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Division S-10—Wetland Soils

The Geochemistry of Phosphorus in Peat Soils of a Semiarid Altered Wetland

^a Dep. of Biotechnology and Environmental Sciences, Tel-Hai College, Upper Galilee 12210, Israel
^b Dep. of Soil Science, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, Rechovot 76100, Israel
^c Dep. of Grassland Science, Technol. Univ. Muenchen, Am Hochanger 1, D-85350 Freising-Weihenstephan, Germany

^* Corresponding author (litaori{at}telhai.ac.il)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
An understanding of P transformations in altered wetlands has mainly developed from temperate and humid regions with neutral to acidic soils. Little is known regarding downstream water quality impact of P transformations in semiarid wetlands that undergone repeated cycles of drying and rewetting. The P geochemistry was studied using the Hedley fractionation scheme in the altered peat soils of the Hula Valley, Israel. The peat soils were sampled according to the peat depth and redox potential characteristics. The mean total P concentration (P_t) in the surface peat horizons (1190 ± 300 mg kg^–1) was significantly higher than in the anaerobic (E_H < –220 mV) peat layers (650 ± 260 mg kg^–1). The concentrations of P in all fractions except the most labile P were significantly higher in the aerobic (E_H = 400 mV) peat layers. The predominant extractable fractions in the surface peat horizons were Ca-P extracted by 1 M HCl (21–60% of P_t), Iron-P extracted by 0.5 M NaOH (11–41% of P_t), and residual P extracted by H₂SO₄ (20–40% of P_t). The source of the Ca in the Ca-P fraction was mainly from gypsum dissolution following the rewetting cycles. The results clearly showed that the drainage of these wetlands facilitated rapid organic matter (OM) oxidation, release of organically bound metals and P followed by sesquioxides and gypsum precipitation. These geochemical transformations enhanced the P_t concentration per mass of altered peat soil and changed the P distribution among the different pools. Most of the P is currently associated with Fe oxides and hydroxides and/or coprecipitated with Ca.

Abbreviations: CBD, citrate-bicarbonate-dithionite • EC, electrical conductivity • OM, organic matter • P_i, inorganic phosphorus • P_o, organic phosphorus • P_t, total phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
FRESH WATER WETLANDS are nutrient sinks that efficiently process and store N and P, thus reducing the potential of eutrophication in downstream lakes (Richardson, 1985; Sharpley and Rekolainen, 1997). The draining of Lake Hula and the elimination of its surrounding swamps, about 20 km upstream from the Jordan River inflow to Lake Kinneret, to increase the arable land in northern Israel during the mid 1950s, removed this crucial nutrient sink. Consequently, increased suspended material and nitrate loading into Lake Kinneret from the Hula Basin via the Jordan River were observed (e.g., Serruya et al., 1969; Stiller, 1979; Inbar, 1982). The biogeochemistry of C and N in the peat soils of the Hula Valley has received much attention in past years (Avnimelch et al., 1978; Brenner et al., 1978) resulting in more precise agricultural practices that reduced nitrate seepage from these soils. However, although P is more important in regulating algal growth in downstream Lake Kinneret (Serruya and Berman, 1976), the only freshwater lake in Israel, which provides 25% of the country's drinking water, relatively little is known on the geochemistry of P in these peat soils.

Other effects of the drainage of the Hula Wetland were rapid oxidation of OM and continuous internal conflagration of the oxidized peat soils, leading to subsidence (approximately 2 m below the original surface) that induced partial reflooding of the drained area. To reverse some of the negative consequences of the drainage of Lake Hula and the surrounding wetlands, a small 100-ha lake (Agmon) was engineered in 1994, covering the least agriculturally productive peat soils in the Hula Valley (Fig. 1) . The creation of the new lake and the elevation of the water table in the center of the Hula Valley by at least 60 cm (Tsipris and Meron, 1998) have transformed the relatively dry oxidized peat soils to wet and anaerobic soil environs. Current management protocol for the valley calls for relatively high ground water level, which ensured that the rewetting of the oxidized peat soils would continue indefinitely. This rewetting practice may increase P discharge into Lake Kinneret, which in turn may enhance eutrophication. Hence, an understanding of the dynamics of P in the altered peat soils of the Hula Valley is required to improve management strategies for the upper catchment of the Jordan River.

