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Published in Soil Sci. Soc. Am. J. 68:1796-1804 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-10—WETLAND SOILS

Microbial Enzyme Activities in a Freshwater Marsh after Cessation of Nutrient Loading

J. P. Prenger* and K. R. Reddy

Wetland Biogeochemistry Lab., Soil and Water Science Dep., Univ. of Florida, Institute of Food and Agricultural Sciences, 106 Newell Hall, P.O. Box 110510, Gainesville, FL 32611

* Corresponding author (jprenger{at}ufl.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Accurate assessment of perturbation and recovery from anthropogenic nutrient inputs in wetland ecosystems is important for resource management decisions. Nutrient availability affects detrital decomposition and sediment accumulation rates, contributing to and helping to maintain changes in plant community structure. In this paper we report patterns of microbial enzyme activities as indicators of change in areas of a subtropical wetland 8 yr after cessation of nutrient loading. Select enzyme (acid phosphatase, ß-glucosidase, and dehydrogenase) activities were assayed on detrital material and surface soils collected from different vegetation communities within impacted and reference (unimpacted) areas. Acid phosphatase activity (APA) did not vary as dramatically as total P (TP) in soil, but was distinctly different in detritus of impacted and reference vegetation communities. The APA in soil and detritus was greatest in the reference sites, particularly in the Panicum area. Beta-glucosidase activity was highest in Typha areas and demonstrated significant temporal variation. Differences in the timing and length of inundation may play a role in observed trends, since the NW Panicum and Cladium reference sites flooded and reached anaerobiosis later in the growing season than did impacted areas. Data from this study indicate ongoing changes in biogeochemical cycling of nutrients in soils and detritus associated with vegetation communities in areas of historic nutrient loading.

Abbreviations: APA, acid phosphatase activity • CTC, 5-cyano-2,3-ditolyl tetrazolium chloride • MUF, methyl-umbelliferyl • Pi, inorganic phosphorus • TN, total nitrogen • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
TRANSFORMATION AND STORAGE of soil nutrients is regulated by the microbial biomass present (Martens, 1995), and flow of nutrients through the soil microbial fraction can be substantial. Microbial communities of wetland soils are important both for the decomposition of organic material, and remobilization and cycling of nutrients (D'Angelo and Reddy, 1994; McLatchey and Reddy, 1998), and thus play a central role in the metabolism of the entire ecosystem (Chrost and Siuda, 2002). Much of the organic accumulation in wetlands is composed of relatively high molecular weight compounds, which must first be hydrolyzed by extracellular enzymes before being used by microbial or plant populations (Burns, 1982; Chrost, 1991; Sinsabaugh et al., 1991). The exogenous enzymes released by microbial cells are integral to the degradation of organic matter and plant detritus (Halemejko and Chrost, 1984; Sinsabaugh et al., 1992), and thus to soil quality and vegetation communities.

Microbial biomass is dynamic and sensitive to changes because of nutrient loading (Powlson and Jenkinson, 1981). Microbial and plant communities and processes in wetlands respond to nutrient impacts (Craft et al., 1995; DeBusk and Reddy, 1998; Wright and Reddy, 2001a), and P enrichment stimulates microbial activity and organic nitrogen mineralization (White and Reddy, 2000). Extracellular enzyme levels are depressed in stream sediments receiving wastewater (Kuhbier et al., 2002) and have been shown to reflect the nutrient status of wetland soils in the Florida Everglades (Wright and Reddy, 2001b).

