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Published in J. Environ. Qual. 32:2436-2443 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Wetlands and Aquatic Processes

Nitrification and Denitrification Rates of Everglades Wetland Soils along a Phosphorus-Impacted Gradient

John R. White* and K. R. Reddy

Wetland Biogeochemistry Laboratory, Soil and Water Science Department, Box 110510, 106 Newell Hall, University of Florida, Gainesville, FL 32611

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

Received for publication November 7, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Little information is available on the effect of phosphorus (P) enrichment on nitrogen (N) biogeochemical cycling in wetland soil. Of particular importance are the coupled nitrification–denitrification reactions that regulate the microbially mediated loss of N from wetland systems. Soils from the northern Florida Everglades have been affected by P loading from surface waters over the past 40 years. Elevated P levels have been show to have an effect on the size and activity of the microbial pool and a decrease in the N to P ratio of the microbial biomass. The objective of the study was to determine if P enrichment in soils affected microbial activities related to nitrification and denitrification in these flooded, peat soils. Potential nitrification rates of soil and detritus were determined using constantly stirred reactors under aerobic conditions while denitrification rates were determined from anaerobic incubations of slurry. Nitrification rates showed two distinct linear phases, a slower initial rate, signifying activity of nitrifiers present, followed by a sharp increase in the NH4+ conversion rate indicative of maximum potential rates. Initial rates of nitrification were highest in the surficial detrital layer decreasing with soil depth and did not correlate to soil total P. The potential rates of nitrification were 13 times greater than the initial rates. Potential denitrification rates were highest in the detritus and 0- to 10-cm soil interval with significantly lower values in the 10- to 30-cm soil interval, significantly correlated to total P of the soil. A significant (P < 0.01) relationship was seen between potential denitrification rates and soil total P suggesting an increased rate of N removal from P-enriched regions of the northern Everglades.

Abbreviations: WCA, Water Conservation Area


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
WETLANDS ARE WIDELY used for their ability to remove N from surface waters. This function is due to the regulation of two coupled processes; nitrification and denitrification (Reddy and Patrick, 1984). Nitrification is the biological oxidation of NH+4 to NO-2 and NO-3 where autotrophic bacteria couple the oxidation of NH+4 to electron transport phosphorylation and utilize dissolved CO2 to synthesize required cellular components. Nitrification can be regulated by NH+4 concentration, O2 concentration, alkalinity, and limiting nutrients. Extent of aeration can be a major rate-limiting factor in the conversion of NH+4 to N2 (coupled nitrification–denitrification) in continuously flooded wetland soils, due to the obligate aerobic requirement of nitrification (Robertson, 1989). Little O2 diffuses into flooded, organic-rich soil profiles due to the slower diffusion rate of O2 in water and high soil oxygen demand of organic flooded soils, favoring increased NH+4 concentrations in wetland soils (White and Reddy, 2000). The concentration gradient and soil characteristics, such as porosity and cation exchange capacity, control transport of N out of the soil into the overlying water column (Reddy and Patrick, 1984). Ammonium, once diffused into the overlying, oxygenated water column is nitrified, increasing the NO-3 concentration of the overlying water (Reddy and Patrick, 1984).

Biological denitrification is the microbially mediated reduction of nitrogenous oxides to N2O and N2 gas. The facultative microbial populations reduce nitrogenous oxides in a stepwise fashion as membrane-bound enzyme systems are engaged in electron transport phosphorylation (Tiedje, 1982). Denitrification rates can be controlled by available carbon, NO-3 concentration, soil aeration status, and limiting nutrients (Robertson, 1989). There is a well-established correlation for a soil's capacity for potential denitrification and the organic carbon content of the soil for upland (Burton and Beauchamp, 1985) and wetland soils (D'Angelo and Reddy, 1999). The presence of O2 in the soil profile has an inhibitory effect on denitrification as denitrifiers utilize NO-3 for respiration in low-O2 environments (Martin et al., 1988). In general, wetland soils have high organic C content and low O2 concentrations, due to poor drainage and presence of water in the soil pores, and NO-3 generally remains as the primary regulator of denitrification in these environments (Schipper et al., 1993; White and Reddy, 1999).

