Report No DWI0741

Dec 1986

Utilisation on agricultural land is the major disposal outlet for sewage sludge in the UK, accounting for about 50% of the sludge produced annually. Metals in sludge applied to agricultural land remain in the cultivated layer of top soil. It is important that repeated applications of sludge do not build up metal concentrations in the soil to levels which could adversely affect crops or the animals, including man, which eat them. Field experiments were established at three sites on soils broadly representative of UK agriculture. Controlled applications of sludge were made and soil and crop concentrations of metals were monitored thereafter over a five year period. The results have defined the effect of sludge applications of known metal content on total and extractable concentrations of metals in soil. The results have defined also the extent of transfer of metals from sludge-treated soil into the leaves and edible parts of six crops of major importance to UK agriculture.

Assist in the formulation of guidelines for the safe disposal of sewage sludge to agricultural land:

1. to determine the uptake of metals (particularly ) by crops;
2. to determine the effect of soil type on the uptake of metals by crops;
3. to determine the yield response to the nutrients and potentially phytotoxic elements in the sludge;
4. to examine the mobility of trace-metal contaminants within the soil after sludge application.

Crops to be grown and harvested on an annual rotation over five successive seasons, with the following variables:

1. four types of digested sludge, each having different concentrations of cadmium;
2. six rates of application, including nil, to give the required range of metal levels in the soil;
3. six crops one being a long term ley of grass;
4. replication of the experimental treatments.

Yield from each plot to be measured, and analysis of the following to be made where appropriate, for nutrients, cation exchange capacity, organic matter, total metals (Ni, Cu, Zn, Cd, Pb) and available metals (Ni, Cu, Zn, Cd, Pb - by the methods of the Agricultural Development and Advisory Service of the Ministry of Agriculture, Fisheries and Food methods):

1. samples of the soil taken before the (once-only) application of the sludge;
2. samples of soil taken after sludging and rotavating, and thereafter annually;
3. samples of each crop taken at maturity, analyses being made on the edible parts of each plant and, where funds approved permit, on other selected parts.

(iv) EFFECTS OF SLUDGE ON SOIL CONCENTRATIONS OF METALS

1. The amount of cadmium applied in liquid sludge to the top 15 cm of three diverse soil types could be entirely accounted for by soil analyses in this profile, approximately five years after sludge application. The increase in soil metal (Mi) above background values was estimated according to soil density, cultivated depth, and metal loading rate as:

Mi = 10 x Ml/(r x s) mg/kg

where M1 is the metal loading rate (kg/ha)

r the soil density (g/cm3), air-dried

s the depth of cultivation (cm)

Where the weight of sludge applied exceeds ten percent of the weight of soil in the cultivated layer, then the effect of sludge on soil density should be taken into account.

Amounts of other metals, Ni, Cu, Zn and Pb applied in liquid sludge were also accounted for in the cultivated profile with the exception of Cu and Zn in the calcareous loam where some mobility may have occurred beyond 15 cm depth or there may have been some lateral movement of sludge-treated soil off the plots following cultivation.

2. The percentages of EDTA-extractable Ni, Cu, Zn and zinc equivalent (ZE) of total in unsludged soils were lower than for metals of sludge origin:

	unsludged soil
	Ni	Cu	Zn	ZE
non-calcareous	13	39	6	13
calcareous	4	20	19	12

3. The percentages of EDTA-extractable Ni, Cu, Zn and ZE of total metal of sludge origin for the liquid and bed-dried sludges in non-calcareous and calcareous soil were as follows:

	Liquid sludges				Bed-dried sludge
	Ni	Cu	Zn	ZE	Ni	Cu	Zn	ZE
non-calcareous	68	74	87	78	62	57	87	68
calcareous	22	37	46	33	12	27	26	21

Crop concentrations of the most plant-available metals (Cd, Zn and Ni) generally followed the pattern of percentage EDTA-tractability being highest on non-calcareous soils and liquid sludge treatments.

(v) EFFECTS OF SLUDGE ON CROP YIELDS

1. Crop yields were not significantly affected by sludge treatments in 60% of all cases studied.
2. Crop yields were significantly increased in 26% of cases of liquid sludge addition and this was attributed to the beneficial effects on soil structure. Maximum dry solids additions to the soil from liquid sludge was 150 tonnes/hectare and 500 tonnes/hectare for bed-dried sludge.
3. Small (6-10% reductions) but statistically significant reductions in wheat grain yield were seen on the clay and calcareous loam soils treated with liquid sludge and the sandy loam and clay soils treated with bed-dried sludge. The results of comparisons between sludge types suggested this was not due to metals and the most likely explanation was lodging of the crop which occurred due to excessive nitrogen in the soil resulting from the single very heavy application of sludge made in the first year of the trial.