Exogenous phosphatases are central to regeneration of inorganic P levels, particularly in P-limited aquatic ecosystems. Phosphatase activity is often repressed by high concentrations of dissolved reactive P (Cembella et al., 1984; Chrost, 1991). Similarly, ß-glucosidase is part of the cellulase complex of enzymes involved in the regeneration of monosaccharides through hydrolysis of glycosides (Eivazi and Tabatabai, 1988), and can be correlated with detrital degradation rates (Sinsabaugh et al., 1994; McLatchey and Reddy, 1998). Other extracellular enzyme activity levels can be useful in predicting the impacts of P loading on wetland or other aquatic ecosystems (Gage and Gorham, 1985; Wetzel, 1991; Newman and Reddy, 1993; Wright and Reddy, 2001a). Most studies have tended to focus on systems with distinct gradients of nutrient levels where impacts are ongoing. As part of an effort to develop indicators of watershed integrity and wetland eutrophication, the current study examines spatial and temporal patterns in the activities of select enzymes in a system where impacts have stopped. The freshwater marsh studied received agricultural runoff from the early 1960s until the 1990s, but is now part of a conservation area in which nutrient inputs have been stopped or reduced. This study, therefore, reflects recycling and assimilation of a nutrient impact in an ecosystem as it moves toward a new equilibrium.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Site Description
The study area is Blue Cypress Marsh Conservation Area, an approximately 8000-ha freshwater marsh in the headwaters of the St. Johns River, in east-central Florida (Fig. 1) . All soils were Histosols, classified as Terra Ceia muck (Taxonomic class: euic, hyperthermic Typic Haplosaprists). The project area is predominately a mosaic of sawgrass (Cladium jamaicense Crantz) stands and maidencane (Panicum spp.) flats, although significant areas of shrub swamp, flag (Thalia geniculata L.) marshes, cattail (Typha spp.) marshes, and deep-water slough communities also exist. Cattail and willow (Salix spp.) are prevalent in nutrient-impacted areas. Approximately 75% of the headwater floodplain has been drained for cattle (Bos taurus), citrus, and row crop usage (Brenner et al., 2001). The project area has inflows from Fort Drum Marsh Conservation Area to the south, and two subbasin watershed tributaries from the west. Lands in the western watersheds have been purchased as part of the Upper St. Johns River Basin Project and removed from agricultural production in the late 1980s. All of the eastern inflows were primarily agricultural and have been diverted away from the marsh since 1992 as part of the water management project. While nutrient runoff into the marsh has largely been eliminated, vegetation changes from native sawgrass stands and maidencane flats to cattail and willow communities are still evident in areas of impact. Brenner et al. (2001) showed that sediment accretion rates at select sites in the Northeast impact area had been altered.



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  		Fig. 1. Sampling sites within Blue Cypress Marsh Conservation Area for spatial (87 sites) and temporal (6 sites) studies. Stippled area indicates aerial extent of January 2001 fire. Triangles represent spatial study sampling sites; stars indicate the six sites chosen for temporal study. Gray arrows indicate waterflow; white arrows indicate historic inflows of elevated nutrient runoff.

 
Soil Sampling
Spatial Study
To determine spatial heterogeneity of soil biogeochemical characteristics, areas were chosen in the northeast and southwest corners, which have both been impacted by agricultural runoff, and the northwest reference area. Soil cores and detritus were collected at 29 sites in each of three grids in the reference (NW) and two impacted (NE and SW) areas (Fig. 1) in September 2000. Sampling was on staggered grids at 15-, 75-, 150-, and 750-m offsets to determine spatial variability within the three study areas. Triplicate samples were collected at approximately 20% of the sites. A composite of two soil cores and a single detritus sample collected from 625 cm2 were obtained at each site. Intact soil cores (0–10 cm) were obtained using a 10-cm-i.d. stainless steel core tube. Soil and detritus were placed in watertight plastic bags and transported on ice to the laboratory, where they were stored at 4°C until analysis.

For purposes of comparing soil characteristics of vegetation communities, sites were assigned to six categories based on dominant (≥90% cover) vegetation communities. These were relatively monodominant sites of Cladium (C), Panicum (P), and Typha (T); mixed communities of Cladium, Salix, and other woody species (C-W); mixed communities of Typha, Salix, and other woody species (T-W); and communities with little or no Cladium, Panicum, or Typha (O).

