Comparison of methods for analysis of organic and inorganic phosphorus in river sediment

War. Res. Vol. 27, No. 1, pp. 77-83, 1993 Printed in Great Britain. All rightsr-~erved

0043.1354/93 $5.00 + 0.00 Copyright © 1993PergamonPre-~ Ltd

COMPARISON OF METHODS FOR ANALYSIS OF ORGANIC AND INORGANIC PHOSPHORUS IN RIVER SEDIMENT LAP.sM. SVENDSEN1., AAGE PxEBSDOI~F1 and PER NORNBERG2 INational Environmental Research Institute, Division of Freshwater Ecology, Vejlszvej 25, P.O. Box 314, DK-8600 Silkeborg and 2Department of Earth Sciences, University of Aarhus, Ny Munkegade bygn. 520, DK-8000 .~rhus C., Denmark (First received January 1992; accepted in revised form May 1992) Atetract--During a study of the dynamics of sediment nitrogen, phosphorus (P) and organic matter in rivers, a common method for analysis of soil P differentiating between total phosphorus O'P), inorganic phosphorus (Pi) and organic phosphorus (Po ffi TP - Pi) revealed serious errors. Negative values of Po were calculated in approx. 40% of 420 river sediment samples and the reproducibifity was low. The method involved a mild extraction from 2 mm wet-sieved samples, respectively ignited (550°C for 2h) and unignited, by shaking for 2h at room temperature with 0.1 M H2SO4. The poor reproducibility of the initial method was greatly improved by grinding the dried sediment to < 250/~m. Even better results were obtained by also changing to stronger extractants (l M HC1 and 7.7 M HCI) and by boiling for 20 rain. The combined effect of grinding and changing extractant yielded approx. 30% more Pi and up to 44% more TP, and good reproducibility was achieved. Further, the number of negative Po values has reduced to 8%. Analysing sediment P in 1094sub-samples, in 282 iron-poor samples only 5% of the Po values were negative, and in 812 iron-rich samples 10% of the Po values became negative. About 75% of the negative Po-values were associated with very low organic contents (<0.6% loss on ignition). W e conclude, that extraction involving 1 M HCI can be recommended for routine determinations of TP and P~ in river sediments, and to determine Po when T P - Pi > I0 m g P kg-1 dw. For very iron-rich sediment, however, some reservation stillmust be made due to some interference,possibly arising during the ignition phase.

Key words--comparison of methods, analysis of phosphorus, organic phosphorus, inorganic phosphorus, river sediment, homogenization, extraction, extractants, sediment iron, organic matter

INTRODUCTION

matter in two Danish lowland streams (Svendsen and Kronvang, 1990, 1993), we discovered some unexpected shortcomings in the procedure for discriminating between organic (Po) and inorganic P (Pi) in a standard agricultural P analysis. In 40% of 420 river sediment samples analysed for total P O'P) and Pi, Po calculated as C I T - Pi), turned out to be negative, Further, the reproducibility was unacceptably low, The procedure used (Landbrugsministeriet, 1972) is based on the internationally recotmized standard methods for soil P analysis (Saunders and Williams, 1955; Jackson, 1958; Kaila, 1962; Williams et al., 1970; Olsen and Sommers, 1982). Courty and Nzrnberg 0985) found that it gave reliable results for analysis of Danish soil samples. In brief, the TP method involves ignition at 550°C for 2 h (Sannders and Williams, 1955) before extraction for 2 h with 0.1 M H2SO4 at room temperature (Kaila, 1962). Pl is extracted with the same extractant on an unignited aliquot. Po is calculated as ( T P - Pl) according to Olsen and Sommers (1982). Analysis of P in lake sediments is based on various modifications of the soft P methods (e.g. Andersen, 1976; Williams et al., 1971, 1976; Hieltjes and

