Compost and Native Seeding Interactively Increase Bulk Soil Carbon, but Compost Addition Favors Nonnative Plants

Rangelands are globally distributed habitats with high potential for supporting nature-based climate solutions. Yet, rangelands will be affected by human disturbances that result in severe habitat degradation and may need intervention to sufficiently recover. Compost addition can help improve soil carbon and water holding capacity, whereas native seeding can help bolster plant cover and richness; synergistically, they could have cascading effects on plant communities and forage quality. To better understand the utility of combining these range improvements, we treated a degraded working grassland (used to stage wildfire-fighting operations) in coastal California with compost broadcasting (1.48 kg · m⁻²) and native seeding (eight California species) treatments annually for 3 consecutive yr (2020–2022). We evaluated plant community composition, forage quality and toxicity, and soil biogeochemistry (soil inorganic carbon [SIC] and soil organic carbon [SOC] concentrations [%] and bulk soil carbon [T C · ha⁻¹]). Compost addition resulted in higher concentrations of soil organic matter (SOM) but did not increase SOC and SIC. Nonnative plant cover increased over time only in compost addition plots. Native cover and richness were unaffected by native seeding, but native cover was lower with compost addition. Compost resulted in higher relative cover of low-quality and less toxic forage, but also lower relative cover of high-quality forage. Bulk soil carbon was unaffected by independent compost addition or native seeding but increased significantly when treatments were combined. Our results demonstrate that compost application and native seeding have the potential to support soil recovery of SOM and bulk soil carbon in a degraded rangeland. The findings also suggest there are trade-offs in these practices, as compost addition could promote nonnative species and higher relative plant cover of low-quality forage. Low success from native seeding indicates that recommended seeding rates may be insufficient to establish plants in a degraded rangeland with compacted soils.