the seasonal pattern of radiocaesium partitioning within swards of agrostis capillaris at two...

Journal ofEnvironmental Radioactivity 45 (1999) 219}234

The seasonal pattern of radiocaesium partitioningwithin swards of Agrostis capillaris at

two defoliation intensities

C.A. Salt!,*, J.W. Kay"

!Department of Environmental Science, University of Stirling, Stirling, Scotland, FK9 4LA, UK"Department of Statistics, University of Glasgow, Glasgow, Scotland, G12 8QW, UK

Received 1 August 1998; received in revised form 6 October 1998; accepted 13 October 1998

Abstract

Many semi-natural ecosystems with potentially high soil-plant transfer of radiocaesium aregrazed by domestic animals. This paper investigates e!ects of grazing intensity over time on thepartitioning of 137Cs in the grass Agrostis capillaris. Two levels of defoliation were simulated ina pot experiment by frequent clipping to sward heights of 3 and 6 cm, respectively. The swardsshowed characteristics similar to "eld swards, but may not have reached equilibrium with the6-cm cutting regime. Between June and December the pool of 137Cs in the standing biomassincreased markedly. Twice as much 137Cs was removed from the 3-cm compared to 6-cmswards through clipping. However, the total 137Cs activity incorporated into standing biomassplus clippings did not di!er between defoliation regimes. The partitioning of 137Cs betweengreen and dead leaves varied with defoliation intensity, time and leaf age. The pool of 137Cs inthe standing biomass reached a maximum in December equivalent to 1.3% of that applied tothe organic soil. A further 0.5% had been removed overall through clipping. The results arediscussed in the context of controlling grazing intensity as a countermeasure. ( 1999 ElsevierScience Ltd. All rights reserved.

1. Introduction

Research activity as a result of the Chernobyl accident has greatly improved ourunderstanding of radioecological processes and raised awareness of the importance of

*Corresponding author. Tel.: 01786 467852; fax: 01786 467843.

0265-931X/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S0265-931X(98)00112-X

semi-natural ecosystems as sources of contaminated food products (Howard& Howard, 1997). Many semi-natural ecosystems with comparatively high soil-plant transfer of radiocaesium are intensively grazed by domestic animals, suchas sheep in the UK. Howard (1993) listed control of grazing pressure, which ispart of normal stock management, as a possible countermeasure for such systemsbut remarked that evidence at the time was insu$cient to justify recommendingthis measure to farmers. Limited evidence from "eld experiments with sheepby Salt and Mayes (1991) and with cattle by Voigt, Rauch and Paretzke(1996) suggests that grazing intensity may a!ect the radiocaesium uptake bypasture plants and thereby the radiocaesium intake by grazing animals. The plantecological changes resulting from defoliation are well documented for grasslands(Pearson & Ison, 1997; Richards, 1993) and detailed sward measurement techniques(Davies, Baker, Grant, & Laidlaw, 1993; Grant, Barthram, Torvell, King, & Smith,1983) are applied in this paper to study the interactions between defoliation andradiocaesium.

Simple concentration ratios are often used to characterise the magnitude ofsoil-plant transfer of radionuclides and provide a measure of comparison (e.g. IAEA,1994). However, these ratios are of limited value in ecological studies sinceactivity concentrations in plants do not solely depend on activity concentrations inthe soil. Site-speci"c factors such as climate, soil bulk density, plant biomass and thedepth distribution of radionuclides in relation to plant rooting can make comparisonsof transfer factors between sites invalid (Ehlken & Kirchner, 1996; Salt & Mayes,1991). It is ecologically more meaningful to measure both activity concentrationand total activity in plant biomass in relation to the total pool of a radionuclide in thesoil (Livens, Horrill, & Singleton, 1991). Plant concentration data are needed tocalculate the radionuclide intake by grazing animals where animal numbers and drymatter intakes are known while total activity data per area for di!erent plantspecies/plant communities are needed to quantify the potentially available pool foranimal intake.

