Nutrient Removal and Greenhouse Gas Fluxes In 19 Year Old Constructed Wetlands: A Temporal Analysis and Comparison.

Tile drainage has improved crop yields by effectively lowering the shallow groundwater table, making the fields more accessible to the famers, but have increased nitrate and phosphorus losses. One way of mitigating this nutrient export is to use an end-of-pipe remediation technique like constructed wetlands to effectively remove and retain N and P. However, little is known about how these N and P removal rates change as wetlands age. There is also little literature that examines the greenhouse gas fluxes. Our study analyzed the N and P removal, as well as greenhouse gas, carbon dioxide, methane, and nitrous oxide, fluxes of two 19 year old constructed wetlands that receive their water from tile drained corn and soybean fields. The average nitrate removal rates for wetlands A and B for the 1995, 1996, and 1997 water years were 543 and 347 kg nitrate N ha^-1 of wetland area yr^-1 respectively. Comparatively, the average nitrate removal rates from wetlands A and B for the 2012 water year were 245 and 185 kg nitrate N ha^-1 yr^-1 respectively. The smaller removal rates in the 2012 water year was due to the lower N loads from the drought in 2012. Fluxes are much higher in 2013, with removal rates still to be calculated. Little total P was removed by the wetlands during either period. The greenhouse gas fluxes for both wetlands A and B varied seasonally with temperature, dissolved gas concentration, and the extent of inundation. Overall, we determined that the greenhouse gas emissions and comparisons of current and past N and P removal efficiencies for wetlands A and B were difficult to establish due to an increased frequency of extreme weather events.