Valtcho D. Zheljazkov (Jeliazkov)*, Phil R. Warman (Department of Environmental Sciences, Nova Scotia Agricultural College, PO Box 550, Truro, NS, Canada, B2N 5E3 Tel: (902) 893 7859, Fax: (902) 893 1404, e-mail: vjeliazkov@cadmin.nsac.ns.ca)
The literature on the bioavailability of compost-born As, Hg and Se to plants is reviewed. It was found, there were very few reports on the reactivity of these elements in the compost-soil-plant system, except for experiments with mushrooms. Mushrooms grown in compost high in As, Hg ad Se tend to accumulate these elements, especially Hg. Absorption and accumulation of compost born As, Hg and Se by plants depends mainly on their concentration and speciation in the compost, media pH, and plant species. From the literature it is not clear whether the application of compost to agricultural land would reduce the bioavailability of these elements.
It is concluded that further research is needed to address a number of questions such as
1) what is the long term effects of these elements as a result of repeated compost application to agricultural crops, 2) what is the dynamic in the speciation of these elements in soils, 3) are the guidelines for maximum acceptable concentration of these elements in compost practically feasible; do the guidelines encourage or restrict the use of composts to agricultural soils.
Arsenic, mercury and selenium are among the most hazardous pollutants in the environment, their volatile compounds (from industrial regions) can be transported over very long distances.
Because of the evidence for toxicity of these elements to plants, animals and humans, guidelines for their maximum concentration in various composts were established in a number of countries. Currently, the Canadian guidelines for maximum concentration of trace elements in compost allow up to 13, 0.8 and 2 mg/kg As, Hg and Se in Type AA compost and 75, 5, and 14 mg/kg (air-dried mass), respectively, in type B compost (CCME 1996). In Europe, different countries have very different standards for these elements in compost; generally the guidelines concerning As, Hg and Se are most restrictive in Netherlands, less restrictive in Switzerland, Germany, Belgium, Austria, and Denmark, and even less restrictive in Spain (Epstein, 1997).
With the increased interest by consumers and
producers towards the use of organic amendments as a substitute for
conventional fertilizers, and as a result of better regulated waste management,
increased compost production and its availability on the market, the use of
compost in agriculture will certainly increase. Although there is literature on the absorption of As, Hg and Se by
plants either from hydroponic, container, or field experiments, the movement of
these elements from compost to soils and plants is far from being completely
understood. The results of container
experiments on plant absorption when As, Hg and Se salts are added to compost
should be interpreted carefully, since such systems do not represent natural
conditions with the application of well matured composts. Therefore, the objective of this paper is to
review the literature on availability of compost-born As, Hg and Se to plants.
DISCUSSION
The As content in edible plants is generally
low, even if the crops are grown on contaminated areas. Roots tend to accumulate more As while
leaves and fruits accumulate much less.
There are reports of up to 3450 mg As/kg in grasses grown on spoil tips,
while similar grasses on uncontaminated soils would have up to 3 mg As/kg
(O’Neill, 1990). Some crops grown on
dredged spoil (35-108 mg As/kg) showed the following decreasing order of As
accumulation: radish > grass > lettuce > carrot > potato tuber >
spring wheat grain (O’Neill, 1990).
The uptake of inorganic mercury from the soil
is thought to be small, methylmercury being more easily taken up by plants
(Cappon, 1981; Woodbury, 1992).
However, there are early reports of significant accumulations of Hg in
plant roots, leaves, and seeds. For instance, Saha et al. (1970) showed that the mercury concentration of
wheat grain grown in CH3Hg- contaminated soils was in direct
correlation with the CH3Hg concentration in soils. Some investigations in Japan (Furutani and
Osajima 1965, 1967) showed that on well drained rice fields, when the Hg
concentration in dry soils was 0.33 mg/kg, the grain had 0.3 mg/kg Hg. The same authors found that in a poorly
drained rice field containing 1.36 mg/kg Hg in the soil, the grain contained
0.8 mg/kg Hg.
