Crop Rotation With Ley and Nitrogen Fertilisation Reduced Soil Carbon Loss in Three Swedish Long-Term Field Experiments

Increasing soil organic carbon (SOC) stocks by improving cropland management practices has great potential to mitigate climate change. Long-term field experiments (LTEs) are valuable to study management effects on soil properties and crop yield. Yet most LTE studies are limited to the topsoil, and farming systems integrating multiple management strategies are often not assessed. This study used three Swedish LTEs to assess the effects of crop rotations and fertilisation on SOC changes. One arable rotation with only annual crops and a ley rotation with annuals, perennial ley and receiving manure were investigated at different application rates of mineral fertilisers. We analysed changes in SOC content and the distribution of SOC content and stocks at multiple soil depths, calculated C inputs and used phospholipid fatty acids (PLFAs) to evaluate how management practices affected SOC in relation to C inputs and microbial communities. Both systems lost carbon in the 0–20 cm topsoil from 1966 to 2019 across sites, but the sandy site lost more than the clayey sites. The ley rotation and nitrogen (N) fertilisation reduced carbon losses. In 2019, SOC stocks in the top 25 cm soil were 3.3 ± 1.6 Mg C ha⁻¹ higher in the ley rotation compared with arable rotation and 2.9 ± 1.6 Mg C ha⁻¹ higher with N fertilisation at the highest rate compared with no N fertilisation. However, the positive effects decreased with depth and became negative at some depths. As a result, differences in SOC stocks to an equivalent depth of 60 cm declined to 0.6 ± 2.4 Mg C ha⁻¹ for rotations and to 1.0 ± 2.4 Mg C ha⁻¹ for N fertilisation. The ley rotation had significantly higher belowground C inputs than the arable rotation, and belowground C inputs were highly associated with changes in SOC. Compared with the arable rotation, total PLFAs, bacterial PLFAs and the ratio of bacteria to fungi in topsoil were significantly higher in the ley rotation, partly attributed to manure application. Our study supports the beneficial effects of leys and manure amendments on SOC compared with systems with only annual crops. It also highlights the risk of losing SOC in the subsoil, especially under mineral N fertilisation. Site characteristics helped to explain the large variation, which must be considered when developing local strategies for SOC accrual in cropland.