Temporal Study
On the basis of results from the spatial sampling, two sites were established in each of the reference and impacted areas for bimonthly sampling across 12 mo to examine temporal variability. Three 2- by 2-m plots were established at each of the six sites. A composite of four soil and four (400 cm2) detritus samples (collected as described for spatial sampling) were collected at randomly selected locations in each plot. Soil and detritus were placed in watertight plastic bags and transported on ice to the laboratory and stored at 4°C until analysis. Soil redox potential was measured bimonthly at three plots at each site to determine aerobic status. Redox was measured at four depths (5, 10, 20, and 30 cm) with Pt electrodes connected to a portable pH meter along with a calomel reference electrode. The sites were chosen to account for the dominant vegetation communities (Table 1) and nutrient impact. The NE Cladium site had slightly elevated total phosphorus (TP) values (based on the spatial sampling), but retained the native vegetation community. In January 2001, a large portion of the marsh burned, including two reference and the two impacted sites in the southwest (Fig. 1). Monitoring began in March 2001, after vegetation regrowth had begun.


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  		Table 1. Dominant vegetation (>90% coverage) in the six temporal study sites. All soils were histosols.

 
Soil and Detritus Characterization
To examine chemical factors affecting nutrient availability and enzyme activity, well-mixed soil samples were analyzed for pH, total carbon, total nitrogen (TN), TP, NaHCO3 extractable inorganic P (extractable Pi), exchangeable NH4–N, and HCl-extractable Ca, Fe, and Al. Soil pH was determined by pH meter on 20 g of wet soil after equilibrating with 10 mL of deionized water. Total C and N content was determined on dried, ground soil and detritus samples by dry combustion (Nelson and Sommers, 1996) using a Carlo-Erba NA-1500 CNS Analyzer (Haak-Buchler Instruments, Saddlebrook, NJ). Total P was determined by combusting approximately 0.2-0.5 g oven-dried, finely ground soil at 550°C for 4 h, digestion of the ash with 6 M HCl and continuous heating on a hot plate, followed by filtration through a No. 41 Whatman filter (Anderson, 1976) and analysis of P by automated ascorbic acid method (Method 365.4, USEPA, 1993). NaHCO3–extractable Pi was determined by extraction of the wet soil equivalent of 0.5 g dry weight with 25 mL 0.5 M NaHCO3 (pH 8.5) shaken for 16 h at low speed and filtration through a 0.45-µm membrane filter (Brookes et al., 1982; Hedley and Stewart, 1982). Exchangeable ammonium (NH4–N) was determined using the method of Mulvaney (1996) by extraction of the wet soil equivalent of 0.5 g (dry weight) soil with 25 mL of 2 M KCl followed by filtration through Whatman No. 41 filter paper. The HCl-extractable cations were determined by extraction of 0.5 g dry soil in 25 mL of 1 M HCl with shaking for 3 h, filtration through a 0.45-µm membrane filter, and analysis by inductively coupled plasma.

