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The effects of sewage sludge application to a heathland site prior to planting with Sitka spruce
Forest Ecology and Management, 66 ( 1994 ) 151-163
151
Elsevier Science B.V.
The effects of sewage sludge application to a heathland site prior to planting with Sitka spruce J. D u t c h ~,* and R. Wolstenholmeb aForestry Commission, Northern Research Station, Roslin EH25 9SY, UK bWRc plc, Beta Centre, Innovation Park, Stirling FK9 4NF, UK (Accepted 5 February 1993)
Abstract
New outlets are being sought for sewage sludge which is currently disposed of at sea. Sewage sludge has long been used as a fertiliser in agriculture, and a research programme is now evaluating the potential of sludge as a forest fertiliser. Single applications of sludge ( 13 and 26 Mg dry solids h a - ~) before planting have enabled a pure Sitka spruce crop to be established on a heathland site where large inputs of conventional fertilisers and herbicides would have been required. Sludge applications reduced the amount of heather in the ground vegetation which would otherwise have resulted in reduced growth of the spruce. At age 7 years the mean tree heights in plots treated with sewage sludge are significantly greater than in those plots receiving conventional treatments. Foliar nitrogen concentrations have now declined from satisfactory to deficient in the sludge treated plots and the conventionally fertilised plots which have not received nitrogen treatments. Phosphorus concentrations remain satisfactory. Metal additions from the sludge applications were well within limits set for agricultural use and could largely be accounted for in the top 75 mm of the soil 5 years after the sludge was applied. Although some increases in the concentration of metals in the spruce foliage were observed, levels were below those at which any phytotoxic effects could be expected. The quality of water draining from the site was also monitored and found to be satisfactory immediately following sludge application. In subsequent months low levels of nutrient leaching occurred only after heavy rainfall, but levels were still within the quality standards for surface water intended for abstraction for drinking water.
Introduction
The disposal of sewage sludge produced as a result of domestic wastewater treatment poses an increasing problem to the U K water industry. New outlets will have to be found for the sludge as the current sea disposal method is to be phased out by 1998. One new outlet for the use of sludge is in forestry as a fertiliser. Sludge contains useful amounts of nitrogen (N) and phosphorus (P), nutrients which frequently limit tree growth. In the USA, forest application of processed sludge has occurred since the mid 1960s and Metro Seattle in Washington has recy*Corresponding author.
© 1994 Elsevier Science B.V. All rights reserved 0378-1127/94/$07.00
SSDI0378-1127(93)06023-X
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cled all of its sludge to land for the past 16 years, with a large proportion going to forestry (Nichols, 1991; H e n r / e t al., this issue, pages 137-149). Agricultural use of sludge as a fertiliser currently accounts for 40% of the UK's sludge production (Department of the Environment (DOE), 1983), but its potential use in forestry received relatively little attention up to the early 1980s. Since then a joint research programme between the Forestry Commission and WRc has established a number of experiments where both the crop response and environmental aspects of sludge applications to forests have been studied (Bayes et al., 1991 ). An estimate made in 1987 suggested that up to 11% of the total U K sludge production of 1.2 X 106 Mg dry solids (ds) could potentially be used in forestry (Taylor and Moffat, 1991 ). The experiment reported here is one of the first to be established as part of the Forestry Commission/WRc research programme. The objectives of the experiment were: (1) to compare sewage sludge applications with conventional fertiliser/ herbicide treatments as a method of establishing a Sitka spruce plantation on a mineral heathland site; (2) to monitor any run-off of sludge or nutrients into the drainage water; (3) to assess any increases in soil and foliar concentrations of metals as a result of sludge addition. Results for the first 7 years of this experiment are reported here. Materials and methods
Site and application description The site, in north Scotland (national grid reference NH 724 794), is at an elevation of 200 m, and is fairly exposed. The ground has a slope of approximately 9% with a northeasterly aspect. At the start of the experiment the area was unplanted, with heather (Calluna vulgaris (L). Hull) dominating the ground vegetation. The soil is a humus-ironpan Stagnopodzol (Avery, 1990) with the depth of peat varying between 100 and 450 m m and a pHH2o of 3.9 in the upper horizon. Rainfall for the area is 900-1000 mm year- 1. A pure Sitka spruce (Picea sitchensis (Bong.) Carr) crop established on such a site would normally receive an application of phosphate at the time of planting with a further application at Year 6-8. In addition, the crop would require some form of treatment to overcome nitrogen deficiency. Chemical control of heather would produce some growth response but this would need to be followed up later with a nitrogen fertiliser (e.g. urea) to allow the crop to attain canopy closure (Taylor and Tabbush, 1990). Six treatments were applied in a randomised block design with four replicates. Treatment plots were 0.1 ha with an internal assessment plot of 0.045 ha. A minimum buffer of 10 m was left between plots. Treatments were:
J. Dutch, R. Wolstenholme / Forest Ecology and Management 66 (1994) 151-163
15 3
( 1 ) P, rock phosphate (50 kg P ha -1 ) in May 1984 after planting, and in June 1989 (60 kgP ha-~); (2) HP, rock phosphate as for (1) plus herbicide (2,4-0 ester at 6 kg a.i. ha-1 ) applied in August 1987 to eliminate heather; (3) NP, rock phosphate as for ( 1) plus urea ( 150 kg N ha- 1) in April 1986 and June 1989; (4) NPH, urea and rock phosphate as for ( 3 ) and herbicide as for (2); ( 5 ) S 1, liquid sludge applied in July 1983 at 365 m 3 ha-1 ( 13 Mg ds ha- 1 ) to supply 445 kg N ha-1 and 128 kg P ha-1 before planting; (6) $2, liquid sludge applied in July 1983 at 728 m 3 ha-~ (26 Mg ds ha-1 ) to supply 893 kg N ha -~ and 256 kg P ha-~ before planting. One further plot (0) received no conventional fertiliser or sludge, but did receive heather control in August 1987. As this plot was unreplicated and was not part of the main experimental design only irregular measurements of tree height have been made. No foliar sampling has been carried out on this plot, however soil analyses for heavy metals and nutrients were made in 1988. The lower rate of sludge application provided 140 kg N ha- 1 in a readily available form as ammonium (NH4). This is approximately equivalent to the standard rate of urea application for forestry use of 150 kg N ha- 1. The sludge used for the experiment was a liquid undigested sludge (cosettled primary sludge consisting of solids from the settlement of sewage together with surplus activated sludge) from Highland Regional Council's Dingwall sewage treatment works. The sludge was stored on site in a lagoon and applied in July 1983 by means of a tractor-drawn slurry tanker. The site was then ploughed in August 1983 using a double mould-board plough to a depth of 600 mm. Planting took place in April 1984 using bare-rooted Sitka spruce transplants of Queen Charlotte Islands origin (average height 150250mm).