View larger version (60K): [in this window] [in a new window]   Fig. 1. The study sites in the Hula Valley showing sampling locations, real-time in situ monitoring stations, Lake Agmon and the drainage canals. The location of the former Hula Lake and swamps (drained) are shown in the middle map. Coordinates are given in meters.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Site
The Hula Valley is the northernmost segment of the Jordan-Arava Rift Valley, and is approximately 70 m above sea level. The Hula Valley measures approximately 175 km² and is currently drained by a system of artificial canals, which empty into the Jordan River at the southern end of the valley. The average thickness of the surface peat layer is 4 to 6 m. The peat soils of the Hula Valley are predominantly Histosols (approximately 1860 ha). Further classification on the basis of decomposition and the occurrence and quantity of CaCO₃ identifies them as Medifibrists, Medihemists, Medisaprists, and ‘Conflagrated Histosols’ without lime, with minimal lime, and with lime (Department of Agriculture, 1986).

On the basis of this soil classification and mapping, we divided the study area into two regions of deep peat soils (>4 m) located mostly north of Lake Agmon and shallow peat soils (<1 m) located adjacent to Lake Agmon (Fig. 1). Three soil cores were hand augured to a depth of 1 to 2 m from the deep peat area (DP-2, DP-12A, and DP-12I), and three from the shallow peat area (SP-8, SP-9, and SP-10). Each intact soil core was divided into 20-cm increments or according to diagnostic layers observed in the field. Hand auguring in the deep peat soils could not be done to a depth >2 m, hence three pits were excavated with a backhoe to a depth of 4 m (approximately 2 m below the regional water table, all at Site BP-11, each 10 m apart) and two soil samples were sampled from each pit at a depth of 3.5 m below the surface. These deep soil samples represent highly reduced peat environs (E_H < –220 mV) that were never altered by the drainage of the swamps and closely mimic the soil redox potential near the surface before the drainage and onset of massive oxidation. The plant tissue and leaf structure of the typical plants that grew in the Hula swamps were easily discernible in these soil samples whereas no plant tissue could be seen with the naked eye in the peat samples collected in the top 1 m. The assumption that the deep soil samples are reasonable proxy to the original surface peat soils was based on the similarities in OM content, degree of relative decomposition of the plant material, amount of CaCO₃ and the redox potential. This assumption is strongly supported by earlier work of Ravikovitch (1945) and Schallinger and Ravikovitch (1962), who suggested that the Hula peat soils were formed by slow decomposition of plant material under mostly anaerobic conditions and the rate of humification of these soils was quite low and plants residue and structure was quite visible within the soils. The soil samples were kept at –15°C until chemical analysis.

General Soil Characterization
Soil pH was determined in 1:10 soil/water suspensions; OM content was determined by the dry combustion method (Nelson and Sommers, 1982), while the CaCO₃ content was determined by a calcimeter apparatus (Nelson, 1982). Soil water content was determined using the gravimetric method with oven drying as described by Topp (1993) and the water-holding capacity was calculated as the difference between the water content at saturation and at complete dryness. Because Fe oxides and hydroxides are the most abundant noncrystalline and crystalline oxides phases in the study area (Litaor et al., 2003), the extractable Fe, Al, and Mn oxides were determined by the citrate-bicarbonate-dithionite (CBD) method described by Jackson et al. (1986). Extractable Al in selected samples was also determined using the ammonium oxalate method described by Ross and Wang (1993) but no significant differences were observed between the two methods. Hence, in the present work only the CBD results were reported. The extractable Fe, Mn, and Al were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES; Spectro, Germany).

