Summer CH4 ebullition strongly determines year-round greenhouse gas emissions from agricultural ditches despite frequent dredging.

Recent studies indicate that greenhouse gas (GHG) emissions from agricultural drainage ditches can be significant on a per-unit area basis, but spatiotemporal investigations are still limited. Additionally, the impact of dredging - a common management in such environments - on ditch GHG emissions is largely unknown. This study presents year-round GHG emissions from nine ditches on a dairy farm in the center of the Netherlands, where each year, approximately half of the ditches are dredged in alternating cycles. We measured monthly diffusive fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), along with ebullitive CH4 emissions, supplemented by diel flux measurements (i.e., 24-h measurements) conducted in summer and winter. Our findings indicate that while diffusive GHG fluxes exhibited low spatiotemporal variation, ebullitive CH4 emissions were significantly higher during warmer periods and marginally elevated at ditch intersections. CH4 ebullition was the dominant pathway of ditch GHG emissions, accounting for 58% of the total annual emissions, followed by CO2 (39%), and N2O (3%). Approximately 80% of the total CH4 emissions occurred through ebullition during spring and summer. The average CH4 emission factor estimated for our ditches (574 kg ha-1 year-1) is ∼40% higher than the Tier-1 value suggested by the IPCC for ditches on mineral soils (416 kg ha-1 year-1). Based on two 24-h measurement campaigns, we concluded that neglecting nighttime diffusive CO2 and CH4 emissions may lead to inaccurate estimates of annual ditch GHG emissions, with ∼12% underestimation in this study. Although dredging led to subtle increases in water-to-atmosphere GHG emissions immediately after the activity, it reduced overall annual GHG emissions by ∼35%. This study highlights the importance of CH4 ebullition and of capturing diel cycles of diffusive emissions to accurately assess ditch GHG emissions and underscores the importance of considering seasonal variations and dredging practices when budgeting ditch GHG emissions.