Assessments and monitoring Total tree height was assessed annually after each growing season from autumn 1984 onwards. Foliar samples were collected each autumn from the top whorl of ten trees per plot and bulked for the plot. Samples were analysed annually for major nutrients, and in 1985 the samples were also analysed for metals. Five years after sludge application ( 1988 ) five soil cores were taken at three depths (0-75 mm, 75-150 mm and 150-200 mm) from each plot and bulked. The litter layer was included in the 0-75 mm depth. Samples were analysed for nutrients and heavy metals. A cut-off drain was installed below the site before sludge application in order to allow collection and monitoring of surface water run-off. Three monitoring points along the cut-off drain were spot sampled by the Highland River
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Purification Board: M 1, receiving drainage from unploughed heathland (the quality of which is indicative of natural run-off); M2, receiving drainage as above plus drainage from an area ploughed and fertilised with rock phosphate (the change in quality from M1 indicates the effects of ploughing and P fertilising); M3, as for M1 and M2 plus drainage from the area containing the experimental plots (the change in quality from M2 indicates the effects of the sludge and urea applications). As no run-off was observed until after snow-melt in February 1984, sampiing was only possible after this date and continued until June 1986. Flow was intermittent and water quality monitoring was restricted to 20 spot samples from the three points during this period. Sampling occurred following at least 10 mm of rainfall in 24 h with zero soil moisture deficit.
Laboratory techniques Foliar samples were dried, milled and digested using a semi-micro Kjeldahl method (Wall et al., 1975). Ammonium and phosphorus from the digests were estimated colorimetrically (Murphy and Riley, 1962; Crooke and Simpson, 1971 ) whilst potassium was analysed using flame emission spectroscopy. To determine foliar metals, oven-dried and milled plant material was ashed at 450°C overnight. This was then digested using nitric acid and the filtrate analysed for metals using atomic absorption spectrophotometry (M.L. Berrow, personal communication, 1991 ). The soil samples were force air dried at 80 °C and then ground to pass a 2 mm sieve and pelletised for metal analysis by X-ray fluorescence spectrometry. The pH was measured using standard Ministry of Agriculture Fisheries and Foods (MAFF) methods with 10 g of dried ground soil added to 25 ml of distilled water (MAFF, 1986). Total nitrogen was determined using a Technicon autoanalyser following sulphuric acid digestion (0.5 g of dried soil added to 10 ml of sulphuric acid, digested for 3 h, cooled and the solution made up to 100 ml) and reaction with hypochlorite and phenate ions in the presence of nitroprusside. Total phosphorus was determined using an auto analyser following catalysed sulphuric acid digest (0.5 g of soil in 10 ml of acid) and reaction with acid molybdate (Anonymous, 1985 ). Water samples were analysed using standard methods for the examination of waters and associated materials (Anonymous, 1980, 198 l a,b).
Statistical analyses The data were subjected to the standard ANOVA test for a randomised block design. Various contrasts and their standard errors were calculated and t-tests used to assess the significance.
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J. Dutch, R. Wolstenholme I ForestEcologyand Management66 (I 994) 151-163
Results Crop response The development of tree height is illustrated in Fig. 1. The mean total tree height in 1990 for the two sludge treatments (2.7 m) were significantly ( P < 0.001 ) greater than the mean height of the conventionally treated plots (2.3 m ). The difference in total tree height between the S 1 and $2 treatments was not significant until 1989, although the trend was for the $2 trees to sustain a higher growth rate than the S 1. The most recent data, 1990, now shows a significant difference ( P < 0.05 ) between total tree height in the $2 treatment (2.9 m) and the S1 treatment (2.55 m). Early height increments (until 1987 ) were significantly ( P < 0.001 ) greater in the S l and $2 plots compared with the P, NP, HP and NPH plots. However, the most recent height data (1990) show that there is now no significant difference in current growth (expressed as the height difference between 1989 and 1990) between the $2 trees (0.55 m) and the trees which had received conventional treatments designed to alleviate nitrogen problems, i.e. HP, NP and NPH (0.61 m). These latter treatments are, however, resulting in significantly ( P < 0 . 0 1 ) greater current growth than the S1 treatment (0.5 m) although the S 1 still has significantly ( P < 0.05 ) greater growth than the P alone treatment (0.28 m). Foliar concentrations of N and P are given in Tables 1 and 2, respectively. 3.00
-4D-p
2.50-
- x - HP --v- NP "-~'" NPH
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-,,--52 2.00.,J c 03
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,w "/
. / - .7 " ~ t"
1.50 -
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..-'" ..w /..-;,'" .- " .x/ .':'/". - ~ f - ~
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T-
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-
.50-
._.~
.00 1984~
C2~_ ''~
I
I
I
I
I
I
t985
t986
1987
1988
1989
1990
'~GQI"
Fig. 1. Tree height ( m ) at the end of the growing season. Sewage sludge applied to S plots 1983, phosphate to P plots 1984 and 1989, herbicide to H plots 1987, and urea to N plots 1986 and 1989. n = 4 for standard error of differences between means ( S E D ) shown.
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Table 1 Foliar nitrogen (percent oven dry weight) concentration in the top whorl of Sitka spruce needles 1. Plots NP and NPH received 150 kg N ha -1 in 1986 and 1989. Plots S1 and $2 received 445 and 893 kg N ha-1, respectively, in 1983 Year
1985 1986 1987 1988 1989 1990
Treatment
SE of mean (n=4)
P
HP
NP
NPH
S1
$2
2.73 1.77 1.44 1.92 1.04 0.77
2.73 1.77 1.43 1.50 1.67 1.24
2.73 1.81 1.69 1.46 1.78 1.49
2.73 1.81 1.69 1.80 2.24 1.62
2.47 1.82 1.64 1.44 1.16 0.85
2.68 1.87 1.59 1.51 1.37 1.01
0.03 0.I0 0.06 0.06 0.06 0.05
1Levels between 1.2 and 1.5 are marginal; levels below 1.2 are deficient (Binns et al., 1980). Table 2 Foliar phosphorus (percent oven dry weight) concentration in the top whorl of Sitka spruce needles 1. Plots P, HP, NP and NPH received 50 kg P ha -1 in 1984 and 60 kg P ha -1 in 1989. Plots SI and $2 received 128 and 256 kg P h a - ~, respectively, in 1983 Year
1985 1986 1987 1988 1989 1990
Treatment
SE of mean (n=4)
P
HP
NP
NPH
S1
$2
0.26 0.18 0.18 0.14 0.18 0.21
0.26 0.18 0.18 0.14 0.23 0.24
0.26 0.22 0.20 0.14 0.21 0.26
0.26 0.22 0.20 0.16 0.25 0.26
0.21 0.20 0.22 0.16 0.19 0.20
0.22 0.20 0.26 0.19 0.28 0.22
0.01 0.01 0.01 0.01 0.02 0.01
1Levels between 0.14 and 0.18 are marginal; levels below 0.14 are deficient (Binns et al., 1980).
Foliar N gradually declined from satisfactory concentrations in all plots in 1985 to being deficient in 1990, except in those plots which had received nitrogen in 1989 (NP and NPH). The mean foliar N values by 1989 for the sludge treated plots were significantly less ( P < 0.001 ) than the mean for all other plots which had received some form of N treatment, i.e. either N fertiliser or heather control. Foliar P concentrations remained satisfactory in all plots throughout the monitoring period. Significant uptake of copper and zinc was detected in the sludge treated trees when sampled for heavy metals in 1985 (Table 3 ). Soil metals Analysis of soil samples taken in autumn 1988 (5 years after application ) showed that the only significant increases in metal concentrations occurred
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Table 3 Foliar metal concentrations in the top whorl Sitka spruce needles two growing seasons after planting Metal
Cu Zn Pb
Treatment means (expressed as mg kg-~ dry matter) P
SE of mean (n--4)
S1
SE of mean (n=4)
$2
SE of mean (n=4)
2.25 21.20 <2.50
0.23 3.32
3.78 35.60 <2.50
0.31 5.64
4.04 37.20 <2.50
0.74 7.46
(M.L. Berrow, personal communication, 1989 ).