The transport and dynamics of river sediment phosphorus (P) are related to factors such as delivery of particulate matter to the river, morphology, hydraulic conditions, hysteresis and exhaustion effects, adsorption--desorption processes, and the stand of macrophytes (Miller et ai., 1977; Harms et aI., 1978; Walling, 1978, 1988; Rigler, 1979; Foster et al., 1988; Klotz, 1991; Svendsen and Kronvang, 1993). Macrophytes and low flow conditions favour sedimentation, so that some of the phosphorusbearing sediment is only later flushed into lakes or coastal areas (Jeppesen et al., 1987; Dorioz et al., 1989; Svendsen and Kxonvang, 1990, 1993). In Denmark, where lowland rivers predominate, the main sources of sediment P transport are erosion of the bottom and the banks (I-Iasholt, 1988) whereas sediment from surface run-off in most cases is imfifnificant (Hausen et al., 1991). In connection with a project on the dynamics of particulate nitrogen, phosphorus and organic *Author to whom all correspondence should be addressed. wR ~ / i - v

77

78

LAgS M. SVI~D~N et al.

Lijldema, 1980; Psenner et aL, 1985; Petterson et al., 1988; Sediment Phosphorus Group, 1988), but we are not aware of any available standardized or recommended method for the analysis of P in river sediment. Harms et al. (1978), who studied the role of stream-bed sediment with respect to the increased P concentrations in the stream water at high flow, determined TP on dry samples < 2 mm ignited at 550°C for 2 h (Saunders and Williams, 1955) and extracted by shaking overnight in a wrist action shaker with 0.5 M H2SO4. Pt was extracted on unignited samples in the same way, and the dissolved reactive P (DRP) was measured in both extracts by the molybdenum blue spectrophotometric method (Murphy and Riley, 1962). In order to elucidate the factors that were responsible for the shortcomings using the Danish standard agricultural method (Landbrugsministeriet, 1972), we decided to investigate the procedure on river sediment samples focusing our attention on two subjects: (1) low reproducibility possibly related to inhomogeneity of the samples (2) negative Po values possibly caused by incomplete extraction. Subject 1 was investigated by grinding the sediment into smaller particles. Subject 2 was examined by increasing the strength of the acid extractant and the extraction time and temperature.

Field methods

Both sediments rich in organic material, and sediment consisting mainly of sand and gravel practically devoid of organic detritus particles, were sampled. The sampling technique is described in Svendsen and Kronvang (1993). The samples were taken in one reach in G-elb~ekand three reaches in the Gjern River by a Kajak corer (diameter 54 mm). The upper 4 cm of the sediment cores were analysed. Initial method (method A)

The samples were pre-treated by wet-sieving through a 2 mm sieve to remove gravel, pebbles and visual macroscopic organic fragments. The sieved samples were dried at 60°C and afterwards the aggregates were broken up with a wooden piston into particles smaller than 2 mm in a porcelain mortar. After mixing, two sub-samples each containing 25 g were taken. Of the first sub-sample 10 g was used for the Pi analysis. Ten grams of the other sub-sample were ignited at 550°C for 2 h in a muffle furnace and then used for the TP analysis (Sannders and Williams, 1955). The extractions were carried out by The Danish Land Development Service according to the method of Landbrugsministeriet (1972) by mixing I0 g of sediment with 250 ml 0.1 M sulphuric acid, and shaking in an end-over-end shaker for 2 h at room temperature. After partial dilution, filtration, neutralization and final dilution the DRIP in both the ignited and unignitcd sub-samples was measured spectropbotometrically by the molybdenum blue method using ascorbic acid as reductant (Murphy and Riley, 1962). Watanabe and Olsen (1965) found this method to be accurate for determining P in extracts from mils. Bla, k.~and standards were treated in the same way as the samples. Po was calculated as: f r p - I'm). Method B

MATERIAI.S AND METHODS Study area Sampling took place in the period June 1987-September 1988 in two Danish lowland streams: the Gelbaek and the Gjern River, both belonging to the Gjern River catchment area (102 km2). The Gjern River is a third order tributary (k-nsu Strahler) of the largest Danish river, the River Gudeni, and GelLnekis a secondary order tributary of Gjern River (Fig. 1). The area is intensively farmed (80% arable land) and drained. Soils in the catchment area are loams or sandy loams derived from glacial tills and fluvio-glacial deposits from the last glaciation (Weichsel).

-!i ~ ...... /'..: ..\

............

