phosphorus release and retention by soils of natural isolated wetlands · 2007-10-12 · phosphorus...

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Int. J. Environment and Pollution, Vol. x, No. x, xxxx 1 Copyright © 200x Inderscience Enterprises Ltd. Phosphorus release and retention by soils of natural isolated wetlands E.J. Dunne*, K.R. Reddy and M.W. Clark Department of Soil and Water Science, Wetland Biogeochemistry Laboratory, University of Florida, P.O. Box 110510, Gainesville, 32611 FL E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author Abstract: Hydrological restoration of historically isolated wetlands may mitigate phosphorus (P) loss. The objectives of this study were to quantify P in soil, determine effect of (1) soil characteristics on P release, and (2) antecedent soil hydrological conditions on P dynamics. Humic/fulvic acid bound P and residual P accounted for majority of P (>78%) in surface soils. Soils with highest nutrient status and labile P fractions released most P during initial flooding. Phosphorus dynamics during additional flooding were dependent on soil characteristics, antecedent soil hydrological conditions, and P levels in the water. Phosphorus retention varied between 0.3 and 8 mg m –2 d –1 . Keywords: phosphorus; isolated wetlands; hydrological restoration; soils; flux; release; retention. Reference to this paper should be made as follows: Dunne, E.J., Reddy, K.R. and Clark, M.W. (xxxx) ‘Phosphorus release and retention by soils of natural isolated wetlands’, Int. J. Environment and Pollution, Vol. x, No. x, pp.xxx–xxx. Biographical notes: Ed Dunne received his PhD from University College Dublin, Ireland, in 2004. He is a Research Associate with the Wetland Biogeochemistry Laboratory at the University of Florida/Institute of Food and Agricultural Science. His research interests include the biogeochemistry of wetlands, water quality and agriculture, and ecological engineering approaches to improve water quality. K.R. Reddy received his PhD at the Louisiana State University in 1976. At present he is departmental chair of the Soil and Water Science Department at the University of Florida/Institute of Food and Agricultural Science. His research interests include biogeochemical cycling of nutrients in wetland and lake ecosystems. He is the author of over 300 peer-reviewed journal papers, and was chairman of the wetland soils division of the Soil Science Society of America in 1992 and 1994. He is a fellow of the Soil Science Society of America and American Society of Agronomy. Mark Clark received his PhD from the University of Florida in 2000. He is an Assistant Professor in Wetland Ecology, and is the University of Florida/Institute of Food and Agricultural Science wetlands and water quality extension specialist. His research interests include wetland nutrient assimilation and storage; vegetation succession dynamics, and ecological engineering design using wetland processes to improve water quality and enhance ecological function of altered landscapes.

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Page 1: Phosphorus release and retention by soils of natural isolated wetlands · 2007-10-12 · Phosphorus release and retention by soils of natural isolated wetlands 3 The Lake Okeechobee

Int. J. Environment and Pollution, Vol. x, No. x, xxxx 1

Copyright © 200x Inderscience Enterprises Ltd.

Phosphorus release and retention by soils of natural isolated wetlands

E.J. Dunne*, K.R. Reddy and M.W. Clark Department of Soil and Water Science, Wetland Biogeochemistry Laboratory, University of Florida, P.O. Box 110510, Gainesville, 32611 FL E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: Hydrological restoration of historically isolated wetlands may mitigate phosphorus (P) loss. The objectives of this study were to quantify P in soil, determine effect of (1) soil characteristics on P release, and (2) antecedent soil hydrological conditions on P dynamics. Humic/fulvic acid bound P and residual P accounted for majority of P (>78%) in surface soils. Soils with highest nutrient status and labile P fractions released most P during initial flooding. Phosphorus dynamics during additional flooding were dependent on soil characteristics, antecedent soil hydrological conditions, and P levels in the water. Phosphorus retention varied between 0.3 and 8 mg m–2 d–1.

Keywords: phosphorus; isolated wetlands; hydrological restoration; soils; flux; release; retention.

Reference to this paper should be made as follows: Dunne, E.J., Reddy, K.R. and Clark, M.W. (xxxx) ‘Phosphorus release and retention by soils of natural isolated wetlands’, Int. J. Environment and Pollution, Vol. x, No. x, pp.xxx–xxx.

Biographical notes: Ed Dunne received his PhD from University College Dublin, Ireland, in 2004. He is a Research Associate with the Wetland Biogeochemistry Laboratory at the University of Florida/Institute of Food and Agricultural Science. His research interests include the biogeochemistry of wetlands, water quality and agriculture, and ecological engineering approaches to improve water quality.

K.R. Reddy received his PhD at the Louisiana State University in 1976. At present he is departmental chair of the Soil and Water Science Department at the University of Florida/Institute of Food and Agricultural Science. His research interests include biogeochemical cycling of nutrients in wetland and lake ecosystems. He is the author of over 300 peer-reviewed journal papers, and was chairman of the wetland soils division of the Soil Science Society of America in 1992 and 1994. He is a fellow of the Soil Science Society of America and American Society of Agronomy.

Mark Clark received his PhD from the University of Florida in 2000. He is an Assistant Professor in Wetland Ecology, and is the University of Florida/Institute of Food and Agricultural Science wetlands and water quality extension specialist. His research interests include wetland nutrient assimilation and storage; vegetation succession dynamics, and ecological engineering design using wetland processes to improve water quality and enhance ecological function of altered landscapes.