The Florida Everglades have been affected by nutrient loading in this historically oligotrophic wetland system (Koch and Reddy, 1992). The effect of the high P loading was observed as increased soil total P in the Water Conservation Areas (WCAs), which are continually flooded except for occasional drought years (DeBusk et al., 1994). Peat accretion rates have increased in areas receiving surface drainage water (Reddy et al., 1993; Craft and Richardson, 1993). The concentration of total P in surface soils increases from approximately 400 mg kg-1 in areas in the center of the marsh to highs of approximately 1600 mg kg-1 at the surface water inflow (Koch and Reddy, 1992).

Phosphorus has been implicated as a factor in ecosystem changes in the Florida Everglades including replacement of the natural sawgrass (Cladium jamaicense Crantz) by monotypic cattail (Typha spp.) stands. The rate of cattail cover in Water Conservation Area 2A (WCA-2A) has increased 1% per year in 1971 to 4% per year in 1987 (Wu et al., 1997). Cattail was found to modify the allocation of biomass from 60:40 (root to leaf) to 40:60 (root to leaf) in areas of elevated P (Miao and Sklar, 1998), perhaps using a competitive advantage for space and light, crowding out the natural sawgrass vegetation and filling in the open slough habitat.

Recent studies have investigated the effect of this elevated soil P on organic matter decomposition rates (DeBusk and Reddy, 1998), extracellular enzyme activity (Wright and Reddy, 2001), and the size and activity of the microbial pool (White and Reddy, 2000; DeBusk and Reddy, 2003). The microbial pool was found to increase in size in response to P loading in the oligotrophic Everglades system.

Few studies have focused on the effect of P concentrations on the biogeochemical cycling of nitrogen in low-P soils, in particular, on the microbially mediated nitrification–denitrification processes (Hue and Adams, 1984; Nair, 1996). These studies looked at short-term changes to addition of P to soils. Organic nitrogen mineralization was found to be significantly increased in a companion study on wetland soils subjected to decades of P loading (White and Reddy, 2000). The goal of this study was to determine the effect of soil P concentrations on potential rates of nitrification and denitrification. The specific objectives were to (i) determine the spatial variability of initial (ki) and potential (km) nitrification rates of soil, (ii) measure potential rates of denitrification (km), and (iii) determine the relationship between measured soil characteristics (including total P) and potential nitrification–denitrification rates.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Area
The sampling site was located in WCA-2A, a tract of the northern Everglades surrounded by an earthen berm in the 1960s for storage of surface water. The 10-km study transect spanned the marsh from a primary water control inflow structure (S-10C) southward and terminated approximately 10 km into the interior of the marsh (Fig. 1) . Eight sampling stations were located at distances of 1.4, 2.3, 3.3, 4.2, 5.1, 7.0, 8.4, and 10.1 km from the inflow structure. The water control structure has been diverting water, primarily draining the Everglades Agricultural Area (EAA), for the past 40 years. Discharges are seasonal, with the highest flow rates during the summer when precipitation is the greatest in southern Florida.



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  		Fig. 1. Station locations along the soil phosphorus gradient in Water Conservation Area 2A (WCA-2A).

 
Field Sampling
Field sampling of detritus and soil was conducted in October 1997. Three soil cores were collected within 1 m of one another at each station by driving a 10-cm-diameter aluminum irrigation pipe into the soil to an approximate 50-cm depth. A probe was inserted into each core to measure the soil surface inside to compare with the soil surface outside to verify that negligible (<5%) compaction had occurred during coring. Cores were removed from the ground, extruded in the field, and separated into soil intervals (0–10 and 10–30 cm) in the field. Detritus (plant litter) was also collected at each station. Detritus consisted of recognizable, loosely associated cattail or sawgrass plant material present on the surface of the more compact, brown, peat soil. Samples were sealed in plastic bags and placed on ice for return to the laboratory where they were transferred into polypropylene containers and stored refrigerated at 4°C until analysis (within four weeks).

Soil Characterization
Gravimetric moisture content was determined by drying approximately 20 g of a field-moist subsample in a forced-air drying oven at 70°C until constant weight. Samples were ground using a ball mill for total C, N, and P analyses. Bulk density was calculated for the soil intervals on a dry weight basis. Bulk density was not determined for detritus. Total C and N contents of detritus and soils were determined on dried, ground samples using a Carlo-Erba (Milan, Italy) NA-1500 CNS analyzer. Total P analysis was performed on subsamples prepared by ashing at 550°C and HCL acid digestion (Anderson, 1976). Total P was determined using an automated ascorbic acid method (Method 365.4; USEPA, 1993).

