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Published in Soil Sci. Soc. Am. J. 69:539-548 (2005).
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Division S-10—Wetland Soils

Nutrient Enrichment of Wetland Vegetation and Sediments in Subtropical Pastures

Stanley M. Gathumbia,*, Patrick J. Bohlena and Donald A. Graetzb

a MacArthur Agro-Ecology Research Center, A Division of Archbold Biological Station, 300 Buck Island Ranch Road, Lake Placid, FL 33852
b Dep. of Soil and Water Science, Univ. of Florida, 106 Newell Hall, Gainesville, FL 32611

* Corresponding author (sgathumbi{at}archbold-station.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
Land use practices exert a major influence on plant productivity, soil and plant nutrient content, and within-stand nutrient cycling in wetlands in agricultural landscapes. We examined differences between improved and seminative pastures in plant and soil nutrient characteristics in seasonally flooded wetlands in subtropical grazing land of south central Florida. The wetlands were embedded within either grazed improved pastures with a long-term history of fertilizer application or seminative pastures with no history of previous fertilizer application. Soil nutrient concentrations decreased with soil depth for both land use types. Total C, N, and P were significantly greater (P < 0.05) in the 0- to 15-cm mineral layer compared with the deeper layers (15–30, 30–45 cm) for both improved and seminative pasture wetland soils. Improved pasture wetlands had greater amounts of total P (22.3 kg P ha–1) in the upper 0- to 15-cm soil layer than did the seminative pasture wetlands (15.7 kg P ha–1). Plant and soil (0–15 cm) N/P and C/P ratios were lower in improved pasture wetlands compared with seminative pasture wetlands, suggesting greater P enrichment in improved pasture wetlands. Microbial biomass C and N decreased with soil depth in both pasture types. Soil microbial biomass C/total C ratios decreased with soil depth and were similar for both improved and seminative pasture wetlands. Our results suggest that plant and soil nutrient enrichment and storage in temporary wetlands may be impacted by adjacent land use practices, which potentially leads to the alteration of the structure and functions of these wetland ecosystems.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
ISOLATED SEASONAL WETLANDS are important ecosystems from the perspective of regional biological diversity, hydrologic functioning, and productivity in many areas of the USA (Bedford and Godwin, 2003; Leibowitz, 2003; Leibowitz and Nadeau, 2003; Tiner, 2003). These wetlands, which are completely surrounded by upland, are not isolated functionally, but rather exist along a continuum of hydrologic and biological links with the surrounding landscape (Leibowitz, 2003). Controversy over the status of isolated wetlands has heightened because of a recent decision by the Supreme Court [Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers, (99-1178) 531 U.S. 159 (2001)] that may have profound implication for legal protection of isolated wetlands (Bryant and Ervin, 2004). This decision creates an urgency to assess the ecological status of isolated wetlands and define their role in the broader aquatic and terrestrial ecosystems where they occur (Nadaeu and Leibowitz, 2003). This urgency is greatest for small isolated wetlands because they are most likely to be lost, despite the fact that they comprise, by far, the greatest number of wetlands, and are extremely important for maintaining biodiversity and other ecosystem functions (Semlitsch and Bodie, 1998).

Within-stand cycling of nutrients is an important feature of wetland systems that provides a basis for understanding their nutrient status, productivity, and degree of eutrophication (Bedford et al., 1999; Jonasson and Shaver, 1999). The length of hydroperiod and intermittent periods of dry-wet conditions can significantly influence the abundance and diversity of soil biota in the seasonal wetlands, which are responsible for most nutrient transformations (Bedford et al., 1999; Jonasson and Shaver, 1999). More importantly, the rate and extent of microbially-mediated nutrient transformation processes, especially for N, fluctuate greatly with temporal soil conditions and organic matter quality in wetlands (Groffman et al., 1996; Patrick, 1982). In addition, quality and quantity of organic matter substrates and timing of nutrient release through mineralization and subsequent uptake by plants determines the extent of nutrient cycling in wetland ecosystems (Verhoeven et al., 2001).

