Can conservation drainage practices contribute to climate change mitigation?

Conservation drainage practices can mitigate water quality impacts of subsurface drainage, but their potential for climate change mitigation remains poorly understood. We summarized processes by which tile-drained croplands impact climate and assessed potential of conservation drainage practices to alter emissions of the greenhouse gases nitrous oxide (N2O) and methane (CH4) and stocks of soil organic carbon (SOC), compared using carbon dioxide equivalents (CO2e). Controlled drainage, bioreactors, saturated buffers, and water quality wetlands can decrease nitrate leaching with little or no increase in on-site N2O emissions, thereby decreasing indirect N2O emissions that would otherwise occur from downstream waters. However, under some conditions, CH4 emissions from bioreactors and especially from wetlands can counteract climate benefits of decreased indirect N2O emissions. Drainage water recycling could potentially increase direct soil N2O emissions while decreasing indirect N2O emissions, but these impacts might be mitigated through sub-irrigation and increased drainage intensity. Many conservation drainage practices are unlikely to markedly increase SOC, aside from saturated buffers. Expressed relative to the area of cropland treated by a given practice, saturated buffers may have the largest climate mitigation potential of examined practices due to the combination of efficient nitrate removal with low N2O emissions, lower risk of CH4 emissions, and high potential for SOC accrual. In sum, available data suggest that several conservation drainage practices can plausibly contribute to climate change mitigation as well as water quality improvement, although more comprehensive studies are needed to better constrain their effectiveness.