Microbial biomass carbon (MBC) was determined by the 24-h chloroform fumigation–extraction technique (Vance et al., 1987). Triplicate samples were extracted with 20 mL of 0.5 M K2SO4, filtered through #42 Whatman (Maidstone, UK) filter paper, and analyzed for total C by combustion in a Dohrmann total organic carbon (TOC) analyzer (Teledyne Tekmar, Mason, OH). Microbial biomass C was determined by subtracting the extractable TOC of the controls from the chloroform-treated samples. An extraction efficiency (kEC) factor of 0.37 was applied using the previous calibration for organic soils by Sparling et al. (1990).

Microbial biomass nitrogen (MBN) was determined by the fumigation–extraction technique (Brookes et al., 1985). Ten milliliters of extract from the microbial carbon procedure was subjected to Kjeldahl-N digestion and analyzed colorimetrically for NH4–N (Method 351.2; USEPA, 1993). Microbial biomass N was determined by subtracting the extractable NH4–N of the triplicate nonfumigated samples from triplicate fumigated samples. A combined extraction efficiency (kEN value) of 0.54 was applied (Brookes et al., 1985).

Initial and Potential Nitrification Rates
Initial and potential nitrification rates were determined for soil and detritus using constantly stirred reactors over 23 to 25 d that were continually bubbled with room air. The reactors were prepared by placing approximately 300 g wet weight soil in each triplicate 1-L Erlenmeyer glass flask and adding 400 mL of water. Flasks were placed on magnetic stirrers, equipped with floating, magnetic stir bars. Continual mixing was employed in combination with continuous aeration with room air, using an aquarium pump connected to glass tubing inserted through a butyl rubber stopper in each flask to maintain aerobic conditions in the reactors. The redox status of the soil slurries was monitored using an Accumet 1002 combination electrode with platinum band (Fisher Scientific, Pittsburgh, PA) and temperature was recorded with mercury thermometers. The average temperature of the reactors was approximately 30°C. Flasks were covered with opaque paper to shield the samples from direct light.

Ten-milliliter soil slurry samples were collected from each reactor and extracted with 10 mL of 2 M KCL. Extracts were mixed on a longitudinal shaker for 30 min and filtered through #42 Whatman filter paper. The samples were refrigerated at 4°C for subsequent colorimetric analysis on an autoanalyzer for NH+4 and (NO-2 + NO-3) (Method 353.2; USEPA, 1993).

Initial nitrification rates (k1) were calculated using linear regression over time for the first two days of NO-3 appearance and represented the activity of the initial population of nitrifiers present in the soil. The potential nitrification rates (km) were calculated by taking the slope of the steepest part of the NO-3 vs. time curve over the course of the 2- to 25-d incubation and signified the maximum rate once the nitrifier populations increased under optimum conditions.

Denitrifying Potential
Laboratory incubations were performed to determine the denitrifying potential of detritus and soils. Approximately 20 mL of soil slurry was obtained from each nitrification reactor after 25 d, placed in glass serum bottles, and sealed with a butyl rubber septum. Headspace air was evacuated to -85 kPa and replaced with 99.99% O2–free N2 gas to achieve anaerobic conditions. Approximately 15% of the headspace N2 was replaced with acetylene gas (C2H2) (Balderston et al., 1976; Yoshinari and Knowles, 1976). Bottles were shaken on a longitudinal shaker in the dark at 30°C for 36 h. Headspace gas was sampled at 2, 8, 12, 24, and 36 h.

Nitrous oxide production was analyzed on a Shimadzu (Kyoto, Japan) GC-14A gas chromatograph equipped with a 3.7 x 108 (10 mCi) 63Ni electron capture detector (300 C). An 1.8-m-long by 2-mm-i.d. stainless steel column packed with Poropak Q (0.177–0.149 mm; 80-100 mesh) was used (Supelco, Bellefonte, PA). Concentration was adjusted for N2O dissolved in the aqueous phase using Bunsen absorption coefficients (Tiedje, 1982). The potential denitrification rate was calculated from the steepest portion of curve produced when cumulative N2O evolution was plotted against time.

