The effect of groundwater depth on topsoil organic matter mineralization during a simulated dry summer in northwestern Europe

Abstract. With climate change expected to intensify the occurrence and severity of droughts, the impacts of the groundwater table (GWT) depth and capillary rise on topsoil moisture may become critical drivers of biological activity. Consequently, the GWT depth could influence topsoil carbon (C) mineralization. In this study, undisturbed 200 cm long soil columns with three different textures (loamy sand, sandy loam and silt loam) were subjected to two artificial GWT depths (−165 and −115 cm) in the laboratory. We examined (1) upward moisture flow by capillary action along the soil profile, specifically into the top 20 cm of soil, and (2) the effect of the GWT on the decomposition of an added 13C-enriched substrate (ryegrass) over a period of 10 weeks, with limited wetting events representing a dry summer. A 50 cm difference in the GWT depth (−165 vs. −115 cm) resulted in different topsoil moisture values for the sandy loam (31 % vs. 38 % water-filled pore space – WFPS) and silt loam (33 % vs. 43 % WFPS) soils. In the loamy sand soil, GWT-induced moisture differences appeared only up to 85 cm above the GWT. The expected acceleration of the mineralization of the added ryegrass under a shallower GWT was not confirmed. In contrast, CO2 efflux pulses after some of the wetting events were even higher for the drier −165 cm GWT than for the −115 cm GWT across all three soil textures. Additionally, a model fitted to cumulative ryegrass mineralization showed a lower mineralization rate for the stable Cryegrass pool in the silt loam soil with the shallowest GWT, where capillary rise contributed most significantly to topsoil moisture, compared with other combinations of soil texture and GWT depth. These findings suggest that the upward capillary moisture flow, along with the resulting increase in topsoil moisture and the anticipated enhancement of biological activity and ryegrass mineralization, might have been counteracted by other processes. One possible explanation could be that rewetting may have triggered a stronger mineralization response, commonly known as the Birch effect, in drier topsoils compared with conditions in which the soil remained consistently wetter with a shallower GWT level. Based on our findings, inclusion of the process of texture-specific capillary supply from the GWT is required to adequately simulate moisture in the topsoil during droughts as they occurred over the past summers in northwestern Europe, depending on the GWT–texture combination. However, the net effect on topsoil C mineralization is complex and warrants further investigation, including the integration of processes related to fluctuations in soil moisture following rewetting.