One region where seasonal isolated wetlands form a dominant landscape feature is the Lake Okeechobee Basin in South Florida, where beef cattle ranching is the dominant land use (Hiscock et al., 2003). Cattle ranching operations in this region interact with some of the most sensitive wetland ecosystems in the United States (Tiner, 2003). During the past several decades, native rangelands in this region have been converted into improved pasture systems (Flaig and Havens, 1995; Hiscock et al., 2003) with the aim of increasing beef cattle production per unit area. Conversion of the native rangelands into intensively managed pasture systems represents a widespread land use change that has consequences for biogeochemical cycles, as well as the links between upland and wetland systems and the eutrophication of wetlands in these agricultural landscapes. Temporary or permanent wetlands account for about 15% of the total land area in the Lake Okeechobee Basin, which is down from 25% 30 yr ago due to more intensive drainage of the landscape primarily for agricultural land use (Flaig and Reddy, 1995; Flaig and Havens, 1995; South Florida Water Management District, 1997).

Seasonally flooded wetlands formed in landscape depressions from collapse of underlying karst material are dominant landscape features in the Lake Okeechobee Basin. These wetlands vary greatly in size, shape, hydroperiod, and vegetation, but have some similar characteristics such as the stratified distribution of vegetation in the wetland area. Dominant plant species include sedges and grasses on the shallower edges, and herbaceous aquatic macrophytes in deeper interior sections (Babbitt, 1996).

Features commonly associated with more intensive land use in the Lake Okeechobee basin include alteration of drainage systems, increased fertilizer use, increased cattle stocking rate, and introduction of high-yielding exotic forage grass species. Such land use practices in or outside the wetlands can affect nutrient-related functions of the wetland ecosystem, such as nutrient retention and transformation processes (van der Peijl et al., 2000). However, little information is available on the effect of ranching practices on the nutrient characteristics of isolated wetlands in south Florida or in other regions, where grazing or other agricultural land uses interface with isolated wetland systems.

We investigated the nutrient characteristics of soils and vegetation in small isolated wetlands embedded in subtropical cattle pastures to assess the impact of cattle grazing and the pasture management history on nutrient pools and concentrations of wetland vegetation and soils. This project was part of a larger integrated project investigating the effects of cattle stocking density on surface water quality and other ecological, agronomic, and ecological parameters in a set of replicated pastures. Wetland studies in these pastures included analysis of wetland invertebrate communities, N cycling processes, and effects of cattle grazing on wetland vegetation. Here, we report on differences in nutrient characteristics of vegetation and soils in isolated wetlands of improved and seminative pastures as they relate to the different management histories of these two major pasture types. This is a first step toward understanding the impact of ranching practices on key functional attributes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 
The study was conducted at the MacArthur Agro-Ecology Research Center, a division of Archbold Biological Station, located in southcentral Florida (27°09' N, 81°11' W). The Center is located within Buck Island Ranch, a 4170-ha commercial cattle ranch. Mean annual rainfall in the area is {approx}1300 mm, of which 75% is received during wet season months (May through October). Mean annual temperature is 22°C with maximum summer temperatures of 33°C. Soils are predominantly Entisols, Spodosols, and Alfisols consisting of poorly drained sands with pH ranging between 3.9 and 6.5.

The Center has a 420-ha array of 16 experimental pastures, eight of which are located in an area of improved pasture and the remaining eight located in an area of semiimproved or seminative pasture. Improved pastures are planted with introduced Bahia grass (Paspalum notatum Flugge), are fertilized annually with N, were historically (at least 20 yr up until 1987) fertilized with P, and are grazed primarily in the summer wet season (May–October). Seminative pastures are comprised of a mixture of Bahia grass and native grasses [e.g., bluestem, Andropogon spp.; Panicum spp.; and carpetgrass, Axonopus fissifolius (Raddi) Kuhlm.], are not fertilized and are grazed primarily during the winter dry season (November–April). Within each pasture type, there were two replicates of four different cattle stocking density treatments established in October 1998: 0, 0.74, 0.99, and 1.73 cow–calf units ha–1 in improved pastures and 0, 0.46, 0.62, and 1.08 cow–calf units ha–1 in seminative pastures. Stocking densities were lower in winter pastures due to lower availability of forage in the winter months. Summer pastures were generally grazed from May through October, and the same herds were rotated to their respective winter pasture from November through April. A total of 24 seasonal wetlands, 12 each in the improved pastures and seminative pastures, and ranging in size from 0.5 to 2.0 ha, were selected for this investigation (Table 1).