Phosphorus Fractionation
We used a modification of Hedley et al.'s (1982) and Tiessen and Moir's (1993) procedure to extract empirically defined pools of P. This method has been widely used to determine bioavailability of P and general pedogenesis (Tiessen et al., 1984; Sharpley et al., 1994; Cross and Schlesinger, 1995), as well as for water-quality issues (Sallade and Sims, 1997a, 1997b). More recently, the suitability of this method for distinguishing labile P and organic P (P_o) fractions in Eutric Histosols and Gleysols was reported (Schlichting et al., 2002). In general, the Hedley sequential extraction procedure removes inorganic and organic P of increasing chemical stability, which can also be interpreted as affinity to various solid-phase soil fractions. The sums of water- and bicarbonate-extractable P_i fractions, which are the most labile P_i forms, represent the water-soluble and moderately sorbed P_i fractions, respectively, that are most available to plants. The NaOH-extracted P_i, which is associated with Fe, Mn, and Al oxides, is considered to be a strongly sorbed P fraction. The 1 M HCl-extracted P_i represents the Ca-P coprecipitated fraction, since Fe- and Al-sorbed P were already removed by the NaOH extraction, whereas the most resistant species to the above extractions and the least available is the residual P removed by boiling in a concentrated H₂SO₄–H₂O₂ mixture.

A 0.5-g sample of wet soil was placed in a 50-mL centrifuge tube and than sequentially extracted with 30 mL each of deionized water, 0.5 M NaHCO₃ (pH 8.5), 0.1 M NaOH and 1 M HCl, to remove water-soluble P, weakly adsorbed P, Fe/Al-P, and Ca-P, respectively. After each extraction, the samples were centrifuged at 17000 x g for 20 min at 0°C, and the supernatant was passed through a 0.45-µm filter. After the final extraction, the residual P in the soil material left in the tubes was determined by extraction with a boiling concentrated H₂SO₄–H₂O₂ mixture. Aliquots from the filtered NaHCO₃ and NaOH extracts were further digested in an autoclave at 103.5 kPa and 121°C with ammonium persulfate to convert all the dissolved P into orthophosphate, to be determined as P_t following the procedure described by Bowman (1989). The P_i and P_t in all the extracts were determined colorimetrically using the method of Murphy and Riley (1962). Organic P was calculated as the difference between P_t and Pi (P_t – P_i). All laboratory analyses were done in triplicate.

Modifications of Hedley's procedure included acidification of the NaHCO₃ and NaOH extracts with 0.9 M H₂SO₄ and centrifugation for 10 min to precipitate the OM before adding the acidic ascorbic ammonium molybdate reagent. In recent work, Marko et al. (1999) made similar modifications to Hedley's method with no reported loss of P from the solution. Indeed, in a series of standard tests involving the addition of P to the acidified extracts of 1M NaOH, Qualls and Richardson (1995) found no P precipitation occurring because of this treatment.

In the present work, we were more concerned with the overestimation of P_o due to the acidification of these extracts, but currently there are no analytical means to quantify the possibly erroneously determined P_o (P_o = P_t – P_i). We further modified Hedley's and Tiessen and Moir's (1993) methods by not using anion-exchange resin to quantify the most labile P in the soil. We assumed that the P in the H₂O extract better represents the mobile P than the resin-exchange P and that it may serve as a better environmental soil-P test as shown by Sharpley et al. (1996) for determining the dissolved P in runoff from an agricultural field. A similar approach was adapted by Sui et al. (1999) in studying fractionated P in soils amended with biosolids.

Ground Water and Soil Solutions
Following the rise of the water table, a program of ground water monitoring was established to periodically examine the level of P and other pertinent chemical constituents. During the last 6 yr over 100 ground water samples and soil solutions were collected from shallow observation wells (0.5- to 5-m depth) located adjacent to the soil sites. The water samples were immediately filtered (0.45 µm), kept in an iced cooler till the end of the sampling day and then transferred to the laboratory. The dissolved P_i and P_t concentrations were determined using the colorimetric standard method of Murphy and Riley (1962), as already described. The electrical conductivity (EC), temperature, and pH were measured in the field. The concentrations of major cations (Ca, Mg, Na, K) and S were determined by ICP-AES, alkalinity was determined by titration to an endpoint pH of 4.5, and Cl concentration was determined potentiometrically.

