Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue14.Dec2000:

(1)

Short communication

Microbial component of radiocaesium retention in highly organic soils

A.L. Sanchez*, N.R. Parekh, B.A. Dodd, P. Ineson

Centre for Ecology and Hydrology, CEH±Merlewood, Grange-over-Sands, Cumbria LA11 6JU, UK

Accepted 22 April 2000

Abstract

The mobility of radiocaesium in the environment and its availability for plant uptake are strongly dependent on the processes controlling its retention in soils. The role of the soil microbial biomass in radiocaesium retention may be important in highly organic soils, yet this role has received little attention. Currently, the techniques used to assess radiocaesium retention tend to ignore the microbial component and, as a result, may compromise assessment of retention. We present here evidence that the microbial component cannot be ignored in such assays and propose changes that recognise the importance of maintaining biological activity in samples of organic soils.q_{2000 Elsevier Science}

Keywords: Radiocaesium; Assay; Retention; Temperature; Autoclaving; Microbial activity

Many studies (e.g. Cremers et al., 1988; Valcke and Cremers, 1994) have shown that radiocaesium interacts mainly with two types of sorption sites on soils: cation exchange sites, such as those on organic matter and planar sites of clays, and frayed edges of illitic clay minerals. The former interactions are reversible, whereas uptake on illitic minerals can be irreversible (Wauters et al., 1996). Laboratory evaluation procedures developed to assess radiocaesium mobility in soils largely consider the process from a physico-chemical perspective, and use standard assays to measure Cs partitioning between soil and solution phases (e.g. Celorie et al., 1989). This involves equilibration of a soil sample with the tracer in a background electrolyte solution to determine the distribution coef®cient, generally using air- or oven-dried soils, with no consideration of the possible involvement of micro-organisms in the process. These model laboratory systems are thus inappropriate for investigating any potential role or contribution of micro-organisms in radiocaesium ®xation.

Largely as a result of the Chernobyl accident highly organic upland soils in the UK became a focus of much investigation, since radiocaesium was found to be persistently available in these soils for uptake by grazing sheep (e.g. Howard et al., 1987). This persistence has been attributed to the retention of radiocaesium on organic sorption sites where it remains readily exchangeable. These soils are

typically low in illite (Livens and Loveland, 1988; Absalom et al., 1995) or, if illite is present, it exists in an expanded state which is unable to ®x radiocaesium (Hird et al., 1996). The unexpectedly strong retention of Chernobyl-derived

137

Cs in the top layers of upland organic soils may be accounted for by active uptake by micro-organisms. Laboratory studies have demonstrated that Cs1is accumu-lated via active K1transport systems in all major groups of micro-organisms and ammonium uptake systems have also been implicated in cyanobacteria (Avery, 1996). Soil fungi commonly found in upland grassland areas have been shown to immobilise radiocaesium (e.g. Dighton et al. 1991) and, furthermore, radiocaesium taken up by fungi in pure culture is predominantly in an exchangeable form (.95% extractable; Shand et al., 1995). The microbial involvement in the interactions between radiocaesium and organic soils deserves further study and may necessitate techniques which take the living microbial component into consideration.

Here, we present the results of an experiment to explore the role of the microbial population on radiocaesium mobility in two highly organic, peat soils. We have adapted the so-called Potassium Adsorption Ratio (PAR) technique (Wauters et al., 1996), used to measure the radiocaesium interception potential of soils, to investigate the partitioning of radiocaesium between soils and solution. A semi-®brous peat was collected from Corney Fell (UK Grid Ref. SD 151 897) and a stagnohumic gley soil from Devoke (Grid Ref. SD 163 969) in Cumbria; the soil classi®cation (Avery, 1990) and ®eld characteristics are shown in Table 1. To

Soil Biology & Biochemistry 32 (2000) 2091±2094

PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 9 9 - 7

www.elsevier.com/locate/soilbio

* Corresponding author. Tel.: 144-15395-32264; fax: 1 44-15395-34705.

(2)

minimise changes in microbial populations, the soils were stored in double polythene bags at 48C in the ®eld moist state and were used shortly after collection. The soils were hand-mixed and passed through a 4-mm sieve; sub-samples were autoclaved (2£1 h at 1208C) to be used as sterile controls. Our basic premise was that if the microbial population was involved in determining radionuclide mobility, then autoclaving should have a signi®cant effect on the process. The heat and high pressure associated with autoclaving contribute to an oxidatively destructive sterilisation resulting in cell lysis and enzyme denaturation. The autoclaving process will thus reduce the biological activity within a given sample. It is still unknown to what extent soil sterilisation by autoclaving will affect other soil properties. To further distinguish between physical and biological processes we also investigated Cs retention at increasing incubation temperatures. Microbial systems are governed by the activity of enzymes and like most chemical processes, a rise in temperature causes an increase in the reaction rate up to the optimum temperature for ef®cient operation of the enzyme (Dawber and Moore, 1980). However, at temperatures above the physiological range, enzymes can be denatured and lose catalytic activity.