Table 4 Soil metal concentrations in the upper 75 mm, 5 years after sludge application Treatment means (expressed as mg kg-t dry matter)
O P HP NP NPH S1 $2 SE of mean (n=4)
Cr
Ni
Cu
Zn
Pb
Cd
14.0 8.2 9.5 8.5 8.7 13.2 13.5 1.6
6.0 3.0 1.5 4.5 2.7 3.0 3.2 0.7
10.0 12.2 8.2 11.2 10.2 31.2 40.5 2.3
34.0 46.0 35.2 47.2 47+0 83.5 99.8 6.7
32.0 35.7 31.2 49.0 94.5 136.5 79.3 25.0
0.4 0.4 0.5 0.4 0.5 0.5 0.6 0.03
in the surface 75 mm. The one exception to this was copper, where a small but significant ( P < 0 . 0 1 ) increase was observed in the 75-150 mm depth (data not shown). Data for the upper 75 mm are given in Table 4. Within this depth, concentrations of chromium and copper were significantly increased in the S1 plots (P<0.001 and 0.01, respectively) and also in the $2 plots ( P < 0.001 ). Zinc concentration was significantly higher only in the $2 treatment ( P < 0.05 ). These comparisons refer to the differences in soil metal concentrations between the conventional treatments and the sludge treated plots. To determine whether the amount of metals added to the plots from the sludge can be accounted for by the increased metal levels detected in the upper 75 mm of soil, the amount of metals present in the single absolute control plot were subtracted from the amount in the sludge treated plots. The absolute control plot was used for this calculation as any impurities of heavy metals in the conventional fertiliser were unknown. Table 5 gives a comparison
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Table 5 Comparison of rates of heavy metals applied in sludge with metals detected in the upper 75 mm of soil 5 years after application (kg ha- ' ) SI
Copper Zinc Lead
$2
Metal application rate
Metal level in soil
Metal application rate
Metal level in soil
3.4 6.5 3.2
3.3 7.9 16.5(4.3)'
6.8 13.0 6.4
4.9 10.4 7.4
'The value recorded for lead in the S1 treatment plots is high owing to one anomalous recorded value. Taking the means of the remaining values, as shown in parentheses, gives a more realistic figure. Table 6 Soil total phosphorus and nitrogen and pH in the upper 75 m m 5 years after sludge application Treatment means
Total P (%) Total N (%) pH
SE of means (n=4)
P
HP
NP
NPH
S1
$2
0.08 1.8 3.9
0.09 1.9 3.8
0.08 2.0 3.8
0.08 1.9 3.9
0.12 1.9 3.9
0.15 2.1 3.9
0.007 0.137
of metal loading to the plots from the sludge (kg ha- ~) and an estimate of elevated metal levels detected in the upper 75 mm of the sludge treated plots. The metal concentrations found in the soil were converted to a mass per unit area basis using a measured surface soil bulk density of 0.21 g cm -3 (N.A. Tenakon, personal communication, 1991 ). This comparison has not been made for cadmium and nickel because the application rates of these metals were small in relation to background soil levels.
Soil nutrients Soil total N and P concentrations in the upper 75 mm are given in Table 6. The analysis of the soil in 1988 showed that the total P present in the sludge treated plots was significantly higher in the 0-75 and 75-150 mm levels ( P < 0.001 and P < 0.01, respectively) than in plots which had received rock phosphate. There was no significant difference for total P in the 150-200 mm level. No significant differences in soil total N concentrations were detected in the 0-75 mm and 150-200 mm levels between the plots treated with sludge
0.17
0.41
1.25
0.11
0.24
0.66
MI Drainage from unploughed heathland M2 As M 1 plus drainage from ploughed and fertilised area M3 As M2 plus drainage from sludge treated plots
95 percentile
Mean
Sample point
Ammoniacal nitrogen mgNl -l
Table 7 Water quality at the Ardross site ( 1984-1986 )
0.43
0.08
<0.06
Mean
0.91
0.27
<0.06
95 percentile
Nitrate nitrogen mgN1-1
l 11
57
32
Mean
297
133
60
95 percentile
Total phosphorus ugP1 - l
39
9
4
Mean
112
23
9
95 percentile
Soluble reactive phosphorus ugP1 - l
%J~t ",O
-~
Z~
~-
-~ ~.
~
160
J. Dutch, R. Wolstenholme / Forest Ecology and Management 66 (1994) 151-163
and the plots which received conventional treatments to overcome N problems (i.e. HP, NP and NPH). Total N concentration in the 75-150 mm level was greater for the sludge treated plots than the HP, NP and NPH plots (P<0.01). However, in all levels all these treatments had significantly (P<0.05) higher concentrations of total N than plots which had received only P.
Water There was no direct run-off of sludge into the cut-off drain following application. No water was detected in the cut-off drain from the time of application in July 1983 until snow melt in 1984. During the following 17 months elevated nutrient levels in the water were experienced only after prolonged rainfall when the soil was at field capacity. Sludge application resulted in an increase in NH4 of 0.4 mg 1-~ (Table 7). Nitrate levels were raised by 0.4 mg 1-1. Soluble reactive P was increased by 30/zg 1-1, and insoluble P by 24 #g 1-1.
Ground vegetation The two sludge treatments eliminated the heather. Heather has re-invaded the S 1 plots 7 years after application. Although heather is now present in the $2 plots, rosebay willow herb (Chamaenerion angustifolium (L.) Scopoli) still dominates the ground vegetation. In the plots which have been treated with herbicide, dead heather still dominates the ground flora. Discussion and conclusions
Data from the first 7 years of this experiment show that a single application of sewage sludge before planting has had an effect on tree growth equivalent to the most intensive conventional treatment of two phosphate applications, two nitrogen applications and herbicide control of heather. The most recent growth data suggest that although growth in the S 1 treatment has begun to decline, the growth rate in the $2 treatment is still equivalent to the other treatments. Continued monitoring of tree height will show whether the rates used provided sufficient nutrients to sustain the crop through to canopy closure. Foliar nitrogen concentrations were similar for all treatments initially but those which have received no N fertiliser since planting, i.e. P, HP, S 1 and $2 are now declining below the deficiency threshold. Trees in these treatments may enter a period of'checked' growth. Foliar P levels show no indication of a deficiency of this element. Increases in foliar metal concentrations were observed for copper and zinc.