According to Mayland et al. (1989) and Shrift
(1973), plants can be divided into the following three groups according to
their Se uptake if grown on high Se substrates: 1) indicators (Astragalus),
2) secondary Se absorbers (Aster, Grindela etc), which accumulate
between 50 and 100 mg Se/kg, and 3) plants that accumulate very little Se even
if grown in high Se substrates (Trifolium repens, Buchloe dactyloides).
Warman et al. (1995) studied the effect of different percentages of biosolid-amended
compost (25, 50, 75, 100 and control) on the bioavailability of As, Cd, Co, Cr,
Cu, Hg, Mo, Ni, Pb, Se, and Zn to swiss chard.
The compost/biosolid mixture contained an average of 0.4 mg As/kg, 1.2
mg Hg/kg and 3.6 mg Se/kg. Plant As
concentration was low, from undetectable amounts to 0.1 mg/kg and not
significantly different between the treatments. Se content in the plants grown on the control soil was undetectable;
however, with an increasing application of biosolid/compost mixture, Se
concentration in the plant tissue also increased, to an average of 0.29
mg/kg. Still, the increase in tissue Se
was not proportional to the increase of media Se concentration. Total and DTPA extractable Se increased with
the increased application of compost/biosolid mixture to the medium and were
highly correlated. The authors found
that the biosolid-amended compost increased swiss chard tissue Hg levels and
uptake (tissue concentration multiplied by dry yield). The highest uptake was found in the 75%
compost treatment; the highest tissue Hg content found in plants was 0.3
mg/kg. Still, the authors showed that
the Hg content in the tissue did not increase proportionally with the increase
of Hg in the growth media.
Cappon (1987) studied the uptake and speciation
of Hg and Se in 16 different vegetable crops grown on garden plots which had
been exclusively treated with residential compost for 6 years. The author found Se to be readily taken up
by plants. Hexavalent Se was around 20%
of the total, the other forms were divalent and tetravalent Se. Crops accumulated Hg and had methyl-mercury
levels of 12.8 % of the total edible tissue Hg content, divalent inorganic Hg
being the only other form identified.
In field experiments with corn and sesame,
Abdel-Sabour et al. (1998) found that
increasing the rate of composted MSW and composted sewage sludge resulted in a
dramatic increase in Hg levels in seeds, sesame accumulated more than
maize. MSW compost enhanced the
accumulation of Hg and other metals in seeds more than sewage sludge
compost.
Rensburg et al. (1998)
evaluated eight amendments applied to the top of a fly ash coal dump with the
objectives to increase the ability of fly ash to support vegetation and to
select a grass species that would be suitable for revegetation. They found that the application of 5 t/ha
compost to be a suitable ameliorant.
Compost application to fly ash did not significantly change As and Se
accumulation in several grasses (Bermuda grass [Cynodon dactilon],
weeping lovegrass [Eragrostis curvula] and annual teff [Eragrostis
tef]) compared to treatments with kraal manure and inorganic
fertilizers. Tissue As was around 1.1
mg/kg and not different from its concentration in the vegetation near the dump,
while tissue Se was between 1.7 and 2.2 mg/kg
and significantly higher than in the vegetation near the dump. The tissue analysis indicated up to 4.4
mg/kg Se accumulation in some naturally occurring grasses.
Karam et al. (1998) grew potatoes on MSW compost-amended growth media at rates of 0, 10, 20 and 30 g/4 L pots. Overall, MSW compost amendment resulted in elevated contents of As and other trace elements in the substrates, and tuber As was also increased proportionally to the MSW compost application rates, still remaining at non-hazardous levels.
In a leaching experiment with perennial flowers (Coreopsis grandiflora), Sawhney et al. (1996) found relatively high mobility of As originating from growth media mixed with 25 to 100 % compost. The As concentration in the leachates was increased with increased compost application to the medium and decreased with time of leaching. The As in the lechate was calculated to be 23-39% of the total As in the compost; however, the authors do not provide data on As uptake by plants.