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2 E.J. Dunne, K.R. Reddy and M.W. Clark

1 Introduction

Loss of nutrients, such as Phosphorus (P), from agricultural practices within watersheds is still a major concern worldwide (Rekolainen et al., 1997; Heathwaite et al., 2000; Schoumans and Groenendijk, 2000; Withers et al., 2000; Smith et al., 2003). In the USA, intensive agricultural practices have maintained a P surplus since 1945 (Sharpley et al., 2001) and this has led to the geographic concentration of nutrient surpluses between and within agricultural watersheds (Sims et al., 2000).

Wetlands such as freshwater swamps and marshes are often a feature of agricultural dominated watersheds, and these wetlands can exist as hydrologically isolated or connected systems. Geographically, isolated wetlands are those wetlands ‘completely surrounded by uplands’ (Tiner, 2003). They occur in depressions that have no permanent connections to aboveground stream or river systems (Stutter and Kral, 1994; Sharitz and Gresham, 1998; Leibowitz and Vining, 2003; Winter and LaBaugh, 2003). However, many isolated wetlands are hydrologically connected to other wetlands and water bodies through ground-water flows and/or by intermittent overflows (Leibowitz and Vinning, 2003; Tiner, 2003; Winter and LaBaugh, 2003).

Phosphorus retention by small wetlands within agricultural watersheds is used as a land management practice in other countries such as Norway and Finland (Braskerud, 2002; Koskiaho, 2003). Thus, hydrological restoration of previously drained isolated wetland ecosystems by ditch management may mitigate some P loss from grazing pastures within agricultural watersheds. Phosphorus retention mechanisms within wetlands include sedimentation of incoming particulate P (Reddy et al., 1999; Braskerud, 2002), sorption and precipitation reactions (Axt and Walbridge, 1999; Reddy et al., 1999; Bridgham et al., 2001) along with biological uptake (Mitsch et al., 1995) and long term P storage processes such as peat accretion (Craft and Richardson, 1993; Reddy et al., 1999).

The impact of agricultural practices in Lake Okeechobee’s Basin on total P (TP) concentrations in the lake, are well-documented (Boggess et al., 1995; Flaig and Reddy, 1995; Haan, 1995; Hiscock et al., 2003). About 52% of the watershed is dominated by agriculture, with the largest agricultural land use being improved grassland pasture (SFWMD et al., 2004). For the period 1985–1989, TP imports to the watershed were estimated at 2,380 t P yr–1. Phosphorus load from 19 contributory basins within the watershed to the lake was previously estimated at 300 t P yr–1 (Boggess et al., 1995). Hiscock et al. (2003) estimated that between 1997 and 2001, P loads from 25 contributory basins to the lake was 303 t yr–1. In 2002, the total P load measured to Lake Okeechobee was 543 tonnes (SFWMD et al., 2004).

Best management practices and regulatory programmes are implemented in the Basin for the past 25 years, in an effort to reduce P loads to the lake (Flaig and Reddy, 1995; SFWMD et al., 2004); however P loads still exceed desired levels (Flaig and Reddy, 1995). The majority of P exports in the Basin are from several sub-basins, which are functionally defined as the ‘four priority basins’. The four priority basins (S-191, S-154, S-65D and S-65E) are dominated by beef and dairy enterprises and account for 12% of the land area in the Okeechobee Basin, while they contribute a disproportionate amount of the P (35%) entering the lake (Flaig and Reddy, 1995).

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Phosphorus release and retention by soils of natural isolated wetlands 3

The Lake Okeechobee Protection Act of 2000 set a Total Maximum Daily Load (TMDL) for the watershed of 140 t P yr–1 to be met by 2015 (SFWMD et al., 2004). To achieve these requirements, P control and mitigation strategies were initiated. One of these strategies is the restoration and/or creation of historically isolated wetlands. Drained isolated wetlands are a landscape feature of the four priority basins, as they occupy about 7% of land surface area in those basins (McKee, 2005). Typically, they are connected by ditches, canals and streams, which convey water off site and ultimately transport water to the lake.

Objectives of our study were to

• quantify isolated wetland soil P forms

• determine the effect of site soil characteristics on P flux from isolated wetland soils to overlying waters

• determine the effect of antecedent soil hydrological conditions on P release and retention during two flooding periods.

2 Materials and methods

2.1 Site description

Two historically drained emergent marsh isolated wetland sites were chosen and each site represented a different historical P loading. The wetlands on Beaty Ranch (N 0270 24.665′, W 0800 56.940’) and Larson Dixie Ranch (N 0270 20.966′, W 0800 56.465′) were within the four priority basins of Lake Okeechobee Basin (Figure 1). Each wetland site was drained by a single ditch, which drained into a larger ditch that transported water off site. Both wetland sites were within cow-calf grazing pastures that were dominated by Bahia grass (Paspalum notatum Flueggé). The Larson Dixie isolated wetland was 2.64 hectares in area and was dominated by Juncus effusus L., Pontedaria cordata var. lancifolia (Muhl.) Torr., Ludwigia repens Forst. and Eleocharis baldwinii (Torr.) Chapm. vegetation. The Beaty isolated wetland had a total area of 1.67 hectares and was dominated by Panicum sp. in deep-water zones and Ludwigia repens Forst., Juncus effusus L., Hydrocotyle ranunculoides L.F. and Andropogon glomeratus (Walter) BSP in shallower areas.