This study aims to:(a) identify e!ects of defoliation intensity on radiocaesium partitioning in Agrostis

capillaris under controlled conditions,(b) quantify seasonal changes in the total pool of radiocaesium residing in the plant

biomass in relation to defoliation intensity and(c) assess the scope for controlling grazing pressure as a countermeasure.A pot experiment was chosen to control environmental factors such as spatialheterogeneity of soil contamination, competition between plant species and changesin soil moisture. Agrostis capillaris was selected as it is the dominant species ofsemi-natural grassland communities most preferred by domestic herbivores in Britain(Hunter, 1962). Grazing was simulated by weekly clipping. The experimental resultsfor radiocaesium activity concentrations are reported in Salt, Kay, Donaldson, andWoolsey (1997). This paper provides a complementary analysis of the e!ectsof biomass development and biomass partitioning into leaf age categories onthe radiocaesium activities in plant tissue components over time in response todefoliation.

220 C.A. Salt, J.W. Kay/J. Environ. Radioactivity 45 (1999) 219}234

2. Materials and methods

2.1. Experimental design

Topsoil from a peaty podzol was collected in August 1993 at Devoke Water,Cumbria, UK (National Grid Reference SD 166973). The soil has pH (CaCl2)3.7, 2.6 mmol

ckg~1 of exchangeable K, 52% organic matter and a 137Cs

background activity of 0.3 kBqkg~1. The "eld moist soil was passed throughan 8-mm riddle and contaminated with 137CsCl equivalent to 490 kBqkg~1

oven-dried soil (stable 133Cs carrier 1.32 lg kg~1 dry soil). The contaminationwas carried out uniformly in small batches of soil, followed by mixing of severalbatches in a plastic barrel designed for cement mixing and then mixing of all batchestogether by spade in a large tank. A homogeneity test on 30 random, dry 3-g samplesshowed a coe$cient of variation of 1.2%. The soil was "lled into 100 pots(volume"2 l, surface area"177 cm2) and Agrostis capillaris L. var Sabovalwas sown in October 1993. To alleviate poor growth, NaNO3 was applied on 24 May(40 kg ha~1). The soil was kept at 80% of "eld capacity by regular surface applicationof rainwater while the pots were outside under a transparent roof. The wateringregime caused no leaching of water from the bottom of the pots thus preventingpotential loss of 137Cs.

There were two defoliation treatments (weekly clipping to sward heights of 3and 6 cm) with 50 pots allocated to each treatment. Clipping began as soon as targetheights were exceeded (6 June for 3 cm, 14 July for 6 cm). All plant tissues removedthrough clipping ("clippings) were collected for determination of dry weight and137Cs activity. Destructive harvesting of 5 pots per treatment was carried outin June 1994 on pots from 3 cm treatments only (2 dates) and from July to September1994 on both treatments (8 dates), leaving 10 pots unsampled. Harvests took placethree days after the last clipping to target sward height. At harvest, above-groundstanding biomass as well as individual biomass of "ve leaf age categories weredetermined: immature (youngest leaf), mature (second youngest leaf), very mature(all other leaves fully green or with less than one-third of laminae senesced), semi-dead(leaves with more than one-third of laminae senesced) and dead leaves (fully senesced).Leaves were separated by peeling o!with their sheaths. In the week preceding harvestdate, tiller densities, leaf extension and senescence rates and speci"c weightsof immature and mature leaves were recorded. The dry matter clipped from eachpot up to the date of harvest was summed over time and is referred to as &cumulativeweight of clippings'. This was added to the standing biomass removed at harvestdate to calculate &total weight of biomass'. The 137Cs activity removed with theclippings per pot was also summed over time (cumulative 137Cs activity in clippings)and added to the activity removed with the standing biomass (total 137Cs activity inbiomass).

All plant tissue samples were counted for 137Cs using a Packard Autogamma 500gamma spectrometer with 3-inch NaI detector. Activities are reported as Bq in drymatter.

A more detailed description of the methods can be found in Salt et al. (1997).

C.A. Salt, J.W. Kay/J. Environ. Radioactivity 45 (1999) 219}234 221

2.2. Statistical methodology

The statistical analysis involved three distinct types of methodology, namely,graphical analysis, linear modelling and multivariate analysis.