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Levels were well below those which would give rise to adverse growth effects (Balsberg Pahlsson, 1989) and as both these elements are trace nutrients required for plant growth, increases may in some cases be beneficial. No increase in foliar lead concentrations was detected. Increases in soil metal levels showed that nearly all of the metals added in the sludge could be accounted for in the top 75 m m of the soil profile 5 years after the sludge was applied. The metals may have formed relatively stable complexes with the organic matter in the upper 75 m m and so be unavailable for leaching. However, no studies have been done on the form of the metals in the soil to confirm this. Concentrations of metals within the soil are well within permissible levels for agriculture (CEC, 1986). These findings are in agreement with those of other experiments where heavy metals have been found to accumulate almost entirely in the raw humus layer (Olesen and Mark, 1991). Soil from the sludge treated plots, when sampled in 1988, showed higher concentrations of P in the upper level than the plots which had received rock phosphate. However, at the time of sampling only one application of rock phosphate had been made to the conventional treatments, with a second application being made 1 year later. The total quantity of P applied to the conventional treatments ( 50 kg P h a - 1) was therefore not comparable with that added in the sludge treated plots (128 and 256 kg P ha -1 in the S1 and $2 treatments, respectively). This difference in the quantity of P applied, rather than any differences in the form of the P, can account for the values recorded in 1988. Unlike total P, total N was not significantly elevated in the upper 0-75 m m level of the soil although markedly higher levels of N had been applied to the S plots than to any of the other treatments and significantly elevated levels of N were found in the 75-150 m m level. Owing to the fact that tree uptake, leaching and gaseous losses of N were either not measured or cannot be calculated on a per hectare basis the importance of the elevated N levels in this mid horizon cannot be assessed in terms of a budgetary approach. However, foliar analysis results for 1989 and 1990, if used to indicate the availability of this additional N, do not suggest that N is being made available to the trees at a sufficient rate to avoid deficiency. The experiment demonstrates that a single liquid sludge application supplying up to 900 kg N and 250 kg P h a - ~ can be applied to a heathland site with only low levels of nutrient leaching being detected in the drainage water. However, not all of the area directly above the drain between points M2 and M3 had received sludge. It is estimated that the sludge treated area amounted to about 20% of the total area contributing to this drainage water. Levels of NH4, NO3 and P in the site drainage did not exceed the quality standards for surface water intended for the abstraction of drinking water (CEC, 1975 ). Drainage from the treatment plots would be further diluted by
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the receiving waters. However, increased levels of nutrients were still evident in the drainage 3 years after application, indicating the continued mineralisation of the sludge. Results were in agreement with findings elsewhere that the greatest potential problem is that of leaching o f N (Zasoski and Edmonds, 1986; Lavergne 1991 ). Sludge application resulted in a change in the ground vegetation with a reduction in the heather component. The negative effect of heather competition on the N status of spruce trees is well documented (Taylor and Tabbush, 1990). The reduction in heather cover as a result of the sludge treatments will therefore have contributed to improved N availability. Hence an additional benefit of sludge application is the financial saving and environmental acceptability associated with reduced herbicide usage on a heathland site. The experiment has clearly demonstrated the potential of sewage sludge as a suitable alternative to conventional fertiliser treatments in the establishment stage of forestry. Possible adverse environmental effects in terms of water quality and soil metal concentrations were monitored and not found to be a problem on this site at the rates and conditions of sludge application employed. The application rates used ( 13 and 26 Mg ds h a - 1) are in line with those used in the USA where research has shown that sludge can be applied at the rate of 34-45 Mg ds h a - 1to forests with a return frequency of 5-7 years (Nichols, 1991 ) . However, suitable rates of application of sludge will need to be planned on a site-by-site basis taking account of local conditions such as topography, drainage and sludge quality. Results from this and other experiments in the joint research programme between the Forestry Commission and WRc are currently being used to develop a manual of good practice for the use of sewage sludge in British forestry.
Acknowledgements The authors wish to thank the Highland Regional Council Water and Sewerage Department, the Highland River Purification Board, Macaulay Land Use Research Institute, and the Forestry Commission research staffwho have planted and maintained the experiment. The financial support of the Scottish Office Environment Department is also gratefully acknowledged.
References Anonymous, 1980. Methods for the Examination of Waters and Associated Materials Phosphorus in Water, Effluents and Sewages. HMSO, London. Anonymous, 198 la. Methods for the Examination of Waters and Associated Materials Ammonia in Waters. HMSO, London. Anonymous, 1981b. Methods for the Examination of Waters and Associated Materials Oxidised Nitrogen in Waters. HMSO, London.
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Anonymous, 1985. Total Nitrogen and Total Phosphorus in Sewage Sludge. Methods for the Examination of Waters and Associated Materials. HMSO, London. Avery, B.W., 1990. Soils of the British Isles. CAB International, Wallingford, 463 pp. Balsberg Pahlsson, A-M., 1989. Toxicity of Heavy Metals (Zn, Cu, Cd, Pb) to vascular plants. Water Air Soil PoUut., 47: 287-319. Bayes, C.D., Taylor, C.M.A. and Moffat, A.J., 1991. Sewage sludge utilisation in forestry; the UK research programme. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 115-138. Binns, W.O., Mayhead, G.K. and Mackenzie, J.M., 1980. Nutrient deficiencies of conifers in British forests. Forestry Commission Leaflet 76, HMSO, London, 23 pp. CEC, 1975. Directive concerning the quality required of surface water intended for the abstraction of drinking water in the Member States, 16 June 1975 (75/440/EEC). Official Journal of the European Communities L 194, 25 July. CEC, 1986. Council directive on protection of the environment and in particular of the soil, when sewage sludge is used in agriculture. Official Journal of the European Communities No L 181/6-12, 4 July. Crooke, W.M. and Simpson, W.E., 1971. Determination of ammonium in Hjeldahl digests of crops by an automated procedure. J. Sci. Food Agric., 22: 9-10. Department of the Environment, 1983. Sewage sludge survey, 1980 data. Standing Committee on the Disposal of Sewage Sludge. DoE, London. Lavergne, G., 1991. Utilisation of dehydrated sludge from Marseille's purification station in forestry. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 167-176. Ministry of Agriculture, Fisheries and Food, 1986. The analysis of agricultural materials. HMSO, London. Murphy, J. and Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural water. Anal. Chim. Acta, 27:31-36. Nichols, C.G., 1991. US forestry uses of municipal sludge. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford. pp. 155-166. Olesen, S.E. and Mark, H.S., 1991. Long-term effects of sewage sludge application in a conifer plantation on a sandy soil. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 177-198. Taylor, C.M.A. and Moffat, A.J., 1991. The potential for utilising sewage sludge in forestry in Great Britain. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge, Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 103-114. Taylor, C.M.A. and Tabbush, P.M., 1990. Nitrogen deficiency in Sitka spruce plantations. Forestry Commission Bulletin 89, HMSO, London, 20 pp. Wall, L.L., Gehrke, C.W., Neuner, J.E., Cathey, R.D. and Rexroad, P.R., 1975. Total protein nitrogen: evaluation and comparison of four different methods. J. Assoc. Off. Chem., 58: 807-811. Zasoski, R.J. and Edmonds, R.L., 1986. Water quality in relation to sludge and wastewater applications to forest land. In: D.W. Cole, C.L. Henry and W.L. Nutter (Editors), The Forest Alternative for Treatment and Utilization of Municipal and Industrial Wastes. University of Washington Press, Seattle and London, pp. 100-109.
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Elsevier Science B.V.