Enzyme Analyses
Enzyme activities were assayed using the fluorescent model substrate 4-methylumbelliferone (MUF) (Chrost and Krambeck, 1986; Hoppe, 1993; Sinsabaugh et al., 1997) at the approximate ambient pH (6.0). All soil enzyme analyses were performed on well-mixed fresh material from which all visible roots and living plant material was removed. All soil enzyme analyses were completed within 2 wk of sampling. Enzyme analyses for detritus samples from the temporal study were completed within 2 wk of sampling, and within approximately 1 mo for the spatial study. Acid phosphatase and ß-glucosidase activities were determined on all soil samples from the spatial and temporal studies; dehydrogenase activities were determined on soils only in the temporal study. For detritus, acid phosphatase, ß-glucosidase, and dehydrogenase activities were determined on field replicate samples from the spatial sampling and all samples from the temporal study. Soil and detritus samples ({approx}1 g) were placed in approximately 9 mL of distilled water, and clumps were broken up by brief agitation with a Tissue Tearor Model 398 (Biospec Products, Bartlesville, OK). Immediately before enzyme assays, a 1/100 or 1/200 dilution of soil or detritus was prepared in water by serial dilution. Two hundred microliters of well-suspended soil slurry was transferred by pipette into eight wells of a 96-well microtiter plate, and 50 µL of substrate solution added to four wells (with four blanks). Samples were incubated (2 h for phosphatase, 24 h for all others) in the dark at room temperature except for dehydrogenase, which was incubated at 30°C. Phosphatase and ß-glucosidase assays were stopped by addition of 10 µL 0.1 M NaOH. Substrate was added to blanks and immediately read on a Bio-Tek Model FL600 fluorometric plate reader (Bio-Tek Instruments, Inc., Winooski, VT). Dehydrogenase assays were stopped with 50 µL acetone, incubated an additional 2 h, and read. Substrate solutions were as follows: for acid phosphatase, 500 µM methyl-umbelliferyl (MUF)-phosphate in 5 mM MES pH 6.0; for ß-glucosidase, 500 µM MUF-glucoside in 5 mM MES pH 6.0; for dehydrogenase, 500 µM 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) in 100 mM Tris pH 7.8. Concentrations were calculated from a standard curve of MUF or CTC-Formazan. Excitation and emission spectra for the two fluorochromes were: Ext. 360 ± 40, Em. 460 ± 40 (MUF-P, MUF-G); and Ext. 530 ± 25, Em. 645 ± 40 (CTC). Enzyme activities were calculated as µmol product g–1 dry soil h–1.

Statistical Analysis
All statistical analyses where performed using JMP version 4.04. All variables were examined for normality and homoscedacity of variance and log transformed where necessary; outliers were identified as observations that fell beyond 1.5 ± interquartile range. Analysis of variance of univariate data included multiple comparison of means by Tukey–Kramer and in all cases was at experimentwise {alpha} = 0.05. Temporal trends in data were fitted using repeated measures in Multivariate Analysis of Variance (MANOVA), site and time effects were tested using an F test that, where appropriate, was adjusted for autocorrelation with the Huynh–Feldt numerator and denominator degrees of freedom modifications (Huynh and Feldt, 1976; Muller and Barton, 1989), the interaction between the effects was tested using Pillai's Trace approximate F test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Spatial Patterns
Comparison by Area
Table 2 provides a summary of chemical analyses of soil and detritus samples from the three areas of the marsh in September 2000. Historically, areas in the NE and SW were impacted by nutrient loading (Brenner et al., 2001). The NW area is least impacted by nutrient loading. Mean TP levels of soil were significantly lower ({alpha} = 0.05) in the NW than in NE and SW, whereas mean TP values of detritus varied significantly ({alpha} = 0.05) in the order NE > SW > NW. The carbon-to-phosphorus ratios varied accordingly; thus, soil and detritus from the NW had the highest ratios, indicating generally lower nutrient levels per unit of biomass. Extractable Pi was significantly greater in detritus (but not the soil) of the NE area than in the NW or SW areas. Soil TP consisted mainly of organic forms (84 ± 6%), while the organic fraction of detritus TP was somewhat less (67 ± 7%). Total N and extractable NH4–N were greatest in both soil and detritus of the NE area. Extractable cations were generally lowest in the NW area. Soil pH was approximately 6 throughout the marsh, with NW soils being slightly more acidic.


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  		Table 2. Summary of soil and detritus chemical analyses for each of the three areas in the spatial study. Values are the means of approximately 40 measurements from each area (samples from 29 sites plus field replicates).