2.2.1. Graphical analysisFor each of the response variables: &weight of standing biomass', &log cumulative

weight of clippings', &activity in standing biomass', &log cumulative activity in clip-pings', &log total weight of biomass', &log total activity in biomass' and &tiller density',separate curves were "tted over time at the two defoliation intensities using theLOWESS method (Cleveland, 1979) available in the statistical package MINITAB(Minitab Inc, 1996). Where results are presented as "gures, a yardstick is given inorder to judge statistical signi"cance; this yardstick was calculated as the product ofthe Bonferroni quantile of Student's t distribution, based on eight comparisons, andthe estimated standard error of the di!erence between the estimated mean responsesat the two defoliation intensities.

If at a particular time the vertical gap between the "tted responses for the twodefoliation intensities is greater than the length of the yardstick, then there isa statistically signi"cant di!erence between the mean responses at the two defoliationintensities.

2.2.2. Linear modellingFor each of the response variables mentioned above a two-way ANOVA was

performed taking height and time as factors. Residual plots were used to check thestandard assumptions of the model and as a result the following transformations andweightings were applied. The variables &weight of standing biomass' and &activity instanding biomass'were analysed on a natural scale but, on that scale, there was strongevidence of heterogeneity; this was corrected by weighting the ANOVA using theinverse estimated variances for each height-time combination. The other four re-sponse variables were modelled by applying the standard ANOVA to the log

%transformed responses.

In the analysis of tiller density, separate polynomial regression models were "ttedfor each defoliation intensity and these provided an adequate description of the data.In the case of the data from the 6-cm swards only a constant was required in themodel.

2.2.3. Multivariate analysisThe variables * &weight of biomass' and &137Cs activity in biomass' * were

recorded for each of "ve categories of leaf-age producing a multivariate response foreach height-time combination. This multivariate response was a set of percentagessumming to 100%* an example of compositional data. The fact that such composi-tional data requires special handling has been well documented in the statisticalliterature; see, for example, Aitchison (1986). The standard approach involves takingone of the categories as the &reference' category* here taken as the &dead' category* and forming ratios of the percentage of the variable in each of the other categories


relative to the dead category and then log-transforming these to produce log-ratios.There are four log-ratios for each specimen forming a four-dimensional observation.These multivariate responses were analysed using multivariate ANOVA with heightand time as factors; these analyses were performed using MINITAB (Minitab Inc,1996).

3. Results

3.1. Weight of standing biomass and clippings

A two-way ANOVA model was "tted to the dry weight of the standing biomass ofA. capillaris. The results show a signi"cant increase in weight per pot over time(P(0.001) (see Fig. 1a). From 15 August onwards, signi"cantly more biomass washarvested from the 6-cm than the 3-cm treatment (P(0.001). Since the di!erencebetween treatments was not constant over time, the interaction between date andheight was also signi"cant (P(0.001).

The temporal changes in cumulative dry weight of clippings were similar to thoseobserved for the standing biomass (see Fig. 1b). Two-way ANOVA of log-transformeddata showed signi"cant main e!ects of time and defoliation and their interaction(P(0.001). Greater cumulative weight was clipped from the 3-cm compared to the6-cm pots on all dates.

When the total dry weight of harvested biomass (standing biomass plus cumulativeclippings per pot) is analysed, two-way ANOVA of log-transformed data suggestsa signi"cant e!ect of time (P(0.001) as well as defoliation (P(0.021). However,Bonferroni 95% con"dence intervals show no statistically signi"cant di!erence be-tween treatments at any date. The "tting of a cubic regression model to the same datashows only a small e!ect of defoliation with greater biomass on 6-cm compared to3-cm swards. Table 1 shows the trends in mean total weight of biomass and therelative contribution of cumulative clippings.

3.2. 137Cs activity in standing biomass and clippings

The temporal trend of 137Cs activity in the standing biomass of A. capillarisis illustrated in Fig. 2a. Weighted two-way ANOVA indicated signi"cant main e!ectsof time and defoliation and their interaction (P(0.001). The "tted modelshows a marked increase in activity in 6-cm swards during July and August whichslowed down in September but picked up again towards the end of the year. Theswards at 3 cm height incorporated 137Cs more steadily up to late October and thenshowed a drop in December. Although statistically there is a signi"cant e!ect ofdefoliation, inspection of Bonferroni 95% con"dence intervals shows that this isrestricted to 29 August and 8 December with signi"cantly more 137Cs in the biomassfrom 6-cm pots.

