Untangling the biological contributions to soil stability in semi-arid shrublands

Biological soil processes that enhance soil stability are important for sustainable ecosystems.  Furthermore, soil stabilization and erosion control are major restoration goals in arid and semi-arid ecosystems at risk from desertification. Communities of plants, biological soil crusts (BSCs), and arbuscular mycorrhizal fungi (AMF) have all been shown to influence soil stability individually, but their relative contributions, as well as combined effects, are not well understood. In a landscape-scale field study in southern Utah, we quantified plant, BSC, and AMF communities along a soil stability gradient to determine their relative contribution to soil stability. We used structural equation modeling to test hypothesized causal effects of plants, BSCs, and AMF on both surface and subsurface soil stability. We found: (1) BSC cover had the strongest direct effect on surface stability, while plant cover had the strongest direct effect on subsurface stability. (2) AMF strongly contributed to both surface and subsurface stability. (3) Plants influenced soil stability both directly and indirectly through their interactions with other soil organisms. (4) Abiotic soil conditions (soil P) influenced the species composition of AMF communities. (5) The relative AMF contribution to soil stability in our study was similar to that of a mesic tallgrass prairie.  Our quantitative models demonstrate how plants, BSCs, and AM fungi work together to both directly and indirectly influence soil stability in semi-arid shrublands.  This research highlights the need for a holistic approach to erosion control and the management of precious soil resources.