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  		Table 1. General description of the number, size, and dominant vegetation of the seasonal wetlands selected for this study in both improved and seminative experimental pastures.

 
The pastures are nearly level (<1% slope) and the water table typically reaches the upper 25 cm of the soil profile during the wet season (Jun.–Oct.) with periodic surface flooding during storm events. Wetlands receive some surface runoff from the adjacent upland and retain water when the water table is high. Subsurface drainage is slow due to the poorly drained status of the soils and relatively impermeable subsurface soil horizons. A system of drainage ditches has been imposed to improve drainage of the sites, and improved pastures are more intensively drained than seminative pastures. The elevation of the pastures ranges from about 7.9 to 8.5 m above mean sea level, and the improved pastures are at a slightly lower (10–15 cm) elevation than the seminative pastures.

Soils in the improved pastures are generally classified as siliceous hyperthermic Arenic Endoaqualfs, while those in seminative pastures are classified as siliceous hyperthermic Arenic Glossaqualfs (USDA, 1989). The parent material of these soils consists of beds of sandy and clayey material that were transported and deposited by ocean currents during several periods of inundation during the Pleistocene period, forming a series of marine terraces. Summary pedon descriptions for the soils found in improved and seminative pastures are shown in Table 2.


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  		Table 2. Summary of site and soil profile descriptions in improved and seminative pastures.

 
South Florida has unimodal annual rainfall distribution, and the rainy season typically begins in May and runs through October, during which most depressional wetlands become flooded. The annual cumulative period between which these wetlands become effectively flooded and when they are completely drawn down following drought is regarded as the hydroperiod. Hydroperiod in the studied wetlands fluctuated considerably as depicted by water level measurements taken between September 2000 and May 2003 (Fig. 1). Construction of improved and extended drainage systems shortens wetland hydroperiod, which is a key factor governing the sustainability of the structure and function of the wetland ecosystem.



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  		Fig. 1. Mean monthly water depth measured in improved pasture and seminative pasture wetlands (September 2000 to May 2003) illustrating the seasonal fluctuation of both water depth and hydroperiod in these wetland systems (modified from Steinman et al. 2003).

 
Plant Sampling
During Fall 2000, we randomly installed three 1.5- by 1.5-m grazing exclosures in each of the selected 24 experimental wetlands to evaluate the influence of cattle grazing on vegetation. The corners of the enclosures were secured firmly onto the ground using iron bars to prevent cattle or other animals from moving or knocking down the cages. We sampled standing crop productivity during fall 2000, spring 2001, and summer 2001 by destructive harvesting of all live and dead materials within a 0.5-m2 quadrat at the center of the enclosures and at randomly selected areas adjacent to enclosures (n = 3 per wetland). Harvested plant materials were separated into live and dead (litter), weighed fresh, and subsampled. Subsamples were oven-dried at 60°C for 72 h, finely ground using a piston ball mill, and analyzed for total C, N, and P using the methods described below. In this paper, only data from vegetation collected inside the grazing exclosure are presented because the influence of grazing on vegetation will be addressed in a separate paper.

Sediment Nutrients
Within each wetland, soils were sampled at four different depths (detrital layer, 0–15, 15–30, and 30–45 cm) along three randomly selected transects running from the center to the edge of the wetland. Sampling locations within each transect included one sample taken within a 5-m radius from the center of the wetland and one point within 5 m of the edge of the wetland. Soils taken from the same depth and relative sampling locations across the three transects were pooled together, resulting in one sample from each depth at center and edge of each wetland (one pooled sample x two locations x four sampling depths = eight samples per wetland). For the detrital layer which contained loosely consolidated material, we adopted a pin block method developed by Johnson et al. (1991) for forest soil detrital layers. In this method, a 15- by 15-cm wooden template with holes in each corner was placed on the soil surface after brushing away any surface plant litter. After first removing recognizable litter from the soil surface, large (15-cm-long) nails were pushed through the holes in the corner of the template and into the soil to hold the template in place. A serrated knife was used to cut through the detrital layer (Oe and Oa horizon) around the edge of the block, and the surrounding detrital layer was pulled back to expose the 15- by 15-cm cut layer. The exposed layer was undercut at the depth of the mineral soil, removed, and any adhering mineral soil shaken off carefully. All soil samples were oven-dried, sieved (4-mm mesh), subsampled, and finely ground for C and N analysis. Separate subsamples were reserved for P fractionation (only total P reported here) and microbial biomass nutrient assays (see below).