In parallel with the standing biomass, the incorporation of 137Cs into clipped plantmaterial increased from July until early September on both defoliation treatments.


Fig. 1. (a) Standing biomass (g dry matter per pot and g m~2) harvested from 3-cm and 6-cmtreatments. Data points with "tted curves (n"5). (b) Cumulative biomass of clippings (g dry matterper pot and g m~2) removed from 3-cm and 6-cm treatments. Data points on a log scale with "tted curves(n"5).

Thereafter, the cumulative activity in clippings from 3-cm swards continued toincrease more noticeably than in the 6-cm swards (see Fig. 2b). Two-way ANOVA oflog-transformed data shows signi"cant e!ects of time and defoliation as well as theirinteraction (P(0.001). However, inspection of individual dates only con"rms signi"-cant di!erences between treatments for mid-July to mid-August when mean activity in


Table 1Total harvested dry biomass of Agrostis capillaris (sum of standing biomass and clippings removed up tothe harvest date) and percentage contribution of cumulative clippings. Means with standard deviations inbrackets; n"5

Harvest date Total biomass Total biomass Clippings Clippings(mg pot~1) (mg pot~1) (%) (%)3-cm swards 6-cm swards 3-cm swards 6-cm swards

6 June 698 (58.2) * 2.8 (0.3) *

21 June 817 (29.4) * 1.9 (0.3) *

14 July 1173 (133) 1148 (116) 4.5 (1.0) 1.3 (0.4)1 August 1637 (187) 1567 (101) 10.0 (2.0) 1.4 (0.8)15 August 2078 (180) 2180 (195) 15.2 (2.8) 4.3 (0.8)29 August 2416 (99.8) 2561 (270) 20.6 (1.4) 8.4 (2.1)12 September 2467 (240) 2847 (162) 20.4 (1.9) 10.0 (3.0)3 October 3042 (349) 3275 (262) 27.5 (2.8) 13.7 (1.9)27 October 3448 (263) 3588 (301) 31.5 (2.2) 14.7 (3.5)8 December 3624 (202) 3911 (286) 32.3 (2.4) 12.8 (3.6)

clippings from 3-cm swards exceeded those from 6-cm swards. Due to the largevariability in the data, di!erences at later dates are not signi"cant.

The 137Cs activities in standing biomass and cumulative clippings were added tocalculate the total activity removed from each pot (Table 2). Two-way ANOVA of log-transformed total activity con"rmed the marked e!ect of time (P(0.001) but noresponse to level of defoliation. The mean cumulative activity removed via the clippingswas equivalent to 10 and 20% of the total activity in the biomass harvested from 6 and3-cm swards, respectively, until late August. Thereafter, this "gure increased up to 20%in 6-cm swards and 40% in 3-cm swards. Thus by December about 20% more activityof the total amount had been removed on average from the short compared to the tallswards through clipping. This calculation does not account for the potential loss of137Cs prior to the harvest date through senescence and leaching.

Although at any given date only a small proportion of the biomass and the 137Csactivity in the biomass is removed through clipping, cumulative o!take by the end ofthe experimental period leads to a signi"cant contribution of the clippings to totalactivity removed. The maximum mean 137Cs activity removed from the soil throughharvest of above-ground standing biomass and clippings was attained in December.This was equivalent to 1.8% of the total activity originally applied to the soil.Standing biomass alone accounted for a mean maximum of 1.3% of the 137Cs activityin soil.

3.3. Partitioning of dry matter into leaf age categories

The distribution of dry matter between the di!erent leaf age categories changedsigni"cantly over time and di!ered between defoliation treatments (P(0.01). On the3-cm swards (see Fig. 3a) the mean proportion of mature leaves decreased from 20%


Fig. 2. (a) Cs-137 activity (Bq in dry matter per pot) in standing biomass defoliated to 3 and 6 cm. Datapoints with "tted curves (n"5). (b) Cs-137 activity (Bq in dry matter per pot) in clippings removed from3-cm and 6-cm treatments. Data points on a log scale with "tted curves (n"5).

in June to 13% in December while that of very mature leaves increased from 5% inJune to 29% in August and then decreased to 15% in December. The mean propor-tion of dead leaves decreased from 50% in June to 25% in September and then rose to56% in December. Compositional changes on the 6-cm sward followed a similarpattern to 3-cm swards but here the interaction between defoliation level and time wasalso signi"cant (P"0.003). On the 6-cm treatment the proportion of dead leaves wasgenerally lower compared to the 3-cm treatment, reaching a mean maximum of 38%in December. Also the proportion of mature leaves was on average higher on the 6-cmcompared to 3-cm swards.

