Nitrous oxide emissions from saturated riparian buffers: Are we trading a water quality problem for an air quality problem?

Reestablishing perennial vegetation along riparian areas in agroecosystems reduces nutrient and sediment losses from agricultural lands. However, subsurface (tile) drains bypass traditional buffers routing the majority of shallow groundwater straight to surface waters, limiting their nutrient removal capabilities. Saturated riparian buffers (SRBs) reconnect subsurface drainage water with the soil profile to remove NO ₃ in tile water through microbial denitrification. One concern of enhancing denitrification on agricultural landscapes is the potential increase in N ₂ O emissions from incomplete denitrification. Our objective was to compare N ₂ O emissions from SRBs to traditional buffers and bordering crop fields at two sites, Bear Creek Site 1 and Iowa Site 1, in Central Iowa. We measured N ₂ O emissions directly from the soil surface and dissolved in shallow groundwater and estimated indirect emissions from downstream denitrification from 2015 through 2017. Nitrous oxide emissions from soil surfaces were greatest from fertilized corn (Zea mays L.). Saturated riparian buffers were only significantly greater (P < 0.05) than traditional buffers in one out of six site-years. Dissolved N ₂ O in shallow groundwater seeping from SRBs was not significantly greater (P < 0.05) than dissolved N ₂ O from the tile outlet among site years. Indirect N ₂ O emissions from rivers and estuaries were significantly reduced from NO ₃ removal in both SRBs. Overall, total N ₂ O emissions from SRBs were similar to those from traditional buffers and less than those from fertilized corn–soybean [Glycine max (L.) Merr.] agriculture. Replacing cultivated land in riparian areas with a SRB has shown potential to subsequently remove NO ₃ from surface waters and reduce N ₂ O emissions from agricultural landscapes.