Significant Emissions From Forest Drainage Ditches-An Unaccounted Term in Anthropogenic Greenhouse Gas Inventories?

AbstractForestry is a natural climate solution for removing atmospheric carbon dioxide (CO2) and reaching net zero emissions. Managed boreal forests typically have extensive drainage ditch networks, and these can be hotspots of anthropogenic greenhouse gas (GHG) emissions, potentially offsetting the terrestrial carbon gain. However, there is a lack of data detailing GHG emissions from ditches on mineral soils, where most boreal forestry occurs. Here, we address this knowledge gap using two approaches. First, we conducted a synoptic campaign to measure summer GHG fluxes from 109 boreal forest ditches draining mineral soils within one local region. We found a clear control of ditch water level on methane (CH4), with zero emissions from dry ditches and variable, but often high, emissions from water‐filled ditches. Almost all ditches acted as sources of CO2, regardless of water status. Second, we reanalyzed a published data set of boreal forest ditches and streams across three regions where GHG concentrations had been repeatedly measured and detailed catchment information was available. Within this data set we categorized 76 ditches into mineral and peatland catchments and detected no difference in mean CH4 and CO2 concentrations between the two soil types. GHG emissions from ditches draining mineral forest soils can be as large as those from peatland forest ditches. Using literature values for forest GHG uptake we demonstrate that ditch CH4 emissions are particularly important and can offset the terrestrial CH4 uptake. Ignoring ditch emissions, which are anthropogenic in origin, will lead to incorrect estimates of the landscape‐scale forest GHG budget.