Sample Analyses
Plant and soil total C and N were analyzed using dry combustion chromatography using a Carlo-Erba NA 1500 C-N analyzer (Haake Buchler Instruments, Inc., Saddle Brook, NJ). Total plant P was determined using procedure for acid extracts of dry ashed plant samples: about 0.5 g of plant tissue was ashed at 450°C for 4 h and the ash extracted using aqua regia extractant (3:1 mixture of concentrated HCl/HNO3) (Allen et al., 1974). The extract was analyzed colorimetrically using the method of Murphy and Riley (1962) on a Technicon Autoanalyzer II (USEPA, 1983).

Soil microbial biomass C and N were determined using a modified fumigation-incubation method using dry soils rewetted to a consistent water-filled pore space (WFPS) (Franzluebbers et al., 1996). Each dry soil sample was weighed into duplicate beakers and rewetted by adding deionized water to achieve 50% WFPS. The amount of water ({theta}m) added per sample was calculated using the formula modified from Gardner (1986):

where WFPS = water-filled pore space (%), {rho}p = particle density (assumed to be 2.65 g cm–3), {rho}b = bulk density (g cm–3), and Ws = soil weight (g). The amount of soil weighed into the beaker was 5 g for the detrital layer, 15 g for the 0- to 15-cm layer, and 25 g for the 15- to 30-cm and 30- to 45-cm layers. All the samples were placed into 1.5-L mason jars and preincubated in the dark at 20 to 25°C for 10 d in presence of NaOH to absorb respired CO2 and {approx}10 mL of deionized water in a scintillation vial to maintain the soil moisture. At 10 d, the NaOH traps were changed and one set of samples containing all treatments were fumigated with ethanol-free chloroform at 20 to 25°C for 24 h. Chloroform vapor was removed by repeated evacuation with a vacuum pump, and the samples were inoculated with 0.2 g of unfumigated soil retrieved from the original sample. All samples were reincubated for another 10 d, and CO2 evolution measured as before. The CO2 production was determined by HCl titration of NaOH after incubation. Microbial biomass C was calculated as follows:

where CF = CO2–C fumigated soil and Cc = CO2–C control soil. The constants 1.73 and 0.56 were used to account for the released microbial C adsorbed by soil during fumigation and were obtained by regressing chloroform fumigation incubation data against microscopically determined biomass C (Horwath et al., 1996). Initial and final soil inorganic N (NO3–N + NH4–N) was determined by extracting 10 g of soil sample with 40 mL 2 M KCl before and after incubation.

Statistical Analyses
Initial analysis showed that there was no influence of the cattle stocking density treatments on nutrients in vegetation or soil. Therefore, we analyzed all data to assess the influence of pasture type on wetland nutrient characteristics. Hence, for the soils data, our statistical model included the pasture type as our main treatment and soil depths as the subtreatments. We also analyzed pasture type x depth interactions. For the vegetation data, our statistical model included only pasture type as a main treatment effect. Although pseudoreplication could not be entirely avoided in this design (Hurlbert, 1984), the study wetlands had sufficient statistical independence because they were randomly selected and spatially located either within the pasture or across the cattle density treatments. All data were analyzed by ANOVA using Genstat statistical software (Payne et al., 1987). Tests of significance between treatment means were performed using the Tukey test at {alpha} = 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 SUMMARY
 REFERENCES
 
Nutrient Distribution with Soil Depth
Soil C, N, and P concentrations decreased with soil depth for all wetlands, and ranged from 3 to 25% for C, 0.2 to 1.7% for N, and 0.004 to 0.06% for P (Table 3). Concentrations of all nutrients were significantly greater for the detrital layer and the 0- to 15-cm soil depths compared with the deeper 15- to 30-cm and 30- to 45-cm depths in both wetland types. Improved pasture wetlands had significantly greater P concentration in the 0- to 15-cm mineral soil layer compared with seminative pasture wetlands, whereas soil C concentration was significantly greater for seminative wetland soils. Nitrogen concentrations were fairly similar for both wetland types. The amount of detritus varied greatly within and across different wetlands and among the two pasture types, but was generally greater in the seminative pasture wetlands (mean = 10.3 ± 0.72 kg m–2) than in the improved pasture wetlands (mean = 6.6 ± 1.05 kg m–2).


