Compost amendments are an increasingly popular rangeland management strategy to increase both soil carbon and forage production. A positive effect of compost on soil carbon has been attributed to direct carbon inputs as well as indirect inputs from resource-stimulated plant growth. However, there is considerable variation in rangeland soil carbon storage after compost amendment. Soil biotic communities may account for this variation. Soil communities alter soil carbon directly through decomposition and indirectly by affecting plant productivity (via both nutrient availability and root predation). Compost is widely used in croplands to shift soil communities away from parasites and toward beneficial taxa, but these effects may vary in rangeland systems as they experience greater environmental variability over space and time.

The premise of our research is that soil communities are key mediators between rangeland compost amendments and soil carbon storage. Taking a soil ecology approach, we are working on: 1) a spatial survey of soil community composition across 15 compost-amended sites in California spanning climate and edaphic gradients; 2) a temporal survey to assess how soil communities shift with carbon pools across a compost amendment chronosequence; and 3) an expanded soil carbon model that explicitly accounts for microbial C pools and an experimental test of the relative inputs from plants and invertebrates to soil carbon in amended versus non-amended soils. Our results will contextualize the mechanisms and conditions under which compost amendents increase soil carbon storage, and our outreach will help stakeholders decide when and where to apply amendments to optimize rangeland soil health.