Composition of N Gases Produced in Riparian Buffers: The Role of Artificial Drainage.

The US Corn Belt is a leading contributor to nitrate (NO₃^-) enrichment and the so-called dead-zone in the Gulf of Mexico. Located at the interface between agricultural fields and surface water bodies, riparian buffers have shown great capacity to remove NO₃^- from agricultural runoff, and thus reduce fertilizer N export to streams. N removal is achieved primarily via denitrification, a process whereby riparian soil microbes convert NO₃^- into nitrous oxide (N₂O) and dinitrogen (N₂). However, from an air quality standpoint, a low N₂O production relative to N₂ (mole fraction of N₂O) would be preferred because N₂O contributes to the greenhouse effect and depletion of the ozone layer. With the hypothesis that frequent water saturation is favorable to the reduction of N₂O into N₂ (thus a low N₂O mole fraction), a study was conducted to identify controlling factors of N₂O mole fraction across various riparian buffers [well-drained (WD), poorly-drained (PD), and tile-drained (TD) sites] in Central Indiana. The relative production of N₂O and N₂ was measured in the laboratory using the acetylene (C₂H₂) block technique. In the absence of C₂H₂, there was no difference in N₂O production rate among the sites. However, in the presence of C₂H₂, N₂O production at PD (30 µg N₂O kg^-1 soil h^-1) was much higher than at the WD site (8.42 µg N₂O kg^-1 soil h^-1). Conversely, the N₂O mole fraction at PD (0.11) was lower than at WD (0.28). These results are consistent with the greater soil moisture, and higher total soil organic C at PD compared to WD. In our on-going studies, we examine the impact of tile drainage on the depth distribution of denitrification, N₂O-reductase and hydrogenase activity in artificially-drained riparian buffers.