POM), mainly through greater crop diversification; however, increases in soil carbon are dependent on its transformation into MAOM. Integrated livestock-forest systems tend to behave similarly to permanent pastures, whereas crop-livestock systems exhibit more dynamic organic matter turnover. In tree-based systems, soil carbon storage depends on the proportion and management of the pasture component, while in systems without trees, soil carbon stabilization is strongly controlled by pasture management, especially nitrogen availability and grazing intensity. This review highlights the importance of distinguishing integrated systems with a tree component from those without trees, as they differ in their effects on soil carbon dynamics. The use of soil organic matter physical fractions is a key approach for understanding soil carbon storage in these systems. However, further studies are needed, particularly in systems designed for pasture recovery and livestock productivity, to better assess their potential to enhance soil carbon storage.