In a long-term field experiment, Kick and Poletschny (1978) did not
find a clear relationship between the rates of compost application and Hg
content in fodder and sugar beets. In another experiment with compost application,
Volkel (1988) found that the contents of Hg, Pb, Cd, Zn, and Cu in the topsoil
were increased proportional to the rate of compost application, but the heavy
metal content of vine leaves, must and wine were not affected.
A particular concern with As, Se and Hg is
their accumulation by mushrooms (Chaney and Ryan, 1993; Woodbury, 1992). Different taxa, however, accumulate unequal
amounts of Hg under natural conditions (Aote and Aote, 1998). The authors found that for 112 accessions of
common wild edible fungi, the average Hg was 1.72 mg/kg, while Agaricus,
Macrolepiota and Lycoperdon had the highest Hg accumulation, averaging 3.02 mg/kg. However, Tuzen et
al. (1998) detected low amounts of Hg accumulation in the mushrooms grown on
heavy metal spiked composts.
The maximum Hg content
in urban compost reported in France was 23.3 mg/kg (Deportes et al. 1995). Because of this high level of Hg, the use of
urban compost for mushrooms in France decreased from 60% to 10% in 5 years and
current regulations target reduction to 5% (Mustin, 1987).
Chaney and Ryan, (1993) summarizing the subject on compost-mushroom transfer of Hg, feel that because of higher toxicity of organic Hg compounds, total Hg content in mushrooms should be interpreted carefully: some mushrooms accumulate relatively high Hg but with very low amounts of it being methyl mercury (up to 3%). Some other mushrooms may accumulate relatively low total Hg, but with up to 36 % of it being methyl mercury. That is why the WHO and US-FDA consider methyl-Hg recommendations and regulations, because of its higher toxicity than inorganic Hg.
Szederkenyi (1997) grew mushrooms on a spiked compost containing 0,
10, 50 and 100 mg/kg Se. With the increase
of Se concentration in the compost, its concentration was also increased
in the mushroom fruiting bodies from first and second flush. The highest yields from the first flush were
obtained from mushrooms grown on compost containing 10 mg/kg Se, while the
highest yield from the second flush was recorded from compost containing 50
mg/kg Se. Increased Se concentration in
the compost from 1 to 5 mg/kg Se lead to an increased Se content in mushrooms,
more in the first flush and less in the second (Toaso et al. 1993). It was also shown that Se enrichment of
compost may significantly affect fruit body concentration not only of Se but
also of Al, Co and V (Toaso et al. 1994).
Frank et al (1974) detected Hg content in fresh mushrooms to be between 0.017 to 0.129 mg/kg while samples of wild mushrooms from the same area had an average content of 0.019 mg/kg. The compost media had 0.42 to 0.61 mg/kg Hg. Authors found correlation between levels of Hg in compost and in marketable mushrooms. All of the Hg detected in the compost, manure and mushrooms was in the inorganic form.
Domsch et al (1976) also found that the addition of 50 % of municipal waste compost to horse manure substrate increased mushroom Hg content 900% relative to the control horse manure compost, resulting in above normal Hg content. In addition to Hg, As was also detected in mushrooms, but in a normal range. The relative increase of mushroom tissue metal content was Hg > Cu = Zn > Pb > As > Cd.
Very few studies indicated hazardous or significant accumulation of these elements in agricultural crops as a result of compost application. Mushrooms grown in compost high in As, Hg ad Se tend to accumulate them, especially Hg.
Absorption and accumulation of compost-born As, Hg and Se by plants depends mainly on their concentration and speciation in the compost, media pH, and plant species.
From the literature it is not clear whether compost application to agricultural land would reduce the bioavailability of these elements.
There is a need for further research on the behaviour of As, Hg and Se in the compost-soil-plant system with commercially available composts, meeting or exceeding the guidelines for maximum trace elements.
Abdel-Sabour
MF, Abdel-Haleem AS, Sroor A, Abdel-Baset N, Zaghloul RA, 1998, J. Environ.
