This experiment tested the hypothesis that the movement of water into the soil due to macrophyte transpiration stimulates nitrate removal from wetlands. The short-term impacts of biomass harvesting upon the ability of wetlands to reduce nitrate concentrations were also investigated. Different amounts of biomass were harvested from Typha latifolia specimens to create three treatments of varying transpiration rates. Harvesting and the resulting different rates of transpiration explained between 10 and 38% of the variation in nitrate reduction during the 10-day experiment and had significant effects upon nitrate concentrations. Greater rates of harvesting and lower rates of transpiration resulted in decreased reductions of nitrate in the surface water of the microcosms. During the first four days of the experiment, 70% and 85% rates of aboveground harvesting reduced transpiration by 63% and 91%, respectively. These changes led to respective decreases in nitrate reduction of 16% and 31% compared to unharvested treatments. Differences in nitrate reduction between the treatments were minimal in the later days of the experiment as nitrate concentrations decreased. Results support the hypothesis that water movement due to plant uptake contributes to nitrogen removal and offers a partial explanation for increased nitrogen removal in vegetated wetland treatment systems. Decreased reduction of nitrate with greater harvesting demonstrates potential negative impacts on water quality improvement when using wetlands as bio-energy production systems.
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