Soils at Larson Dixie Ranch site are classified as Siliceous, hyperthermic Spodic, Psammaquents (Basinger series) and Beaty Ranch site soils are classified as Sandy, siliceous, hyperthermic Typic humaquepts (Placid series). Both soils are deep, poorly drained, rapidly permeable and are formed in beds of sandy marine sediments (Lewis et al., 2001).

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4 E.J. Dunne, K.R. Reddy and M.W. Clark

Figure 1 Priority sub-basins of Okeechobee watershed north of the lake with dairy and improved pasture land uses outlined. Isolated wetland study sites at Larson Dixie and Beaty ranches are not to scale

2.2 Soil sampling

For site soil characterisation, three intact soil columns were taken at random from shallow water zones of each site. Polycarbonate tubes (10 cm internal diameter × 0.3 cm wall depth × 60 cm in length) were sharpened at one end and hammered down to a soil depth of 30 cm. Tubes (with adhered soil) were then extracted from the substrate using shovels. Soils were extruded from tubes using an extruder. Soils were sectioned at 0–5 cm, 5–10 cm, 10–20 cm and 20–30 cm depth increments at Larson Dixie, and 0–7 cm, 7–18 cm and 18–30 cm depth increments at Beaty. Differences in sectioning depths were due to differences in soil horizon. For column studies, 36 intact soil columns were taken at random from each site. After soil columns were extracted, site floodwater was siphoned off soil and the underside of tubes were capped and sealed. Columns were placed on ice, transported back to laboratory and stored at 4°C for one week until column experiments were conducted.

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Phosphorus release and retention by soils of natural isolated wetlands 5

2.3 Physicochemical characterisation

Bulk density was determined using a coring method (Blake and Hartage, 1986). A known mass of wet soil was dried for 72 hours at 70°C and the net percentage difference between wet and dry weights was quantified to estimate soil water content (Gardner, 1986). pH was measured in a 1:2 soil to water ratio. To determine inorganic P, soils (0.5 g of finely ground and sieved to 2 mm) were extracted with 25 ml of 1 M HCl. After this, samples were then centrifuged for 10 minutes at 6,000 rpm and filtered through 0.45 µm filter paper using a vacuum filtration system. We analysed for soluble reactive P (SRP) using an automated (Technicon AA II) ascorbic acid method (Method 365.1; USEPA, 1993). Cations such as iron (Fe), aluminium (Al), calcium (Ca) and magnesium (Mg) were quantified using inductively coupled argon plasma spectrometry (ICP). We used 1 M HCl extracted solutions for the ICP analyses. Soil TP content was determined on 0.5 g of finely ground dry soil that was combusted at 550°C in a muffle furnace for four hours. Ash was then dissolved in 6 M HCl (Andersen, 1976) and digestate analysed for P using the automated ascorbic acid method. To determine amorphous and poorly crystalline forms of Fe and Al, soils were extracted with 0.175 M ammonium oxalate and 0.1 M oxalic acid at a soil solution ratio of 1 : 40 for four hours (McKeague and Day, 1966). Extracts were centrifuged, filtered to 0.45 µm and analysed for Fe and Al using ICP. Depending on soil textural components, 5–60 mg of dried finely ground soil was analysed for Total Carbon (TC) and Total Nitrogen (TN) by dry combustion using a CNS analyser (Carlo Erba Model NA-1500). A single point P sorption isotherm was used to measure P sorption capacity of soils equilibrated with 1000 mg P kg–1 (P1000) (Reddy et al., 1998). Soils were extracted for 24 hours on a reciprocating shaker, centrifuged, filtered to 0.45 µm and analysed for P using the automated ascorbic acid method. Phosphorus sorbed by soil was calculated as determined in equation (1) (Reddy et al., 1998).

[( ) ( )] /o tS C V C V M= × − ×′ (1)

where S′ was the phosphorus sorbed by solid phase (mg kg–1); Co was the initial concentration of P added to solution (mg l–1); V was the volume of extracting solution (l); Ct was the concentration of P in solution after 24 hours of equilibration (mg l–1); and M was the mass of dry soil (kg). A single point P sorption isotherm was used instead of a sequential P adsorption isotherm, as sequential isotherms are time consuming and labour intensive to conduct. Other researchers (Bache and Williams, 1971; Simard et al., 1994) previously used single point P sorption indices as an indicator for P sorption capacity.

2.4 Soil phosphorus fractionation

Inorganic soil P fractions were determined using a modified Chang and Jackson (1957) fractionation scheme initially described by Hieltjes and Lijkelma (1980). An additional step was integrated into the scheme to differentiate between Al and Fe bound P, as it was hypothesised that Al bound P was an important inorganic P fraction in isolated wetland soils. Therefore, we extracted 0.5 g of dry weight equivalent soil with 0.5 M ammonium fluoride (NH4F) for one hour at a soil to solution ratio of 1:25 to determine Al bound P (Olsen and Summers, 1982). The overall fractionation scheme included five different extractions on field-moist soil (0.5 g dry-weight equivalent). Soil was initially equilibrated (continuous shaking) for 2 hours with 1 M KCl. Solutions were then

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6 E.J. Dunne, K.R. Reddy and M.W. Clark

centrifuged, filtered to 0.45 µm and analysed for SRP as described above. Residual soil was then used for the NH4F extraction. After equilibriation with NH4F, residual soil was extracted with 1 M NaOH for 17 hours. Solutions were centrifuged, filtered and analysed for SRP as described previous. Solutions were also analysed for TP (APHA, 1992). Solutions analysed for SRP were regarded as Fe-bound P with the difference between TP and SRP in the NaOH solution considered as P associated with humic and fulvic acids. The residual soil from this step was then extracted with 0.5 M HCl for 24 hours and SRP was analysed on centrifuged and filtered solutions. Phosphorus in these extracted solutions was thought to represent P associated with Ca and Mg compounds. Finally, TP was quantified in the remaining soil (Andersen, 1976), which we regarded as residual P.