3.4. Partitioning of 137Cs into leaf age categories

The partitioning of 137Cs into the di!erent leaf age categories is illustrated in Fig.4a and b. The main e!ects of time and defoliation intensity (P(0.001) as well as their


Table 2137Cs activity in total harvested biomass of Agrostis capillaris (sum of standing biomass and clippingsremoved up to the harvest date) and percentage contribution of cumulative clippings. Means with standarddeviations in brackets; n"5

Harvest date 137Cs activity

Total biomass (Bq pot~1) Clippings (%)

3-cm sward 6-cm sward 3-cm sward 6-cm sward

6 June 46.8 (5.5) * 9.4 (3.0) *

21 June 49.4 (14.2) * 6.3 (2.6) *

14 July 125 (62.0) 88.6 (50.2) 7.4 (2.2) 3.8 (2.9)1 August 603 (342) 470 (230) 8.6 (2.1) 1.3 (0.9)15 August 1289 (438) 1323 (814) 14.5 (3.6) 5.4 (1.2)29 August 2076 (517) 2955 (301) 21.0 (3.6) 11.6 (3.1)12 September 2510 (340) 2944 (563) 22.6 (4.6) 13.6 (3.9)3 October 3765 (667) 3655 (664) 31.5 (3.4) 20.7 (4.8)27 October 4251 (1230) 3884 (661) 36.4 (3.9) 20.0 (3.4)8 December 4569 (709) 4951 (713) 41.4 (3.7) 19.5 (4.8)

Note: The activity contained in the soil per pot was about 300 kBq.

interaction (P"0.021) are statistically signi"cant. On most sampling dates, 80}90%of the 137Cs in the standing biomass was contained within live green tissues. However,at the beginning and end of the experimental period, dead matter contributedsigni"cantly as a reservoir of 137Cs. On the 3-cm swards highest values for deadmatter were reached in early June (mean 41%) and again in December (mean 25%),whilst on the 6-cm swards a mean of 14% was reached in July. Under both defoliationtreatments the contribution of mature leaves to the total activity in biomass decreasedwith time while that of very mature leaves increased.

3.5. Tiller density

The tiller density on the 6-cm swards did not change over time while on the 3-cmswards the e!ect of time was signi"cant (P(0.05) with tiller numbers peaking in lateAugust (see Fig. 5). Over the entire measurement period tiller numbers were higher onthe 3 compared to 6-cm swards on all dates except the "rst (14 July) and the last date(8 Dec). The quadratic regression model "tted to the data showed a good "t with anR2 value of 31%. Since at each date, the variation amongst the replicates may beconsidered to be &pure error', the maximum amount of variation any model couldexplain is 36.6%. This means that the model explains about 85% of the variabilitythat could possibly be explained by any model.


Fig. 3. Distribution of dry matter within "ve leaf age categories of swards defoliated to (a) 3 cm and(b) 6 cm. (n"5 per leaf category).

4. Discussion and conclusions

4.1. The state of the sward

It is important to assess how far the results of this pot experiment are relevant to the"eld situation. Since similar detailed measurements of the partitioning of 137Cs ina grass sward are lacking, the state of the sward itself can be used as an indicator. If thecharacteristics of the experimental swards are in agreement within the range reported


Fig. 4. Distribution of 137Cs activity within "ve leaf age categories of swards defoliated to (a) 3 cm and (b)6 cm (n"5 per leaf category).

in the literature for "eld conditions, this provides some support for the realism of the137Cs behaviour. The aim was to simulate a continuous grazing regime throughfrequent but lenient clipping to mimic controlled management (Grant, King, & Bar-thram, 1987). Continuously grazed swards show characteristic responses to seasonand level of defoliation in terms of tiller density, sward height, standing biomass, agedistribution of leaves, and growth rate.