The effects of sewage sludge application to a heathland site prior to planting with Sitka spruce J. D u t c h ~,* and R. Wolstenholmeb aForestry Commission, Northern Research Station, Roslin EH25 9SY, UK bWRc plc, Beta Centre, Innovation Park, Stirling FK9 4NF, UK (Accepted 5 February 1993)
Abstract
New outlets are being sought for sewage sludge which is currently disposed of at sea. Sewage sludge has long been used as a fertiliser in agriculture, and a research programme is now evaluating the potential of sludge as a forest fertiliser. Single applications of sludge ( 13 and 26 Mg dry solids h a - ~) before planting have enabled a pure Sitka spruce crop to be established on a heathland site where large inputs of conventional fertilisers and herbicides would have been required. Sludge applications reduced the amount of heather in the ground vegetation which would otherwise have resulted in reduced growth of the spruce. At age 7 years the mean tree heights in plots treated with sewage sludge are significantly greater than in those plots receiving conventional treatments. Foliar nitrogen concentrations have now declined from satisfactory to deficient in the sludge treated plots and the conventionally fertilised plots which have not received nitrogen treatments. Phosphorus concentrations remain satisfactory. Metal additions from the sludge applications were well within limits set for agricultural use and could largely be accounted for in the top 75 mm of the soil 5 years after the sludge was applied. Although some increases in the concentration of metals in the spruce foliage were observed, levels were below those at which any phytotoxic effects could be expected. The quality of water draining from the site was also monitored and found to be satisfactory immediately following sludge application. In subsequent months low levels of nutrient leaching occurred only after heavy rainfall, but levels were still within the quality standards for surface water intended for abstraction for drinking water.
Introduction
The disposal of sewage sludge produced as a result of domestic wastewater treatment poses an increasing problem to the U K water industry. New outlets will have to be found for the sludge as the current sea disposal method is to be phased out by 1998. One new outlet for the use of sludge is in forestry as a fertiliser. Sludge contains useful amounts of nitrogen (N) and phosphorus (P), nutrients which frequently limit tree growth. In the USA, forest application of processed sludge has occurred since the mid 1960s and Metro Seattle in Washington has recy*Corresponding author.
© 1994 Elsevier Science B.V. All rights reserved 0378-1127/94/$07.00
SSDI0378-1127(93)06023-X
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cled all of its sludge to land for the past 16 years, with a large proportion going to forestry (Nichols, 1991; H e n r / e t al., this issue, pages 137-149). Agricultural use of sludge as a fertiliser currently accounts for 40% of the UK's sludge production (Department of the Environment (DOE), 1983), but its potential use in forestry received relatively little attention up to the early 1980s. Since then a joint research programme between the Forestry Commission and WRc has established a number of experiments where both the crop response and environmental aspects of sludge applications to forests have been studied (Bayes et al., 1991 ). An estimate made in 1987 suggested that up to 11% of the total U K sludge production of 1.2 X 106 Mg dry solids (ds) could potentially be used in forestry (Taylor and Moffat, 1991 ). The experiment reported here is one of the first to be established as part of the Forestry Commission/WRc research programme. The objectives of the experiment were: (1) to compare sewage sludge applications with conventional fertiliser/ herbicide treatments as a method of establishing a Sitka spruce plantation on a mineral heathland site; (2) to monitor any run-off of sludge or nutrients into the drainage water; (3) to assess any increases in soil and foliar concentrations of metals as a result of sludge addition. Results for the first 7 years of this experiment are reported here. Materials and methods
Site and application description The site, in north Scotland (national grid reference NH 724 794), is at an elevation of 200 m, and is fairly exposed. The ground has a slope of approximately 9% with a northeasterly aspect. At the start of the experiment the area was unplanted, with heather (Calluna vulgaris (L). Hull) dominating the ground vegetation. The soil is a humus-ironpan Stagnopodzol (Avery, 1990) with the depth of peat varying between 100 and 450 m m and a pHH2o of 3.9 in the upper horizon. Rainfall for the area is 900-1000 mm year- 1. A pure Sitka spruce (Picea sitchensis (Bong.) Carr) crop established on such a site would normally receive an application of phosphate at the time of planting with a further application at Year 6-8. In addition, the crop would require some form of treatment to overcome nitrogen deficiency. Chemical control of heather would produce some growth response but this would need to be followed up later with a nitrogen fertiliser (e.g. urea) to allow the crop to attain canopy closure (Taylor and Tabbush, 1990). Six treatments were applied in a randomised block design with four replicates. Treatment plots were 0.1 ha with an internal assessment plot of 0.045 ha. A minimum buffer of 10 m was left between plots. Treatments were:
J. Dutch, R. Wolstenholme / Forest Ecology and Management 66 (1994) 151-163
15 3
( 1 ) P, rock phosphate (50 kg P ha -1 ) in May 1984 after planting, and in June 1989 (60 kgP ha-~); (2) HP, rock phosphate as for (1) plus herbicide (2,4-0 ester at 6 kg a.i. ha-1 ) applied in August 1987 to eliminate heather; (3) NP, rock phosphate as for ( 1) plus urea ( 150 kg N ha- 1) in April 1986 and June 1989; (4) NPH, urea and rock phosphate as for ( 3 ) and herbicide as for (2); ( 5 ) S 1, liquid sludge applied in July 1983 at 365 m 3 ha-1 ( 13 Mg ds ha- 1 ) to supply 445 kg N ha-1 and 128 kg P ha-1 before planting; (6) $2, liquid sludge applied in July 1983 at 728 m 3 ha-~ (26 Mg ds ha-1 ) to supply 893 kg N ha -~ and 256 kg P ha-~ before planting. One further plot (0) received no conventional fertiliser or sludge, but did receive heather control in August 1987. As this plot was unreplicated and was not part of the main experimental design only irregular measurements of tree height have been made. No foliar sampling has been carried out on this plot, however soil analyses for heavy metals and nutrients were made in 1988. The lower rate of sludge application provided 140 kg N ha- 1 in a readily available form as ammonium (NH4). This is approximately equivalent to the standard rate of urea application for forestry use of 150 kg N ha- 1. The sludge used for the experiment was a liquid undigested sludge (cosettled primary sludge consisting of solids from the settlement of sewage together with surplus activated sludge) from Highland Regional Council's Dingwall sewage treatment works. The sludge was stored on site in a lagoon and applied in July 1983 by means of a tractor-drawn slurry tanker. The site was then ploughed in August 1983 using a double mould-board plough to a depth of 600 mm. Planting took place in April 1984 using bare-rooted Sitka spruce transplants of Queen Charlotte Islands origin (average height 150250mm).
Assessments and monitoring Total tree height was assessed annually after each growing season from autumn 1984 onwards. Foliar samples were collected each autumn from the top whorl of ten trees per plot and bulked for the plot. Samples were analysed annually for major nutrients, and in 1985 the samples were also analysed for metals. Five years after sludge application ( 1988 ) five soil cores were taken at three depths (0-75 mm, 75-150 mm and 150-200 mm) from each plot and bulked. The litter layer was included in the 0-75 mm depth. Samples were analysed for nutrients and heavy metals. A cut-off drain was installed below the site before sludge application in order to allow collection and monitoring of surface water run-off. Three monitoring points along the cut-off drain were spot sampled by the Highland River
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Purification Board: M 1, receiving drainage from unploughed heathland (the quality of which is indicative of natural run-off); M2, receiving drainage as above plus drainage from an area ploughed and fertilised with rock phosphate (the change in quality from M1 indicates the effects of ploughing and P fertilising); M3, as for M1 and M2 plus drainage from the area containing the experimental plots (the change in quality from M2 indicates the effects of the sludge and urea applications). As no run-off was observed until after snow-melt in February 1984, sampiing was only possible after this date and continued until June 1986. Flow was intermittent and water quality monitoring was restricted to 20 spot samples from the three points during this period. Sampling occurred following at least 10 mm of rainfall in 24 h with zero soil moisture deficit.
