Priming effects decrease with quantity of cover crop residues – implications for net soil carbon sequestration

Recent meta-analyses suggest a global potential of cover crops to increase soil organic carbon (SOC), thus contributing to climate change mitigation. However, some studies also found that cover cropping did not affect or even reduced SOC, thus it is uncertain how this effect is controlled. Here we aimed at comprehensively evaluating the potential and mechanisms of carbon (C) sequestration from cover crops in a Danish long-term crop rotation field experiment (LTE) initiated in 1997. We quantified SOC to 1-m depth, and also operationally divided soil organic matter into fractions of particulate organic matter (POM) and mineral associated organic matter (MAOM) to investigate the C saturation status of soils. Moreover, we performed a mescosm study with topsoils where the fate of varying doses (0.1-1.6 mg C g^-1 soil) of ¹⁴C-labeled cover crop residues (fodder radish, FR; Raphanus sativus L.) and SOC priming were traced in two texturally similar soils having the same long-term management, but different SOC contents (2.0 vs. 2.6% SOC). Our LTE results showed that cover cropping for up two decades had negligible effect on SOC contents in POM and MAOM fractions in the topsoil and in the subsoil. However, the mesocosm study showed considerable net C increases (20-25% of added) when the cover crop C input exceeded 0.3 and 0.6 mg C g^-1 in soils with 2.0 and 2.6% SOC, respectively. This was due a combination of new SOC formation and priming effects shifting from positive to negative. Collectively the LTE and mesocosm study suggests that buildup of SOC stock was not essentially constrained by soil C saturation, but rather by the low productivity and C input from cover crops. Our study suggests that agricultural management practices should be adopted (e.g., species choice and sowing time) to achieve a cover crop C input that exceeds a certain threshold to ensure effective C sequestration.