2.5 Column studies

Two separate column studies were conducted. The first experiment was an initial 60-day flooding period to investigate the effect of site soil characteristics on P flux from soil to water using a laboratory soil/water column study. The second column study was undertaken to determine the effect of antecedent soil hydrological conditions (soils that were flooded to 20 cm, +20 cm; surface saturated, 0 cm; and water levels that were drawn down to 5 cm below soil surfaces, –5 cm; for 60-days) on P dynamics during two 28-day flooding periods (soils were flooded to 20 cm above soil surfaces). The two 28-day flooding periods were used to try and simulate two months of wetland flooding.

In the first experiment, 12 intact soil columns were placed at random in an insulated water bath (2.4 × 1.05 × 0.95 m) within a laboratory and incubated in the dark at room temperature. Columns were flooded to a water depth of 20 cm with filtered (0.45 um) Kissimmee River water and then spiked at 0.1 mg l–1 or 1 mg SRP l–1, as KH2PO4 to represent a low and a high P level, respectively. The initial incubation period was 60-days and each treatment (P load and site) was replicated three times. To measure P dynamics during the 60-days of flooding, overlying floodwaters in columns were sampled (20 ml floodwater samples) at 10 cm below the floodwater surface, on days 0, 2, 4, 7, 14, 21, 28, 42 and day 60. All samples were filtered to 0.45 µm using membrane disc filters and analysed for SRP, as previously described. On days 0, 28 and 60, water samples were also analysed for total dissolved P (TDP) on 0.45 µm-filtered samples and TP on unfiltered samples after potassium persulfate digestion. We measured P in solution using the ascorbic acid method previously described.

In the second experiment, 36 soil columns that were previously incubated in the dark for 60-days that were flooded (+20 cm), surface saturated (0 cm) or had overlying floodwaters drawn down (–5 cm) at the start of the 60-days period were flooded to a water depth of 20 cm for two 28-day flooding periods. During each flooding period, overlying floodwaters were spiked at respective P concentrations (0.1 and 1 mg SRP l–1). At the end of the first 28-day flooding period, soil columns were drained and re-flooded with 0.1 and 1 mg SRP l–1 spiked water and incubated for an additional 28-days. During the two flooding periods, 20 ml water samples were taken on days 0, 2, 4, 7, 14, 21 and day 28, filtered through 0.45 µm filter paper and analysed for SRP to determine effect of antecedent hydrological conditions on P release and retention by soils. On days 0 and 28 of each period, water samples were also analysed for TDP and TP as previously described. After flooding periods, columns were removed from water bath; soils were extruded from columns, sectioned and analysed for soil physicochemical parameters as outlined previously.

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Phosphorus release and retention by soils of natural isolated wetlands 7

Redox potential (Eh) of soils was monitored and recorded on a continuous basis using platinum redox probes and a datalogger (Campbell Scientific Inc.) during both column studies to determine whether soils were aerobic or anaerobic. For each experimental treatment, three redox probes were placed into columns at soil depths of 5 cm. Floodwater pH, temperature and dissolved oxygen concentrations were measured using a portable multimeter (YSI; Model 556 multiprobe system).

To determine P release/retention during flooded periods, water column P mass per unit area was determined using equation (2) (Pant and Reddy, 2003):

P retention/release ( ) /o t tC C V A= − × (2)

where P retention/release was the cumulative P retained or released per unit surface area of sediment-water interface (mg m–2); Co was the water column initial P concentration (mg l–1); Ct was the water column P concentration at time t (mg l–1); Vt was the volume of surface water within column at time t (l); and A was the surface area of the sediment-water interface (m2). The relationship between P retention/release and P concentration typically follows a linear relationship (equation (3)):

P retention/release a t kK C P= × + (3)

where Ka was the constant representing P assimilation or buffering capacity coefficient (l m–1); Ct was the concentration of floodwaters at time t; and Pk was the initial desorbed P per unit surface area of soils (mg m–2). Equilibrium P concentration of floodwaters (EPCw), which is determined as the water column concentration where retention equals release, was determined using equation (4) (Reddy et al., 1999):

EPC / .w a kK P= (4)

Environmental conditions during flooding periods. Floodwater temperatures within columns during flooding periods were 22 ± 0.1°C. Column floodwater pH across treatments averaged 6.8 ± 0.2. Floodwaters were aerated to ensure overlying water turnover and dissolved oxygen concentrations averaged 7.3 ± 0.1 mg l–1 of O2. SRP concentrations in floodwaters accounted for 90% and 82% of TDP and TP, respectively (P < 0.05).

2.6 Data analyses

Experimental treatment significant differences (P < 0.05) were determined using t-tests and Analysis Of Variance (ANOVA). Comparison of means was by paired student t-tests, whereas in ANOVA all pairs were compared using Tukey-Kramer’s Honest Significant Difference (HSD). Normal quantile plots were used to determine whether data was normally distributed. Where data was not normally distributed, it was log transformed and tested for normality. All treatment significant differences were determined on transformed data. Multivariate linear correlations (Pearson product-moment correlation) were used to determine relationships between P release/retention parameters of overlying floodwaters and soil physicochemical properties. Regression and multiple regression analyses were performed to investigate predicative relationships for P release and retention data in overlying floodwaters. These analyses used the standard least squares method and used non-transformed data. All statistical analyses were performed using the JMP software programme (SAS Institute Inc., Cary, North Carolina).

