Methane emission responses to drainage ditch cleaning in forested peatlands

Long-term anoxic conditions enable natural peatlands to accumulate carbon in peat soils over millennia and are generally described as CO2 sinks and CH4 sources. Drainage of these ecosystems is known to reduce CH4 emissions from soils. However, drainage ditches can partly offset this reduction by acting as hotspots for CH4 emission. Many of these ditches are periodically cleaned to maintain their drainage efficiency. The effect of this procedure on ditch CH4 fluxes is unclear.To address this knowledge gap, we measured CH4 fluxes from forestry drainage ditches before and after ditch cleaning between April and October 2025 at 58 measuring points across four drainage system catchments in western Estonia. The peat layer at the study sites was approximately 2 meters thick and underlain by clay, resulting in multiple locations where clay was exposed in the ditch bottom after cleaning. The clay-bottom ditches were filled with sediment prior to cleaning. No significant change in CH4 emissions was observed within the full dataset. However, when separating the data by ditch bottom substrate, peat-bottom ditches showed a nearly fivefold increase in CH4 fluxes (from 9.01 to 45.07 nmol m-2 s-1), while fluxes from clay-bottom ditches remained similar (12.95 to 11.37 nmol m-2 s-1). Prior to ditch cleaning, CH4 emissions did not differ significantly between peat- and clay-bottom ditches.The mechanisms for this separation post-cleaning are unclear. Firstly, ditch water parameters (depth, pH, dissolved oxygen, redox potential, electrical conductivity, temperature) measured alongside fluxes showed no significant differences between uncleaned and cleaned ditches. Furthermore, a multiple linear regression model based on measured water parameters explained nearly 40% of the variability in peat-bottom ditch CH4 fluxes prior to cleaning. This explanatory power was lost following ditch cleaning, indicating a change in mechanism. Increased lateral inflow of dissolved CH4 may contribute to post-cleaning fluxes. Although neither CH4 fluxes nor dissolved concentrations increased further downstream with greater catchment size, the contribution of lateral transport cannot be excluded. In clay-bottom ditches, where most organic substrate was removed during cleaning, a substantial proportion of CH4 emissions may originate from lateral inputs rather than in situ methanogenesis. The removal of vegetation and sediment during cleaning may have disrupted a long-established stability in the system, enhancing methanogenesis in peat-bottom ditches while suppressing in situ methanogenesis in clay-bottom ditches due to substrate limitations.