Agricultural ponds are a surprising greenhouse gas sink

Nitrous oxide is a greenhouse gas 300 times as potent as carbon dioxide, but it’s unclear just how much N₂O is produced and soaked up by human activities such as farming. Agricultural runoff—the movement of nitrogen-rich fertilizers and other nutrients into lakes and streams—is thought to encourage the growth of N₂O-producing microbes. Now, researchers led by Kerri Finlay at the University of Regina have complicated this picture, at least for small water reservoirs built on farmland. These bodies of water tend to soak up, not emit, N₂O. The research team measured 101 small Saskatchewan ponds, called dugouts, and found that their N₂O production has been overestimated by 7- to 33-fold (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1820389116). There are about 16 million similar reservoirs on farms around the world, and they are understudied, says Jackie Webb, a water scientist at the University of Regina. Previously, scientists extrapolated N₂O production for these ponds using data from larger bodies of water, estimating that the reservoirs contribute from 4 to 45% of total landscape N₂O emissions. Webb says the farm dugouts in the study were stratified, with an oxygen-poor layer of water on the bottom, most likely creating conditions for microbes to convert fertilizers into inorganic nitrogen, instead of N₂O.