In the PAR technique, 100 mM Ca is added to the mixed solution (MS) to mask the regular exchange sites, so that radiocaesium retention is directed to the speci®c sites. We used a less concentrated MS containing 10 mM Ca(NO3)211 mM KCl to avoid exceeding the normal

ionic strengths found in upland pore waters, as this could potentially affect the viability of the microbial population. Known amounts (1 g air-dry weight) of the moist autoclaved and non-autoclaved soils were packed in dialysis bags (Medicell, 12±14,000 Da) and incubated with the MS in 50 ml centrifuge tubes at different temperatures (5, 10, 15, 20, 358C), using 3 replicates per treatment. Following three rinses with MS over a 48 h period, approximately 20 Bq 134Cs in MS was equilibrated with each soil bag at

the appropriate incubation temperature. The amount of

134

Cs tracer taken up by the soils after 24 h was measured using a Minaxig Autogamma Counter 5500 (Canberra).

Each soil bag was then suspended in a solution of 1 M NH4Cl and extracted three times to determine the fraction

of exchangeable 134Cs. These extractions involved shaking for 24 h at room temperature and the amount of radio-caesium in the pooled extracts was determined as above.

Signi®cance of treatment effects on percentage 134Cs retention and the exchangeable fraction were assessed with the SAS statistical software package (SAS, 1990) using analyses of variance (ANOVA) followed by a posteriori comparison of means using Duncan's multiple range. Percentage data were arc-sin transformed prior to analysis (Sokal and Rohlf, 1969).

Fig. 1a and b shows the percentage134Cs retention by the autoclaved and non-autoclaved soils from Corney and Devoke at the ®ve incubation temperatures. A signi®cant differencep,0:001in

134

Cs retention was found between the autoclaved and non-autoclaved soils at the four lower incubation temperatures, with retention being approxi-mately 30% higher in the non-autoclaved soils. However, at 358C the total134Cs retention for both non-autoclaved and autoclaved soils was very similar. For Corney soil, the amount of radiocaesium retained across the different incubation temperatures for the non-autoclaved soils was not signi®cantly different except for the highest temperature

p,0:001; with retention decreasing from 90 to 50%

(Fig. 1a). Retention by the autoclaved soils was similar at all temperatures and there was a signi®cantp,0:01

inter-action between autoclaving and temperature. Radiocaesium retention by autoclaved and non-autoclaved samples of Devoke soil was generally lower than that for the Corney soil at all incubation temperatures. Corney soil is composed of ®ne humi®ed peaty material in contrast to Devoke soil, which is a gley soil. The intrinsic differences in the proper-ties of both soils (Table 1) as well as differences in the

A.L. Sanchez et al. / Soil Biology & Biochemistry 32 (2000) 2091±2094

2092 Table 1

Soil types and ®eld characteristics

Devoke water Corney fell

Soil type Cambic stagnohumic gley Raw oligo-®brous peat Slope None, 0.5 m from stream 38

Vegetation Grassland Grassland

Presence of roots Common, ®brous Common, ®brous

Root mat F/H layer; 6 cm 9 cm

Soil depth Only measured to 20 cm 30 cm depth Ð hits rock Structure Blocky/prismatic

Texture Sandy loam Organic

Degree of water logging Gleying evident; seasonal saturation

Seasonal saturation Presence of horizons No horizon differentiation Humi®ed peat layer 5 cm;

Dark brown semi-®brous layer, drier consistence.

pH (solution) 4.0 3.7

(3)

indigenous micro¯ora may account for the variation in their radiocaesium retention. The two soils showed similar trends of higher retention on the non-autoclaved samples, except at 358C. These observations support the hypothesis that micro-bial activity, rather than purely physico-chemical processes, is involved in the temperature dependent responses of both non-autoclaved soils.

A further indication of microbial involvement in the retention process can be seen from the results of the extraction experiment (Fig. 1c and d). Ammonium ions (NH41) readily

displace Cs1ions from exchangeable sorption sites in soils and sediments (Kennedy et al., 1997) and we used 1 M NH4Cl to extract the sorbed Cs from both autoclaved and

non-autoclaved soils. The majority of the 134Cs sorbed on the non-autoclaved Corney soils (incubated at 5±208C) was not extractable, with signi®cantly more p_,0:001being

extracted for the 358C incubation than for the other tempera-tures (Fig. 1c). In marked contrast, any radiocaesium sorbed on the autoclaved soil, at all incubation temperatures, was readily extractable. There were small, but signi®cantp_,

0:01; differences in the amount of non-extractable

radio-caesium remaining after incubation at different temperatures (Fig. 1c) which must re¯ect the direct effects of temperature on abiotic exchange. The Devoke soil did not show the same trend since the majority of sorbed radiocaesium in

the autoclaved and non-autoclaved soils was lost on extraction, regardless of incubation temperature (Fig. 1d).