During the drying procedure (at 60°C) some flakes were formed, which could not be totally mixed with the rest of the sediment particles. Therefore, the pretreatment procedure was changed to achieve a better homogenization of the dried sediment. After wet sieving (2 mm) and drying (at 60°C) the total sample (about 125 g) was ground in a steel mortar for approx. 7 rain allowing it to pass through a 250/~m sieve. The extraction was by the same mild method as in method A. Methods C and D Methods C and D involved stronger extractants and boiling for 20 rain. In method C the extractant was 7.7 M (25%) HCI, and in method D 1 M HCI (2.9%). The extractions were carried out on ground ~liment samples (<250/~m sieve). After the extraction DRP was an~!ysed as in method A. Comparison of methods A, B, C and D

, /:...:,........ /

".,... ........... ,

I .J

.-

~

.... "..)

" . .•.".... ,':). . .......... . . . .) . .

• gonltork~station "" • Sediment ssmplng reach

~..."

~......... ,

,

0

1

,

5kin

Fig. I. Location of monitoring stations and sediment rumpling reaches in the Gelbmk and the Gjern River. The inset shows the location of the catchment area in De-m~rk. The monitoring stations were used for automatic collection of water samples.

In the present study, we compared the four methods, A, B, C and D on three sediment rumples (1, II and HI), which represented a wide range of organic content. The analyses used approx. 0.5 g sub-utmples in 25 ml of extractant. All analyses were triplicated. After testing the four methods, we decided to use method D on the same 420 samples, which had been nnalysed unsuccessfully by method A, and on all the river rudiment samples (n = 1094). The ~mples were analysed by the Danish Land Development Service on 5g sediment sub-samples. RI~ULTS

Measuring TP and Pi by method A resulted in negative values o f Po in 40% o f 420 samples (Table I).

79

Analysis of phosphorus in fiver sediments Table 1. Mean values of TP in river-bed sediment (location of the reaches are in Fig. 1) measured by method A, as well as the percentage of negative Po values (Po ffi TP - Pi)

increased (by 16-23% for TP and by 15-20% for Pt) except for one sample where Pi decreased by 18% (Table 2). The calculated values of Po for the three samples clearly increased, although one sample yields a negative Po-Value even after grinding, and the CVs were reduced.

Method A Number of TP Negative Po samples (ms P kg-t dw) (%) Gelb~k River Reach 1

107

460

8

Gjern River Reach 2 Reach 3 Reach 4 Gjern River in all

110 95 108 313

492 544 682 569

45 39 62 49

Total

420

540

40

Effect of different extracting agents and extraction procedures

In the Gelb~ek 8% of the calculated Po values were negative, while in the Gjern River 39 to 62% of the results were negative.

Effect of grinding To investigate the effect of grinding, tests were performed on three samples to compare method A and B. The random errors were significantly reduced using method B, as the coefficient of variation (CV) for both TP and Pt is much smaller in method B than in method A. The extracted amounts of TP and Pt all

To investigate the effects of the extracting agents, we compared methods B, C and D on the three samples (Table 3). The results show an acceptable reproducibility for Pi and TP for all three methods (CV range 0.2-3.2%). Methods C and D yielded positive Po for the three samples, while method B gave a negative value in sample III. Methods C and D yielded almost identical results (Tables 3 and 4). Method D extracted about 15% more Pi and 14-28% more TP than method B. The calculated Po values were clearly higher using methods C and D (Tables 2 and 3). The combined effect of grinding and changing extractant yielded approx. 30% more Pi (with one exception: sample I - 1%) and 27-44% more TP (Table 4).