(vi) EFFECTS OF SLUDGE ON CROP COMPOSITION

1. Increases in metal concentrations in soil due to sludge added, produced significant increases in Cd, Ni, Cu and Zn concentrations in the edible portion of most of the six crops grown: wheat, potato, lettuce, red beet, cabbage and ryegrass.
2. Lead was relatively unavailable to crops from the soils. In the vast majority of cases there was no significant increase of Pb in crop tissue in relation to Pb added to the soil in sludge.
3. Increases in Cd concentrations in crops were most frequently directly proportional to the total Cd concentration in the soil. Thus they were estimated by a linear regression model fitted to Cd in crops plotted against 'total' Cd in soil. Increases in crop concentrations of Cd (mg/kg dm) per 1 mg/kg increase in Cd content of liquid sludge-treated soil (air dry) are summarised below (see also Tables 29 and 42):

 

Crop	Soil*	Estimated increase in crop tissue concentration per 1 mg/kg increase in soil content.
Wheat	sl	0.097
grain	c	0.105
	cl	0.046
Potato	sl	0.042
tuber	c	NS
	cl	0.028
Lettuce	sl	0.75
	c	1.2
	cl	0.34
Red beet	sl	0.22
root	c	0.24
	cl	0.07
Cabbage	sl	0.034
	c	0.051
	cl	0.044
Ryegrass	sl	0.084
	c	0.079
	cl	0.040

 

* sl = sandy loam, c = clay, cl = calcareous loam NS = relationship not statistically significant

4. Increases in Zn concentrations in crops were most frequently proportional to the logarithm of the EDTA-extractable Zn concentration in the soil. Zn levels in crops were best estimated by fitting a logarithmic curve to data when plotted against EDTA extractable Zn in soil

(vii) EFFECTS OF TIME

1. In general terms agreement between actual and predicted concentrations of metals in soil was best in the last two years of the trial. m e explanation for this may be that the repeated cultivations gradually led to better mixing of sludge and soil and hence reduced the error associated with soil sampling.
2. There was little evidence that sludge metals were lost from the cultivated horizon except where sludge was originally incorporated below this depth. Apparent losses of less than ten percent of total Cu and Zn applied to the calcareous loam were indicated. Downward or lateral loss of sludge-treated soil from the plots as a result of cultivation could explain this effect.
3. The EDTA-extractable fraction of total Cu, Ni and Pb from the two non-calcareous soils increased with time.
4. Cadmium and Zn availability to crops over 5 years showed only three definite significant seasonal variations but less than ten percent of cases indicated definite trends over 5 years. Overall there was no trend in crop availability on any soil over S years. There was no trend of reducing plant availability of metals over the 5 year period.

(viii) EFFECTS OF SOIL CONDITIONS AND SLUDGE TYPE

1. The EDTA-extractable fraction of total Ni, Cu and Zn was less from the calcareous loam than the two non-calcareous soils. The relative differences were: Ni one fifth, Cu one half, and Zn four tenths.
2. The rate of increases in Cd concentrations in crops grown on non calcareous soils was on average double the rate on calcareous soil. m is was attributed to the difference in pH of 6.5 for non calcareous soils in contrast to 8.0 for the calcareous soil.
3. Although the non-calcareous soils had the same pH, Cd was 1.6 times more available from the clay than the sandy loam to lettuce and cabbage. Even on unsludged treatments Cd was most available from the clay. m e explanation for this may lie in the low content of iron and manganese sesquioxides of the clay soil.
4. Metals expressed as zinc equivalent were about 10% less extractable from soil treated with bed-dried sludge than from soil treated with liquid sludge (Table 40, Page 95).
5. The availability of metals to crops was in general terms lower in soil treated with bed-dried sludge compared with liquid sludge. This effect depended on the crop in question. Cadmium availability to crops from bed-dried sludge was found to be a half to an eighth of that from liquid sludges (Table 42, Page 97). This was probably because physically discrete bed-dried sludge particles remained in the soil during the trial reducing the likelihood of contact with crop roots. However, EDTA analysis indicated that metals may also be in less available chemical forms. Physical effects would be expected to decrease with time particularly on cultivated land.

(IX) RELATION TO CURRENT RECOMMENDATIONS FOR SLUDGE UTILISATION

1. Nickel, copper and zinc concentrations in soil produced by the upper rates of liquid and bed-dried sludge were close to the maximum permissible levels set out in the EC Directive for the use of sewage sludge in agriculture. In terms of additions of zinc equivalent, upper rates of sludge application to soil exceeded the maximum permitted in UK guidelines by about 2x and 4x for liquid sludge and bed-dried sludge respectively (Table 34, Page 85). Nevertheless, no phytotoxic effects of metals were evident in terms of reduced yields, toxic tissue concentrations or visible symptoms. The only exception to this finding concerned concentrations of copper and zinc in lettuce grown on the clay soil which exceeded upper critical concentrations at high rates of sludge application.
2. In general terms there was no significant increase in crop concentrations of lead according to sludge treatment. Sludge applications did not influence the lead content of crops by the plant uptake route up to a soil concentration of Pb of at least 330 mg/kg, the highest recorded in the trial. This is the comparison to the EEC upper limit of 300 mg/kg.
3. In general terms there was a linear relationship between concentrations of cadmium in soil and concentrations of cadmium in the edible parts of the crops studied. An increase of 2.5 g/kg in the soil concentration of cadmium produced the following increases in crop tissue content of cadmium (mg/kg dm):

 

Crop	Non-calcareous soils	Calcareous soils
Wheat grain	0.25	0.12
Potato tuber	0.11	0.07
Lettuce	2.4*	0.85
Red beet root	0.58	0.18
Cabbage	0.11*	0.11
Ryegrass	0.20	0.10

* see (viii) 3.

4. The percentage increase in crop cadmium content on calcareous soil was usually less than that seen on the two non-calcareous soils. Extractability of the zinc equivalent metals by EDTA from calcareous soils treated with sludge was about 40% of extractability from non-calcareous soils treated with sludge.

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