Hydrological controls on greenhouse gas fluxes and soil carbon quality in a Belgian floodplain

Floodplain hydrology regulates soil carbon dynamics and is a key factor in ecosystem restoration strategies for climate regulation. In Belgium, floodplains have been extensively modified by drainage and land-use change, yet the combined effects of hydrology, land use, and soil carbon quality on greenhouse gas (GHG) fluxes remain unclear. In the Dijle valley, located in central Belgium within the Belgian loess belt, we conducted a comprehensive study combining in situ GHG flux measurements with soil carbon quality characterization.

We measured soil carbon dioxide (CO₂) and methane (CH₄) fluxes during the wettest year on record across three hydrological zones: (i) a moderately drained floodplain with fluctuating water table (Fluctuating WT), (ii) a poorly drained floodplain with shallower water table (High WT), and (iii) freely drained soils of the adjacent plateau. Representative land uses included forest, grassland, cropland, and marsh. Temperature sensitivity (Q₁₀) varied with soil moisture and water regime: under Fluctuating WT, Q₁₀ decreased with increasing moisture, whereas under High WT, Q₁₀ increased as moisture declined. CH₄ contributed substantially to total GHG emissions only in High WT sites, though its relative impact declined under very wet conditions. Maintaining water tables below but close to the soil surface through rewetting could therefore reduce soil CO₂ emissions and their temperature sensitivity, primarily by imposing environmental constraints on microbial activity. This highlights the importance of floodplain rewetting as a management strategy for climate regulation through the control of soil GHG emissions.

We also explored soil carbon quality and its possible relationship to C-specific basal respiration (R₁₀, µg CO₂-C gC^-1 h^-1). Soil samples (0–10 cm) were analyzed for organic carbon (C), nitrogen (N), and C/N ratio. Thermal properties were assessed using differential scanning calorimetry (DSC), including Energy Density (ED, J mgC^-1) and T₅₀ (temperature at 50% energy release). ED varied along the hydrological gradient (High WT < Fluctuating WT < Plateau), while T₅₀ followed High WT < Plateau < Fluctuating WT. In addition, R₁₀ was negatively correlated with C/N, slightly positively correlated with ED, but not with T₅₀. These patterns suggest that hydrology shapes the energetic quality and thermal stability of soil carbon, which may partially explain variations in respiration.

Combining field flux measurements with energetic characterization provides a novel perspective on links between hydrology, land use, and soil carbon quality. Appropriate floodplain management, including rewetting, can enhance their contribution to climate regulation by limiting soil GHG emissions and influencing carbon cycling.