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  		Table 3. Total soil C, N, and P concentrations determined in improved and seminative pasture wetlands. The values represent the means plus or minus the standard errors for all samples taken from the interior and edge of the wetlands for each respective soil depth (n = 24). Values (mean ± SE) in a column followed by different letters are significantly different by Tukey test ({alpha} = 0.05).

 
Total C, N, and P content (concentration x mass) in mineral soil layers beneath the detrital layer for both wetland types were markedly greater for the 0- to 15-cm depth compared with the deeper soil depths (15–30 and 30–45 cm), which did not differ significantly in nutrient content (Table 4). The detrital layer had the lowest total C and N of all soil layers due to the relatively small mass of the detrital layers relative to mineral soil layers. Total C and N in wetland soils were similar for improved and seminative pastures at each respective soil depth. Total P accumulation in the 0- to 15-cm soil depth was greater in soils from improved pasture than in seminative pasture wetlands. Total microbial biomass C and chloroform extracted N were greater in the detrital and the 0- to 15-cm layers than in the deeper soil layers in wetlands in both pasture types (Table 4). Soil respiration decreased with increasing soil depth for both land uses, and C mineralization per unit organic matter was positively correlated with the measured C concentration (Fig. 2). Soil microbial biomass C/total C ratios decreased with soil depth and were similar for both improved and seminative pasture wetlands.


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  		Table 4. Distribution of sediment total organic C, N, P, microbial biomass C, and Chloroform extracted N in the soil profile for seasonal wetlands surrounded by improved and seminative pastures. The values represent the means ± the SE for all samples taken from the interior and edge of the wetlands for each respective soil depth (n = 24). Values (mean ± 1 SE) in a column followed by different letters are significantly different by Tukey test ({alpha} = 0.05).

 


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  		Fig. 2. Relationship between total soil C concentration and cumulative total CO2–C per unit organic matter for both improved and seminative pasture wetlands

 
C/N/P Stoichiometry in Soils and Plants
Soil C/N, N/P, and C/P ratios in the top 0- to 15-cm of soil were greater (P < 0.05) in seminative pasture wetlands than in improved pasture wetlands (Fig. 3). Wetland soil nutrient ratios were not significantly different between pasture types at depths > 15 cm. The C/N ratio for the detrital layer was lower in improved pasture wetlands than in seminative pasture wetlands, but detrital layer N/P and C/P were similar for both land use types. Depth distribution of N/P and C/P ratios indicated that in the improved pasture wetlands, both the detrital and 0- to 15-cm soil layers were enriched in P relative to deeper soil layers; but in the seminative pasture wetlands, apparently only the detrital layer was enriched in P relative to deeper mineral soil layers. There was no significant difference in soil C/N ratio among the four soil depths in wetlands in either pasture type. Soil C and N were positively correlated, with r2 values ranging between 0.53 and 0.97 for improved pasture wetlands and 0.34 to 0.98 for seminative wetlands (Table 5). Significant relationships were also observed between soil C and P, except for the detrital layer in seminative wetlands. For soil N and P, significant positive correlations were observed only for the subsoil (>15 cm).



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  		Fig. 3. Nutrient stoichometric ratios for improved and seminative wetland soil wetland soils. Bars represent standard errors of the sample means.

 

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  		Table 5. Correlation (r2) of C, N, and P concentrations in soil samples from various depths in wetlands of improved or seminative pastures. Values for nutrient concentrations from the interior and edge of the wetlands were used in the regression analysis (n = 24).