The gradual rise and fall in tiller density of A. capillaris on the 3-cm swardsresembled those reported by Grant et al. (1987) for ¸olium perenne at 3.5 cm, sugges-ting that tiller numbers were at optimum density and responding to changes in light


Fig. 5. Tiller density (per pot and per m2) on 3 cm and 6-cm defoliation treatments (n"5).

level. The comparatively constant tiller density on the 6-cm swards may indicate thatthe original sowing density was already close to optimum. Detailed measurements(not presented here) show a steady increase in weight of mature leaves up to Octoberon the 6-cm swards while the weight of mature leaves on the 3-cm swards remainedconstant. The measured densities of up to 70 000 tillers m~2 are similar to thoserecorded in a "eld study of frequently grazed A. capillaris which attained a maximumdensity equivalent to 60 000 m~2 (Jones, 1967).

Examination of the relationship between sward height and standing biomass(Armstrong, Gordon, Grant, Hutchings, Milne, & Sibbald, 1997) suggests that thebiomass attained by the end of the experiment was within the expected range, buttending more towards the low end of the spectrum on the 6-cm swards. Between Juneand December the standing biomass increased markedly on both treatments while thesward height was kept constant. This was due to the accumulation of dead matter inautumn, in agreement with "eld evidence (Jones, Collett, & Brown, 1982; Powell& Malcolm, 1974), and to an increase in the speci"c weight of leaves (weight per lengthof lamina). However, the magnitude of increase in standing biomass in the experi-mental swards was greater than expected from "eld (Salt & Mayes, 1991) or modellingevidence (Armstrong et al., 1997). A comparison with studies of unimproved grass-lands containing A. capillaris (Job & Taylor, 1978; Powell & Malcolm, 1974) showsthat the percentage of green material in the experimental sward in summer was similarto a "eld situation. In winter, however, the green biomass may be as low as 25%, muchless than the minimum of 44% measured in this experiment. The ratio of green to deadtissues of A. capillaris, expected to peak around June/July, was highest on theexperimental swards in August.

It appears that by the end of the experiment the 3-cm swards were well establishedwhile the 6-cm swards had not yet reached equilibrium with the cutting regime.


4.2. Ewects of defoliation and season on radiocaesium

On the less severely defoliated swards the 137Cs pool in the standing biomass wason average greater and less activity was removed with the clippings compared to themore severely defoliated swards. However, due to the variability of the replicates,these di!erences were statistically only signi"cant on some of the sampling dates.When the cumulative biomass of clippings and the standing biomass were addedtogether, no di!erences between defoliation treatments were evident either in terms ofbiomass or sward content of 137Cs. Equally Oesterheld (1992) detected no e!ect ofdefoliation intensity on "nal aerial biomass of two grass species when o!take due todefoliation was accounted for. However, root biomass, not measured in this study,was negatively a!ected by a severe defoliation regime.

Independent of defoliation intensity, the proportion of 137Cs removed via thecumulative clippings was relatively greater than the proportion of biomass removed.This can be attributed to the higher content of mature and immature leaves inclippings compared to the whole sward (Barthram & Grant, 1984) and the higher137Cs concentrations in these young leaf categories compared to the sward average(Salt et al., 1997).

The increasing incorporation of 137Cs into the sward in summer and earlyautumn is a result of both increasing biomass and activity concentration. Forinstance, the mean activity concentration in unseparated biomass of short swards rosefrom around 60 kBqkg~1 in June to 1000 kBqkg~1 in mid-September. Similarincreases occurred in all green leaf fractions on both 3 and 6-cm swards. Thefurther increase in 137Cs content up to the end of the year is solely based on abiomass increase since activity concentrations in standing biomass and clippingslevelled o!. The activity concentrations are reported in detail in a previous paper(Salt et al., 1997).

Higher 137Cs activity concentration and total activity in autumn compared tosummer may be related to a second growth peak which typically occurs once grassesswitch from reproductive to vegetative growth (Job & Taylor, 1978). Although A.capillaris in this experiment never produced visible reproductive tillers due to thenature of the clipping regime, tillers would be developing reproductive apices insummer. Also a decline in available potassium in the soil may have promotedincreased uptake of 137Cs (Salt, Kay, & Jarvis, 1996).

