Submitted to: Society of Wetland Scientists
Publication Type: Abstract Only
Publication Acceptance Date: 4/7/2009
Publication Date: 6/21/2009
Citation: Fox, R.J., Gustafson, A.B., Fisher, T.R., Kana, T., Ator, S., Denver, J., Jordan, T., Lang, M.W., McCarty, G.W. 2009. Denitrification in restored and constructed wetlands adjacent to crop fields on the Mid-Atlantic coastal plain [abstract]. Society of Wetland Scientists. 2009 CDROM. Interpretive Summary:
Technical Abstract: Fertilizer applications on crop fields are a significant source of nitrate (NO3), and groundwater concentrations are frequently 500-1000 µM. We show that groundwater transport of agricultural NO3 results in significant denitrification in adjacent wetlands in the Choptank Basin on the Delmarva Peninsula. We use piezometers for groundwater sampling, and two gas collection methods for vadose zone sampling. We quantify gases associated with soil respiration using quadrapole mass spectrometers and gas chromatographs. Excess N2 and CO2 are defined as the difference between observed concentrations and those expected or measured at equilibrium with air at recharge temperatures. In transects from fields to wetlands, we have observed large horizontal concentration gradients of NO3 (e.g., 7-54 µmoles L-1 m-1) and large accumulations of excess N2 gas, CO2, N2O, and CH4. Accumulation of excess N2 and N2O was negatively associated with declining O2 and NO3, whereas accumulation of CH4 (20 - 400 µM) increased rapidly following depletion of O2 and NO3. Excess N2 frequently represented 10 to >100% of the measured NO3, and N2O was in the range of 1-5% of excess N2. These data strongly support the use of natural, restored, or constructed wetlands in the vicinity of agricultural fields as a mitigation strategy to remove agricultural NO3. However, since the greenhouse gases N2O and CH4 can be generated under conditions of NO3 depletion, there may be optimal ratios of crop field to wetland to ensure adequate NO3 removal without excess production of greenhouse gases.