Priming effects decrease with the quantity of cover crop residues - Potential implications for soil carbon sequestration

Meta-analyses suggest a global potential of cover crops to increase soil organic carbon (SOC) stocks, yet with a large variation across studies, which underlines the need to understand the effect of cover crops on carbon (C) sequestration under specific soil and climate conditions. We studied the C sequestration potential from cover crops, based on a Danish long-term field experiment (LTE) initiated in 1997, where SOC and C in the fractions of particulate organic matter (POM) and mineral associated organic matter (MAOM) was measured to 1-m depth. Next, we performed a mesocosm study where the fate of ¹⁴C-labeled cover crop residues (fodder radish, Raphanus sativus L.) and SOC priming were traced in two texturally similar soils from the LTE with different SOC concentrations (2.0 vs. 2.6% SOC). The results showed that cover cropping for up two decades had negligible effect on SOC in POM and MAOM fractions. Yet, the mesocosm study showed considerable overall SOC increases (20–25% of added C) when the cover crop C input exceeded rates of 0.2–0.3 mg C g⁻¹ in the two soils. This was due to a combination of new SOC formation and priming effects shifting from positive to negative. The input rates of 0.2–0.3 mg C g⁻¹ correspond to the C input from cover crops with an aboveground yield of approximately 0.7–1.1 Mg dry matter ha⁻¹, which is a level not always achieved at the field site. The combined observations from the field and mesocosm study suggest that SOC buildup was not constrained by soil C saturation, but rather by low cover crop productivity and/or positive priming effects. Therefore, agricultural management practices (e.g., species choice and sowing time) should be adopted to achieve a sufficient cover crop C input to secure that the positive priming effect is not exceeding the rate of SOC formation.