 
In general, enzymatic activities showed no definitive trends with regard to nutrient impact when compared among the three areas. Mean acid phosphatase activity (APA) was significantly lower ({alpha} = 0.05) in soils from the SW, but was highest in the NE (Fig. 2) . Mean APA in detritus was highest in NW but the difference was not significant. Beta-glucosidase showed no significant trends by area (Fig. 3) . Microbial activity in the detrital layer, as measured by dehydrogenase activity levels, was greatest in the NE (Fig. 4) , where Typha was dominant. Enzyme activities in detritus and soils showed significant correlations with select physicochemical parameters (Table 3). Both ß-glucosidase and dehydrogenase were found to be significantly correlated with extractable Pi, extractable NH4–N, TP, and TN, with coefficients in the range of 0.46 to 0.56, while APA showed poor correlation with these parameters.



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  		Fig. 2. Comparison of means for acid phosphatase activities (APA) in soil and detritus samples from the three sampling areas and six major vegetation communities in the spatial study. C, Cladium; C-W, Cladium and woody mix; O, others; P, Panicum; T, Typha; and T-W, Typha and woody mix. Values are means of 29 sites from each area, and 18 (C), 10 (C-W), 23 (O), 10 (P), 14 (T), and 12 (T-W) sites from each vegetation class. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples. The line in the body of each box represents the median; top and bottom of the box represent the 75th and 25th quantiles; the lines above and below each box represent the 10th and 90th quantiles.

 


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  		Fig. 3. Comparison of means for ß-glucosidase activity in soil or detritus samples from the three sampling areas and six major vegetation communities in the spatial study. C, Cladium; C-W, Cladium and woody Mix; O, others; P, Panicum; T, Typha; and T-W, Typha and woody Mix. Values are means of 29 sites from each area, and 18 (C), 10 (C-W), 23 (O), 10 (P), 14 (T), and 12 (T-W) sites from each vegetation class. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples.

 


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  		Fig. 4. Comparison of means for dehydrogenase activity in detritus samples from the three sampling areas and six major vegetation communities in the spatial study. C, Cladium; C-W, Cladium and woody mix; O, others; P, Panicum; T, Typha; and T-W, Typha and woody mix. Values are means of 29 sites from each area, and 18 (C), 10 (C-W), 23 (O), 10 (P), 14 (T), and 12 (T-W) sites from each vegetation class. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples.

 

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  		Table 3. Pairwise correlations of enzyme activities with detritus and soil chemical properties. Number of samples indicated in parentheses. R values in italic are at {alpha} < 0.002.

 
Variation in enzyme activities between different vegetation communities confounded direct comparisons between areas. Field replicates were obtained to assess variance within sampling sites (Table 4). Coefficients of variance averaged between 0.17 and 0.33. Areas of highest variability for APA in soils were SW Cladium/woody mix and others (coefficients of variance of 0.79 and 0.54, respectively). In detritus, highest variability was in Typha/woody mix and Cladium/woody mix areas of SW and NE, ranging from (0.61 to 0.68). Within site variability was greatest in detrital dehydrogenase activities of the SW Typha/woody mix area (0.92) due to one extremely high value. In both soils and detritus, ß-glucosidase activities demonstrated the smallest variability in replicate samples, averaging 0.17 and 0.26, respectively.


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  		Table 4. Coefficients of variance for enzyme activity analyses of triplicate samples obtained at 20% of field sites.

 
Comparison by Vegetation Type
When the spatial data were analyzed to compare among vegetation types, differences related to nutrient loading became more apparent. The significantly higher TP, TN, and NH4–N levels in soils and detritus were in areas dominated by Typha in the NE and a mix of Typha and woody species in the SW (Table 2). Extractable Pi showed a similar pattern, but differences were significant only in detritus. Enzyme activities in soil generally showed little difference between areas or vegetation types, although both APA and ß-glucosidase activity were significantly higher in soils of Typha areas than those of Cladium/woody mix areas (Fig. 2 and 3). The APA was not significantly different in detritus of any vegetation type, but data in Fig. 3 indicate that ß-glucosidase activity associated with Typha detritus was elevated relative to that in detritus of other vegetation types. This difference was reflected in marginally higher activity of ß-glucosidase in soils. Dehydrogenase in detritus also showed a significant difference between Typha areas and other communities (Fig. 4).