To determine the redox potential in the peat soils, we established two automated in situ redox stations installed in deep peat soil (at Site DP-12I) and shallow peat (at Site SP-10, see Fig. 1). The stations consist of pH and E_H electrodes (pH combined electrodes for semisolids, model U-05998-20, and ORP combined electrodes with Pt band, model U-05990-55, Cole-Parmer, IL) that were installed at three depths (above and below the water table) at each site. The electrodes were inserted into PVC sleeves that fit snuggly around the electrode body for protection, leaving the very tip of the electrodes exposed to the soil and interstitial waters. Each electrode calibration was performed after the electrode had been inserted into its protective sleeve. The E_H electrodes were calibrated with standard pH buffer solutions mixed with quinhydrone (0.2 g 100 mL^–1). The electrodes were connected to a data logger (Multilog, Fourier Systems, Inc., Israel, model DB-526) equipped with internal batteries and recharged by an external solar panel.

Data Analysis
Data analyses were conducted using SPSS/PC version 11. Empirical relationships between the various P fractions and soil properties were established using Pearson correlation and multiple stepwise regression analysis. Differences between groups of soils according to location and geochemical conditions were tested using nonparametric tests such as Kolmogorov-Smirnov Z because the data of the different groups of soils were not normally distributed with equal variances. The saturation indices of solutions with respect to various solid phases were computed with the widely used geochemical model PHREEQC, ver. 2, software (Parkhurst and Appelo, 1999) and its PHREEQC.DAT thermodynamic database, which was extended to include additional Ca-P minerals according to stability constants from Lindsay (1979), and Ca-bicarbonate-P minerals (CBP) from Salingar et al. (1993).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
General Soil Characteristics
Significant differences were observed between the physicochemical attributes of the altered surface peat layers and those in the unaltered subsurface peat layers (Table 1). Surface peat soils exhibited an average OM content of 42.8% compared with 64% observed in the anaerobic deep peat layers (Fig. 2) . None of the surface peat soils showed the fibric or hemic decomposition stages reported in an earlier soil survey (Department of Agriculture, 1986) while the fibric character of the anaerobic subsurface peat horizons was clearly evident in the field. A steady decline in OM content has been observed in the cultivated surface peat layers over the years. Before the drainage of the Hula swamps, the peat soil exhibited an OM content of 50 to 70%, whereas a decade after the drainage in 1970 these soils contained 30 to 50%, and 25 to 35% in 1985 (Litaor et al., 2003). The surface peat soils of the present study were collected in 1998 and 2002, hence the amount of OM content in the surface soils of the present study suggest that the decomposition rate has slowed considerably, partially because of management practices such as raising the ground water level around Lake Agmon and intensive wetting of the surface soils before cultivation. The amount of OM in the anaerobic subsurface peat horizons supported our working notion that the peat samples collected at depth of 3 m and more are quite similar to the original surface layers before the drainage.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Comparison between surface (0–130 cm) and subsurface peat soils (>130 cm) using organic matter (OM) (%) and sesquioxides (g kg–1) contents. The circle and asterisks are outlier and extreme value, respectively.

The most obvious difference between the highly oxidized surface peat soils and the barely decomposed anaerobic layers found at depths of 150 to 350 cm is the capacity to hold water (Table 1). The hydrophilic nature of the deep peat layers is clearly demonstrated by their large water-holding capacity (370 ± 106%), compared with the more mineralized surface peat layers (84 ± 40%) that undergo successive drying and rewetting cycles every season. This phenomenon was clearly demonstrated by Giskin and Levin (1978) who studied the decrease in moisture-retention curves and saturation percentages of the surface peat soils of the Hula Valley two decades after the drainage, using three consecutive drying and rewetting cycles.

The amount of CaCO₃ varied greatly in all the peat soils, from zero to more than 25% (Table 1). No significant differences were observed in CaCO₃ content between surface and subsurface peat layers because the altered wetlands of the Hula Valley receive ground water enriched with Ca²⁺ and HCO^–₃ originating from the karstic calcareous mountainous terrain surrounding the valley.

