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Wetland Biogeochemistry Institute, Louisiana State Univ., Baton Rouge, LA 70803
* Corresponding author (faulkner{at}lsu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BLH, bottomland hardwood • chl, chloramphenicol • DEA, denitrification enzyme activity • NAT, natural mature BLH wetlands • RWH, restored BLH wetlands with hydrology reestablished • RWOH, restored BLH wetlands without hydrology reestablished • SOC, soluble organic C • TOC, total organic C
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In the Mississippi River alluvial valley, <25% of forested wetlands remain (Abernathy and Turner, 1987) and loss of these wetlands affects water quality in the watershed, wildlife populations, and reduces flood control (Walbridge, 1993). One type of forested wetland, BLH wetlands, performs many valuable functions including nutrient uptake and transformations, sediment retention, floodwater storage, and organic C export to downstream ecosystems (Mitsch and Gosselink, 1993). Although mature BLH wetlands perform many useful functions within a watershed, there are few data to indicate whether restored BLH wetlands perform these same functions (Vellidis et al., 1993; Martin et al., 1999).
One important water quality function of natural riparian wetlands is denitrification, a microbial process which occurs in anoxic soils and may be a useful indicator of functioning in a restored wetland when compared with its natural counterpart. We used the denitrification enzyme activity (DEA) assay, a measure of the potential of a microbial population to produce the nitrate reductase enzyme, to evaluate denitrification in restored BLH wetlands.
Our study examines how restoration affects biogeochemical functions in BLH wetlands. The objective of this research was to compare denitrification potentials of natural and restored wetlands with and without hydrologic regime reestablished and relate these potentials to specific soil characteristics. It was hypothesized that denitrification would be higher in natural wetlands than in restored wetlands because of longer hydroperiods and greater soil organic matter in the former.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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85% of these forests to cropland resulted in a decrease in water quality of the Tensas River which was caused by runoff of sediments and nutrients from adjacent agricultural fields (USDA–NRCS, 1995). Recently, agricultural croplands are being restored back to BLH wetlands to improve water quality of this river and to provide wildlife habitat and flood storage.
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Soils in the Tensas Basin are alluvial clays and fine silts and the study wetlands chosen for this research contained primarily Sharkey and Sharkey-Tunica soil associations. Sharkey and Tunica are listed as hydric soils. Sharkey is a very-fine, smectitic, thermic Chromic Epiaquerts and this map unit consists of poorly drained clayey soils with slopes of generally 0 to 3%. Tunica is a clayey over loamy, smectitic over mixed, superactive, nonacid, thermic Vertic Epiaquepts. The Sharkey-Tunica map unit consists of poorly drained and somewhat poorly drained clayey soils with slopes of 0 to 3%. Sharkey soils, occurring in swales, and Tunica soils, occurring on low ridges, make up 58 and 26% of this map unit, respectively (USDA–NRCS, 1995).
Experimental Design
A 20 by 30 m grid system was established in each of the study wetlands. The grid consisted of three columns 10 m apart and four rows 10 m apart. The exception to this grid system was one of the NAT wetlands (Little Fork) which had a 15 by 30 m grid system installed in which the columns were 5 m apart and the rows were 10 m apart. In all wetlands, the grid system was set up so that rows were parallel to the water source. In restored wetlands, the water source was overflow from ditches draining agricultural fields. All wetlands were chosen for their proximity to cropland so that surface flow to the wetlands included runoff from adjacent fields. A mature stand (
70 yr) is present on NAT wetlands. The RWH sites were both restored in 1990 and the RWOH sites were restored in 1993 and 1994.
Rainfall and groundwater levels were monitored monthly in each study wetland. For groundwater depth measurements, one 90-cm groundwater well was installed every 10 m in the middle row of each site. Water levels were determined monthly by measuring the depth to water in the well using a flashlight and tape measure. Rainfall was measured using tipping bucket rain gauges installed in Dorsey (RWH), Brown (RWOH), Windham (RWOH), and Greenlea (RWH) sites. Because Windham and Canon (NAT) were located beside each other, as were Dorsey and Little Fork (NAT), only one rain gauge was installed for each of these pairs of study wetlands.