Seasonal Variation
Surface water was not present during March, but had returned to some areas by June. Because of slightly higher topography, the NW area typically is most shallow and last to reflood. Reduced conditions were not observed until July in NE and SW areas, and not until September in the NW. A fire occurred in the marsh in January 2001 and burned four of the six sampling sites (Fig. 1). The most dramatic effects of the fire were the loss of detrital material and much of the standing plant biomass; no loss of peat soils was observed.

Nutrient levels varied among the six test plots of the temporal study. There were no significant differences in soil TP between the unburned NE Typha area and the burned SW area, or between the unburned NE Cladium and reference sites; however, soils from the two SW sites and the NE Typha site contained significantly more TP than the NE Cladium and reference sites (Table 5; {alpha} = 0.05). Detritus from the SW Typha/woody mix area contained the most TP (Table 5; significant at {alpha} = 0.05), followed by the NE Typha and SW Typha/Cladium/woody sites. Extractable Pi was significantly lower in detritus of NW Cladium and NW Panicum than the NE Typha and SW Typha/woody mix sites, and NH4–N was significantly lower in NW Cladium than the NE Typha and SW Typha/woody mix sites (Table 5). Detritus from the NE Typha and SW Typha/woody mix areas contained significantly greater TN (Table 5).


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  		Table 5. Summary of soil and detritus chemical analyses for each site in the temporal study. Values are the means of 18 measurements (three composite samples taken bimonthly for one year).

 
The effect of increased N and P in the nutrient impacted areas on microbial metabolism can be seen in the increased activity of APA, particularly in detritus. In soils, APA was significantly higher in NW Panicum soils than those of either SW site, but was not significantly different among the remaining sites (Fig. 5 ; {alpha} = 0.05). In detritus, APA was significantly higher in NW Panicum than all other sites and NW Cladium was higher than the SW Typha/woody mix site (Fig. 5).



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  		Fig. 5. Comparison of means for acid phosphatase activity in 18 soil and detritus samples from each of the six study sites and bimonthly sampling times in the temporal study. NW-C, northwestern Cladium; NW-P, northwestern Panicum; NE-C, northeastern Cladium; NE-T, northeastern Typha; SW-TCW, southwestern Typha, Cladium, and woody mix; SW-TW, southwestern Typha and woody mix. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples.

 
Soil APA demonstrated a marginal temporal trend with lowest values in summer and highest in fall and winter (Fig. 5). The APA in detritus showed significant differences across time, with lowest values in June and highest values in December and January (Fig. 5); however, overall this trend was not significant (Table 6). Beta-glucosidase activity was not significantly different in soils or detritus of the various areas, but it did show a trend across the year, with lowest values in June and higher values during the winter months (Fig. 6) . Although somewhat more variable, dehydrogenase activity showed a similar pattern in soils and detritus (Fig. 7) .


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  		Table 6. Repeated measures analysis of seasonal and spatial variation in enzyme levels. Differences in enzyme activities across time and between sites were tested for significance. Probability values were obtained by Pillai's Trace Criterion test. Data were transformed where necessary.

 


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  		Fig. 6. Comparison of means for ß-glucosidase activity in 18 soil and detritus samples from each of the six study sites and bimonthly sampling times in the temporal study. NW-C, northwestern Cladium; NW-P, northwestern Panicum; NE-C, northeastern Cladium; NE-T, northeastern Typha; SW-TCW, southwestern Typha, Cladium, and woody mix; SW-TW, southwestern Typha and woody mix. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples.

 


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  		Fig. 7. Comparison of means for dehydrogenase activity in soil and detritus for the 18 bimonthly samples in the temporal study. There were no significant differences between sites. Different letters indicate significant differences ({alpha} < 0.05). Dashed line indicates mean for all samples.

