Soil carbon accrual in ley rotations compared with cereal monoculture after four decades at four long-term experiments in Sweden

Long-term field experiments (LTEs) in which the effects of agricultural practices on soil organic carbon (SOC) dynamics have been quantified for the full soil profile are rare. We analysed 40–50-year data from four meta-replicated LTEs across Sweden comparing grass-clover ley-dominated rotations (LR) and cereal monocultures (CM; with or without crop residue retention) at four N fertilization rates, to quantify their effects on grain yield, C input and SOC dynamics in top- and subsoil. We found that grain yield was significantly higher in LR (3.5 ± 1.1 Mg ha⁻¹) than in the CM (2.7 ± 1.1 Mg ha⁻¹). Belowground C (BGC) inputs were identified as the major driver for SOC change. They were about three times higher in LR than in CM and explained 97 or 98 % of the variation in SOC change between rotations and N levels, respectively. Topsoil (0–20 cm) SOC content was consistently higher in LR than in CM and this difference increased over time. Topsoil SOC increased also with N application in CM, and 1 kg of N fertilizer was associated with approximately 1 kg of SOC increase. After 4–5 decades, we found significantly higher SOC stocks along the soil profiles in LR than in CM, 19.8 and 2.8 Mg C ha⁻¹ at 0–25 and 25–60 cm depth, respectively. The results also show that SOC stocks are driven by interactions between agricultural management practices and site-specific soil properties. This should be considered when designing management strategies for SOC sequestration in agricultural systems.