In Situ Denitrification in Saturated Riparian Buffers

Excess NO3 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 NO3 flux to surface waters. Edge-of-field practices focus on maximizing microbial denitrification, the conversion of NO3 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 NO3-rich tile drainage water into riparian buffers soils. The SRBs in this study removed between 27 and 96% of the total diverted NO3 load. Measured cumulative average denitrification rate for each SRB sample year accounted for between 3.7 and 77.3% of the total NO3 removed. Both the cumulative maximum and 90% confidence interval denitrification rates accounted for all of the NO3 removed by the SRBs in three of the five sample years, indicating that denitrification can be a dominant NO3 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 NO3 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 NO3 and C could limit denitrification in these SRBs.