Statistical Analysis
For the DEA assay, the experimental design was a split-plot design with wetland type as the whole plot treatment and DEA treatment as the subplot treatment. Distance from the surface water source (i.e., drainage ditches in the restored wetlands) was also included as a block treatment. Data were analyzed using the Proc Mixed procedure of SAS Institute (1994). For the SOC and soil moisture data, the experiment was evaluated as a block design with wetland type as the main treatment and distance from the water source as the block.
Main effects were considered significant if they had a P 
0.10. Differences among means within each main effect and within interaction effects were evaluated using Fischer's Protected LSD test (
= 0.10). Relationships between dependent and independent variables were examined using Pearson's correlation coefficients.
Collection of Soil Samples
Twelve soil samples were collected from the upper 15 cm at each study site using a 5-cm diam. soil auger. Samples were collected seasonally for 1 yr. After collection, the soils were stored on ice, brought to the Wetland Biogeochemistry Institute, and refrigerated (4°C). Each soil sample was homogenized by passing it through mesh with 1.25-cm2 holes and then analyzed for gravimetric soil moisture, SOC, and denitrification potential.
Soluble Organic Carbon
From each sample, 10 g of field moist soil was shaken in 100 ml deionized water for 30 min and left standing for 18 h (Kaiser and Zech, 1996). Each solution was then shaken by hand and 40 ml was poured into a centrifuge tube and centrifuged at 7500 x g in a refrigerated Surafuge 22 (Heraeus Sepatech, Germany) for 10 min at 25°C. Twenty milliliters of the supernatant was filtered through a 0.45-µm polysulfone membrane filter into a scintillation vial and stored at 4°C until analysis could be completed. Samples were analyzed for nonpurgable organic C using a Shimadzu TOC-5000A Total Organic C (TOC) analyzer (Shimadzu Scientific Instruments, Inc., Columbia, MD). Nonpurgable organic C concentration in each sample was measured by acidifying the sample with 40 µl of HCl and then purging for 8 min with TOC grade (CO2 free) compressed air. Acidification converts inorganic C to primarily CO2 in these samples and purging volatilizes CO2 out of solution. Samples were then analyzed for organic C concentrations. Results were given as the mean of three replicates per sample. Results were also corrected for soil moisture so that final results were expressed as mg SOC g-1 soil on a dry weight basis.
Denitrification Enzyme Activity Assay
Potential denitrification rates were determined using the DEA procedure of Tiedje (1982). Four 25-g subsamples were taken from soil samples and placed in four 125-ml incubation flasks. The four subsamples were treated with 25 ml of one of the following solutions: Treatment A is 1 g L-1 solution of chloramphenicol (chl); Treatment B is 1 mM KNO3 and 1 g L-1 chl; Treatment C is 1 mM glucose and 1 g L-1 chl; and Treatment D is 1 mM KNO3, 1 mM glucose, and 1 g L-1 chl. Chloramphenicol inhibits protein synthesis so that the microbial population size is the same as at the time of field sampling. By applying these different treatments, it can be determined if denitrification is limited by substrate (NO-3), energy source (glucose), or both. The mixtures were shaken vigorously to obtain a slurry. The flasks were capped with gas impermeable stoppers and made anaerobic by flushing with Ar for 1 min. Ten milliliters of purified acetylene was added to each flask to achieve a final concentrations of 10% (10 kPa) in the gas phase. The soil slurries were placed on a rotary shaker for 1 1/2 h and headspace gas was sampled by syringe at 30 and 90 min. Gas samples were stored in evacuated 10-ml vacutainer vials until N2O could be measured by gas chromatography. Nitrous oxide concentrations were measured using a Tremetrics 9001 gas chromatograph (Tretmetrics, Inc., Austin, TX) with an electron capture detector. Nitrous oxide dissoved in sample water was corrected with the Bunsen relationship, M = Cg
sc, where M is the total amount of N2O in water plus gas phase, Cg is the concentration of N2O in gas phase, Vg is the volume of gas phase, Vl is the volume of liquid phase, and D is the Bunsen absorption coefficient (at 25°C). Denitrification rates were expressed as ng N2O evolved per gram dry soil per day. Because this assay measures DEA, these values give an index of denitrifier population size in soils; this index is strongly related to annual soil denitrification rates (Ambus, 1993; Hanson et al., 1994; Groffman et al., 1996).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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104 d for NAT and RWH wetlands and 58 d RWOH wetlands, although saturation was not continuous for the RWOH wetlands.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The NAT and RWH wetland soils were continuously saturated for 104 d while the RWOH soils were saturated for 44 d, after which time water levels in the RWOH wetlands dropped 30 cm below soil surfaces (Fig. 3). After 2 to 3 wk, water levels were again within 15 cm of soil surfaces in the RWOH wetlands for 2 wk. Because rainfall had decreased from the previous month, this data indicates endosaturation was ocurring in these wetlands. One of the RWOH wetlands was directly influenced by the Mississippi River, in which water levels rise during the spring. Thus, groundwater influences will cause saturation to occur in RWOH soils, but the effects are briefer than surface flooding in NAT and RWH wetlands and also did not reach soil surfaces in this sampling year.
