Measurement of Exchangeable Inorganic Phosphate in Lake Sediments Wan C. Li and David E. Armstrong’ Water Chemistry Laboratory, University of Wisconsin, Madison, Wis. 53706

Robin F. Harris Department of Soil Science, University of Wisconsin, Madison, Wis. 53706

H The amounts of isotopically exchangeable inorganic phosphate in a range of Wisconsin lake sediments were measured in two contrasting (long- and short-term) equili- bration systems. The systems differed in equilibration times, oxidation-reduction conditions, and the degree of agitation. Levels of total exchangeable inorganic P were similar for the two systems, indicating that the simplified short-term equilibration method was suitable for routine measurements of exchangeable inorganic P. Differences between the two systems in the distribution of exchange- able P between the solid and solution phases were appar- ently related to differences in oxidation-reduction condi- tions. Exchangeable inorganic P ranged from 18-65% of the total inorganic P in the sediments investigated.

The role of phosphorus in lake eutrophication is of widespread concern because of the importance of P as an essential and often limiting plant nutrient. Lake eutrophi- cation is in part a problem of the presence of a sufficient quantity of available P to allow lakes to reach a high level of productivity of algae and high plants, The interchange of P between sediments and lake water may play an im- portant role in determining the available P status of lakes. The uptake and release of P by sediments is a function of interacting physical, chemical, and biological processes (Syers e t al., 1973) and cannot be accurately predicted a t the present time. The ability of sediments to sorb inor- ganic P is well documented (Harter, 1968; Williams et al., 1970; Shukla et al., 1971).

The direction of net P transport is generally from the lake water to the sediments (Megard, 1973), presumably due to particle settling and sorption processes. However, the direction of transport of dissolved inorganic P will often be from the sediments to the overlying water due to direct biological uptake and the higher levels of inorganic P in the sediment interstitial water than in the overlying lake water (see review by Syers e t al., 1973). Exchange- able inorganic P represents the pool of sediment inorganic P characterized by a high potential for interaction with the sediment interstitial water and the overlying lake water (Li e t al., 1972).

Exchangeable P, as determined by isotope dilution has been used as an index of available P in soils (Baker, 1964; Tandon and Kurtz, 1968). Recently, exchangeable sedi- ment inorganic P was investigated under controlled condi- tions (oxygen status, pH, and temperature) in a long-term equilibration system (Li e t al., 1972). Although the long- term system provided information on the exchangeable sediment inorganic P under certain limnological condi- tions, the method was time-consuming and not well suited for the routine determination of exchangeable inorganic P in lake sediments. This investigation was conducted to evaluate the applicability of a relatively simple short-term equilibration system for exchangeable P measurements.

1 To whom correspondence should be addressed.

Materials and Methods Sediments. The sediments were obtained from eight

Wisconsin lakes. The methods of sampling and storage of the sediments used in this investigation have been de- scribed elsewhere (Williams et al., 1970; Li et al., 1972). Detailed characteristics of sediment samples previously obtained from these lakes are presented by Williams et al. (1970; 1971a; 1971b; 1971~) .

Phosphorus Measurements. Total sediment P was de- termined by sodium carbonate fusion (Shukla e t al., 1971), total organic P by the Mehta extraction procedure (Mehta et al., 1954; Sommers et al., 1970), and total inor- ganic P as the difference between total P and total organic P. Dissolved inorganic P in sediment extracts and in sedi- ment-water equilibration systems was measured by the method of Murphy and Riley (1962).

Sediment-Water Equilibration Systems. Systems des- ignated “long-term equilibration systems” were described previously (Li et al., 1972). Briefly, sediments were equili- brated as a 1% suspension (5 liters) in 10-liter containers. Light was excluded, and aerobic or anaerobic conditions were maintained by purging with air or nitrogen, respec- tively. Mixing was achieved by stirring with a magnetic stirrer for 15-min periods twice each day and prior to each sampling.

Short-term equilibration systems involved equilibration of sediments (170 suspension) in a 0.1M NaCl (noncalcar- eous sediments) or 0.001M Ca(HC03)z (calcareous sedi- ments) on a wrist action shaker. The sediment sample (0.4 gram dry wt) was placed in a polypropylene centri- fuge tube (50 ml), and either water (37 grams) and 2M NaCl (2 grams) or water (29 grams) and 0.004M Ca(HC03)z (10 grams) were added. After equilibration of the suspension for approximately 48 hr, 1 ml of carrier- free 32P-inorganic P (0.5 pglml) was added, and equili- bration was continued for an additional 24 hr. The sedi- ment was removed by centrifugation, and the supernatant solution was filtered (0.45 p M ) and analyzed for 31Pso~n and 32P,oln as described previously (Li et al., 1972).