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8 E.J. Dunne, K.R. Reddy and M.W. Clark

3 Results and discussion

3.1 Soil physicochemical characterisation

Isolated wetland soils at Larson Dixie had highest inorganic P and TP values in surface soils (0–5 cm) with subsurface soil depths generally having lower values (Table 1). Larson Dixie surface soils (0–5 cm) had about four to five times the inorganic P, TP, TC and TN content of Beaty surface soils (0–7 cm) suggesting that Larson Dixie soils had a higher nutrient status than Beaty soils. This may be due to previous nutrient loading events within grazing pastures surrounding the isolated wetland ecosystems.

Oxalate extractable Fe was significantly higher in Larson Dixie than in Beaty soils (P < 0.05; Table 2). Surface soils of both sites tended to have highest values (P < 0.05). Oxalate extractable Al was somewhat uniformly distributed at Larson Dixie, but not at Beaty (Table 2). Surface soils at Beaty had significantly higher oxalate extractable Al than subsurface depths (P < 0.05). High values of Fe in surface soils are often associated with high total organic carbon content (Gale et al., 1994; Reddy et al., 1995 and 1998). Aluminium and Fe can complex with surface accumulations of organic matter that can indirectly affect P sorption capacity (Syers et al., 1973).

There was a good relationship between soil inorganic P and oxalate extractable soil P (R2 = 0.71; n = 16; P < 0.01). In surface soils, especially at Larson Dixie, oxalate extractable P was about twice the inorganic P fraction (Tables 1 and 2). This suggests that oxalate extractions were accounting for soil P components, which the 1 M HCl extractions were not and this is not surprising. Ammonium oxalate extractions typically represent amorphous or poorly crystalline forms and can be a major component of total soil P in acid soils. Oxalate extractable P can also represent forms of phosphate such as calcium phosphate in calcareous soils (Freese et al., 1992). At low pH values (<5.0) stable complexes of metallic ions with organic matter may exist (Goodman, 1987). Haynes and Swift (1989) suggested that interactions between increased P sorption and Al organic matter complexes may also exist. Oxalate extractions dissolve both amorphous and organically bound Fe and Al (Parfitt and Childs, 1988) of which may be bound to phosphate in surface soils.

Phosphorus sorption capacity, as measured by P1000 varied with soil depth in Larson Dixie, but not in Beaty soils. Subsurface soils deeper than 20 cm at Larson Dixie had highest values (P < 0.05; Table 1), which may suggest that surface soils are undergoing P saturation.

Both Larson Dixie and Beaty surface soils had similar fractions of TP stored as labile P (1 M KCl extractable P) (Table 3), which was somewhat higher than what other researchers have shown for wetland soils in the Okeechobee Basin (Reddy et al., 1995). Labile P increased to nearly 25% at the 20–30 cm soil depth at Larson Dixie. Aluminium bound P increased from 0.5% of TP in Larson Dixie surface soils to 22% of TP at the 20–30 cm soil depth. Iron bound P along with Ca/Mg bound P also increased with depth in Beaty soils (Table 3) suggesting that soil mineral components increased with depth. There was no difference in Fe bound P in Larson Dixie soils deeper than 5 cm (Table 3). Reddy et al. (1995) found that Fe bound P ranged between 17 and 43% of total P in selected wetland soils of the Okeechobee Basins, which was somewhat similar to this study. We found a range between 7.5% and 48%, which is independent of site and soil depth.

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Phosphorus release and retention by soils of natural isolated wetlands 9

Table 1 Physicochemical characteristics of isolated wetland soils at Larson Dixie and Beaty sites ± one standard error (n = 21)

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10 E.J. Dunne, K.R. Reddy and M.W. Clark

Table 2 Oxalate and 1 M HCl extractable metals of Larson Dixie and Beaty isolated wetland sites ± one standard error (n = 21)

Oxalate extractable HCl extractable Wetland site Depth (cm) P (mg kg–1) Fe (mg kg–1) Al (mg kg–1) Ca (mg kg–1) Mg (mg kg–1)

0–5 170 ±24 999 ±157 238 ±15 1953 ±1078 416 ±248 5–10 65 ±11 265 ±53 246 ±31 2381 ±1713 513 ±402

10–20 60 ±3 164 ±13 334 ±43 1422 ±1107 244 ±223

Larson Dixie

20–30 53 ±9 52 ±13 256 ±42 401 ±219 102 ±3 0–7 31 ±9 127 ±62 77 ±21 342 ±178 97 ±72 7–18 <4† ND‡ <2 ND <6 ND 132 ±41 7 ND

Beaty

18–30 <4 ND <2 ND <6 ND 131 ±70 67 ND

† < is less than minimum detection limits for particular parameter. ‡ND is non determinable.