Data Analysis
Soil characteristics and microbial processes were statistically related using Pearson's product–moment correlation and regression analysis. All data were checked for homogeneity of variances and log-transformed before statistical comparisons where appropriate. Analysis of variance (ANOVA), Student's t test, and Fisher's least significant difference (LSD) tests were used to make comparisons between treatments (Manugistics, Rockville, MD).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characterization
The organic soils contained 90 to 96% moisture contents and dry weight bulk densities averaging 0.066 and 0.088 g cm-3 for the 0- to 10- and 10- to 30-cm soil intervals, respectively. Total C and N did not vary significantly along the transect, yielding mean values of 426, 439, and 444 g C kg-1, and 21.9, 26.5, and 27.5 g N kg-1, respectively, for detritus, 0- to 10-, and 10- to 30-cm soil intervals (Table 1). Values are similar to those found in previous studies in WCA-2A (Koch and Reddy, 1992; DeBusk et al., 1994; White and Reddy, 1999).


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  		Table 1. Select physicochemical properties of detritus and soils collected from along the study transect in Water Conservation Area 2A (WCA-2A) in October 1997. Dates are mean values (n = 3).

 
Total P decreased with distance from the surface water inflow point for all samples (Table 1). Total P for detritus and the 0- to 10-cm soil depth, taken separately, were significantly negatively correlated with distance (r = -0.78 and -0.93, respectively). Total P concentration in the 10- to 30-cm soil exhibited no significant trend with distance from inflow. Total P was negatively correlated (P < 0.01) with depth (r = -0.68; Table 2) and results of a one-way ANOVA revealed that total P was significantly higher (P < 0.05) in both detritus and 0- to 10-cm soil when compared with the underlying 10- to 30-cm soil.


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  		Table 2. Correlation matrix of soil characteristics and microbial processes for detritus and soil samples collected along the transect in Water Conservation Area 2A (WCA-2A) in October 1997. Significance level (P < 0.05) for r = 0.404.

 
Extractable NH+4, determined from the 0-d samples, was shown to be an excellent indicator of heterotrophic N mineralization potential of wetland soils (Williams and Sparling, 1988; Ross et al., 1995; White and Reddy, 2000). Consequently, the distribution of extractable NH+4 might provide a useful measure of other N cycling processes in wetland soils. Extractable NH+4 (Time 0 samples from the nitrification assay) was negatively correlated with depth (r = -0.74; P < 0.01) and positively correlated with total P (r = 0.56; P < 0.01; Table 2). Others have reported decreasing concentrations of extractable NH+4 in wetland peat soils with increasing depth (Humphrey and Pluth, 1996). There was no detectable NO-3 present in the soil initially, due to the high denitrifying enzyme activity of these soils (White and Reddy, 1999).

Microbial Biomass
The size and activity of the microbial pool are important measures of N transformation processes in soils (Wardle, 1992). Microbial biomass C and N were significantly correlated (P < 0.01) with total P (r = 0.60 and 0.56, respectively). Both microbial components were also significantly negatively correlated with depth (r = -0.63 and 0.65, respectively; Table 2). Microbial biomass C averaged 12.1, 2.02, and 0.92 g C kg-1 and microbial biomass N averaged 1093, 299, and 144 mg N kg-1 for the detritus, 0- to 10-, and 10- to 30-cm soil depths, respectively.

Microbial biomass C and N were also significantly correlated (r = 0.92) with one other at P < 0.01, demonstrating that either determination can be effectively used in these wetland soils to determine relative microbial biomass pool sizes as they co-vary. Extractable NH+4 was significantly correlated (P < 0.01; r = 0.72) with both microbial biomass C and N (Table 2).

Initial Nitrification Rates
Nitrifying organisms in a flooded, wetland soil are likely to be concentrated closest to the surface of the soil due to the obligate O2 requirement. As such, initial nitrification rates (ki) of soil and detritus in WCA-2A were significantly negatively correlated with depth at P < 0.01 (r = -0.66). Mean initial nitrification rates of detritus, 0- to 10-, and 10- to 30-cm soil depths were 28.5, 12.8, and 2.13 mg N kg d-1, respectively (Table 3). The differences in initial nitrification rates are likely due to the presence of differences in active nitrifying populations in each soil depth. The presence of nitrification activity in the subsurface soil (10–30 cm) is likely due to the presence of a small nitrifier population associated with the rhizosphere of macrophytes (Reddy et al., 1989). There was no significant relationship of initial nitrification rates with distance along the transect or total P concentrations, suggesting no significant effect of P on nitrification after the O2 limitations were removed in the reactors (Table 3).


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  		Table 3. Initial and potential nitrification rates for soil and detritus collected from along the transect in Water Conservation Area 2A (WCA-2A) in October 1997. Data are mean values (n = 3) and one standard deviation in parentheses.

