Rapid increase in soil organic carbon and structural stability in a sandy loam soil following conversion from long-term arable to semi-natural grassland irrespective of initial soil conditions

Converting arable land to semi-natural grassland has the potential to restore soil structural stability (SSS) and increase soil organic carbon (SOC) storage. However, the effect of the initial soil C level and conditions on the changes following introduction of grassland is poorly understood. We quantified changes in SSS and SOC in topsoil from the Askov straw disposal and cover cropping experiment initiated in 1981 in Denmark before and after its conversion to semi-natural grassland in 2020. Due to different straw input and cover crop use in the arable phase, the grass was established on soils with different initial conditions (11.9–17.9 g C kg−1 oven-dry soil). Soils sampled in 2019 and 2021 from plots subjected to different treatments in the arable phase were analyzed for clay dispersibility measured on two macro-aggregate sizes (ClayDis 1–2 mm and ClayDis 8–16 mm), wet stability of aggregates (WSA) and SOC. Irrespective of initial soil conditions, introduction of semi-natural grassland resulted in a rapid increase in SOC, a decline in ClayDis 1–2 mm and no change in WSA. The relative change in ClayDis 1–2 mm was larger than the change in SOC, and the slope for the relationship between SOC and ClayDis 1–2 mm decreased after conversion to grassland. Based on these observations, we suggest that the additional driver in play was aggregate-binding and -bonding agents being persistent due to the absence of tillage following the conversion to grassland. Notably, changes in SOC and SSS were similar regardless of initial soil conditions, which may be related to a similar productivity in the semi-natural grassland and soil C saturation