In situ denitrification in saturated riparian buffers

Excess NO ₃ leaching from the agricultural Midwest via tile drainage water has contributed to both local drinking water and national Gulf of Mexico benthic hypoxia concerns. Both in-field and edge-of-field practices have been designed to help mitigate NO ₃ flux to surface waters. Edge-of-field practices focus on maximizing microbial denitrification, the conversion of NO ₃ to N ₂ gas. This study assessed denitrification rates from two saturated riparian buffers (SRBs) for 2 yr and a third SRB for 1 yr, for a total of five sample years. These SRBs were created by diverting NO ₃ –rich tile drainage water into riparian buffers soils. The SRBs in this study removed between 27 and 96% of the total diverted NO ₃ load. Measured cumulative average denitrification rate for each SRB sample year accounted for between 3.7 and 77.3% of the total NO ₃ removed. Both the cumulative maximum and 90% confidence interval denitrification rates accounted for all of the NO ₃ removed by the SRBs in three of the five sample years, indicating that denitrification can be a dominant NO ₃ removal mechanism in this edge-of-field practice. When adding the top 20 cm of each core to the cumulative denitrification rates for each SRB, denitrification accounted for between 33 and over 100% of the total NO ₃ removed. Buffer age (time since establishment) was speculated to enhance denitrification rates, and there was a trend of the soil closer to the surface making up the majority of the total denitrification rate. Finally, both NO ₃ and C could limit denitrification in these SRBs.