Laboratory techniques Foliar samples were dried, milled and digested using a semi-micro Kjeldahl method (Wall et al., 1975). Ammonium and phosphorus from the digests were estimated colorimetrically (Murphy and Riley, 1962; Crooke and Simpson, 1971 ) whilst potassium was analysed using flame emission spectroscopy. To determine foliar metals, oven-dried and milled plant material was ashed at 450°C overnight. This was then digested using nitric acid and the filtrate analysed for metals using atomic absorption spectrophotometry (M.L. Berrow, personal communication, 1991 ). The soil samples were force air dried at 80 °C and then ground to pass a 2 mm sieve and pelletised for metal analysis by X-ray fluorescence spectrometry. The pH was measured using standard Ministry of Agriculture Fisheries and Foods (MAFF) methods with 10 g of dried ground soil added to 25 ml of distilled water (MAFF, 1986). Total nitrogen was determined using a Technicon autoanalyser following sulphuric acid digestion (0.5 g of dried soil added to 10 ml of sulphuric acid, digested for 3 h, cooled and the solution made up to 100 ml) and reaction with hypochlorite and phenate ions in the presence of nitroprusside. Total phosphorus was determined using an auto analyser following catalysed sulphuric acid digest (0.5 g of soil in 10 ml of acid) and reaction with acid molybdate (Anonymous, 1985 ). Water samples were analysed using standard methods for the examination of waters and associated materials (Anonymous, 1980, 198 l a,b).
Statistical analyses The data were subjected to the standard ANOVA test for a randomised block design. Various contrasts and their standard errors were calculated and t-tests used to assess the significance.
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J. Dutch, R. Wolstenholme I ForestEcologyand Management66 (I 994) 151-163
Results Crop response The development of tree height is illustrated in Fig. 1. The mean total tree height in 1990 for the two sludge treatments (2.7 m) were significantly ( P < 0.001 ) greater than the mean height of the conventionally treated plots (2.3 m ). The difference in total tree height between the S 1 and $2 treatments was not significant until 1989, although the trend was for the $2 trees to sustain a higher growth rate than the S 1. The most recent data, 1990, now shows a significant difference ( P < 0.05 ) between total tree height in the $2 treatment (2.9 m) and the S1 treatment (2.55 m). Early height increments (until 1987 ) were significantly ( P < 0.001 ) greater in the S l and $2 plots compared with the P, NP, HP and NPH plots. However, the most recent height data (1990) show that there is now no significant difference in current growth (expressed as the height difference between 1989 and 1990) between the $2 trees (0.55 m) and the trees which had received conventional treatments designed to alleviate nitrogen problems, i.e. HP, NP and NPH (0.61 m). These latter treatments are, however, resulting in significantly ( P < 0 . 0 1 ) greater current growth than the S1 treatment (0.5 m) although the S 1 still has significantly ( P < 0.05 ) greater growth than the P alone treatment (0.28 m). Foliar concentrations of N and P are given in Tables 1 and 2, respectively. 3.00
-4D-p
2.50-
- x - HP --v- NP "-~'" NPH
/ /" -'/"
-,,--52 2.00.,J c 03
~d
,w "/
. / - .7 " ~ t"
1.50 -
"" ~
..-'" ..w /..-;,'" .- " .x/ .':'/". - ~ f - ~
v-""
/""
T-
1.00
-
.50-
._.~
.00 1984~
C2~_ ''~
I
I
I
I
I
I
t985
t986
1987
1988
1989
1990
'~GQI"
Fig. 1. Tree height ( m ) at the end of the growing season. Sewage sludge applied to S plots 1983, phosphate to P plots 1984 and 1989, herbicide to H plots 1987, and urea to N plots 1986 and 1989. n = 4 for standard error of differences between means ( S E D ) shown.
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z Dutch, R. Wolstenholme / Forest Ecology and Management 66 (1994) 151-163
Table 1 Foliar nitrogen (percent oven dry weight) concentration in the top whorl of Sitka spruce needles 1. Plots NP and NPH received 150 kg N ha -1 in 1986 and 1989. Plots S1 and $2 received 445 and 893 kg N ha-1, respectively, in 1983 Year
1985 1986 1987 1988 1989 1990
Treatment
SE of mean (n=4)
P
HP
NP
NPH
S1
$2
2.73 1.77 1.44 1.92 1.04 0.77
2.73 1.77 1.43 1.50 1.67 1.24
2.73 1.81 1.69 1.46 1.78 1.49
2.73 1.81 1.69 1.80 2.24 1.62
2.47 1.82 1.64 1.44 1.16 0.85
2.68 1.87 1.59 1.51 1.37 1.01
0.03 0.I0 0.06 0.06 0.06 0.05
1Levels between 1.2 and 1.5 are marginal; levels below 1.2 are deficient (Binns et al., 1980). Table 2 Foliar phosphorus (percent oven dry weight) concentration in the top whorl of Sitka spruce needles 1. Plots P, HP, NP and NPH received 50 kg P ha -1 in 1984 and 60 kg P ha -1 in 1989. Plots SI and $2 received 128 and 256 kg P h a - ~, respectively, in 1983 Year
1985 1986 1987 1988 1989 1990
Treatment
SE of mean (n=4)
P
HP
NP
NPH
S1
$2
0.26 0.18 0.18 0.14 0.18 0.21
0.26 0.18 0.18 0.14 0.23 0.24
0.26 0.22 0.20 0.14 0.21 0.26
0.26 0.22 0.20 0.16 0.25 0.26
0.21 0.20 0.22 0.16 0.19 0.20
0.22 0.20 0.26 0.19 0.28 0.22
0.01 0.01 0.01 0.01 0.02 0.01
1Levels between 0.14 and 0.18 are marginal; levels below 0.14 are deficient (Binns et al., 1980).
Foliar N gradually declined from satisfactory concentrations in all plots in 1985 to being deficient in 1990, except in those plots which had received nitrogen in 1989 (NP and NPH). The mean foliar N values by 1989 for the sludge treated plots were significantly less ( P < 0.001 ) than the mean for all other plots which had received some form of N treatment, i.e. either N fertiliser or heather control. Foliar P concentrations remained satisfactory in all plots throughout the monitoring period. Significant uptake of copper and zinc was detected in the sludge treated trees when sampled for heavy metals in 1985 (Table 3 ). Soil metals Analysis of soil samples taken in autumn 1988 (5 years after application ) showed that the only significant increases in metal concentrations occurred
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Table 3 Foliar metal concentrations in the top whorl Sitka spruce needles two growing seasons after planting Metal
Cu Zn Pb
Treatment means (expressed as mg kg-~ dry matter) P
SE of mean (n--4)
S1
SE of mean (n=4)
$2
SE of mean (n=4)
2.25 21.20 <2.50
0.23 3.32
3.78 35.60 <2.50
0.31 5.64
4.04 37.20 <2.50
0.74 7.46
(M.L. Berrow, personal communication, 1989 ).