Soil moisture for all sites was lower in the fall than in any other season, which is typically the period of greatest moisture deficit. Rainfall was measured monthly, and for the 1997 and 1998 sampling period, highest rainfalls were recorded in January. Lowest rainfalls for the sampling year were recorded in August and September 1997, causing soils to be dried out by October 1997, when fall samples were collected. Soil moisture was lower in NAT soils than in the RWH soils in fall probably because transpiration by mature trees growing in these study sites caused a drop in the water table.
Soluble Organic Carbon Concentrations
Contrary to expectations, no differences were seen in SOC concentrations among wetland types during three out of the four sampling seasons. It was assumed that SOC would be greater in the NAT wetlands than in restored because the former wetlands have a thick leaf litter layer on soil surfaces while the restored do not (Hunter, unpublished data, 1999–2000). Restored wetlands were once cultivated and organic matter that accumulated while the area was wetland may have been oxidized when soils were aerated upon drainage for agriculture and during tillage. Cultivation over a long period of time will reduce concentrations of soluble sugars in soils (Boyer and Groffman, 1996).
Soluble organic C concentrations were higher in winter than in any other season in all wetland types. Litterfall, dead herbaceous vegetation, and root biomass may be decomposed in late fall and early winter, thus creating higher concentrations of soluble C when these soils were sampled in January. In addition, the highest rainfall of the year was recorded in January (Fig. 3) and in the NAT wetlands excessive rainfall may have leached C out of the litter layer and into the upper soil surface (Moore, 1997). The restored wetlands did not have an accumulated litter layer and therefore, had no SOC peak in January due to leaching.
Denitrification Enzyme Activity Assay
There was a large amount of sample variation for N2O evolution measured within each wetland type and these results are consistant with many studies measuring both denitrification rates and potentials (Parkin, 1987; Groffman et al., 1992; Groffman et al., 1996; Verchot et al., 1998; Martin et al., 1999). Study wetland soils have a high clay content (NAT is 59%, RWH is 68%, and RWOH is 47%) and the small particle size of clay can create microsites within soil that exhibit extreme variation in O and organic matter concentrations and water content which lead to differences in microbial populations (Christensen et al., 1990; Hill, 1996). Relationships between N2O evolution and SOC and soil moisture were very weak and this is most likely because the high N2O sample variation measured in the DEA assay prevented any significant relationships from being detected.
Higher DEA was measured in all soils receiving Treatment D than in those receiving the other three treatments because Treatment D supplied both NO-3 and an energy source (glucose) for denitrifiers, while the other treatments were limited by NO-3 (Treatment B), energy source (Treatment C), or both (Treatment A) (Fig. 5). Because no significant differences were found between Treatments C and D, soils of these wetlands may be C limited relative to the available soil NO-3 (Beauchamp et al., 1989; Hill, 1996; Schnabel et al., 1996; Luo et al., 1998; Verchot et al., 1998).
