Infrequent compost applications increased plant productivity and soil organic carbon in irrigated pasture but not degraded rangeland

Improved agricultural soil management can facilitate the removal of carbon dioxide from the atmosphere to help keep planetary warming at or below 2 °C as outlined in the Paris Agreement. The application of compost to agricultural soils increases soil carbon by directly fertilizing the soil with carbon (no net carbon dioxide removal), and by stimulating plant productivity and plant-derived carbon inputs (net carbon dioxide removal). In semi-arid managed grasslands, it is unclear how compost application may impact plant production and soil carbon stocks. We conducted a multi-year experiment measuring plant and soil carbon and nitrogen responses following additions of compost in two adjacent semi-arid grasslands: an irrigated cool season perennial pasture and a degraded rangeland. Compost application altered plant production and soil carbon and nitrogen dynamics in the irrigated pasture, but not in the degraded rangeland. Aboveground biomass increased approximately 40% under the compost amendment in the irrigated pasture, while belowground biomass only differed between treatments in the first experimental year. Over eight years, soil organic carbon stocks increased 0.6 Mg OC ha⁻¹ yr⁻¹ in the 0–10 cm depth of the irrigated pasture, which was a net increase of 0.3 Mg OC ha⁻¹ yr⁻¹ after accounting for the carbon supplied by the compost. Soil inorganic carbon was highly variable at both experimental sites contributing to about half or more of total soil carbon at depth. Soil inorganic carbon decreased over the 8 year study period through the entire profile (0–50 cm) in the compost amended irrigated pasture (P = 0.01) and but not in the other systems, demonstrating the importance of accounting for soil inorganic carbon in carbon stock change estimates under improved soil management practices in semi-arid climates. We conclude that moderate, infrequent applications of high-quality compost to irrigated pastures can promote plant productivity and soil organic carbon, but degraded rangelands may be less responsive to this practice.