Short-term leys and mineral fertilisation increase potential nitrogen mineralisation rate and stable carbon and reduce bulk density

Inclusion of short-term leys in rotations has been shown to reduce soil organic matter losses and increase yields at lower nitrogen (N) inputs. However, it remains unclear how grass-only or grass-legume leys, in the absence of manure, affect topsoil potential N mineralisation rate, the abundance of alkyl carbon (C) as an estimate of stable C, bulk density, and C stocks under different N fertilisation regimes. This study used a long-term agricultural field experiment series at three sites in southern and central Sweden comprising three crop rotations and four levels of mineral N fertiliser. Potential N mineralisation rates were significantly higher in soils from the ley rotations than from the no-ley rotation across sites. At Stenstugu, characterised by lower initial soil C and coarser texture, N mineralisation was further enhanced in the grass-legume ley rotation compared to the grass ley rotation. Mineral N fertilisation increased potential N mineralisation relative to no N addition. Across all sites, ley rotations, particularly the grass-legume ley, showed a higher abundance of stable C, as indicated by alkyl-C, whereas mineral N fertilisation mainly increased stable C in the pure grass ley and no-ley rotations. Bulk density was lower with N fertilisation than without, and at Stenstugu also in the ley rotations. Topsoil C stocks were higher in ley rotations and under high N fertilisation. Overall, these results indicate that short-term leys can contribute to improved aspects of soil fertility, but that the magnitude of these effects is site-specific and may be influenced by initial soil properties.