 
The results of the repeated measures analysis (Table 6) indicate that enzyme levels varied significantly across time in both soil and detritus from the seasonal study, with the exception of APA in detritus. Dehydrogenase and ß-glucosidase activities did not vary significantly in detritus between the different sites (i.e., vegetation). The interaction of site (plot) and time showed significance only in ß-glucosidase and dehydrogenase of soil, and marginal significance in APA of detritus. The lack of significance in the interaction terms indicates that seasonality affects all six plots in a similar way with regard to enzyme activity levels.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Nutrient impacts on freshwater wetlands have long-lasting effects (Newman et al., 1997). Phosphorus inputs to Blue Cypress Marsh were largely removed by the early 1990s, but vegetation changes and internal cycling of elevated P concentrations continue in the NE and SW areas of the marsh. Microbial enzyme activities such as those of APA, ß-glucosidase, and dehydrogenase were highly variable when compared by area, probably because of the heterogeneity of plant communities, particularly in the NE where slough areas and willow were common. When the spatial data were compared by vegetation community, differences in enzyme activities were more discernable.

Acid phosphatase activity was shown to vary somewhat, but most differences were not significant because of the overall variability. In contrast, carbon metabolism in detritus (as indicated by ß-glucosidase) did show significant differences between the Typha communities and other areas (Fig. 3). The higher level of ß-glucosidase activity was not apparent in data from the temporal study (Fig. 6), probably because of a drastic die-back of Typha in 2001 caused by drought conditions that reduced the amount of fresh litter material, and to the fire that removed older detritus from the NW and SW areas. Increased dehydrogenase activity in Typha communities was also consistent with greater degradation in these areas (Fig. 4).

Increased ß-glucosidase and dehydrogenase activities without a concomitant increase in APA may be a reflection of the higher P and N levels in detritus of Typha dominated communities (Tables 2 and 5). Discrimination of soils and detritus by enzyme activities is likely a result of the more labile nature and higher nutrient content of some species, particularly Typha, growing in the P-enriched areas of the marsh (Rejmánková, 2001). Decomposition is affected by soil properties and litter quality (Coûteaux et al., 1995; DeBusk and Reddy, 1998). Phosphorus enrichment increases foliar P concentrations, resulting in higher P in detritus and, consequently, higher rates of decomposition (Vitousek, 1998), as well as decreased foliar phenolic content leading to increased herbivory and fungal infection (Richardson et al., 1999). While increased decomposition has been observed in Cladium grown under enriched P conditions (Richardson et al., 1999), observations in the current study were mixed. Comparison of ß-glucosidase levels in Cladium of NW and NE areas during the temporal study (Fig. 6) did not show significant differences, possibly because of loss of detritus in the NW in the fire. The marginally higher APA levels in NW Cladium detritus may be due to the lower levels of extractable Pi (Table 5). The correlations of enzyme activities with chemical parameters in the spatial data (Table 3) suggest that microbial activities related to organic matter decomposition were influenced by bioavailable N and P. Poor correlations with APA suggest that a large part of Blue Cypress Marsh is not limited by P availability. However, significant differences in APA among the temporal study sites (Fig. 5, Table 6) suggest that the reference area may still be limited for available P.

The six sites selected for the seasonal study were chosen to reflect the major vegetation communities and differences in nutrient levels. While both soil and detrital TP levels were significantly higher in the impacted communities of NE and SW, values in detritus were most variable, particularly in the NW Panicum and SW Typha/Cladium/woody communities. The APA did not vary as dramatically as TP in soil, but demonstrated significant variation in detritus among the communities (Fig. 5). The difference in APA levels of detritus from the two Cladium areas is especially noteworthy, given that TP values do not exhibit the same magnitude of difference. Extractable Pi was significantly lower in the NW Cladium detritus (Table 5) which would reduce inhibition and explain the higher APA values. This difference may have been lessened by the January 2001 fire, since the NW sites burned while the NE sites did not. Nutrient levels are greater in sawgrass culms during early regrowth after fire, but decrease after 3 to 5 mo (Steward and Ornes, 1975).

