ARS | Publication request: Denitrification Activity, Wood Loss, and N2O Emissions Over 9 Years From a Wood Chip Bioreactor

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 7, 2010
Publication Date: September 15, 2010
Citation: Moorman, T.B., Parkin, T.B., Kaspar, T.C., Jaynes, D.B. 2010. Denitrification Activity, Wood Loss, and N2O Emissions Over 9 Years From a Wood Chip Bioreactor. Ecological Engineering. 36:1567-1574.

Interpretive Summary: Loss of nitrate from agricultural fields in subsurface drainage water is an important problem in the Midwestern United States and elsewhere. One possible strategy for reducing nitrate export is the use of denitrification bioreactors. A variety of experimental bioreactor designs have been shown to reduce nitrate losses in drainage water for periods up to several years. This research reports on the denitrification (coversion of nitrate to nitrogen gas) activity of a wood chip-based bioreactor operating in the field over a nine year period. Denitrification activity was sustained over the nine year period, which agreed with sustained nitrate removal from drainage water in the field. Populations of denitrifying bacteria were greater in the wood chips than in adjacent subsoil. Loss of wood through decomposition reached 75% at the 3 to 3.5 ft depth, but loss was less than 20% at 5 to 5.5 ft depth. The differential wood loss at these two depths appears to result from sustained anaerobic conditions below the drainage line at 4 ft depth. Pore space concentrations of oxygen were reduced as soil depth increased and methane concentrations increased, supporting this conjecture. Nitrous oxide exported in tile water was not significantly increased over exports in the untreated control drainage water. This work provides important information about the duration of the wood chip media and the emission of nitrous oxide, a greenhouse gas, that will be useful to scientists, engineers and conservationists for future design of these reactors.

Technical Abstract: Loss of nitrate from agricultural fields in subsurface drainage water is an important problem in the Midwestern United States and elsewhere. One possible strategy for reducing nitrate export is the use of denitrification bioreactors. A variety of experimental bioreactor designs have been shown to reduce nitrate losses in drainage water for periods up to several years. This research reports on the denitrification activity of a wood chip-based bioreactor operating in the field over a nine year period. Denitrification enzyme activity was sustained over the nine year period, which agreed with sustained nitrate removal from drainage water in the field. Populations of denitrifying bacteria were greater in the wood chips than in adjacent subsoil. Loss of wood through decomposition reached 75% at the 90-100 cm depth, but loss was less than 20% at 155-170 cm depth. The differential wood loss at these two depths appears to result from sustained anaerobic conditions below the drainage line at 120 cm depth. Pore space concentrations of oxygen and methane support this conjecture. Nitrous oxide exported in tile water was not significantly increased over exports in the untreated control drainage water.