 
Phosphorus concentration in wetland plant tissues was much greater in improved pasture wetlands (range = 0.16–0.24%) than in seminative pasture wetlands (range = 0.07–0.09%) (P < 0.05 for mean value). Likewise, mean N concentration in wetland plant tissues was greater in improved pasture (1.9%) than in seminative pasture (1.4%) wetlands. The relationship between N and P concentration in live materials was stronger for improved pasture wetlands (r2 = 0.65, slope = 0.16) than seminative pasture wetlands (r2 = 0.45, slope = 0.06) (Fig. 4). Plant tissue C/N, N/P, and C/P ratios determined in live and dead (litter) materials were consistently lower for improved pasture than seminative wetlands (Fig. 5). The C/N ratio of live vs. dead material was not significantly different in wetlands of either pasture type, but the N/P ratio and C/P ratio of dead material was significantly greater than in live material for all wetlands, indicating possible translocation of P to belowground tissues before senescence in the perennial wetland species.



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  		Fig. 4. Relationship between N and P concentrations for live plant materials harvested in both improved and seminative pasture wetlands.

 


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  		Fig. 5. Plant tissue nutrient stoichometric ratios for improved and seminative pasture wetlands. Bars represent the standard error of the sample means.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
SUMMARY
 REFERENCES
 
Wetland Sediment and Plant Nutrient Characteristics
The most salient observation made in this study was that upper soil layers and aboveground tissues of plants were more enriched in P in improved pasture wetlands than in seminative pasture wetlands. The N/P and C/P ratios in soils and live and dead (litter) plant materials were consistently lower in wetlands surrounded by improved pastures than in those surrounded by seminative pastures, implying greater enrichment of P in the improved pasture wetlands (Fig. 3 and 5). For most soil variables, there were no significant differences between seminative and improved pasture wetlands in the subsoil (>15 cm), which implies that the differences observed in the topsoil (0–15 cm) for P concentration, total soil P, and nutrient ratios between the two pasture types were due to the differences in the management practices in the two pasture types. As discussed previously, improved pastures were fertilized regularly with complete (NPK) fertilizer for at least 20 yr before 1987, and are still fertilized annually with N fertilizer; whereas the seminative pastures have never, to our knowledge, been fertilized. These differences in fertilization regime likely contributed to the greater enrichment of wetland sediments with N and P in the improved pastures relative to seminative pastures.

The decline in total soil nutrients with increasing soil depth has been previously reported for various soils and wetland sediments. For example, in a study of wetland soil in the Florida Everglades, there was a decline in total soil nutrients, especially P, and microbial biomass C and N with increasing soil depth (Qualls and Richardson, 1995). Haynes and Williams (1999) also observed a similar trend with organic C and microbial biomass C with soils taken from cattle pastures on a Typic Dystrochrept soil in New Zealand. The study also found that differences in soil C measurements between heavily and lightly impacted pasture patches were apparent only in the upper soil layer. Everglades wetland sediments enriched in P from agricultural runoff show a pattern of decreasing P enrichment with increasing soil depth (Reddy et al., 1999). To describe P enrichment of soil through time, Reddy et al. (1999) presented a conceptual model in which the detrital layer is influenced in the short term (<3 yr), the 0-10 cm mineral soil in the intermediate term (<10–15 yr) and the 10- to 30-cm layer in the longer term (>10–15 yr). Our data support this general model with respect to the enrichment of wetland sediments with P in improved pastures; the P enrichment was restricted to the detrital layer and upper 15 cm of soil, with some impacts evident at the 15- to 30-cm layers, and no enrichment of the soil below 30 cm. The N/P and C/P ratios in the top 0- to 15-cm soil depth of our wetlands were greater (38 and 495 for improved pasture wetlands; 49 and 830 for seminative pasture wetlands) than the N/P ratio of 19 and C/P ratio of 270 reported for the nutrient-impacted areas of the Florida everglades, suggesting that the isolated wetlands in improved pastures are not as heavily impacted by P enrichment as are some of the impacted Everglades soils.

Measurements of P adsorption parameters for the soil samples collected in study site revealed that P sorption capacities increased with soil depth and were generally greater in improved pasture compared with seminative pasture wetland mineral soils, but were below the critical threshold value of 30% in both cases (Sperry, 2004). The degree of P saturation observed in the detrital layer was greater for improved pasture wetlands (62%) compared with seminative pasture wetlands (44%). This implies that although the improved pasture wetlands have been more impacted by the adjacent pasture management activities than have the seminative pasture wetlands, the mineral soil still has significant nutrient assimilative and storage capacity.

