Oecologia
DOI 10.1007/s00442-014-2908-8

Global change ecology - Original researchHighly reduced mass loss rates and increased litter layer
in radioactively contaminated areas
Timothy A. Mousseau · Gennadi Milinevsky ·Jane Kenney‑Hunt · Anders Pape Møller

Received: 24 June 2013 / Accepted: 13 February 2014
© Springer-Verlag Berlin Heidelberg 2014

Abstract The effects of radioactive contamination from
Chernobyl on decomposition of plant material still remain
unknown. We predicted that decomposition rate would be
reduced in the most contaminated sites due to an absenceor reduced densities of soil invertebrates. If microorganisms were the main agents responsible for decomposition, exclusion of large soil invertebrates should not affectdecomposition. In September 2007 we deposited 572 bagswith uncontaminated dry leaf litter from four species oftrees in the leaf litter layer at 20 forest sites around Chernobyl that varied in background radiation by more than afactor 2,600. Approximately one quarter of these bags weremade of a fine mesh that prevented access to litter by soil
invertebrates. These bags were retrieved in June 2008,
dried and weighed to estimate litter mass loss. Litter massloss was 40 % lower in the most contaminated sites relative to sites with a normal background radiation level for
Ukraine. Similar reductions in litter mass loss were estimated for individual litter bags, litter bags at different sites,and differences between litter bags at pairs of neighboringsites differing in level of radioactive contamination. Litter
mass loss was slightly greater in the presence of large soil
invertebrates than in their absence. The thickness of the forest floor increased with the level of radiation and decreased
with proportional loss of mass from all litter bags. Thesefindings suggest that radioactive contamination has reduced
the rate of litter mass loss, increased accumulation of litter,and affected growth conditions for plants.
Keywords Background radiation · Chernobyl ·Decomposition · Invertebrates · Microorganisms

Communicated by Jason P. Kaye.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s00442-014-2908-8) contains supplementarymaterial, which is available to authorized users.
T. A. Mousseau
Department of Biological Sciences, University of South Carolina,
Columbia, SC 29208, USA
G. Milinevsky
Space Physics Laboratory, Taras Shevchenko National Universityof Kyiv, 64, Volodymyrska Street, Kyiv 01601, Ukraine
J. Kenney‑Hunt
Department of Biology and Environmental Science, Westminster
College, Fulton, MO 65251, USA
A. P. Møller (*)
Laboratoire d’Ecologie, Systématique et Evolution, CNRS UMR
8079, Université Paris-Sud, Bâtiment 362, 91405 Orsay Cedex,
Francee-mail: anders.moller@u‑psud.fr

Introduction
Climate has a crucial influence on litter decomposition
rates, both directly and indirectly through effects on litterchemistry (e.g., Aerts 1997; Berg et al. 1993; Robinson
2002). Bacteria and fungi play key roles as decomposers of
newly dead plant material (Staaf 1980; Howard and Howard 1974, 1980; Berg and Ekbohm 1991). Initial decomposition is followed by decomposition of resistant tissues likecellulose, lignins and cutins by fungi of various kinds (Staaf
1980; Howard and Howard 1974, 1980; Berg and Ekbohm
1991). For example, many studies have demonstrated theimportance of lignin as a regulating factor in the decomposition of leaf litter (review in Osono 2007). Detritivores andmicrobivores follow suit and consume both detritus and its
microflora (Staaf 1980; Howard and Howard 1974, 1980;
Berg and Ekbohm 1991), and their interactions affect the13

Oecologia

abundance and the diversity of species (review in Brown
1995). Decomposition of organic matter is a function ofleaching, fragmentation, and mineralization, making C, N,
P, and other nutrients available to plants (e.g., Attiwill andAdams 1993; Kalbitz et al. 2000). High rates of release of
N and P during decomposition benefit growing plants, andthousands of plant species are completely dependent on theoutcome of competition with microbes for acquisition of
essential nutrients such as N and hence for growth and survival (van der Heijden et al. 2008). Furthermore, free-living
microbes strongly regulate plant productivity through mineralization of organic matter (van der Heijden et al. 2008).
Soil invertebrates play a significant role in litter decomposition (e.g., Cornelissen et al. 1999; Pouyat et al. 1994; Gonzalez and Seastedt 2001). Therefore, when the communityof soil microbes and invertebrates is seriously perturbed,
as in the case of a major nuclear accident, such perturbation can have dramatic indirect effects on performance ofplants and hence herbivores, but also on the mineralizationof organic matter.
Although decomposition and mineralization of organic
matter are of utmost importance for soils, there are only
rudimentary analyses of the effects of radiation on soil
communities at Chernobyl, Fukushima or other sites withelevated background radiation (Møller and Mousseau2006). Pigeon and Odum (1970) experimentally irradiateda forest at Luquillo in Puerto Rico with gamma radiationand monitored the effects on litter decomposition. This
study of acute high dose irradiation is of general interest
although there was no replication of irradiated and control
sites, exposures were limited to 3 months, the radiation was
only from a single external source, and there was no ability
to control for the many potentially confounding effects ofradiation on other components of the study system. Hence
it is difficult to make any generalization from this study.
Rafferty et al. (1997) showed that fungus-mediated translocation of 137Cs to fresh litter may explain the persistenceof Chernobyl radiocaesium in the upper layers of forest
soils. In contrast, a laboratory study of decomposition ofirradiated wheat straw showed only weak effects of ionizing radiation (Niedree et al. 2012), perhaps suggesting thatthe consequences of short-term radiation in the lab differ
from chronic levels of radiation for numerous generationsof soil microorganisms and invertebrates under field conditions. Most soil and litter invertebrates were exposed tohigh levels of radiation for long periods after the Chernobyl disaster due to most radionuclides being concentrated
in the topmost 10 cm of the soil. Krivolutski et al. (1999)
showed that within months of the accident invertebrates at
a distance up to 7 km from the nuclear power plant werereduced by up to a factor of 30. This reduction arose fromextensive reproductive failure and heavy mortality of eggs
and juvenile stages. The ratio of adult to juvenile stages