The amount of exchangeable sediment inorganic P was calculated from isotope dilution as follows:

sed exch 32P P S O I ”

Sed exch P, = 31Psoln X 32

where sed exch 31P, = exchangeable sediment inorganic P,

sed exch 32P = the 32P in the sediment, expressed as

3lPsoln = inorganic P in solution, expressed as

3*Psoln = the 32P in solution, expressed as ’70 of

Total exchangeable inorganic P (total exch P,) was cal-

expressed as pg/g sediment

70 of added 32P

pg/g sediment

added 32P

culated as the sum of 31Psoln and sed exch P,.

Results and Discussion Measurements of exchangeable P were conducted in

short-term and long-term equilibration systems to deter- mine whether measurements obtained in a relatively sim-

454 Environmental Science & Technology

Table I . Comparison of Exchangeable Inorganic P in Short-Term and Long-Term Equilibration Systems

Sediment Condition

Inorganic P in solution.

N4Ig Tomahawk 5 Short-term system 9

Long-term system (aerobic) 25 Long-term system (anaerobic) 242

Mendota 5 Short-term system 9 Long-term system (aerobic) 38 Long-term system (anaerobic) 31 3

Exchangeable sediment inorganic P

"gig %

620 37 480 29 420 25 79 0 61 602 46 452 35

Tota! exchangeable P ~ _ _ _ _ .

KJIs %

629 38 505 3 1 662 4 0 799 6 2 640 50 765 59

ple equilibration system (short-term) were comparable to values obtained from more controlled conditions (long- term). In addition, the simplicity of the short-term system allowed investigation of a large number of sediments. Major differences between the two systems were the equil- ibration times, oxidation-reduction conditions, and the degree of agitation of the sediment-water system.

Preliminary experiments indicated that values for sed exch 31Pl in short-term systems were somewhat higher than values obtained in long-term equilibrations. T o allow di- rect comparisons of the same sediment samples, measure- ments of exchangeability were made for Tomahawk 5 and Mendota 5 sediments in both short-term and long-term equilibration systems (Table I ) .

Exchangeable P values for short-term and long-term systems were closely related but some differences were ob- served. In long-term equilibration systems, the level of sed exch 31Pl was greater under aerobic than 'anaerobic condi- tions (Table I ) . Apparently, release of inorganic P to solu- tion under anaerobic conditions caused a decrease in the amount of exchangeable P associated with the solid phase. However, total exch P, (solid phase exchangeable P plus inorganic P released into solution) was slightly greater for anaerobic than aerobic systems. This result was similar to tha t reported by Li et al. (1972) for sediments and by Lar- sen (1967) for paddy soils. The short-term systems showed a n increase of 12-2670 over long-term systems in the pro- portion of sediment inorganic P held in an exchangeable form in spite of the shorter equilibration time. However, total exch P, values were comparable for short-term and anaerobic long-term systems. Levels of inorganic P in so- lution in short-term systems were lower than in long-term anaerobic or aerobic systems.

The increase in sed exch 31P, in short-term as compared to long-term equilibration systems suggests t ha t either differences in oxidation-reduction conditions between the two systems resulted in a difference in the forms of inor- ganic P present or t ha t physical characteristics (such as the degree of agitation) of the equilibration system were important. Both systems appeared to have reached equi- librium as continued incubation did not result in an in- crease in exchangeable P. consequently, the major differ- ences between the two systems were the oxidation-reduc- tion conditions and the degree of agitation. The more vig- orous agitation provided by the wrist-action shaker in short-term systems may have promoted penetration of inorganic P into the sediment components and a corre- sponding increase in exchangeability. In addition, sedi- ments in the short-term systems are likely partially re- duced, although not to the extent attained in anaerobic long-term systems.

The agreement between total exchangeable P levels in short-term and anaerobic long-term systems suggests t ha t oxidation-reduction status may have been an important

Table I I . Amounts of Exchangeable Sediment Inorganic P in Several Wisconsin Lake Sediments

Total Total Inorganic Exchangeable

P P solution inorganic P inorganic organic P in sediment

Sediment

Minoqua 5 Little John 5 Tomahawk 5 Crystal 3 Trout 6 Devil's 5

Mendota 5 Wingra 8

10 11 12 13

g g l g Noncalcareous Sediments

551 1 493 29 2969 867 11 1662 395 9 2679 1253 2

389 637 3 909 41 5 6

Calcareous Sediments 1298 460 9

431 24 1 9 316 249 2 353 232 6 335 215 6 326 244 2

ggrg

2410 1170

620 632 239 595

790 77 98 70 65 87

O h

44 39 37 24 61 65

61 18 31 20 19 27

factor in accounting for the differences in exchangeable sediment P in these systems. Apparently, the major dif- ference was in the distribution of P between the sediment and solution phases. Oxidation of surface Fe in the short- term system due to oxygen diffusion through the polypro- pylene tubes (Browman et al., 1972) apparently increased the proportion of P retained in the sediment phase with little alteration of total exch P, as compared to the anaer- obic long-term system. In short-term systems, a n attempt was made t o equilibrate sediments with minimal alter- ation of native condition. Consequently, exchangeable P measurements in short-term systems may be more repre- sentative of exchangeability under natural conditions than measurements made in long-term systems. In terms of simplicity and precision, the short-term system is well suited to routine determination of exchangeable sediment inorganic P .