The N/P ratios in soils and plant materials are occasionally used to assess the N and P status in a given site. Concentration of both N and P in plant tissues is considered a robust indicator of the limiting or nonlimiting nutrient conditions of the soil, possibly because the N and P concentration in plant tissues tend to reflect the status of available soil N and P present in the soil (Bedford et al., 1999; Tessier and Raynal, 2003). Studies conducted in 45 European wetlands concluded that wetland sites with plant N/P < 14 were N limited, those with N/P > 16 were P limited, while those with 14 ≤ N/P ≤ 16 exhibited colimitation of both N and P (Koerselman and Meuleman, 1996; Verhoeven et al., 1996). These N/P values were used in a more recent review to classify a wide range of North American wetlands and give an overview of nutrient status of a those wetlands (Bedford et al., 1999). If a plant N/P < 14 indicates N limitations, then the improved pasture wetlands reported in this study would be classified as N limited, whereas the seminative wetlands would not be N limited. The greater concentration of P in the plant tissues in improved pasture wetlands were likely responsible for the low N/P values in those wetlands. However, our data showed that dead (litter) tissue N/P ratios were higher than those reported for live plant tissues for both wetland types which concurred with previous findings that wetland plants could be more conservative in recycling P than N (Bedford et al., 1999).

Total soil organic C storage across all soil layers was about 19% greater in wetlands surrounded by seminative pastures than in wetlands surrounded by improved pastures possibly due to vegetation type and inherent soil organic matter build up (Table 4). The greater amounts of total soil C and organic matter in the surface layers of the seminative pasture wetlands could be attributed to the dominance in those wetlands of maidencane (Panicum hemitomon Schult.) which is a prolific producer of detritus and contributes to accumulation of a litter layer consisting of dead rhizomes, roots, and shoots. The thickest litter layer in improved pasture wetlands were in the few wetlands dominated by maidencane grass in that pasture type. Also, it is possible that the amount of detrital material is influenced by the presence of cattle, since cattle trample the litter layer, incorporate it into the mineral soil, and enhance its mineralization, particularly during the dry period when the organic matter decomposition is enhanced by promoting of microbial activity through aeration (Battle and Galloday, 2001). Most of the improved pasture wetlands were not dominated by maidencane, but rather by more succulent herbs that produce less litter with a faster decomposition rate than maidencane litter, or by soft rush (Juncus effusus L.), which produces dense bunches of standing detritus that do not create distinct soil organic layers.

Although the seminative pasture wetlands had greater concentrations of soil C in the upper 0- to 15-cm soil layer than did improved pasture wetlands, the reverse was true for microbial biomass C. The greater concentration of microbial C in improved pasture wetlands may be partly due to higher rates of C mineralization favored by greater nutrient enrichment in these soils, or simply to the mixing of the detritus into the soil by cattle trampling, enriching the microbial pool in the upper mineral soil. Also, there is evidence that cattle dung mixed with soil increases the activity and size of microbial biomass in the short term (Haynes and Williams, 1999; Lovell and Jarvis, 1996). Cattle may impact improved pasture wetlands more than seminative pasture wetlands over the long term because cattle stocking rates are greater on improved pastures.

Nutrient Enrichment in Improved Pasture Wetlands
The establishment of improved pastures in the Lake Okeechobee watershed involves the introduction of exotic pasture grass (mainly Bahia grass), construction of artificial drainage systems, increased cattle stocking rates, and fertilizer application, all of which can influence the structure and function of associated wetlands. Conversion of native rangelands into more intensively managed pasture systems is more likely to affect nutrient enrichment in the uppermost surface soil than the subsoil. Increased use of inorganic fertilizers associated with agricultural activities is known to increase both particulate and dissolve nutrients, particularly P, in sediments and surface water and overland flow (Drexler and Bedford, 2002; Tilman et al., 2001). In agricultural areas where wetlands are an important feature of the landscape, fertilizer applied to field crops or pastures may enter adjacent wetlands via inadvertent overspreading of fertilizer or as particulate runoff or dissolved forms in surface runoff or groundwater. The main forms in which the applied nutrients could have moved into the depressional wetlands in this study are either through dissolved or particulate form in underground water seepage or overland flow, and to a lesser extent, through grazing cattle.

