Nitrous Oxide Emission from Cropland and Adjacent Riparian Buffers in Contrasting Hydrogeomorphic Settings

Riparian buffers are important nitrate (NO₃⁻) sinks in agricultural watersheds, but limited information is available regarding the intensity and control of nitrous oxide (N₂O) emission from these buffers. This study monitored (December 2009–May 2011) N₂O fluxes at two agricultural riparian buffers in the White River watershed in Indiana to assess the impact of land use and hydrogeomorphologic (HGM) attributes on emission. The study sites included a riparian forest in a glacial outwash/alluvium setting (White River [WR]) and a grassed riparian buffer in tile-drained till plains (Leary Weber Ditch [LWD]). Adjacent corn (Zea mays L.) fields were monitored for land use assessment. Analysis of variance identified season, land use (riparian buffer vs. crop field), and site geomorphology as major drivers of N₂O fluxes. Strong relationships between N mineralization and N₂O fluxes were found at both sites, but relationships with other nutrient cycling indicators (C/N ratio, dissolved organic C, microbial biomass C) were detected only at LWD. Nitrous oxide emission showed strong seasonal variability; the largest N₂O peaks occurred in late spring/early summer as a result of flooding at the WR riparian buffer (up to 27.8 mg N₂O–N m⁻² d⁻¹) and N fertilizer application to crop fields. Annual N₂O emission (kg N₂O–N ha⁻¹) was higher in the crop fields (WR: 7.82; LWD: 6.37) than in the riparian areas. A significant difference (P < 0.02) in annual N₂O emission between the riparian buffers was detected (4.32 vs. 1.03 kg N₂O–N ha⁻¹ at WR and LWD, respectively), and this difference was attributed to site geomorphology and flooding (WR is flood prone; no flooding occurred at tile-drained LWD). The study results demonstrate the significance of landscape geomorphology and land–stream connection (i.e., flood potential) as drivers of N₂O emission in riparian buffers and therefore argue that an HGM-based approach should be especially suitable for determination of regional N₂O budget in riparian ecosystems.