Effects of Agricultural Management on Water Retention via Changes in Organic Carbon in Topsoil and Subsoil

Background: Increases in soil organic carbon (SOC) through agricultural management can alter soil water retention. This is determined via changes in pore size distribution, namely, air capacity (AC), plant-available water capacity (PAWC), and hygroscopic water capacity (HWC). However, it is uncertain how much soil water retention in topsoil and subsoil can be increased with agricultural management practices. Aims: The aim of this study was to quantify the influence of different agricultural management practices commonly used in Central Europe on AC, PAWC, and HWC in topsoil and subsoil as a result of SOC changes in different soil types. Methods: We sampled topsoil and subsoil at 11 sites across Germany that showed a management-induced SOC increase and a wide range of soil texture and measured the volumetric water content at different pressure heads from saturation to permanent wilting point. Results: In topsoil, AC increased on average by 1.3 mm 100 mm⁻¹ (+17%), PAWC by 1.6 mm 100 mm⁻¹ (+8%), and HWC by 1.6 mm 100 mm⁻¹ (+15%) per 10 g kg⁻¹ increase in SOC. In subsoil, SOC was increased by incorporating compost or topsoil material, which increased AC by 10.0 mm 100 mm⁻¹ (+134%), PAWC by 0.7 mm 100 mm⁻¹ (+3%), and HWC by 0.5 mm 100 mm⁻¹ (+7%) per 10 g kg⁻¹ increase in SOC. The total average increase in PAWC was 1 mm in topsoil and 1 mm in subsoil, with SOC increases by 4 and 5 g kg⁻¹, respectively. Conclusions: More than 50 years of additional farmyard manure addition resulted in an average SOC increase of 3.4 g kg⁻¹, but PAWC increased only slightly by 2.5%, showing the limited impact of agricultural management on water retention capacities.