Global importance of methane emissions from drainage ditches and canals

Abstract

Globally, there are millions of kilometres of drainage ditches which have the potential to emit the powerful greenhouse gas methane (CH4), but these emissions are not reported in budgets of inland waters or drained lands. Here, we synthesise data to show that ditches spanning a global latitudinal gradient and across different land uses emit large quantities of CH4 to the atmosphere. Area-specific emissions are comparable to those from lakes, streams, reservoirs, and wetlands. While it is generally assumed that drainage negates terrestrial CH4 emissions, we find that CH4 emissions from ditches can, on average, offset ∼10% of this reduction. Using global areas of drained land we show that ditches contribute 3.5 Tg CH4 yr−1 (0.6–10.5 Tg CH4 yr−1); equivalent to 0.2%–3% of global anthropogenic CH4 emissions. A positive relationship between CH4 emissions and temperature was found, and emissions were highest from eutrophic ditches. We advocate the inclusion of ditch emissions in national GHG inventories, as neglecting them can lead to incorrect conclusions concerning the impact of drainage-based land management on CH4 budgets.

Small artificial waterbodies are widespread and persistent emitters of methane and carbon dioxide

“…There were significantly higher CH 4 fluxes in waterbodies with the largest TP concentrations, in keeping with a wide body of literature showing higher emissions in nutrient-rich waterbodies including ditches and artificial ponds (Audet et al, 2020;Beaulieu et al, 2019;Herrero Ortega et al, 2019;Ollivier et al, 2019a;Peacock et al, 2017Peacock et al, , 2019Webb, Hayes, et al, 2019). However, high TP concentrations did not guarantee high fluxes, and this is in agreement with a recent synthesis of ditch CH 4 emissions (Peacock et al, 2021 Note: Soil type had a significant effect on CO 2 concentrations (p < 0.001, d = 0.71, effect size = medium) and CH 4 concentrations (p = 0.041, d = 0.19, effect size = very small). For land use, significant differences were found for CO 2 between settlement and unmanaged land and forest (p = 0.003, d = 0.73, effect size = medium) and settlement and grassland (p = 0.003, d = 0.8, effect size = large).…”

“…Smaller ponds are difficult to accurately map (Sivanpillai & Miller, 2010), and therefore, uncertainty remains in upscaled estimates, from the regional to the global scale. Similar uncertainty exists for mapping ditches, rendering upscaled ditch GHG emissions uncertain (Peacock et al, 2021), although recent developments in methods appear promising (e.g. Connolly & Holden, 2017).…”

“…for Great Britain, total ditch length is more than double that of streams and rivers; Brown et al, 2006). Yet ditches are largely unexplored ecosystems (Koschorreck et al, 2020), and recent evidence suggests that they can have high CH 4 emissions, with implications for landscape to global‐scale CH 4 budgets (Evans et al, 2016; Koschorreck et al, 2020; Peacock et al, 2021).…”

“…Grinham et al (2018) found that artificial ponds in the state of Queensland emit the equivalent of 10% of the GHG emissions from the state's land use, land use change and forestry (LULUCF) sector emissions. For ditches, two syntheses (one of peatland studies, and one of all studies) found that they act as hotspots for CH 4 emission and, despite their limited areal extent, have the potential to offset carbon uptake by the terrestrial portion of the ecosystem (Evans et al, 2016; Peacock et al, 2021). Although other studies of carbon emissions from artificial ponds and ditches exist, they are often limited with either no temporal replication (i.e.…”

Worms and submersed macrophytes reduce methane release and increase nutrient removal in organic sediments

“…This topic seems central in a period of ecological transition and restoration of impacted ecosystems. The conversion of floodplains into arable lands has resulted in loss of biodiversity and ecosystem services and in the creation of extended networks of artificial canals (Foley et al 2005;Peacock et al 2021). Their chemical and biological quality is generally poor, due to generalized N and P excess from fertilizers and organic enrichment (Leip et al 2015).…”