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) equilibration 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 exchangeable P between the solid and solution phases were apparently related to differences in oxidation-reduction conditions. 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 eutrophication 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 important 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 et al., 1973) and cannot be accurately predicted a t the present time. The ability of sediments to sorb inorganic 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 et al., 1973). Exchangeable 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 et 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 sediment inorganic P was investigated under controlled conditions (oxygen status, pH, and temperature) in a long-term equilibration system (Li et al., 1972). Although the longterm system provided information on the exchangeable sediment inorganic P under certain limnological conditions, 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.
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 described 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; 1 9 7 1 ~ ) . Phosphorus Measurements. Total sediment P was determined by sodium carbonate fusion (Shukla et al., 1971), total organic P by the Mehta extraction procedure (Mehta et al., 1954; Sommers et al., 1970), and total inorganic P as the difference between total P and total organic P. Dissolved inorganic P in sediment extracts and in sediment-water equilibration systems was measured by the method of Murphy and Riley (1962). Sediment-Water Equilibration Systems. Systems designated “long-term equilibration systems” were described previously (Li et al., 1972). Briefly, sediments were equilibrated 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, respectively. 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 (170suspension) in a 0.1M NaCl (noncalcareous sediments) or 0.001M Ca(HC03)z (calcareous sediments) on a wrist action shaker. The sediment sample (0.4 gram dry wt) was placed in a polypropylene centrifuge 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 carrierfree 32P-inorganic P (0.5 pglml) was added, and equilibration was continued for an additional 24 hr. The sediment was removed by centrifugation, and the supernatant solution was filtered (0.45 p M ) and analyzed for 3 1 P s o ~ 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 P, = 31Psoln X
sed exch 32P 32 PSOI”
where sed exch 31P, = exchangeable sediment inorganic P, expressed as pg/g sediment sed exch 32P = the 32P in the sediment, expressed as 70of added 32P 3 l P s o l n = inorganic P in solution, expressed as pg/g sediment 3*Psoln= the 32P in solution, expressed as ’70 of added 32P Total exchangeable inorganic P (total exch P,) was calculated as the sum of 31Psolnand sed exch P,. Results and Discussion
1 To whom
454
correspondence should be addressed.
Environmental Science & Technology
Measurements of exchangeable P were conducted in short-term and long-term equilibration systems to determine whether measurements obtained in a relatively sim-
Table I . Comparison of Exchangeable Inorganic P in Short-Term and Long-Term Equilibration Systems
Sediment
Tomahawk Mendota
5
Condition 5
Short-term system Long-term system (aerobic) Long-term system (anaerobic) Short-term system Long-term system (aerobic) Long-term system (anaerobic)
ple equilibration system (short-term) were comparable to values obtained from more controlled conditions (longterm). 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 equilibration 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 direct comparisons of the same sediment samples, measurements 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 observed. In long-term equilibration systems, t h e level of sed exch 31Pl was greater under aerobic t h a n 'anaerobic conditions (Table I ) . Apparently, release of inorganic P to solution 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 t h a n aerobic systems. This result was similar to t h a t reported by Li et al. (1972) for sediments and by Larsen (1967) for paddy soils. The short-term systems showed a n increase of 12-2670 over long-term systems in the proportion of sediment inorganic P held in a n 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 solution in short-term systems were lower t h a n 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 h a t either differences in oxidation-reduction conditions between the two systems resulted in a difference in the forms of inorganic P present or t h a t physical characteristics (such as the degree of agitation) of the equilibration system were important. Both systems appeared t o have reached equilibrium as continued incubation did not result in a n increase in exchangeable P. consequently, the major differences between the two systems were the oxidation-reduction conditions and the degree of agitation. The more vigorous agitation provided by the wrist-action shaker in short-term systems may have promoted penetration of inorganic P into the sediment components and a corresponding increase in exchangeability. In addition, sediments in the short-term systems are likely partially reduced, 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 h a t oxidation-reduction status may have been a n important
Inorganic P in solution. N4Ig 9 25 242 9 38 31 3
Exchangeable sediment inorganic P
Tota! exchangeable P
~
_
_
_
"gig
%
KJIs
%
620 480 420 79 0 602 452
37 29 25 61 46 35
629 505 662 799 640 765
38 31 40 62 50 59
_
.