Table 4 Soil metal concentrations in the upper 75 mm, 5 years after sludge application Treatment means (expressed as mg kg-t dry matter)
O P HP NP NPH S1 $2 SE of mean (n=4)
Cr
Ni
Cu
Zn
Pb
Cd
14.0 8.2 9.5 8.5 8.7 13.2 13.5 1.6
6.0 3.0 1.5 4.5 2.7 3.0 3.2 0.7
10.0 12.2 8.2 11.2 10.2 31.2 40.5 2.3
34.0 46.0 35.2 47.2 47+0 83.5 99.8 6.7
32.0 35.7 31.2 49.0 94.5 136.5 79.3 25.0
0.4 0.4 0.5 0.4 0.5 0.5 0.6 0.03
in the surface 75 mm. The one exception to this was copper, where a small but significant ( P < 0 . 0 1 ) increase was observed in the 75-150 mm depth (data not shown). Data for the upper 75 mm are given in Table 4. Within this depth, concentrations of chromium and copper were significantly increased in the S1 plots (P<0.001 and 0.01, respectively) and also in the $2 plots ( P < 0.001 ). Zinc concentration was significantly higher only in the $2 treatment ( P < 0.05 ). These comparisons refer to the differences in soil metal concentrations between the conventional treatments and the sludge treated plots. To determine whether the amount of metals added to the plots from the sludge can be accounted for by the increased metal levels detected in the upper 75 mm of soil, the amount of metals present in the single absolute control plot were subtracted from the amount in the sludge treated plots. The absolute control plot was used for this calculation as any impurities of heavy metals in the conventional fertiliser were unknown. Table 5 gives a comparison
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Table 5 Comparison of rates of heavy metals applied in sludge with metals detected in the upper 75 mm of soil 5 years after application (kg ha- ' ) SI
Copper Zinc Lead
$2
Metal application rate
Metal level in soil
Metal application rate
Metal level in soil
3.4 6.5 3.2
3.3 7.9 16.5(4.3)'
6.8 13.0 6.4
4.9 10.4 7.4
'The value recorded for lead in the S1 treatment plots is high owing to one anomalous recorded value. Taking the means of the remaining values, as shown in parentheses, gives a more realistic figure. Table 6 Soil total phosphorus and nitrogen and pH in the upper 75 m m 5 years after sludge application Treatment means
Total P (%) Total N (%) pH
SE of means (n=4)
P
HP
NP
NPH
S1
$2
0.08 1.8 3.9
0.09 1.9 3.8
0.08 2.0 3.8
0.08 1.9 3.9
0.12 1.9 3.9
0.15 2.1 3.9
0.007 0.137
of metal loading to the plots from the sludge (kg ha- ~) and an estimate of elevated metal levels detected in the upper 75 mm of the sludge treated plots. The metal concentrations found in the soil were converted to a mass per unit area basis using a measured surface soil bulk density of 0.21 g cm -3 (N.A. Tenakon, personal communication, 1991 ). This comparison has not been made for cadmium and nickel because the application rates of these metals were small in relation to background soil levels.
Soil nutrients Soil total N and P concentrations in the upper 75 mm are given in Table 6. The analysis of the soil in 1988 showed that the total P present in the sludge treated plots was significantly higher in the 0-75 and 75-150 mm levels ( P < 0.001 and P < 0.01, respectively) than in plots which had received rock phosphate. There was no significant difference for total P in the 150-200 mm level. No significant differences in soil total N concentrations were detected in the 0-75 mm and 150-200 mm levels between the plots treated with sludge
0.17
0.41
1.25
0.11
0.24
0.66
MI Drainage from unploughed heathland M2 As M 1 plus drainage from ploughed and fertilised area M3 As M2 plus drainage from sludge treated plots
95 percentile
Mean
Sample point
Ammoniacal nitrogen mgNl -l
Table 7 Water quality at the Ardross site ( 1984-1986 )
0.43
0.08
<0.06
Mean
0.91
0.27
<0.06
95 percentile
Nitrate nitrogen mgN1-1
l 11
57
32
Mean
297
133
60
95 percentile
Total phosphorus ugP1 - l
39
9
4
Mean
112
23
9
95 percentile
Soluble reactive phosphorus ugP1 - l
%J~t ",O
-~
Z~
~-
-~ ~.
~
160
J. Dutch, R. Wolstenholme / Forest Ecology and Management 66 (1994) 151-163
and the plots which received conventional treatments to overcome N problems (i.e. HP, NP and NPH). Total N concentration in the 75-150 mm level was greater for the sludge treated plots than the HP, NP and NPH plots (P<0.01). However, in all levels all these treatments had significantly (P<0.05) higher concentrations of total N than plots which had received only P.
Water There was no direct run-off of sludge into the cut-off drain following application. No water was detected in the cut-off drain from the time of application in July 1983 until snow melt in 1984. During the following 17 months elevated nutrient levels in the water were experienced only after prolonged rainfall when the soil was at field capacity. Sludge application resulted in an increase in NH4 of 0.4 mg 1-~ (Table 7). Nitrate levels were raised by 0.4 mg 1-1. Soluble reactive P was increased by 30/zg 1-1, and insoluble P by 24 #g 1-1.
Ground vegetation The two sludge treatments eliminated the heather. Heather has re-invaded the S 1 plots 7 years after application. Although heather is now present in the $2 plots, rosebay willow herb (Chamaenerion angustifolium (L.) Scopoli) still dominates the ground vegetation. In the plots which have been treated with herbicide, dead heather still dominates the ground flora. Discussion and conclusions
Data from the first 7 years of this experiment show that a single application of sewage sludge before planting has had an effect on tree growth equivalent to the most intensive conventional treatment of two phosphate applications, two nitrogen applications and herbicide control of heather. The most recent growth data suggest that although growth in the S 1 treatment has begun to decline, the growth rate in the $2 treatment is still equivalent to the other treatments. Continued monitoring of tree height will show whether the rates used provided sufficient nutrients to sustain the crop through to canopy closure. Foliar nitrogen concentrations were similar for all treatments initially but those which have received no N fertiliser since planting, i.e. P, HP, S 1 and $2 are now declining below the deficiency threshold. Trees in these treatments may enter a period of'checked' growth. Foliar P levels show no indication of a deficiency of this element. Increases in foliar metal concentrations were observed for copper and zinc.