The difference between the TP and SRP measured in the NaOH extracted solutions is supposed to represent living and dead portions of organic P associated with humic and fulvic acid (Reddy et al., 1998a). This fraction accounted for 84% and 80% of TP in Larson Dixie and Beaty surface soils, respectively. Humic acid bound P is considered resistant to biological breakdown; however fulvic acid bound P is biologically active and can be hydrolysed to bioavailable forms. The processes and rates at which organic P compounds are transformed (hydrolysed into bioavailable forms) are controlled by physical, chemical and biological factors along with abiotic processes at soil water interfaces (Reddy et al., 1995). Residual P, which is considered resistant to biological uptake or unavailable mineral bound P (Reddy et al., 1999, 2005) decreased to 14% at the 20–30 cm depth of Larson Dixie soils, whereas it increased to 45% of TP at the 18–30 cm soil depth in Beaty soils (Table 3). Reddy et al. (1995) observed similar values of residual P (43–51% of TP) in wetland soils of Okeechobee Basin.

3.2 Column studies

Redox conditions during soil/water column studies

During the 60-days of antecedent hydrological conditions, which attempted to simulate different hydrological regimes in soils, redox values (which were measured at 5 cm below soil surfaces) were 114 ± 7, 132 ± 9 and 152 ± 7 mV for soils that were incubated at +20 cm, 0 cm and –5 cm water depths, respectively. This implies that all soils had a low reduction intensity; with all soils being below Fe reduction threshold, which is about 200 mV at pH 7 (D’Amore et al., 2004). During the two 28-day flooding periods, soil columns that had water levels drawn down to –5 cm below soil surfaces during antecedent hydrological conditions and were then flooded, had higher redox values (41 ± 10 mV) than columns that were previously flooded to 20 cm (–29 ± 16 mV) or saturated at soil surfaces (0 cm) (–17 ± 13 mV) (P < 0.05) with all soils in columns being in a reduced state.

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Phosphorus release and retention by soils of natural isolated wetlands 11

Table 3 Depth specific phosphorus fractions of Larson Dixie and Beaty isolated wetland sites ± 1 standard error (n = 21)

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12 E.J. Dunne, K.R. Reddy and M.W. Clark

Phosphorus flux from continuously flooded soils

There was generally an increase in water column SRP concentration during the 60-days of antecedent flooding at both P levels (P < 0.05; Figure 2). Average SRP release rates from soil to overlying water was highest from Larson Dixie columns initially spiked at 1 mg SRP l–1 (7 ± 1.9 mg m–2 d–1) and lowest from Beaty columns (1 ± 1.5 mg m–2 d–1) at the same P level. Flooded Beaty columns had similar P release rates at both P levels. These findings suggest that site soil characteristics such as greater labile P, inorganic P and TP in Larson Dixie soils relative to Beaty soils had an increasing effect on P release rates. It is well established that site soil characteristics such as those described are an important controlling factor in P dynamics within wetland soils (Richardson, 1985; Reddy et al., 1998; Bridgham et al., 2001; Pant and Reddy, 2003). In addition to previous nutrient loading, redox potential is an important factor in governing P dynamics between soil and water (Patrick and Khalid, 1974) and it also plays an important role in the overall biogeochemistry of flooded soils (Gotoh and Patrick, 1974). All soils had a redox potential value below 200 mV; therefore this may indicate suitable conditions for the reduction of Fe from ferric (Fe3+) to ferrous (Fe2+) forms. In comparison to other systems around the world, isolated wetland soils released similar amounts of SRP relative to flooded agricultural soils in other areas of the Okeechobee Basin (Pant and Reddy, 2003) and some Everglade studies (Fisher and Reddy, 2001). van der Moel and Boers (1994) found that shallow freshwater lakes within the Netherlands had somewhat comparable P release rates. They ranged between 4 mg m–2 d–1 and 57 mg m–2 d–1. The Norfolk Broads, which is a wetland complex in south east England had the potential to release much greater amounts between 16 mg m–2 d–1 and 278 mg m–2 d–1 (Phillips et al., 1994), while a flux study of shallow lakes in Denmark, indicated that sediments released nearly 200 mg m–2 d–1 (Sondergaard, 1989). Changes in water column P during flooding periods

Mean water column P independent of soil and initial SRP spike decreased from 170 ± 7 mg m–2 during the first 28-day flooding period to 106 ± 4 mg m–2 during the second 28-day flooding period. Under high P levels, as number of flooding periods increased, the standing mass of P in the water column decreased. Greatest decreases were observed in both site columns that were previously saturated and dry (Figure 3). Previously flooded soils had a smaller decrease, possibly due to source depletion of soluble P (Pant and Reddy, 2003); as previous to this, flooded columns underwent a 60-day flooding, of which, P was fluxed from soil to water. At low P levels, water column P tended to both increase and decrease, with an increase in flooding period (Figure 4). No obvious relationship existed at these low P levels (0.1 mg SRP l–1).

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Phosphorus release and retention by soils of natural isolated wetlands 13

Figure 2 Change in water column SRP concentration during 60-days of flooding. (a) Larson Dixie columns spiked at 0.1 mg l–1; (b) Beaty columns spiked at 0.1 mg l–1; (c) Larson Dixie columns spiked at 1 mg l–1 and (d) Beaty columns spiked at 1 mg l–1

Figure 3 Boxplots of water column SRP content (mg m–2) of flooded (+20 cm), saturated (0 cm) and drawn down (–5 cm) soil/water columns during the two flooding periods (T1) 28-days and (T2) an additional 28-days for (a) Larson and (b) Beaty soils with overlying water spiked at 1 mg SRP l–1

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14 E.J. Dunne, K.R. Reddy and M.W. Clark

Figure 4 Boxplots of water column SRP content (mg m–2) of flooded (+20 cm), saturated (0 cm) and drawn down (–5 cm) soil/water columns during the two flooding periods (T1) 28-days and (T2) an additional 28-days for (a) Larson and (b) Beaty soils with overlying water spiked at 0.1 mg SRP l–1