Restored wetlands without hydrology reestablished consistently had denitrification potentials which were lower than the other two wetland types because these sites were saturated in the upper 15 cm for about half the time that soils in NAT and RWH sites were saturated. Length of saturation will affect O concentrations and, thus, microbial activity and composition (Mitsch and Gosselink, 1993). Surface flooding will therefore indirectly affect numbers of denitrifiers because it directly affects length of soil saturation.
Another probable reason that denitrification potential was lower in the RWOH wetlands than in the other wetlands is related to the source of surface floodwater. Hydrologic inputs to the NAT and RWH wetlands come from runoff from agricultural fields, rainfall, and groundwater while the only inputs to RWOH are groundwater and rainfall. Agricultural runoff contains N, P, and organic matter which will cause increases in soil microbial populations. Verchot et al. (1998) found that microbial populations in subsoils of vegetated filter zones were significantly impacted by exposure to agricultural runoff, with the number of denitrifiers increased over preexposure numbers. Drury et al. (1998) studied the long-term effects of fertilization on clay loam soil and found that it resulted in 35% higher denitrification capacity and 65% higher CO2 production than in unfertilized soils, indicating that microbial populations were higher with fertilization.
It is unclear why higher denitrification potentials were measured in fall than in any other season. Because soil moisture was lower in fall than during the rest of the year, oxidation of organic matter may have been higher than in months when soils were saturated or flooded. Because denitrifiers are primarily heterotrophs, elevated C concentrations may increase numbers of these microbes. Although this hypothesis is not supported by SOC data, it must be emphasized that SOC includes water soluble sugars, amino acids, fulvic acids, and humic acids (Dalva and Moore, 1991), but does not include other C sources that are not water soluble such as cellulose, which is readily utilized by heterotrophs once it has been hydrolyzed into smaller subunits (i.e., cellobiose and glucose) by soil fungi (Wagner and Wolf, 1998). Concentrations of SOC were higher in winter than in other seasons, but lower temperature in January may have inhibited microbial growth.
There has been extensive research investigating denitrification capacity of riparian areas, demonstrating that these areas are capable of providing a buffer zone for removal of NO-3 from agricultural runoff water (Ambus and Lowrance, 1991; Groffman et al., 1992; Lowrance, 1992; Hanson et al., 1994; Schipper et al., 1993; Maag et al., 1997; Jordan et al., 1998; Verchot et al., 1998). On an annual basis, the NAT, RWH, and RWOH wetlands could remove 8.2, 5.7, and 1.4 g NO3-N kg-1 soil yr-1, respectively. These figures were calculated based on mean results for Treatment B in the DEA assay because this treatment measured denitrification activity that occurs with addition of NO-3, simulating conditions in late winter and early spring when NO-3 is present in surface runoff from adjacent cropland. Thus, even though DEA was lower in the RWOH wetlands than in the other two types, these wetlands did have the capacity to remove NO-3 through denitrification. Surface runoff never flooded these restored wetlands, however, it bypassed them because it was confined to drainage ditches. Because these wetlands were removed from the surface hydrology of the watershed, RWOH wetlands would not provide improvements in water quality even though denitrifying microorganisms were present in soils.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Weak relationships were detected between N2O evolution and soil moisture and SOC concentration, however we believe this is because of the large amount of N2O sample variation within each wetland study site. It is well documented that soil denitrification rates are directly influenced by soil moisture, O, and organic matter concentrations.
To fully restore BLH wetlands, it is necessary to modify drainage ditches around restored wetlands to imitate the natural hydrology of BLH wetlands in the Tensas River Basin. As demonstrated by this research, flashboard risers can provide surface flooding onto restored wetlands at a duration equivalent to surface flooding in natural wetlands. By installing water control structures, the natural cycle of winter and spring flooding can be emulated and restored wetlands will be connected to the surface hydrology of the watershed. If runoff water bypasses wetlands via drainage ditches then even if denitrifying microorganisms are present in soils, the wetlands will not be improving water quality before runoff water reaches nearby rivers and streams.
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ACKNOWLEDGMENTS |
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Received for publication August 30, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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