 
Initial nitrification rates were most strongly correlated with extractable NH+4 of soil and detritus (Fig. 2 ; Table 2). Organic N mineralization rates were found to be highest in the surface soil in a previous study and therefore, the concentration of extractable NH+4 was also highest at the surface (White and Reddy, 2000). There were likely no substrate limitations (NH+4) on these initial rates as these soils had high extractable NH+4 concentrations (Table 1). The initial nitrification rates consumed, on average, only 10, 12, and 3% of the initial NH+4 over the first two days for the detritus, 0- to 10-, and 10- to 30-cm soil intervals, respectively. Therefore, nitrifier activity was likely the limiting factor for the nitrification rate, as both O2 (redox averaged 455 mV) and NH+4 were not limiting at this point in the experiment. Additionally, the aerobic N mineralization rate led to a doubling of the extractable NH+4 concentrations on average over the first week providing even more substrate for nitrification (White and Reddy, 2001).



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  		Fig. 2. Initial nitrification rates vs. extractable NH4–N for detritus, 0- to 10-, and 10- to 30-cm soil intervals from along the transect in Water Conservation Area 2A (WCA-2A).

 
Potential Nitrification Rates
Potential rates of nitrification (km) averaged 159, 295, and 119 mg N kg d-1 for detritus, 0- to 10-, and 10- to 30-cm soil depths, respectively (Table 3). There were no significant differences detected in potential nitrification rates between the detritus and 0- to 10-cm depths; however, there was a significant (P < 0.05) difference between nitrification rates for the 0- to 10- and 10- to 30-cm depths, likely related to the larger nitrifier populations at the soil surface.

On average, potential nitrification rates were 13 times greater than the initial (ki) nitrification rates (Table 3). Initial slow nitrification rates reflect the lag phase in buildup of nitrifying populations (Fig. 3) . At the initiation of the experiment, soils were anaerobic and low in nitrifying populations as a result of the O2 limitation. However, incubation of these soils under aerobic conditions promoted the activity of nitrifying organisms, resulting in a rapid rate of nitrification after the initial lag phase. Thus the initial slow rate reflects the activity of ambient nitrifying populations in these soils. After the O2 limitation was overcome by bubbling with room air and constant stirring in the reactors, then the nitrifier populations appeared to limit potential nitrification as indicated by the significantly higher potential nitrification rates when compared with the initial rates at each station (Fig. 3). Potential rates of nitrification demonstrated a significant correlation (P < 0.01; r = 0.50) with total P of the soil and detritus (Table 2).



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  		Fig. 3. Changes in nitrate concentration over time for the 0- to 10-cm soil interval at Station 1 showing the range used for calculation of the initial and potential nitrification rates.

 
Denitrification Potential
The potential denitrification rates were highest and not significantly different in the detritus and 0- to 10-cm soil depths averaging 13.1 and 14.1 mg N kg-1 h-1, respectively (Fig. 4 ; Table 4). Rates of potential denitrification were significantly lower (P < 0.001) in the 10- to 30-cm soil interval with a mean value of 0.51 mg N kg-1 h-1 (Table 4). In addition, potential denitrification rates were significantly correlated to distance from inflow in the detrital and 0- to 10-cm soil depths (Fig. 4). Denitrification rates were significantly (P < 0.01) correlated with microbial biomass C, microbial biomass N, and extractable NH+4 (Table 2).



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  		Fig. 4. Potential denitrification rates of detritus, 0- to 10-, and 10- to 30-cm soil intervals from along the transect in Water Conservation Area 2A (WCA-2A). Means and one standard error are plotted.

 

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  		Table 4. Potential denitrification rates of detritus and soils collected along the transect in Water Conservation Area 2A (WCA-2A) during the October 1997 sampling. Rates were determined in anaerobic bottle incubations after 25 d of aerobic conditions. Data are mean values with one standard deviation in parentheses.