Table 2. Test of the effect of grinding by comparing the P content determined by method A and method B on three sediment samples I, II and III

Sample

Loss on

Method A

Method B

ignition <250#m (%)

Pi TP (mS ks -t dw)

Pi TP (ms ks -l dw)

I

2.36 (7,6) 4.18 (6.0) 6.91 (10.I)

II III

382 (4.6) 704 (18.6) 1075 (24.2)

312 (8.9) 681 (1.5) 988 (2.6)

325 (0.8) 832 (0.5) 1345 (0.2)

A Po < 2 mm (rag kg-I dw)

370 (2.1) 866 (0.3) 1279 (2.0)

B Po < 250 t~m (rag kg-I dw)

-70 (58) -23 (545) -81 (297)

(B -- A)/B Pi (%)

(B - A)/B TP (%)

-18

16

45 (21) 34 (9.2) -66 (35)

15

21

20

23

All analyses were triplicated. The figures are mean values with the coefficient of variation (CV) in % given in parentheses. Table 3. The effect of different extractants and extraction procedures on the three same samples as used in Tables 2 and 4 Method B 0.1 M H2SO4 Pi

Method C 7.7 M HCI

Pi

TP

TP

Sample

Method D 1 M HCI

Method

B Po

Pi TP (rag kg -I dw)

C Po

I

D Po

325 370 386 423 377 428 45 37 51 (0.8) (2.1) (0.8) (1.7) (2.5) (3.2) (21) (12) (33) II 832 866 1003 1118 989 1110 34 115 121 (0.5) (0.3) (1.8) (1.2) (1.4) (0.9) (9.2) (5.4) (48) III 1345 1279 1580 1773 1537 1776 -66 193 239 (0.2) (2.0) (0.7) (0.6) (1.4) (0.2) (35) (9.8) (4.5) All analyses were triplicated. The methods employed are B, C and D. The figures are mean values with CV (%) given in parentheses. Table 4. The percentage gain in Pi, TP and Po values in changing from methods A, B and C to method D using the figures from Tables 2 and 3

(D - X)/D Pi Sample I Sample II Sample III

(%)

-1.0 29 30

03 - B)/D

0~ - C')/D

TP

Pi

TP (%)

Po

Pi

TP (%)

Po

27 39 44

14 16 12

14 22 28

12 70 129

-2.4 - 1.4 -2.8

1.2 -0.7 0.7

28 5.0 19

Laps M. SV~qDS&~ et al.

80

Table 5. Comparison of the number of negative Po values (determined by method D) with TP and loss on ignition (with CV in %) on sub-samples from the 420 samples initially analysed with method A Percentages of samples with negative Po

Loss on ignition (%)

Method D, 420 samples all 1094 samples Ground Unground

Number of samples

"IT (rag P kg -1 dw)

Method D, 420 samples

Gelbcek River Reach 1

282

469

3

5

2.40 (17)

2.49 (16)

Gjern River Reach 2

284

572

10

8

Reach 3

260

663

II

10

Reach 4

268

878

9

II

2.15 (22) 3.92 (39) 3.64

2.21 (22) 3.85 (31) 3.66

(30)

(31)

812

702

10

10

2.20 (30)

2.21 (28)

1094

640

8

8

3.00 (27)

3.02 (25)

Gjern River in all Total

Method D used on all samples As methods C and D extract almost the same amount of TP and Pi (Table 4), method D, with the more convenient extractant, was chosen as the routine procedure for all samples (n = 1094). A comparison for sub-samples of the 420 samples initially analysed with method A showed that method D markedly reduced the number of negative Po values: in the Gelbeek from 8 to 3% and in the Gjern River from 49 to 10% (Tables 1 and 5). Further, only 8% of all samples (n = 1094) gave negative Po values using method D. Method D extracts 2% more TP than method A on Gelbeek samples, and 16-29% more TP on Gjern River samples (Tables 1 and 5). Grinding the sediment samples to <250#m reduced the loss on ignition by 4% in C-elbmk samples and practically no changes occurred in Gjern River samples (Table 5). These variations, however, do not differ significantly from the uncertainty of the ignition method. Further, the distribution of the loss on ignition values is almost the same for ground and unground samples.

revealed a significant positive correlation for method D (Table 7). In fact 75% of the negative Po values from Gelb~ek, and 61-65% of the negative Po values from Gjern River samples were found in the samples with a low loss on ignition (<0.6%). In 607 water samples collected at the two water monitoring stations (Fig. 1) the mean content of total iron was twice as high in the Gjern River as in the Gelb~ek. The mean content of total iron in resuspended sediment (sampled during stormflows) also was about twice as high in the Gjern River (Table 8). Further, there were twice as many negative Po values in the Gjern River as against the Gelb~ek (10 vs 5%). DISCUSSION

For the analysis of P in river sediments shortcomings have been identified in a standard soil chemical extraction method, which in different modifications (Olsen and Sommers, 1982) is widely used for routine 1800

/

1600

Effect of the boiling period Prolonging the bolting period from 20 rain to 4 h on 35 sediment samples did not increase the amount of extracted Pi and TP. On the contrary, there was a significant reduction (Table 6 and Fig. 2).

