Differential Controls of Greenhouse Gas (CO ₂, CH ₄, and N ₂O) Concentrations in Natural and Constructed Agricultural Waterbodies on the Northern Great Plains

Abstract

Inland waters are hotspots of greenhouse gas (GHG) cycling, with small water bodies particularly active in the production and consumption of carbon dioxide (CO ₂), methane (CH ₄), and nitrous oxide (N ₂O). However, wetland ponds are being replaced rapidly by small constructed reservoirs in agricultural regions, yet it is unclear whether these two water body types exhibit similar physical, chemical, and environmental controls of GHG content and fluxes. Here, we compared the content and regulatory mechanisms of all three major GHGs in 20 pairs of natural wetland ponds and constructed reservoirs in Canada's largest agricultural region. Carbon dioxide content was associated primarily with metabolic indicators in both water body types; however, primary production was paramount in reservoirs, and heterotrophic metabolism a stronger correlate in wetland ponds. Methane concentrations were correlated positively with eutrophication of the reservoirs alone, while competitive inhibition by sulfur‐reducing bacteria may have limited CH ₄ in both waterbody types. Contrary to expectations, N ₂O was undersaturated in both water body types, with wetlands being a significantly stronger and more widespread N ₂O sink. Varying regulatory processes are attributed to differences in age, depth, morphology, and water‐column circulation between water body types. These results suggest that natural and constructed water bodies should be modeled separately in regional GHG budgets.

Plain Language Summary

Small inland water bodies, ponds and reservoirs, are hotspots of greenhouse gas (GHG) production. However, modern farming practices replace natural wetlands by artificial reservoirs, without consideration on the effects on landscape GHG fluxes. Here we measured concentrations and controls of carbon dioxide (CO ₂), methane (CH ₄), and nitrous oxide (N ₂O) in locally‐paired wetland ponds and agricultural reservoirs and found that while GHG concentrations were similar in the two waterbody types, underlying mechanisms differed, owing to the differences in shape, depth, and age of individual water bodies. Shallower wetland ponds were typically more fully mixed, exhibited more oxygen at depth, and had higher nutrients and organic carbon concentrations reflecting their ages. Findings suggest that the balance between wetland drainage and reservoir construction may have a pronounced effect on regional budgets of GHG fluxes, particularly in the face of forthcoming climate changes.

Key Points

• CO ₂ content was associated with metabolism; primary production in reservoirs, but bacterial respiration in wetlands

• CH ₄ concentrations were correlated positively with either eutrophication (reservoirs) or competitive inhibition of methanogenesis (both)

• N ₂O undersaturation was ubiquitous, but wetlands were stronger and more widespread N ₂O sinks