Clearly, the evidence derived from the autoclaved/non-autoclaved comparisons presented here suggests microbial involvement in the process of radiocaesium sorption and retention by organic soils. The chemical changes resulting from autoclaving could also have exerted some in¯uence on the physico-chemical processes associated with retention. Nevertheless, the non-linear temperature response for retention in non-autoclaved soils supports the hypothesis of an inhibition at higher temperatures, indicative of a biologically-mediated process.

Laboratory assays designed to determine radiocaesium retention in organic soils should recognise the importance of maintaining biological activity in soil samples taken for assay and be performed on ®eld fresh soil or soils where the biological status has been maintained as far as possible. We recommend that any sample handling which compromises biological activity prior to, or during, the assay (e.g. air-drying, osmotic shock, high temperatures and manual handling) should be avoided. Additionally, it is clearly necessary that a more detailed understanding and quanti®-cation of the role of microbial radiocaesium retention is needed to enable accurate model predictions of radio-caesium mobility in organic soils.

A.L. Sanchez et al. / Soil Biology & Biochemistry 32 (2000) 2091±2094 2093

Fig. 1. The mean percentage initial retention (a) and (b) and subsequent percentage of134_{Cs remaining after extraction (c) and (d) in two organic soils}

incubated at different assay temperatures. Comparisons of autoclaved (shaded) and non-autoclaved (clear) soils are shownn3:Bars represent standard

(4)

Acknowledgements

We would like to thank CEH and NERC for funding this work; R. Creamer for help with soil collection and classi®cation; S. Wright, J. Smith and D. Singleton for their assistance in the experiments, and B. Howard and M. Hornung for helpful comments.

References

Absalom, J.P., Young, S.D., Crout, N.M.J., 1995. Radiocesium ®xation dynamics Ð measurement in six Cumbrian soils. European Journal of Soil Science 463, 461±469.

Avery, B.W., 1990. Soils of the British Isles, CAB International, Wall-ingford, Oxon, UK.

Avery, S.V., 1996. Fate of caesium in the environment: distribution between the abiotic and biotic components of aquatic and terrestrial ecosystems. Journal of Environmental Radioactivity 30, 139±171. Celorie, J.A., Woods, S.L., Vinson, T.S., Istok, J.D., 1989. A comparison

of sorption equilibrium distribution coef®cients using batch and centrifugation methods. Journal of Environmental Quality 18, 307±313. Cremers, A., Elsen, A., De Preter, P., Maes, A., 1988. Quantitative analysis

of radiocaesium retention in soils. Nature 335, 247±249.

Dawber, J.H., Moore, A.T., 1980. Chemistry for the Life Sciences, Mac-millan, London.

Dighton, J., Clint, G.M., Poskitt, J., 1991. Uptake and accumulation of Cs-137 by upland grassland soil fungi: a potential pool of Cs immobiliza-tion. Mycological Research 959, 1052±1056.

Hird, A.B., Rimmer, D.L., Livens, F.R., 1996. Factors affecting the sorption and ®xation of caesium in acid organic soil. European Journal of Soil Science 471, 97±104.

Howard, B.J., Beresford, N.A., Burrow, L., Shaw, P.V., Curtis, E.J.C., 1987. A comparison of caesium 137 and 134 activity in sheep remaining on upland areas contaminated by Chernobyl fallout with those removed to less active lowland pasture. Journal of the Society of Radiological Protection 72, 71±73.

Kennedy, V.H., Sanchez, A.L., Oughton, D.H., Rowland, A.P., 1997. Use of single and sequential chemical extractants to assess radionuclide and heavy metal availability from soils for root uptake. The Analyst 1228, R89±R100.

Livens, F.R., Loveland, P.J., 1988. The in¯uence of soils properties on the environmental mobility of caesium in Cumbria. Soil Use and Manage-ment 43, 69±75.

SAS, 1990. SAS/STAT User's Guide, Version 6, 4th ed., SAS Institute Inc., Cary NC, USA.

Shand, C.A., Cheshire, M.V., Smith, S., Campbell, C.D., Anderson, P., Davidson, C.M., Littlejohn, D., Jamieson, N., 1995. Radiocesium in an organic soil and the effect of treatment with the fungicide Captan. Plant and Soil 1702, 315±322.

Sokal, R.R., Rohlf, F.J., 1969. Biometry, Freeman, San Francisco. Valcke, E., Cremers, A., 1994. Sorption±desorption dynamics of

radio-caesium in organic matter soils. The Science of the Total Environment 157, 275±283.

Wauters, J., Elsen, A., Cremers, A., Konoplev, A.V., Bulgakov, A.A., Comans, R.N.J., 1996. Prediction of solid/liquid distribution coef®-cients of radiocaesium in soils and sediments. Part one: a simpli®ed procedure for the solid phase characterisation. Applied Geochemistry 11, 589±594.

A.L. Sanchez et al. / Soil Biology & Biochemistry 32 (2000) 2091±2094