Effect of organic matter and iron in the sediment Of Po Regression analysis between the content of organic matter and Po using methods A and D on 420 samples

/

1,oo

j

•

j

~, 12oo

1000

•

y

j,oo

•

y.,

,I"

i'°°l

Y" '

Table 6. Regre~on between the results from boiling 20 rain or 4 h (method D) Expre~on r TP, u - 24 + 0.84.TP2e .~. Pgu " - 9 . 7 + 0.84.Pro, m

0.9689 0.9906

r - coefficient of determination. A F.test yielded a confidence level P ql0.001 for the expre~ons of TP and P,. All concentratiom in m g P k g - I dw.

0

'r

0

,

,

200

,

,

400

,

,

600

,

,

,

,

,

,

,,

,

,,

,

,

,

800 1000 1200 1400 1600 1800

Boilng20 minutes: TP(ragkg4) Fig. 2. The extracted a m o u n t s o f total phosphorus (TP)

obtained after boiling for 4h with I M HCl (y) plotted against the results from boiling for only 20 min (x). The full-drawn line is the line o f full agreement. The test was performed on 35 samples u l e c t e d from the 4 sediment sampling reaches (Fig. I).

Analysis of phosphorus in river sediments

81

Table 7. Linear regression analysis (Po ffi k + ~t. LI) between loss on ignition (LI) and organic phosphorus 0Po) using methods A (A) and D (D) Reach 1 k ct r

Reach 2

Reach 3

A

D

A

D

57 20.7 0.308

29 17.6 0.497

18 -5.5 0.077

8.7 25.0 0.486

A

D

13 30 10.3 9.7 0 , 2 6 0 0.570

Reach 4 A

D

-16 -9.2 0.138

10 22.6 0.686

k is the intercept value, ~ is the regression coefficient and r the correlation coefficient. The underlined values (r) indicate a significant relation on a 1% confidence level. None of the interception values are significantly different from zero.

