Nixing nitrate flow from the farm
The researchers also filled mesh bags with wood chips and buried the bags at depths of 2 feet and 5 feet in a nearby trench that was also filled with wood chips. The fields above this trench were also cropped with a corn-soybean rotation. Establishing this extra trench allowed them to dig up wood chips to see how fast they decomposed without removing wood from the experimental trenches.
The team found that over the 9-year study period, the wood chip “bioreactors” consistently removed nitrates from the field leachate, with removal rates remaining steady in the last 5 years. From 2001 to 2008, annual nitrate loss in plots with conventional drainage averaged 48.6 pounds per acre, but losses dropped to 21.8 pounds per acre in plots with the denitrification walls.
The data also indicated that, compared to subsoil, the average denitrification potential of wood increased from 31-fold in 2003 to 4,000-fold in 2004. These findings supported an earlier laboratory study by Greenan that indicated denitrification by microbes is the main mechanism in wood chip bioreactors responsible for removing nitrate from leachate.
The scientists also found that the population of denitrifying microbes exceeded 454 million per pound of wood, compared to 45 million per pound of surface soil and 4.5 million per pound of subsurface soil—strong evidence that the wood chips provided a habitat that favored the denitrifying organisms.
The scientists periodically checked the bagged wood samples over the 9-year study period to see how quickly the wood was decomposing. They found that 50 percent of the wood buried between 35 and 39 inches deep had decomposed 5 years after it was buried, and 75 percent of the wood buried at this depth decomposed after 9 years.
However, less than 13 percent of the wood buried between 61 inches and 70 inches deep had decomposed after 9 years. The decreased decomposition rates at greater depths was probably due to lower oxygen levels in the subsoil, which was saturated with water for longer interludes than the subsoils at shallower depths. These findings can help in the design of denitrifying wood trenches, since wood decomposition rates will be needed to calculate the functional life expectancy of a denitrification wall after it is installed.
Denitrification also results in the production of the greenhouse gas nitrous oxide, and the team was concerned that the bioreactors might increase these emissions. But they found that overall nitrous oxide emission rates did not notably change with increasing denitrification in the bioreactor. This is partly because overall soil nitrate losses were reduced, which prevented nitrates from leaching out of the ground and into nearby waterways, where discharged nitrates are converted into nitrous oxide. “Until this study, very little work had been conducted on nitrous oxide loss from these bioreactors,” Moorman says.
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