Denitrification Potentials in Restored and Natural Bottomland Hardwood Wetlands

functioning in a restored wetland when compared with its natural counterpart. We used the denitrification enWetland restoration projects are frequently evaluated by their hyzyme activity (DEA) assay, a measure of the potential of drologic roles and vegetation characteristics, but their success in rea microbial population to produce the nitrate reductase storing biogeochemical processes, such as denitrification, is less well known. To determine how restoration of structure affects specific enzyme, to evaluate denitrification in restored BLH wetecosystem processes, denitrification potential, soluble organic C (SOC) lands. concentrations, and soil moisture were measured seasonally over a Our study examines how restoration affects biogeo1-yr period in replicated natural bottomland hardwood (BLH) wetchemical functions in BLH wetlands. The objective of lands (NAT), restored BLH wetlands with hydrology reestablished this research was to compare denitrification potentials (RWH), and restored BLH wetlands without hydrology reestablished of natural and restored wetlands with and without hy(RWOH). Denitrification potential was significantly higher in NAT drologic regime reestablished and relate these potentials wetlands (657 ng N2O-N g 1 soil h 1 ) than in RWOH (167 ng N2O-N to specific soil characteristics. It was hypothesized that g 1 soil h 1 ) (P 0.07). Soil moisture was highest in NAT wetlands denitrification would be higher in natural wetlands than and lowest in RWOH (P 0.01), but no differences were measured in restored wetlands because of longer hydroperiods in SOC concentrations in three out of the four seasons sampled. Because RWOH wetlands exhibited denitrification potentials which and greater soil organic matter in the former. were significantly lower than NAT wetland sites, these results demonstrate that a BLH wetland restored without the natural hydrologic MATERIALS AND METHODS regime reestablished will not be a replacement for a natural BLH wetland in terms of biogeochemical processes such as denitrification. Site Description Study wetlands were located in the Tensas River Basin in northeastern Louisiana (Fig. 1), an area where agriculture W are often created or restored to mitigate is the primary land use (USDA–NRCS, 1995). The 291 500 the loss of wetland functions caused by conversion hectare Tensas River Basin was once over 90% BLH forests. Conversion of 85% of these forests to cropland resulted in to another land use. Compensatory mitigation is required a decrease in water quality of the Tensas River which was under Section 404 of the Clean Water Act (USEPA, caused by runoff of sediments and nutrients from adjacent 1997) and may occur onsite or through the purchase of agricultural fields (USDA–NRCS, 1995). Recently, agriculacreage credits in a mitigation bank. Millions of dollars tural croplands are being restored back to BLH wetlands to have been spent through governmental programs such improve water quality of this river and to provide wildlife as the Wetland Reserve Program (WRP) to restore wethabitat and flood storage. lands on marginal croplands (USDA–NRCS, 1995). AlThree types of BLH wetlands were studied: natural mature though structure (e.g., hydrology and vegetation) is rewetlands (NAT), wetlands restored with hydrology reestabstored in these wetlands, very few studies have been lished (RWH), and wetlands restored without hydrology reesconducted to determine if functions have been restored tablished (RWOH). Restored wetlands were once natural BLH wetlands that were converted to agricultural fields by clearing as well. vegetation and digging drainage ditches around the wetlands. In the Mississippi River alluvial valley, 25% of forBoth RWH and RWOH sites were replanted with BLH species ested wetlands remain (Abernathy and Turner, 1987) (Quercus lyrata Walt., Quercus nigra L., Quercus phellos L., and loss of these wetlands affects water quality in the Acer rubrum L.). Hydrologic restoration of the RWH sites watershed, wildlife populations, and reduces flood conconsisted of installing a water control structure in the drainage trol (Walbridge, 1993). One type of forested wetland, ditch adjacent to each site. The water height in the ditch was BLH wetlands, performs many valuable functions incontrolled using a flashboard riser so that surface runoff from cluding nutrient uptake and transformations, sediment adjacent agricultural fields would inundate each site. By using retention, floodwater storage, and organic C export to the water control structure, the hydrologic regime of NAT downstream ecosystems (Mitsch and Gosselink, 1993). wetlands in this watershed can be simulated. Neither RWOH site had a flashboard riser installed in the adjacent drainage Although mature BLH wetlands perform many useful ditch and, therefore, agricultural runoff simply bypassed these functions within a watershed, there are few data to insites. Study wetlands were chosen for their proximity to agdicate whether restored BLH wetlands perform these ricultural fields as well as for similarities in soil type, slope, same functions (Vellidis et al., 1993; Martin et al., 1999). and topography. Two replicates of each type of wetland were One important water quality function of natural riparchosen. Textural classes were clay for NAT and RWH soils ian wetlands is denitrification, a microbial process which and silty clay for RWOH soils. occurs in anoxic soils and may be a useful indicator of Soils in the Tensas Basin are alluvial clays and fine silts Abbreviations: BLH, bottomland hardwood; chl, chloramphenicol; R.G. Hunter and S.P. Faulkner, Wetland Biogeochemistry Institute, DEA, denitrification enzyme activity; NAT, natural mature BLH Louisiana State Univ., Baton Rouge, LA 70803. Received 30 Aug. wetlands; RWH, restored BLH wetlands with hydrology reestab1999. *Corresponding author ([email protected]). lished; RWOH, restored BLH wetlands without hydrology reestablished; SOC, soluble organic C; TOC, total organic C. Published in Soil Sci. Soc. Am. J. 65:1865–1872 (2001).