analyses of total and inorganic soil P. Inhomogeneity than the 20 mg P kg-I dw proposed by Olsen and of dried sub-samples sieved through a 2 mm sieve Sommers (1982). It was applied based on results (method A) revealed severe reproducibility problems, from a series of Danish freshwater intercalibrations which were overcome by grinding the samples to (Kronvang and Robsdorf, 1988), using the method <250;tm (method B) (Table 2). The total surface proposed by Korolcff (1983). area of the particles in contact with the extractant There was a significant positive relation between was enlarged by grinding, which probably was the the contents of organic matter and Po using method cause for the increase in the extracted amounts of P~ D (Table 7). We observed that most of the negative and TP (Table 2). Further, the grinding ensured that values of Po occurred when there was only little more representative sub-samples could be taken. The accumulated fine material and the samples therefore aggregates formed during drying apparently had no consisted largely of inorganic fiver sands (Svondsen effect on the values or on the reproducibility of the and Kronvang, 1993). In such cases (TP - Pi) is near loss on ignition, in analysing 420 samples (Table 5). to or below the detection limit, and consequently for Grinding, however, did not solve the other short- statistical reasons, some of the calculated Po values coming---the calculated negative Po values (Table 2). necessarily will be negative. Still, a low content of Changing to stronger extractants and changing from organic matter in the samples cannot explain all room to boiling temperature (methods C and D) negative Po values. Po could also be underestimated provides an adequate, if not perfect, solution to this because of volatilization of PeOs or formation of problem (Tables 3 and 4). acid-insoluble P-compounds with AI, F¢ and Si HC1 was chosen instead of H2SO4 as HCI has a minerals during ignition. Incomplete ignition of the stronger dissolving effect on Fe-bound P than H eSO4, organic matter can be excluded as this was controlled which has an oxidizing potential and therefore also by visual inspection. Hydrolysis of organic matter may destroy part of the organic matter, thus convert- during extraction with strong acid could load to ing some Po into Pi in unignited samples. Boiling increased Pi (Anderson, 1960, 1975; Williams et al., sediments with 7.7 M HC1 (method C) had previously 1970; Olsen and Sommers, 1982; Condron et al., been shown to be more effective in dissolving iron 1990). We suspect that an important factor contributfrom iron-rich lake sediment than boiling with 1 M ing to the negative Po values was a high iron content HCI (method D) (Rebsdorf, unpublished), while in the river sediment. In all the river-bed samples Andersen (1976) recommended boiling with 1 M HCI (n ffi 1094) 10% of the Po values were negative in the to determine TP in lake sediments. As there were Gjern River, as against 5% in the Gelb~ek (Table 5). no significant differences between methods C and The mean total iron concentration, both in the stream D in our study, we chose the 1 M HC1 for routine water (NERI, unpublished) and in the resuspended use. Methods C and D were much more effective fiver sediment, was about twice as high in the Gjern than method B in extracting TP and Pi (Tables 3 River as against the Geib~ek (Table 8). and 4). Extending the boiling period from 20 rain to 4 h did After using method D on the main bulk of not improve the extraction efficiency; on the contrary sediment samples, 8% of the total number (1094) the amount of extracted P (TP, P~) decreased (Table 6 of samples still had negative calculated Po values and Fig. 2). Andersen (1976) found a recovery of 97, (Table 5). Some of these are due to the detection limit 98, 98, 99 and 100% when boiling for 1, 2, 5, 15 and for TP and Pi of 10 mg P kg -1 dw. This limit is lower 30 win with 1 M HCI, and as a compromise we chose to use a 20 rain boiling time for the extractions. Table 8. The median total iron content in 607 water samples from C~lbmk and the Gjern rivers and the content of total iron in rmug~nded ~liment from the two streams compared with the percentage of negative Po values (method D) in all (n ffi 1094) the stream.bed samples Total Fe in Total Fe in Samples with water samples suspended sediment negative Po (mg I- i ) (g kg- ndw) (%) Gzlbmk River Gjern River

1.17 2.19

31.0 78.6

5 10

CONCLUSION

The common standard procedure for soil agricultural P-analysis in Denmark, involving extraction at room temperature for 2 h in an end-over-end shaker with 0.1M H2SO4 on 2 m m wet-sieved samples, turned out to be inapplicable in determining TP and P, in fiver sediment, because the reproducibility was

82

LxRs M. Sw~rOS~Net al.

low and because the calculated Po-values became negative in 40% of all samples. Grinding dry sub-samples < 2 5 0 # m greatly improved the reproducibility and increased the extracted amount of Pi by up to 20% and of TP by approx. 20%. By changing to stronger extractants (7.7 M or 1 M HCI) and by boiling for 20 min both unignited samples (for Pi analysis) and ignited (for TP analysis), the yield further increased by approx. 15% for Pi and approx. 25% for TP. The number of negative Po values was reduced to 8% in all the samples (n = 1094). About 75% of the negative Po values were related to samples with a low content of organic matter (<0.6% loss on ignition) i.e. where

TP~Pi. We recommend the use of method D for routine determinations of both TP and Pi in river sediment

grinding to at least <250/zm and boiling for 20 min with 1M HC1. When T P - P i > 10mg P kg -~ dw, TP - Pi can be taken as a reproducible estimate of Po, although a high content of iron in the sediment increases the risk of achieving negative Po values. Po calculated as T P - Pi is uncertain when the organic content is very low. Acknowledgements--The authors wish to thank the National Agency of Environmental Protection for their major financial support and The Danish Land Development Service which analysed the 1094 sediment samples with methods A and D. The authors are grateful to C. AubRobinson, Department of Earth Sciences, Arhns University, for critically reviewing the manuscript. We also express our indebtedness to laboratory technicians Susanne Luth Hnsted, Birte Laostsen and Birgit Toft for their patience and good humour during the preparation of the samples.

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