Qual.10: 245-251.
Aote
VJ, Aote BE. 1998. Moklogiai Kezlemenyek. 37: 1-3, 71-80.
Canadian Council
of Ministers of the Environment, 1996, Guidelines for Compost Quality. Minister
of Public Works and Government Services, Ottawa, Canada. Cat#
En108-/106E
Cappon CJ, 1981, Arch.Environ. Contam. Toxicol 10: 673-689.
Cappon
CJ, 1987, Water, Air Soil Pollut. 34: 353-361.
Chaney R, Ryan
JA, 1993, In: Science and Engineering of Composting: Desing Environmental,
Microbiological and Utilization Aspects. (HAJ Hoitink, HM Keener, Editors). Renaissance Publications, USA, pp 451-506.
Deportes I, Benoit-Guyod JL, Zmirou D, 1995, Sci. Total Environ. 172:
197-222.
Domsch KH, Grabbe
K, Fleckenstein J. 1976, Zeitschrift fur Pflanzenernahrung und Bodenkunde. 4:
487-501.
Epstein
E, 1997, The science of composting.
Technomic Publ. Comp. Inc., Lancaster, Pensilvania.
FrankR, Rainforth
JR, Sangster D, 1974, Can. J. Plant Sci. 54: 3, 529-534.
Furutani
S, Osajima Y, 1967, Study on residual components from agricultural chemicals in
food. III. Value of mercury in wheat, some vegetables and paddy field soil. Kyushu Daigaku Nogakubu. Gakugei Xasshi 22: 45.
Furutani
S, Osajima Y, 1965, Study on residual components from agricultural chemicals in
food. Mercury content in rice and other foods, Nippon Shkuhin Kogyo Gakkaishi
14: 15-19.
Karam NS, Ereifej
KI, Shibli RA, AbuKudais H, Alkofahi A, Malkawi Y, 1998, J. Plant Nutrition.
21: 4, 725-739.
Kick H,
Poletschny H, 1978, Landwirtschaftliche Forsching. Sonderheft 35: 412-418.
Mayland HF, James
LF, Panter KE, Sonderegger JL, 1989, In: Selenium in Agriculture and the
Environment, (LW Jacobs Editor), SSSA Special Publications, no 23, pp 5-50.
Mustin M, 1987, Le compost: Gestion de la matiere organique. F.Dubose,
Paris pp 953.
O’Neill P, 1990, In: Heavy
Metals in Soils. (BJ Alloway, Editor), New York, Halsted Press, pp 83-99.
Rensburg L van, Sousa-Correira RI de, Booysen J, Ginster M, 1998, J.
Environ. Qualit. 27: 6, 1479-1486
Saha JG, Lee YW,
Tnline RD, Chinn SHF, Austnson HM, 1970, Can. J. Pl. Sci. 50: 597-599.
Sawhney BL,
Budbee GJ, Stilwell DE, 1996, Compost Sci. Utilization. 4: 4, 35-39.
Shrift A, 1973,
In: Organic selenium compounds: Their chemistry and biology, DL Klayman, WHH
Gunther, (Editors), New York Wiley Interscience, pp 763-814.
Szederkenyi T, Schmidt R, Toaso G, Szalka E, 1997, Acta Agronomica
Ovariensis. 39: 1-2, 21-33.
Toaso G, Schmidt
R, Fodor P, 1993, Acta Agronomica Ovariensis. 35: 1, 73-86.
Toaso G, Schmidt
R, Fodor P, 1994, Champignon. 378: 76-77.
Tuzen M, Ozdemir M, Dmirbas A, 1998, Food Res. Technology. 206: 6,
417-419.
Volkel
R, 1988, In: Proceedings of the VDLUFA congress. Sept. 1987, Koblenz Germany.
No. 23, 467-476.
Warman PR, Muizelaar T, Termeer WC, 1994, Compost Sci. Utilization 3: 4,
40-50.
Woodbury PB,
1992, Biomas.Bioenerg. 3: 38-41.