Assessing the Impact of Capillary Moisture on Topsoil Carbon Mineralisation in Flemish Cropland Using a Physical Barrier

To accurately predict soil organic carbon (SOC) stocks under a changing climate, models must properly integrate soil hydrological controls. In Northwest Europe, prolonged droughts are expected, during which capillary moisture transport may help maintain topsoil moisture. At present, the importance of capillary moisture supply for topsoil moisture during dry summers remains uncertain, and so it is not clear if account thereof during SOC stock simulation is needed. This study aimed to determine whether a ~2-m deep groundwater table supplies moisture to topsoil during a dry summer and its effect on SOC mineralisation in six croplands across common textures (loamy sand, (sandy) loam and silt loam) in Flanders. We adopted a novel approach by installing repacked topsoil columns in situ within the plough layer, with or without a gravel layer below to act as a hydraulic barrier. In the loamy sand and (sandy) loam soils, groundwater tables of up to 2.3-m depth appeared too deep to affect topsoil through capillary moisture transport in our setup since topsoil was even slightly wetter in the gravel treatment, likely due to impeded downward water redistribution following rainfall. This artefact could be avoided with deeper barrier placement. In contrast, in the silt loam fields with groundwater tables up to 2-m depth, soil at a 15-cm depth was significantly wetter (25% volumetric water content (VWC) and a matric suction of −405 cm water height (WH)) when upward water transport was unobstructed compared to when it was blocked by the gravel barrier (18% VWC and −445 cm WH) during a 1-month drought period with very limited rainfall (5.8 mm). Surprisingly, this moisture increase via capillary rise did not enhance C mineralisation. Apparently, in these silt loam soils, C mineralization did not strongly depend on moisture, whereas in the coarser loamy sand soil, temporal moisture fluctuations had a greater impact on C mineralisation. This suggests that if capillary rise were to reach the topsoil, for instance with shallower groundwater, it could potentially influence C mineralisation. However, further research is needed to confirm this effect. Overall, whether groundwater moisture supply significantly impacted topsoil moisture and C mineralisation could only be evaluated in the silt loam croplands. Nonetheless, the proposed hydraulic barrier, with a recommended deeper installation, offers a promising tool for further testing conditions where capillary wetting may influence SOC dynamics.