Phosphorus release and retention by isolated wetland soils during flooding periods

During the first 28-day flooding period, there was large variability, with water columns initially spiked at 1 mg SRP l–1 tending to retain and/or release P (0.4 ± 2 mg m–2 d–1); whereas those at 0.1 mg SRP l–1 all released P (4.7 ± 1 mg m–2 d–1). This suggests that lower SRP concentrations of floodwater may facilitate P release from soil to water, as stores of P are greater in soil relative to overlying waters, causing disequilibria (Diaz et al., 1994; Reddy et al., 1995, 1999). Similarly, Pant and Reddy (2003) found that initial SRP concentrations of floodwater less than 0.2 mg SRP l–1 had negative impacts on P release from soils to water. Phosphorus can flux from soil to overlying water by processes that include diffusion and mass flow (Reddy et al., 2005).

With regard to site specific effects, Larson Dixie soils released P at higher rates than Beaty soils (Tables 4 and 5), as Larson Dixie soils had a higher nutrient status (Table 1) and more labile P fractions (Table 3) than Beaty soils. Pre-flooded (+20 cm) Larson Dixie columns initially spiked at 0.1 mg SRP l–1, had lower P release rates than pre-saturated (0 cm) and pre-drawn down soils (–5 cm) of the same site (P < 0.05; Table 4). Thus, it appears that at low P levels and at this particular site, antecedent hydrological conditions were affecting P release rates. However, in Beaty columns spiked at 0.1 mg SRP l–1, there was similarity between P release rates by soils, which underwent

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Phosphorus release and retention by soils of natural isolated wetlands 15

different antecedent hydrological conditions (Table 4). These results imply that depending on site-specific soil characteristics, antecedent soil hydrological conditions may affect P release rates. Pre-flooded Larson Dixie and Beaty columns spiked at 1 mg SRP l–1, had similar P retention rates. Thus, antecedent conditions and P concentration in overlying floodwaters appear to have a combined effect on P retention at this higher P concentration, with site characteristics having little impact. However, P release rates were similar between pre-saturated and pre-drawn down Beaty soils, during the first 28-day flooding period (Table 5).

During the second 28-day flooding period, soils with overlying floodwaters initially spiked at 1 mg l–1 SRP generally retained P (Table 5). Pre-drawn down Larson Dixie columns released P during the second 28-day flooding period, possibly an artefact of antecedent hydrological conditions and/or site soil characteristics, as Larson Dixie soils had highest initial nutrient status and pre-drawn down soils were the least flooded soils; therefore there was a larger soluble P pool to deplete, in comparison to pre-flooded columns. There was no difference in P release rates between Beaty columns spiked at 0.1 mg l–1 SRP (Table 5) implying that during the second 28-day flooding period, antecedent hydrological conditions had little impact on P release rates.

Table 4 Phosphorus parameters of isolated wetland soil/water columns initially spiked at 0.1 mg SRP l–1 ± one standard error

Antecedent hydrological conditions (cm) † Site

Flooding period‡

Retention/release (mg m–2 d–1) § EPCw (mg l–1)

+20 1 0.65 ±0.77 0.17 ±0.002 +20 2 –0.34 ±3.21 0.57 ±0.196

0 1 8.60 ±3.55 0.27 ±0.073 0 2 5.15 ±1.95 0.13 ±0.005

–5 1 10.25 ±2.21 0.37 ±0.019 –5

Larson

2 3.74 ±2.82 0.12 ±0.007 +20 1 0.71 ±1.34 0.18 ±0.019 +20 2 1.68 ±1.05 0.12 ±0.011

0 1 5.71 ±2.00 0.28 ±0.080 0 2 2.79 ±2.40 0.25 ±0.089

–5 1 2.44 ±1.67 0.27 ±0.027 –5

Beaty

2 2.88 ±2.11 0.27 ±0.015

†Antecedent soil hydrological conditions: +20 cm are soil columns, which were pre-flooded for 60-days; 0 cm are soil columns, which were pre-saturated for 60-days; and – 5 cm are soil columns, which were drawn down for 60-days.

‡Flooding period: 1 and 2 are the first and second 28-day flooding periods, respectively. §Retention/release: negative values indicate retention; positive values indicate release.

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16 E.J. Dunne, K.R. Reddy and M.W. Clark

Table 5 Phosphorus parameters of isolated wetland isolated wetland columns initially spiked at 1 mg SRP l–1 ± one standard error

Antecedent hydrological conditions (cm) † Site

Flooding period‡

Retention/release (mg m–2 d–1) § EPCw (mg l–1)

+20 1 –3.82 ±2.90 1.08 ±0.063 +20 2 –4.60 ±2.02 0.79 ±0.074

0 1 3.59 ±6.67 1.18 ±0.020 0 2 –5.08 ±4.82 0.97 ±0.006

–5 1 –1.75 ±6.90 1.30 ±0.210 –5

Larson

2 2.90 ±4.12 0.81 ±0.121 +20 1 –7.73 ±4.87 1.07 ±0.027 +20 2 –3.87 ±2.26 0.94 ±0.139

0 1 4.35 ±5.23 1.30 ±0.101 0 2 –3.15 ±2.14 0.89 ±0.052

–5 1 2.79 ±2.15 1.22 ±0.065 –5

Beaty

2 –1.60 ±3.22 0.84 ±0.086

†Antecedent soil hydrological conditions: +20 cm are soil columns, which were pre-flooded for 60-days; 0 cm are soil columns, which were pre-saturated for 60-days; and – 5 cm are soil columns, which were drawn down for 60-days.