Acid phosphatase activity was significantly lower in soils of the SW Typha areas than in NW Panicum, while that in detritus from NW Panicum was higher than all other sites (Fig. 5). While the availability of P is a determinant of soil phosphatase activity (Wright and Reddy, 2001b), the amount of microbial and root biomass present will also determine the level of enzyme produced. The APA has been shown to be significantly related to fine root density in forest soils (Grierson and Adams, 2000; Schneider et al., 2001). Panicum has a finer root structure than Cladium, Typha, and the woody species in Blue Cypress Marsh, which may explain the differences between sites to some extent. The increase in APA of detritus across time is probably related to the accumulation of recently senesced material with more labile organic phosphates from primary productivity during the growing season. Depletion of Pi by growing vegetation may be the cause of increased APA in soils across time. Differences in the timing and length of inundation may play a role as well, since the NW Panicum and Cladium sites flooded and reached anaerobiosis later in the growing season. The increased variability of APA in soils in July (Fig. 5) may be due to reflooding of some sites but not others. While increased moisture in histosols generally increases microbial activity, once saturation is reached aerobic activity is inhibited and anaerobic processes are stimulated (Reddy and Patrick, 1975; Tate and Terry, 1980).

Beta-glucosidase activities did not vary significantly among sites, but varied significantly with time, particularly in detritus (Fig. 6). While the January 2001 fire in SW and NW sites likely affected carbon availability, the trends were consistent throughout the six sites, regardless of burning. The greater activity in soils early in the growing season (March) may be partially because of higher microbial biomass. This may be related to decreased saturation and anoxia, as well as translocation of carbon and other nutrients to roots during senescence the previous fall (Aerts, 1996) providing a greater availability of carbon in soils early in the growing season. Plant growth and increasing water levels in April and May probably decreased substrate and oxygen availability in soils, reducing microbial biomass and activity. The increased activity throughout the growing season may be because of the new productivity and availability of dissolved organic carbon leaching from growing and senescing plants.

On the basis of results from the spatial and seasonal studies, we conclude that microbial enzyme activities may be useful in understanding ongoing changes in biogeochemical cycling of nutrients after disturbance. Soil microbial enzymes demonstrated only marginal changes between areas of historic impact and reference sites, indicating little difference in biogeochemical cycling within the peat soils of these areas. However, cycling of nutrients within the detrital layer appears to be mainly influenced by changes in vegetation communities because of impact. Loss of detritus in some areas because of fire confounds interpretation of these results. However, APA levels in detritus of burned sites in impacted and reference areas and enzyme activities of the various vegetation communities before burning suggest that nutrient cycling within decomposing vegetation may be a dominant factor in the system as it continues to assimilate historic nutrient impacts. Changes in detrital decomposition rates have implications for sediment accretion, consolidation, and burial of nutrients, which may influence persistent alterations in vegetation communities.

This study demonstrated a relationship between vegetation communities and the activities of microbial enzymes in detritus and soils of a nutrient impacted wetland. The relationship among vegetation and activities of nutrient cycling enzymes are particularly interesting in light of the length of time since exogenous nutrient inputs have been removed from the marsh. The activities in soils and detritus of vegetation communities associated with historic nutrient loading suggest ongoing assimilation and recycling of exogenous nutrients within marsh communities. This ongoing process may place a limit on the spatial extent of community changes and on the likelihood of reestablishment of native vegetation communities.


   ACKNOWLEDGMENTS
 
Florida Agricultural Experiment Station Journal Series No. R-10181. This project was supported in part by a grant from the USEPA under grant No. R-827641-01.

Received for publication January 14, 2003.


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