The shallow peat soils (SP-8, SP-9, and 10) are located within a pedogenic transitional area that separated the former Lake Hula from the swamps to the north (Fig. 1). The spatial boundaries of this area are quite fuzzy because of seasonal migration of the swamp/lake boundaries which produced two distinct parent-materials; the plant residue of the former swamps and the calcareous marl of the former lake. Hence, some of the soils in this area (e.g., SP-10) are characterized by highly calcareous horizons (>60%) at shallow depth (60 cm), that was originated from the limnitic marl parent material covered with altered peat layers from the old swamps, which exhibited high OM content (Table 1). Some of the layers in the shallow peat soils exhibiting high OM content (>60%) were quite acidic, reminiscent of the acidic conditions that prevailed in the swamps before the drainage. A sharp increase in pH from 5.6 to higher than 7 was observed (e.g., SP-10) across the depositional boundary between the altered peat horizons and the limnic calcareous layers of the old Lake Hula.

Phosphorus Fractionation
In general, the P_t concentrations in all surface peat soils decreased with depth (Table 2). From this P_t, the smallest fraction of P was extracted by distilled water (<0.1%) in the peat layers and <1% in the limnitic marl layers at the bottom of SP-10. Because the concentrations of this fraction were so insignificant, we extracted it from only a small but representative subset of the soil samples. The surface peat layers exhibited relatively low concentrations of the moderately sorbed P extracted by NaHCO₃ (<10%). Similar results were observed by Gephen et al. (1985) in their assessment of P availability to typical crops for this area, which led to the recommendations for high P fertilization rates, which have been practiced by the farmers from the mid 1980s to the late 1990s (20–50 kg ha^–1 of P annually).

View this table: [in this window] [in a new window]   Table 2. Concentrations of sequentially extracted P in surface and subsurface peat layers. Because of large similarity and close proximity between SP-8 and 9 only SP-9 was analyzed.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Saturation index (SI) of calcite and gypsum in soil interstitial water and shallow ground water indicated that most water samples were unsaturated with respect to gypsum but saturated with respect to calcite.

Comparison between Aerobic and Anaerobic Peat Layers
To test the hypothesis that oxidation of the peat soils increased the sesquioxide content followed by adsorption of P by Fe oxides and hydroxides, we compared the P-fractionation results in the oxidized surface peat soils (E_H > 400 mV) with the anaerobic subsurface peat layers (E_H < –220 mV) (Fig. 4) . As expected, there was no significant difference in the concentrations of labile P in these two soil environs, as represented by the NaHCO₃–extracted P fractions, even though P_t was significantly higher in the surface soils. The higher P_t concentration is attributed to the high-affinity sorption of P to the fresh amorphic precipitates of ferric oxides and hydroxides formed during peat oxidation. On the other hand, there was a significant increase in the surface soils at all levels of confidence in the least labile and recalcitrant P fractions in these two soil environs (Fig. 4). For example, the mean NaOH-P_i fraction in the oxidized surface soils was 177 ± 84 vs. 26 ± 13 mg kg^–1 in the anaerobic subsurface peat layers. Similar results were obtained for the NaOH-P_o and residual P fractions (Fig. 4). These results support our notion that OM oxidation followed by Fe transformation strongly affected the P pool size as well as its availability. The increase of the Ca-P pool size in the oxidized surface layers compared with the anaerobic subsurface layers was also statistically significant (Fig. 4). However, some subsurface samples exhibited surprisingly high Ca-P levels, attesting to the high natural heterogeneity of the peat soils. This increase may reflect stabilization and fixation of percolating P that was released from reductive-dissolved ferric hydroxides by gypsum- or calcite-originated Ca in the subsurface soils. Similar high natural variability was evident in the CaCO₃ contents observed in the subsurface peat layers (see Table 1).

View larger version (16K): [in this window] [in a new window]   Fig. 4. A comparison between the concentrations of the various P pools extracted from surface (0–130 cm) and subsurface peat soils (>130 cm). NS = not significant; * is P < 0.01 and ** is P < 0.001.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	ACKNOWLEDGMENTS

 
This research was supported in part by the EU project PROWATER, EVK1-CT1999-00036 and in part by a grant (GLOWA- Jordan River) from the Israeli Ministry of Science and Technology; and the German Bundesministerium fuer Bildung und Forschung (BMBF).