Table I I. Amounts of Exchangeable Sediment Inorganic P in Several Wisconsin Lake Sediments Total inorganic
P Sediment
Total Inorganic organic P in P solution
Exchangeable sediment inorganic P
gglg
ggrg
Oh
Noncalcareous Sediments Minoqua 5 Little John 5 Tomahawk 5 Crystal 3 Trout 6 Devil's 5
551 1 2969 1662 2679 389 909
493 867 395 1253 637 41 5
29 11 9 2 3 6
2410 1170 620 632 239 595
44 39 37 24 61 65
790 77 98 70 65 87
61 18 31 20 19 27
Calcareous Sediments Mendota 5 Wingra 8 10 11 12 13
1298 431 316 353 335 326
460 24 1 249 232 215 244
9 9 2 6 6 2
factor in accounting for the differences in exchangeable sediment P in these systems. Apparently, the major difference was in the distribution of P between the sediment and solution phases. Oxidation of surface Fe in the shortterm system due to oxygen diffusion through the polypropylene 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 t h e anaerobic long-term system. In short-term systems, a n attempt was made t o equilibrate sediments with minimal alteration of native condition. Consequently, exchangeable P measurements in short-term systems may be more representative 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,, biological immobilization of inorganic P may influence the distribution of 32P and thereby contribute to error in the measurement. Previous investigations (Olsen, 1958; Li et al., 1972) have concluded t h a t this effect is small. The sediment microorganisms present were apparently not P deficient as shown by the relatively high amounts of inorganic P in solution (>20 kg/l.). Further, P deficiency is not expected in sediments containing large amounts of nonoccluded P (Sagher and Harris, 1972). T h e rate of P uptake by non-P deficient "microorganisms is slow and unlikely to have influenced appreciably the distribution of Volume 7, Number
5,
May 1973
455
32P in the systems investigated. Furthermore, subsequent experiments have shown t h a t the 32P added to the sediment prior to equilibration was recovered completely in sediment inorganic P fractions (Li et al., 1973). These results 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 sediment 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 exchangeable P (2000 pg/g) and that Crystal is an oligotrophic lake. In the calcareous group, Wingra sediments exhibited a lower proportion of exchangeable P (18-3170) than the Lake Mendota 5 sediment (61%). The relative proportions of exchangeable P are consistent with the higher proportion of NaOH-P (nonoccluded P; Syers et al., 1973) in Mendota than in Wingra sediments (Williams et al., 1971b). It 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 1931% of sediment inorganic P even though Wingra sediments are considered to be fairly uniform in P characteristics (Williams et al., 1970). The differences in exchangeable P levels among Wingra sediments may be related to seasonal organic P deposition-mineralization and inorganic 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 Wingra 11 and 12). Wingra is a shallow eutrophic lake characterized by dense macrophyte and plankton algae populations (Williams et al., 1970; Kluesener, 1972; Koonce, 1972). The relatively high proportion of exchangeable inorganic 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 sediments from these lakes (Williams et al., 1971a; 1971b).
456
Environmental Science & Technology
The inorganic P in these fractions exhibits a high degree of exchangeability (Li et 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.
Literature Cited Baker, D. E., Soil Sci. SOC.Amer. Proc., 28,511 (1964). Browman, M. G., Syers, J . K., Abstr. 15th Intern. Ass. Great Luke Res. Coni., Madison, Wis., April 5-7, Abstr., 1972. Dunbar, A. D., Baker, D. E., Soil Sci. SOC.Amer. Proc., 29, 259 (1965). Harter, R. D., ibid.,32, 514 (1968). Kluesener, J . W., PhD Thesis, University of Wisconsin, Madison, Wis., 1972. Koonce, J . F., ibid. Larsen, S., Plant Soil, 27, 401 (1967). Li, .W. C., Armstrong, D. E., Syers, J. K., Soil Sci. SOC.Amer. Proc., 37, in press (1973). Li, W. C., Armstrong, D. E., Williams, J. D. H., Harris, R. F., Syers, J. K., ibid., 36,279 (1972). Megard, R. O., Trans. ASAE, in press (1973). Mehta, N . C., Legg, J. O., Goring, C. A. I., Black, C. A., Soil Sci. SOC. Amer. Proc., 18,443 (1954). Murphy, J., Riley, J. P., Anal. Chim. Acta., 27,31(1962). Olsen, S., Verh. Int. Ver. Limnol., 13,915 (1958). Sagher, A,, Harris R. F., Abstr. 15th Intern. Ass. Great Lakes Res. Conj., p 913, 1972. Shukla, S. S., Syers, J . K., Williams, J. D. H., Armstrong, D. E., Harris, R. F., Soil Sci. SOC.Amer. Proc., 35, 244 (1971). Sommers, L. E., Harris, R. F., Williams, J. D. H., Armstrong, D. E., Syers, J. K., Limnol. Oceunogr., 15,301 (1970). Syers, J. K., Harris, R. F., Armstrong, D. E., J. Enuiron. Qual., 2, 1 (1973). Tandon, H. L. S., Kurtz, L. T., Soil Sci. SOC.Amer. Proc., 32, 799 (1968). Williams, J. D. H., Syers, J. K., Armstrong, D. E., Harris, R. F.,, ibid., 35,556 (1971a). Williams, J. D. H., Syers, J. K., Harris, R. F., Armstrong, D. E., p 250 (1971b). Williams, J. D. H., Syers, J. K., Shukla, S. S., Harris, R. F., Armstrong, D. E., Enoiron. Sci. Technol., 5 , 1113 (1971~). Williams, J. D. H., Syers, J. K., Harris, R. F., Armstrong, D. E., ibid., 4, 517 (1970). Received for review July 26, 1972. Accepted February 5, 1973. This investigation was supported in part by Office of Water Resources Research Projects No. 14-01-001-1961 (B-022WIS) and N o . 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 Station. Approved for publication by the Director of the Research Division, College of Agricultural and Life Sciences, University of Wisconsin, Madison, Wis.
