During dry conditions, traditional surface-drained grasslands support higher earthworm abundances than contemporary deeply drained grasslands

Excess water has traditionally been removed from farmland using surface drainage systems. In surface-drained grasslands, excess water flows from raised rigs toward lower laying grips, from where it is transported to surrounding canals. With the intensification of agricultural practices in the 20th century, many of these surface-drained grasslands were replaced with levelled, deep-drained grasslands. This transformation must have impacted the soil moisture dynamics of topsoils, as well as earthworms (Lumbricidae), an important group of soil ecosystem engineers that also serve as prey for meadow birds. To study how earthworms respond to surface-drained versus deep-drained grasslands across a meadow bird breeding season (March-June 2023, divided into an ‘early’, ‘middle’, and ‘late’ sampling period), we compared the (1) soil moisture content, (2) earthworm abundance, and (3) estivation of individuals. We found that for both grassland types, the soil moisture content declined over the course of the season, with the soils of surface-drained grasslands retaining more moisture than the soils of deep-drained grasslands. In the late sampling period, the abundances of epigeic and endogeic earthworms were higher in surface-drained grasslands, in which fewer individuals were estivating. This means that during the dry summer of 2023, earthworms in surface-drained grasslands have temporarily contributed more to soil processes while also being more abundant prey for meadow birds compared to those in deep-drained grasslands.