Received for publication October 28, 2003.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Avnimelch, Y., S. Dasberg, A. Harpaz, and I. Levin. 1978. Prevention of nitrate leakage from the Hula Basin, Israel: A case study in watershed management. Soil Sci. 125:233–239.
• Bowman, R.A. 1989. A sequential extraction procedure with concentrated sulphuric acid and dilute base for soil organic phosphorus. Soil Sci. Soc. Am. J. 53:362–366.[Abstract/Free Full Text]
• Brenner, S., R. Ikan, A. Nicki, D. Argon, and A. Niessenbaum. 1978. Hula Valley peat: Review of chemical and geochemical aspects. Soil Sci. 125:226–232.
• Cross, A.F., and W.H. Schlesinger. 1995. A literature review and evaluation of the Hedley fractionation: Application to the biogeochemical cycle of soil phosphorus in natural ecosystems. Geoderma 64:197–214.
• Department of Agriculture. 1986. The Hula: A soil survey report. (In Hebrew.) Ministry of Agriculture/Water Commissioner, the Department of Soil Conservation and Drainage, Lake Kinneret Authority, Lake Kinneret, Israel.
• Gephen, U., I. Levin, and A. Shlomi. 1985. Improvement in crop yield in peat soils of the Hula Valley by adding phosphorous fertilizer. (In Hebrew.) 5th series, No 1655. The Agriculture Research Administration, Kiryiat Shemona.
• Giskin, M., and I. Levin. 1978. Alternate drying and rewetting effects on chemical and physical properties and moisture salinity relationships of a Histosol. Agron. J. 70:445–447.[Abstract/Free Full Text]
• Hedley, M.J., J.W.B. Stewart, and B.S. Chauhan. 1982. Changes in inorganic and organic soil phosphorous fractions induced by cultivation practices and laboratory incubations. Soil Sci. Soc. Am. J. 46:970–976.[Abstract/Free Full Text]
• Inbar, M. 1982. Spatial and temporal aspects of man-induced changes in the hydrological and sedimentological regime of the upper Jordan River. Isr. J. Earth Sci. 31:53–66.
• Jackson, M.L., C.H. Lim, and L.W. Zelazny. 1986. Oxides, hydroxides and aluminosilicates. p. 101–150. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. No. 9. ASA and SSSA, Madison, WI,
• Lindsay, W.L. 1979. Chemical equilibrium in soils. John Wiley & Sons, New York.
• Lindsay, W.L., P.L. Vlek, and S.H. Chien. 1989. Phosphate minerals. p. 1089–1130. In J.B. Dixon and S.B. Weed (ed.) Minerals in soil environments. 2nd ed. SSSA, Madison, WI.
• Litaor, M.I., O. Reichmann, M. Beltzer, K. Auerswald, A. Nishri, and M. Shenker. 2003. Spatial analysis of phosphorus sorption capacity in a semi-arid altered wetland. J. Environ. Qual. 32:335–343.[Abstract/Free Full Text]
• Marko, J.B., R.J. Buresh, and P.C. Smithson. 1999. Soil phosphorus fractions in unfertilized fallow-maize systems on two tropical soils. Soil Sci. Soc. Am. J. 63:320–326.[Abstract/Free Full Text]
• Murphy, J., and J.P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27:31–36.
• Nelson, R.E. 1982. Carbonate and gypsum. p. 159–164. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. 2nd ed. Agron. Monogr. No. 9. ASA and SSSA, Madison, WI.
• Nelson, D.W., and L.E. Sommers. 1982. Total carbon, organic carbon, and organic matter. p. 539–577. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. 2nd ed. Agron. Monogr. No. 9. ASA and SSSA, Madison, WI.
• Parkhurst, D.L., and C.A.J. Appelo. 1999. User's guide to PHREEQC (version 2)—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. U.S. Dep. of the Interior, U.S. Geological Survey, Water-Resources Investigations Report 99–4259. U.S. Gov. Print. Office, Washington, DC.