 
Total P exhibited a significant correlation with potential denitrification at r = 0.85 (Fig. 5) . In a previously published study, denitrifying enzyme activity (DEA) of soils was strongly correlated to total P in WCA-2A. However, the correlation of DEA with total P was demonstrated to be a coincidental relationship, as NO-3 was determined to be the limiting factor for DEA in the field (White and Reddy, 1999). Initial NO-3 solution concentrations used in the denitrification study were relatively high average (±one standard deviation) initial concentrations of 1848 ± 544, 906 ± 441, and 367 ± 290 mg NO3–N kg-1 for the detritus, 0- to 10-, and 10- to 30-cm soil intervals, respectively. Both nitrification of initial NH+4 and mineralization of organic N and subsequent nitrification contributed to the NO-3 levels developed during aerobic incubation of these soils (White and Reddy, 2000, 2001). Consequently, NO-3 was no longer a limiting factor in any of the soil treatments and it was likely that another nutrient was limiting to the potential denitrification rate under these high-NO-3 conditions. From earlier studies, C did not appear to be a limiting factor for denitrification in these high-organic-peat soils (White and Reddy, 1999). Therefore, P was the limiting nutrient to potential denitrification rates that increased in regions of high total P (r = 0.85) and can potentially affect the N balance in this northern Everglades wetland (Table 2).



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  		Fig. 5. Potential denitrification rates vs. total P for detritus, 0- to 10-, and 10- to 30-cm soil intervals from along the transect in Water Conservation Area 2A (WCA-2A).

 
Coupled nitrification–denitrification rates in wetlands play a major role in regulating N loss (Reddy and Patrick, 1984). In the Everglades WCA-2A, high levels of NH4–N and negligible levels of NO3–N in these soils suggest that nitrification rate is the key regulator of overall N loss from the system. Earlier studies have shown that coupling of nitrification and denitrification was regulated by diffusion of NH+4 to the aerobic zones and diffusion of NO-3 to anaerobic zones (Reddy and Patrick, 1984). The ratio of potential nitrification rates to potential denitrification rates was significantly correlated to total P content of the soil (r = 0.66; P = 0.01). The ratio was >1 at total P concentrations of >740 mg P kg-1. These results suggest that denitrifiers are more sensitive to P limitations than nitrifiers. Previous work (DeBusk and Reddy, 1998; Wright and Reddy, 2001) has shown increased microbial activity as a result of P enrichment. High rates of organic matter turnover also promoted the productivity of vegetation and ultimately availability of organic C. Thus, high denitrification rates in nutrient-enriched areas were due to a combination of high P availability and an adequate supply of available labile organic C (White and Reddy, 1999; Burton and Beauchamp, 1985). However, nitrification rates were more dependent on inorganic substrates such as NH+4 and high alkalinity, with minimal effect seen from P availability. Although both nitrification and denitrification rates decreased with distance from inflow, the effect of P enrichment was much greater on the denitrifiers than NH+4 oxidizers.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Total P was significantly correlated with depth and distance from the surface water inflow point along transect in WCA-2A, demonstrating that continual P loading over the past 40 years has resulted in a soil total P gradient. Microbial biomass C and N were significantly higher in the surface soils and were correlated to extractable NH+4 concentrations. Initial nitrification rates were low, likely limited by a low, active nitrifier population present in the flooded wetland soils. Potential nitrification rates were, on average, 11 times greater than initial rates and were positively correlated to total P. Potential denitrification rates appeared to be limited by the total P content of the soil and decreased by first-order decay with increasing distance from the inflow. Results suggest that O2 availability to the soil was the greatest limiting factor for nitrification in WCA-2A. Consequently, this limitation on nitrification controls the rate at which NH+4 is removed from the wetland by the coupled processes of nitrification–denitrification during the dry season, which permits aerobic conditions to develop in the surface soil layer.

These results have consequences for water management for this southern Florida wetland. The presence and depth of the water column is critical in maintaining a stable N balance in the ecosystem, as aerobic surface soil conditions could increase the rate of inorganic N removal out of the system into the atmosphere by coupled nitrification–denitrification processes. Therefore, it is critical that the wetland soil should be kept hydrated despite the great pressure for surface water for nearby agricultural and urban areas. Soils from within the P-impacted region, closest to the surface water inflow point, demonstrated highest rates of denitrification potential, which could shift this area toward N limitation while the rest of the marsh is P limited. This N limitation could affect the plant communities that can ultimately influence ecosystem structure and function.


   ACKNOWLEDGMENTS
 
The South Florida Water Management District provided logistical and financial support. This research was also supported, in part, by the Florida Agricultural Experiment Station and is approved for publication as Journal Series #R-09128. Special thanks to Yu Wang for her assistance with laboratory procedures and Dr. Susan Newman from the South Florida Water Management District for logistical support.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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