Groundwater-Level Response of Annual CO₂ Budgets from a Dutch Eddy Covariance Network: Comparison with European Temperate Peatlands across Land Use and Sites

Abstract. Peatlands in the Netherlands hold significant cultural, agricultural, and ecological value but also contribute substantially to national greenhouse gas (GHG) emissions as a result of drainage. This necessitates urgent mitigation, leading to the implementation of various land and water management strategies. As part of the Dutch national GHG research program (NOBV), eddy covariance (EC) measurements of carbon dioxide (CO₂) fluxes were conducted at 20 sites, encompassing managed peat meadows, wet nature areas, and paludiculture systems over periods of 1 to 3 years. A novel mobile EC set-up was utilized at some locations to enhance spatial coverage.

Net Ecosystem Exchange (NEE) of CO₂ demonstrated pronounced seasonal dynamics across different land-use types. Wet and highly productive systems exhibited both elevated uptake and emissions, leading to intervals of net CO₂ uptake. Managed pastures were typically net emitters on a daily timescale, except during early summer when assimilation peaked. Although annual net ecosystem carbon balance (NECB) estimates were subject to uncertainty due to data gaps and adjustments for harvest, grazing, and manure application, systematic differences between land-use types remained evident. To assess the sensitivity of CO₂ emissions to groundwater dynamics, ecological response functions (ERFs) were employed to relate NECB to groundwater depth, and these relationships were compared with findings from the literature. Across all sites, NECB exhibited only a weak association with mean annual groundwater level when referenced to the soil surface (ERF slope approximately 0.5 t CO₂ ha^-1 yr^-1 cm^-1, R² = -0.08), and an even weaker association with mean summer groundwater level. When groundwater depth was referenced to the clay layer, the inferred ERF sensitivity increased, resulting in steeper slopes of about 0.8 t CO₂ ha^-1 yr^-1 cm^-1 (R² = 0.26), although the overall explanatory power remained limited. These ERF-based sensitivities align with previously published ERFs, including studies that report non-linear groundwater–CO₂ responses and threshold behavior.

Groundwater management interventions produced mixed effects on NECB. Of the pasture-oriented measures, only the Active Water Infiltration System (AWIS) resulted in a detectable shift in NECB beyond background interannual variability, while other interventions were not distinguishable from within-land-use variability. By contrast, clearer categorical patterns emerged across land-use and soil classes, and paludiculture systems showed lower net CO₂ emissions than drained managed grasslands, and sites with a clay cap consistently emitted less CO₂ than pure peat under comparable management. Overall, these results indicate that although mean groundwater metrics have limited predictive power at the site-year level, CO₂ emissions at landscape scales are primarily governed by land use and the presence of a clay-layer, which defines the baseline against which water-management measures operate.