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Levels were well below those which would give rise to adverse growth effects (Balsberg Pahlsson, 1989) and as both these elements are trace nutrients required for plant growth, increases may in some cases be beneficial. No increase in foliar lead concentrations was detected. Increases in soil metal levels showed that nearly all of the metals added in the sludge could be accounted for in the top 75 m m of the soil profile 5 years after the sludge was applied. The metals may have formed relatively stable complexes with the organic matter in the upper 75 m m and so be unavailable for leaching. However, no studies have been done on the form of the metals in the soil to confirm this. Concentrations of metals within the soil are well within permissible levels for agriculture (CEC, 1986). These findings are in agreement with those of other experiments where heavy metals have been found to accumulate almost entirely in the raw humus layer (Olesen and Mark, 1991). Soil from the sludge treated plots, when sampled in 1988, showed higher concentrations of P in the upper level than the plots which had received rock phosphate. However, at the time of sampling only one application of rock phosphate had been made to the conventional treatments, with a second application being made 1 year later. The total quantity of P applied to the conventional treatments ( 50 kg P h a - 1) was therefore not comparable with that added in the sludge treated plots (128 and 256 kg P ha -1 in the S1 and $2 treatments, respectively). This difference in the quantity of P applied, rather than any differences in the form of the P, can account for the values recorded in 1988. Unlike total P, total N was not significantly elevated in the upper 0-75 m m level of the soil although markedly higher levels of N had been applied to the S plots than to any of the other treatments and significantly elevated levels of N were found in the 75-150 m m level. Owing to the fact that tree uptake, leaching and gaseous losses of N were either not measured or cannot be calculated on a per hectare basis the importance of the elevated N levels in this mid horizon cannot be assessed in terms of a budgetary approach. However, foliar analysis results for 1989 and 1990, if used to indicate the availability of this additional N, do not suggest that N is being made available to the trees at a sufficient rate to avoid deficiency. The experiment demonstrates that a single liquid sludge application supplying up to 900 kg N and 250 kg P h a - ~ can be applied to a heathland site with only low levels of nutrient leaching being detected in the drainage water. However, not all of the area directly above the drain between points M2 and M3 had received sludge. It is estimated that the sludge treated area amounted to about 20% of the total area contributing to this drainage water. Levels of NH4, NO3 and P in the site drainage did not exceed the quality standards for surface water intended for the abstraction of drinking water (CEC, 1975 ). Drainage from the treatment plots would be further diluted by
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J. Dutch, R. Wolstenholme / bbrest Ecology and Management 66 (1994) 151-163
the receiving waters. However, increased levels of nutrients were still evident in the drainage 3 years after application, indicating the continued mineralisation of the sludge. Results were in agreement with findings elsewhere that the greatest potential problem is that of leaching o f N (Zasoski and Edmonds, 1986; Lavergne 1991 ). Sludge application resulted in a change in the ground vegetation with a reduction in the heather component. The negative effect of heather competition on the N status of spruce trees is well documented (Taylor and Tabbush, 1990). The reduction in heather cover as a result of the sludge treatments will therefore have contributed to improved N availability. Hence an additional benefit of sludge application is the financial saving and environmental acceptability associated with reduced herbicide usage on a heathland site. The experiment has clearly demonstrated the potential of sewage sludge as a suitable alternative to conventional fertiliser treatments in the establishment stage of forestry. Possible adverse environmental effects in terms of water quality and soil metal concentrations were monitored and not found to be a problem on this site at the rates and conditions of sludge application employed. The application rates used ( 13 and 26 Mg ds h a - 1) are in line with those used in the USA where research has shown that sludge can be applied at the rate of 34-45 Mg ds h a - 1to forests with a return frequency of 5-7 years (Nichols, 1991 ) . However, suitable rates of application of sludge will need to be planned on a site-by-site basis taking account of local conditions such as topography, drainage and sludge quality. Results from this and other experiments in the joint research programme between the Forestry Commission and WRc are currently being used to develop a manual of good practice for the use of sewage sludge in British forestry.
Acknowledgements The authors wish to thank the Highland Regional Council Water and Sewerage Department, the Highland River Purification Board, Macaulay Land Use Research Institute, and the Forestry Commission research staffwho have planted and maintained the experiment. The financial support of the Scottish Office Environment Department is also gratefully acknowledged.
References Anonymous, 1980. Methods for the Examination of Waters and Associated Materials Phosphorus in Water, Effluents and Sewages. HMSO, London. Anonymous, 198 la. Methods for the Examination of Waters and Associated Materials Ammonia in Waters. HMSO, London. Anonymous, 1981b. Methods for the Examination of Waters and Associated Materials Oxidised Nitrogen in Waters. HMSO, London.
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Anonymous, 1985. Total Nitrogen and Total Phosphorus in Sewage Sludge. Methods for the Examination of Waters and Associated Materials. HMSO, London. Avery, B.W., 1990. Soils of the British Isles. CAB International, Wallingford, 463 pp. Balsberg Pahlsson, A-M., 1989. Toxicity of Heavy Metals (Zn, Cu, Cd, Pb) to vascular plants. Water Air Soil PoUut., 47: 287-319. Bayes, C.D., Taylor, C.M.A. and Moffat, A.J., 1991. Sewage sludge utilisation in forestry; the UK research programme. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 115-138. Binns, W.O., Mayhead, G.K. and Mackenzie, J.M., 1980. Nutrient deficiencies of conifers in British forests. Forestry Commission Leaflet 76, HMSO, London, 23 pp. CEC, 1975. Directive concerning the quality required of surface water intended for the abstraction of drinking water in the Member States, 16 June 1975 (75/440/EEC). Official Journal of the European Communities L 194, 25 July. CEC, 1986. Council directive on protection of the environment and in particular of the soil, when sewage sludge is used in agriculture. Official Journal of the European Communities No L 181/6-12, 4 July. Crooke, W.M. and Simpson, W.E., 1971. Determination of ammonium in Hjeldahl digests of crops by an automated procedure. J. Sci. Food Agric., 22: 9-10. Department of the Environment, 1983. Sewage sludge survey, 1980 data. Standing Committee on the Disposal of Sewage Sludge. DoE, London. Lavergne, G., 1991. Utilisation of dehydrated sludge from Marseille's purification station in forestry. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 167-176. Ministry of Agriculture, Fisheries and Food, 1986. The analysis of agricultural materials. HMSO, London. Murphy, J. and Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural water. Anal. Chim. Acta, 27:31-36. Nichols, C.G., 1991. US forestry uses of municipal sludge. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford. pp. 155-166. Olesen, S.E. and Mark, H.S., 1991. Long-term effects of sewage sludge application in a conifer plantation on a sandy soil. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge. Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 177-198. Taylor, C.M.A. and Moffat, A.J., 1991. The potential for utilising sewage sludge in forestry in Great Britain. In: J.E. Hall (Editor), Alternative Uses for Sewage Sludge, Proceedings of the WRc International Workshop, 5-7 September 1989, University of York, UK. Pergamon Press, Oxford, pp. 103-114. Taylor, C.M.A. and Tabbush, P.M., 1990. Nitrogen deficiency in Sitka spruce plantations. Forestry Commission Bulletin 89, HMSO, London, 20 pp. Wall, L.L., Gehrke, C.W., Neuner, J.E., Cathey, R.D. and Rexroad, P.R., 1975. Total protein nitrogen: evaluation and comparison of four different methods. J. Assoc. Off. Chem., 58: 807-811. Zasoski, R.J. and Edmonds, R.L., 1986. Water quality in relation to sludge and wastewater applications to forest land. In: D.W. Cole, C.L. Henry and W.L. Nutter (Editors), The Forest Alternative for Treatment and Utilization of Municipal and Industrial Wastes. University of Washington Press, Seattle and London, pp. 100-109.