‡Flooding period: 1 and 2 are the first and second 28-day flooding periods, respectively. §Retention/release: negative values indicate retention; positive values indicate release.

In general, P retention by isolated wetland soils varied from 0.3–8 mg SRP m–2 d–1. Similarly, P retention by other systems such as riparian forests, freshwater marshes and constructed wetland systems is reported to vary between 0.8 and 9 mg m–2 d–1 (Peterjohn and Correll, 1984; Craft and Richardson, 1993; Niswander, 1994; Mitsch et al., 1995). Methods used to determine these rates varied depending on researchers; however it is interesting that estimates have similar scales, irrespective of system type and method used.

Equilibrium phosphorus concentrations. Equilibrium P concentration of floodwaters (EPCw) is often used as an estimate of P concentration in wetland floodwaters, where P retention equals release (Reddy et al., 1995, 1999). It is analogous to equilibrium P concentration at zero sorption (EPC0) determined from sequential P sorption isotherm data, where adsorption equals desorption. As P concentrations within floodwaters decrease, EPCw and P retention also decrease (Diaz et al., 1994). Regression analysis of our data indicates that EPCw values were largely controlled by water column P concentrations (R2 = 0.62; P < 0.05; n = 61) rather than site soil characteristics. In theory, if incoming wetland waters have higher SRP concentrations than wetland EPCw values determined (Tables 4 and 5), wetland soils may act as a sink for P, whereas if incoming waters have lower SRP concentrations, soils may act as a source (Pant and Reddy, 2003; Reddy et al., 2005). Generally, EPCw values ranged from 0.12 to 1.3 mg l–1 SRP during the two 28-day flooding periods, with columns initially spiked at 0.1 mg l–1 SRP having lower EPCw values than columns spiked at 1 mg l–1 SRP (P < 0.05). The range of EPCw values in this study were similar to those determined by Reddy et al. (1995) for wetlands within the Lower Kissimmee River Basin, Florida. Pant and Reddy (2003) found that

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Phosphorus release and retention by soils of natural isolated wetlands 17

EPCw values for a range of flooded agricultural soils that were previously under different management practices in Okeechobee Basin varied from 1.3 to 3.4 mg l–1 SRP with land use being an important factor. Equilibrium P concentration at zero sorption (EPC0), is an estimate of the P concentration maintained in a solution when rates of adsorption equal desorption (Pierzynski et al., 1994). Young and Ross (2001) found that EPC0 values (which are somewhat comparable to EPCw) ranged from 0.02 to 7.2 mg l–1 for seasonally flooded wetland and agricultural soils from the Lake Champlain Basin, New York.

4 Conclusion

Our findings indicate that labile and inorganic P fractions increased as a percentage of TP with soil depth in isolated wetland soils. Humic/fulvic acid bound P and residual P fractions accounted for the majority of TP in surface isolated wetland soils. We found that isolated wetland soils with a higher nutrient status and higher labile fractions of soil P (site soil characteristics) fluxed P to overlying waters at highest rates during 60-days of continuous flooding.

During an additional 28-day flooding period, soils with overlying water spiked at 1 mg SRP l–1 retained and/or released P, whereas those spiked at 0.1 mg SRP l–1 generally released P. Pre-flooded Larson Dixie columns spiked at 0.1 mg SRP l–1 had lower P release rates than pre-saturated and pre-drawn down soils of the same site, suggesting an effect of antecedent hydrological conditions. However, in Beaty columns spiked at 0.1 mg SRP l–1; P release rates from soils were similar. Pre-flooded columns from the different sites spiked at 1 mg SRP l–1, had similar P retention rates suggesting that initial site soil characteristics were of little importance during this flooding period. During the second 28-day flooding period, soils with overlying floodwaters initially spiked at 1 mg l–1 SRP generally retained P, indicating that during the second 28-day flooding period, antecedent hydrological conditions had little impact on P dynamics with P levels in overlying waters governing P dynamics.

The practical implications of this work indicate that increasing an isolated wetland system’s flooding periods by hydrological restoration for the purposes of P storage and retention, should take into account short-term effects of site-soil P characteristics and antecedent soil hydrological conditions. If soils in isolated wetland ecosystems are saturated or water levels are below soils surfaces and are then flooded due to heavy rainfall events, or during seasonal changes from a dry to a wet season, it may be necessary to store wetland waters for P retention by ditch management. Alternatively, if soils are previously flooded (going from wet season to dry season) it maybe appropriate to allow water flow from isolated wetland to ditch, by management of wetland water levels to provide additional landscape storage. During wetland flooded periods, the P retained within the system may be governed (in part) by the P load in the overlying water column. Further experimentation is required at the isolated wetland ecosystem-scale to determine if the rates of P release and retention we observed between soil and water are present at the wetland-scale such that appropriately scaled management options can be evaluated.

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18 E.J. Dunne, K.R. Reddy and M.W. Clark

Acknowledgements

This research was funded in part by the Florida Department of Agriculture and Consumer Services. The other part was funded by the USA/Ireland Co-operation Programme in Agricultural Science and Technology. The authors acknowledge the help of Wetland Biogeochemistry Laboratory staff Ms. Yu Wang, Mr. Ron Elliot, Mr. Gavin Wilson and Mr. Charles Campbell.

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