In the procedure used to determine sed exch SIP,, biologi- cal immobilization of inorganic P may influence the dis- tribution of 32P and thereby contribute to error in the measurement. Previous investigations (Olsen, 1958; Li et al . , 1972) have concluded tha t this effect is small. The sediment microorganisms present were apparently not P deficient as shown by the relatively high amounts of inor- ganic P in solution (>20 kg/l.). Further, P deficiency is not expected in sediments containing large amounts of non- occluded P (Sagher and Harris, 1972). The rate of P up- take by non-P deficient "microorganisms is slow and un- likely to have influenced appreciably the distribution of

Volume 7, Number 5, May 1973 455

32P in the systems investigated. Furthermore, subsequent experiments have shown tha t the 32P added to the sedi- ment prior to equilibration was recovered completely in sediment inorganic P fractions (Li et al., 1973). These re- sults indicate that the contribution of organic P to total exchangeable P was not significant.

Levels of sed exch 3IP1 ranged from 18-6570 of the sedi- ment total inorganic P for the 12 sediments investigated (Table 11). Sediments containing higher levels of inorganic P (>1500 pg/g) exhibited a lower proportion of exchange- able P (<50%) than shown by the other sediments except Wingra. The lowest proportion of exchangeable P (24%) in the noncalcareous sediments was shown by the Crystal 3 sediment. While the reason for the lower degree of exchangeability of the Crystal sediment inorganic P is not clear, it may be significant that Crystal contained a high proportion of sediment organic P compared to other sedi- ments characterized by a high level of total P (>2000 pg/g) and that Crystal is an oligotrophic lake. In the cal- careous group, Wingra sediments exhibited a lower pro- portion of exchangeable P (18-3170) than the Lake Men- dota 5 sediment (61%). The relative proportions of ex- changeable P are consistent with the higher proportion of NaOH-P (nonoccluded P; Syers e t al., 1973) in Mendota than in Wingra sediments (Williams et al., 1971b). I t has been shown that NaOH-P exhibits a high degree of exchangeability in soils (Dunbar and Baker, 1965) and sediments (Li et al., 1973).

Exchangeable P in Wingra sediments ranged from 19- 31% of sediment inorganic P even though Wingra sedi- ments are considered to be fairly uniform in P character- istics (Williams et al., 1970). The differences in exchange- able P levels among Wingra sediments may be related to seasonal organic P deposition-mineralization and inorgan- ic P release cycles. Wingra 10 and 13 were sampled during the winter-spring period (April 1971 and February 1972, respectively) and Wingra 8, 11, and 12 during the summer period (June 1970 for Wingra 8 and August 1971 for Win- gra 11 and 12). Wingra is a shallow eutrophic lake charac- terized by dense macrophyte and plankton algae popula- tions (Williams et al., 1970; Kluesener, 1972; Koonce, 1972).

The relatively high proportion of exchangeable inorgan- ic P in the sediments investigated (Table 11) is consistent with the high levels of nonoccluded P (NaOH-P and CB-P for calcareous sediments; "IF-P and NaOH-P for noncalcareous sediments) previously reported for sedi- ments from these lakes (Williams et al., 1971a; 1971b).

The inorganic P in these fractions exhibits a high degree of exchangeability (Li e t al., 1973). The results show that a high proportion of the inorganic P in a large group of Wisconsin sediments is in a form potentially available to aquatic organisms and for interaction with the overlying lake water.

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Dunbar, A. D., Baker, D. E., Soil Sci. SOC. Amer. Proc., 29, 259

Harter, R. D., ibid., 32, 514 (1968). Kluesener, J . W., PhD Thesis, University of Wisconsin, Madison,

Koonce, J . F., ibid. Larsen, S., Plant Soil, 27, 401 (1967). Li, .W. C., Armstrong, D. E., Syers, J. K., Soil Sci. SOC. Amer.

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Received for review July 26, 1972. Accepted February 5, 1973. This investigation was supported in part by Office of Water Re- sources Research Projects No. 14-01-001-1961 (B-022WIS) and No. 14-31-0001 (A-C4OWIS) and by Environmental Protection Agency Project No. WP-01470-01, administered through the University of Wisconsin Water Resources Center. Acknowledgment is made of the cooperation and support of the Engineering Experiment Sta- tion. Approved for publication by the Director of the Research Division, College of Agricultural and Life Sciences, University of Wisconsin, Madison, Wis.

456 Environmental Science & Technology