Nitrogen removal and greenhouse gas fluxes from integrated buffer zones treating agricultural drainage water

Integrated buffer zones (IBZ) are novel mitigation measures designed to decrease the loading of nitrogen (N) transported by subsurface drainage systems from agricultural fields to streams. In IBZ, drainage water flows into a pond with free water surface followed by an inundated, vegetated filterbed. This design provides an environment favorable for denitrification and thus a decrease in nitrate concentration is expected as water flow through the IBZ. However, due to the establishment of anaerobic conditions, there is a risk for increasing emissions of the greenhouse gases nitrous oxide (N₂O) and methane (CH₄). In this year-long study, we evaluated the N removal efficiency along with the risk of N₂O and CH₄ emissions from two pilot-scale IBZs (IBZ1 and 2). The two IBZs had very different yearly removal efficiencies, amounting to 29% and 71% of the total N load at IBZ1 and 2, respectively. This was probably due to differences in infiltration rates to the filterbed, which was 22% and 81% of the incoming water at IBZ1 and 2, respectively. The site (IBZ2) with the highest removal efficiency was a net N₂O sink, while 0.9% of the removed nitrate was emitted as N₂O at IBZ1. Both IBZs were net sources of CH₄ but with different pathways of emission. In IBZ1 CH₄ was mainly lost directly to the atmosphere, while waterborne losses dominated in IBZ2. In conclusion, the IBZs were effective in removing N three years after establishment, and although the IBZs acted as greenhouse gas sources, especially due to CH₄, the emissions were comparable to those of natural wetlands and other drainage transport mitigation measures.