Since P fertilizer use in the improved pastures was discontinued 14 yr before this study, it is likely that the greater sediment P enrichment in improved wetlands was due to accumulation and recycling of earlier deposits and possibly also to continued movement and uptake of dissolved P in runoff or shallow groundwater from surrounding pastures. Phosphorus loads in surface runoff were about seven times greater from improved pastures than from seminative pastures, and this difference has been linked to past use of P fertilizer in the improved pastures (J.C. Capece, K.L. Campbell, P.J. Bohlen, D.A. Graetz, and K.M. Portier, 2005, unpublished data). Likewise, P concentrations in the surface water of wetlands used in this study were much greater in improved pasture wetlands (370 µg L–1) than in seminative pasture wetlands (53 µg L–1), for samples taken in September 2001 (Steinman et al., 2003).

These depressional wetlands form microcatchments by accumulating collecting runoff from a contiguous pasture area between constructed drainage ditches and may also interact with adjacent pastures via lateral groundwater flow. Hence, these wetlands become the primary recipients of nutrients from those hydrologically separated microcatchments, before making their way into the larger watershed drainage systems. These numerous wetlands may function as initial nutrient interceptors, assimilating and storing nutrients within the pasture systems and thus reducing the load that eventually reaches the main water bodies. Alternatively, once these wetlands become nutrient saturated, they could become sources rather than sinks for nutrients in the landscape. A greater understanding of their hydrologic interaction with surrounding pastures and drainage systems is needed before such impacts can be fully assessed.


   SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
REFERENCES
 
The nutrient assimilative and storage capacity of isolated seasonal wetlands embedded within agricultural landscapes determines their role as nutrient sinks, but also as potential nutrient sources within the landscape. The potential for nutrient retention within these wetlands is perhaps their most critical role in sustaining wetland vegetation productivity and reducing the amounts of nutrients in surface runoff and thus reducing nonpoint-source water pollution. Since the main sources of nutrients in these seasonally flooded wetlands is mainly by internal recycling of native nutrients through biomass and sedimentation, any modification in land use activities within and in areas adjacent to the wetlands may alter the amount and rates of nutrient transformations within the wetlands (Neill et al., 2001; Qualls and Richardson, 1995).

Wetlands embedded in grazing lands provide multiple ecosystem functions, including high quality forage production, areas for cattle to cool themselves during hot weather, important wildlife habitat, nutrient storage and assimilation, and greater vegetation productivity than surrounding pastures. Conversion of native rangelands into more intensively managed pasture systems increases plant and soil nutrient concentrations and also alters seasonal plant production patterns of the embedded wetlands. Changes in pasture land use practices also lead to alteration in wetland plant community composition and structure and seasonal net primary productivity, which has a direct impact on within-stand nutrient cycling (Gathumbi and Bohlen, 2003, unpublished data). The greater nutrient enrichment of improved pasture wetlands may increase forage production or quality for grazing cattle, but this has yet to be determined. Intensive grazing in improved pasture wetlands appears to shift the plant community toward less palatable species, such as J. effusus, so that the benefits of nutrient enrichment may be offset by shifts in plant community composition.

The increase in nutrient enrichment, especially P, in soils and sediments in improved pastures and the resulting high P concentrations in wetland surface water raise questions about how well these wetland systems will serve as nutrient sinks within the landscape. It is not certain whether the nutrient enrichment of these wetlands exceeds their nutrient assimilative capacity. However, the fact that P loads in surface runoff were five to seven times greater from improved than from semiimproved pastures, and were not significantly affected by the percentage of wetlands within a given pasture (J.C. Capece, K.L. Campbell, P.J. Bohlen, D.A. Graetz, and K.M. Portier, 2005, unpublished data), suggests that these isolated wetlands do not significantly effect P loads in surface runoff from these pastures under current management.


   ACKNOWLEDGMENTS
 
This work was supported by funds from USDA-CSREES Competitive Grant No. 00-35101-9282 and matching contributions from Archbold Biological Station. We thank Mike McMillian, Lourdes Rojas, and Julie Conklin for their assistance with field and laboratory work.

Received for publication February 17, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 SUMMARY
 REFERENCES
 




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P. J. Bohlen and S. M. Gathumbi
Nitrogen Cycling in Seasonal Wetlands in Subtropical Cattle Pastures
Soil Sci. Soc. Am. J., May 16, 2007; 71(3): 1058 - 1065.
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