• Qualls, R.G., and C.J. Richardson. 1995. Forms of soil phosphorus along a nutrient enrichment gradient in the northern Everglades. Soil Sci. 160:183–198.
• Ravikovitch, S. 1945. The peat soils and enriched organic matter soils of the Hula Valley. (In Hebrew.) Agric. Rep. 1:23–54.
• Richardson, C.J. 1985. Mechanism controlling phosphorus retention capacity in freshwater wetlands. Science 228:1424–1426.[Abstract/Free Full Text]
• Ross, G.W., and C. Wang. 1993. Extractable Al, Fe, Mn, and Si. p. 239–246. In M.R. Carter (ed.) Soil sampling and methods of analysis. Can. Soc. Soil Sci. Lewis Publishers, London.
• Salingar, Y., Y. Geifman, and M. Aronowich. 1993. Orthophosphate and calcium carbonate solubilities in the upper Jordan watershed basin. J. Environ. Qual. 22:672–677.[Abstract/Free Full Text]
• Sallade, Y.E., and J.T. Sims. 1997a. Phosphorus transformations in the sediments of Delaware's agricultural drainageways. I. Phosphorus forms and sorption. J. Environ. Qual. 26:1571–1579.[Abstract/Free Full Text]
• Sallade, Y.E., and J.T. Sims. 1997b. Phosphorus transformations in the sediments of Delaware's agricultural drainageways. II. Effect of reducing conditions on phosphorus release. J. Environ. Qual. 26:1579–1588.[Abstract/Free Full Text]
• Schallinger, K.M. and S. Ravikovitch. 1962. Organic soils in the Hula Valley, occurrence, classes, and properties. Bull. Res. Council of Israel. Section 9. 11G:163–164.
• Schlichting, A., P. Leinweber, R. Meissner, and M. Altermann. 2002. Sequentially extracted phosphorus fractions in peat-derived soils. J. Plant Nutr. Soil Sci. 165:290–298.
• Serruya, C., and T. Berman. 1976. Phosphorus, nitrogen and the growth of algae in Lake Kinneret. J. Phycol. 11:155–162.[ISI]
• Serruya, C., S. Serruya, and T. Berman. 1969. Preliminary observations on the hydromechanics nutrient cycles and eutrophication status of Lake Kinneret. Verh. Int. Ver. Theor. Angew. Limnol. 17:342–351.
• Sharpley, A.N., and S. Rekolainen. 1997. Phosphorus in agriculture and its environmental implications. p. 1–53. In H. Tunney et al. (ed.) Phosphorus loss from soil to water. CAB International. Wallingford, UK.
• Sharpley, A.N., S.C. Chapra, R. Wedepohl, J.T. Sims, T.C. Daniel, and K.R. Reddy. 1994. Managing agricultural phosphorus for protection of surface waters: Issues and options. J. Environ. Qual. 23:437–451.[Abstract/Free Full Text]
• Sharpley, A.N., T.C. Daniel, J.T. Sims, and D.H. Pote. 1996. Determining environmentally sound soil phosphorus levels. J. Soil Water Conserv. 51:160–169.
• Stiller, M. 1979. The influence of Lake Hula drainage on the 14C record and on the sedimentation rate in lake Kinneret. Isr. J. Earth Sci. 28:1–12.
• Sui, Y., M.L. Thompson, and C. Shang. 1999. Fractionation of phosphorus in Mollisol amended with biosolids. Soil Sci. Soc. Am. J. 63:1174–1180.[Abstract/Free Full Text]
• Tiessen, H., and J.O. Moir. 1993. Characterization of available phosphorus by sequential extraction. p. 75–86. In M.R. Carter (ed.), Soil sampling and method of analysis. Can. Soc. Soil Sci., Lewis Publishers, Ann Arbor, MI.
• Tiessen, H., J.W.B. Stewart, and C.V. Cole. 1984. Pathways of phosphorus transformations in soils of differing pedogenesis. Soil Sci. Soc. Am. J. 48:853–858.[Abstract/Free Full Text]
• Topp, G.C. 1993. Soil water content. p. 541–557. In M.R. Carter (ed.), Soil sampling and method of analysis. Can. Soc. Soil Sci., Lewis Publishers, Ann Arbor, MI.
• Tsipris, J., and M. Meron. 1998. Climatic and hydrological aspects of the Hula restoration project. Wetland Ecol. Manage. 6:91–100.

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