Despite the accumulation of dead matter in the swards in autumn, the pool ofradiocaesium in dead leaves remained small because during senescence a largeproportion of radiocaesium is lost (Clint, Harrison, & Howard, 1992). The relativeimportance of leaching and retranslocation in this process is still uncertain. Increaseddefoliation overall led to a greater proportion of dead matter in the 3-cm compared to6-cm sward in agreement with studies of ¸. perenne (Hodgson, 1985). The smallertillers of A. capillaris may have been expected to be less a!ected by defoliation at 3 cmcompared to ¸. perenne. The particularly high proportion of dead matter on the 3-cmsward in December was re#ected in a decline of 137Cs in the sward as a whole. Overallthe greatest e!ect of defoliation was seen in the distribution of 137Cs in the leaf agecategories.


4.3. Relative radiocaesium pools in plant and soil

A maximum of 1.3% of the 137Cs applied to the soil was present in the standingbiomass of A. capillaris (6-cm sward in December). On a peaty podzol dominated by ¸.perenne and Festuca rubra at 5 cm sward height and similar biomass, a maximum of1.9% of the activity was incorporated into the standing biomass (Salt & Mayes, 1991).Compared to grasses growing in mineral soils these values are high and re#ect thegreater availability of radiocaesium for plant uptake on organic soils. However, someplant species, e.g. Amaranthus retro-exus, are able to remove a similar proportion ofradiocaesium from mineral soils (Lasat, Fuhrmann, Ebbs, Cornish, & Kochian, 1998).

A survey of a range of European vegetation communities in 1989 by Livenset al. (1991) found (0.1%}2.5% of the 137Cs inventory (to 10 cm soil depth) in theground cover vegetation. Samples included dwarf shrubs, grasses, forbs and mosses invarying proportions. There was a clear tendency of increased 137Cs content in thevegetation on soils with higher organic matter content. This is also con"rmed byinventories of radiocaesium made in 1988 (Harrison, Clint, Jones, Poskitt, Howard,Howard, Beresford, & Dighton, 1990) and 1989 (Miller, Horrill, Paterson, Thomson,& McGowan, 1991) on a range of soils supporting heather moorland in the UK.Between 8 and 20% of 137Cs was present in the vegetation, with lowest values onpodzols and highest values on peats. Dwarf shrub communities dominated by com-mon heather (Calluna vulgaris) generally show high values because of the combinede!ect of large biomass and high radiocaesium concentration compared to grass-dominated communities (Salt & Mayes, 1993). Di!erences in life history and rates oftissue turnover between grasses and dwarf shrubs may be contributing factors.A signi"cant cover of mosses below the dwarf shrubs also contributes to highradiocaesium inventories (Harrison et al., 1990).

4.4. Implications for countermeasures

Examination of evidence from "eld (Salt & Mayes, 1991; Voigt et al., 1996)and pot experiments (Salt et al., 1997) together with results presented here, indicatesthat e!ects of defoliation on pasture contamination are likely to be small comparedto seasonal e!ects and are not consistent over time. Also "eld results did notsupport the theory that higher stocking density will increase radiocaesium contamina-tion of vegetation through greater availability of radiocaesium due to recyclingto pasture via faeces and urine and through soil adhesion. Crout, Beresford andHoward (1993) stressed that &soil adhesion is unlikely to be a signi"cant dietary sourceof available radiocaesium, unless the soil concerned exhibits an unusually highbioavailability of radiocaesium'. Higher stocking density is more likely to depressradiocaesium intake by grazing animals through lower herbage intake (Salt, Mayes,& Elston, 1992). It is concluded that control of grazing pressure in itself is not likely tobe an e!ective countermeasure. However, increasing stocking rates on less con-taminated pastures within a farm could still be bene"cial where due to the presence ofdi!erent soil and vegetation types some areas within a farm have a lower soil-plant-animal transfer than others.


Acknowledgements

This work was supported by the Natural Environment Research Council of theUK. The authors would like to thank Lana Donaldson and Jack Woolsey for theirassistance with the experiments and Bill Jamieson for the production of the graphs.Thanks also go to S. A. Grant, B. J. Howard, C. A. Marriott and A. N. Tyler for theirhelpful comments on the manuscript.

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the seasonal pattern of radiocaesium partitioning within swards of agrostis capillaris at two...

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