13

among soil invertebrates was restored in most areas after
2.5 years, although biodiversity still remained severely
reduced due to the loss of many radio-sensitive species(Victorov 1993; Krivolutski and Pokarzhevsky 1992). For
example, the abundance and diversity of soil invertebrates
were severely reduced by up to a factor 30 at distances
exceeding 3 km from the reactor just after the accident(Krivolutski and Pokarzhevsky 1992), and abundanceswere still depressed 20 years after the accident (Maksimova
2005). Effects on soil invertebrates in agricultural areas
were weaker than in forests although radiation also affected
abundance, diversity and reproduction of soil invertebrates
in these habitats (Krivolutski et al. 1999). Therefore, it is
not surprising that bird species with a main diet of soil
invertebrates had highly reduced abundance in the mostcontaminated areas either due to contamination from soil
invertebrates or due to reductions in the abundance of food(Møller and Mousseau 2007). Evidence suggests that theabundance of saprophytic bacteria in contaminated soils is
reduced (Zymenko et al. 1995; Romanovskaya et al. 1998)
although analyses of bacterial biofilms on building surfaces
at Chernobyl and elsewhere found no reduction in abundance or biodiversity (Ragon et al. 2011). However, such
surfaces are peculiar because of the general effects of ultraviolet radiation on biodiversity (Ragon et al. 2011).
The objectives of this study were to quantify litter massloss from areas varying in level of contamination withradionuclides in the vicinity of Chernobyl. First, we placed
litter bags with uncontaminated leaf litter in the litter layer
in forest habitats varying in degree of contamination andcompared the effects of radiation on leaf litter loss to variation in radiation within sites. Furthermore, we deposited
155 leaf litter bags with fine nylon mesh to prevent large
invertebrates from gaining access to leaves while 400
bags in the same sites with coarse nylon mesh created thecontrast between decomposition by the combination of
invertebrates and microorganisms or by microorganisms
alone. Second, we tested the extent to which differences
in background radiation levels between a high- and a lowradiation site predicted differences in mass loss from litterbags between such pairs of sites. Such pairs of sites arelikely to be very similar in many respects because theytend to share microclimate, soil type, soil pH, soil moisture, vegetation and many other characteristics. Third, we
related forest floor thickness to radiation level, litter massloss from the litter bag experiments and stand composition. We predicted that the forest floor would be thicker
in sites with higher level of background radiation, in sites
with reduced proportional mass loss, and in the presenceof pine stands surrounding the area where we placed thelitter bags because conifer needles can sometimes requirea long time to decompose (Prescott et al. 2000; Albers
et al. 2004).

Oecologia

Fig. 1  Sites used for decomposition experiments in relation to background radiation level. Adapted from Shestopalov (1996)

Materials and methods
Study sites
We (wearing radiation protection suits in the most contaminated areas) deposited litter bags during September 2007
within the Chernobyl exclusion zone or in areas adjacenton the southern and western borders. Sites were selected
to ensure a wide range of radiation levels (Fig. 1). Locations of the 20 study sites are shown in Fig. 1. The study
sites were forests or successional stages of farmland thatare presently reverting to forests and are located in dry soil
with little variation in altitude or climate.
Litter mass loss experiments
We were interested in the early stages of decomposition because large amounts of accumulated litters suggest an effect of radiation on decomposition. In addition
such early effects can be linked directly to the abundanceof vertebrates that consume soil invertebrates. We collected freshly abscised leaves of oak Quercus robur, maple
Acer platanoides, or birch Betula pendula and needles of
Scots pine Pinus sylvestris on the ground at a number of
uncontaminated sites near Kiev, Ukraine during September
2007. Litter samples for each species were subsequently
mixed to avoid any effects of origin of samples before
deposition in the litter layer in the forests. Backgroundradiation levels at the sites from where the leaves werecollected ranged from 0.05 to 0.12 μSv/h, and were hence
within the range of natural levels of background radiationin Ukraine. Leaves were air dried at room temperature forabout 2 weeks followed by additional drying in an ovenat 60 °C for 24 h immediately before being weighed ona calibrated electronic balance to the nearest 0.01 g. Litter bags were then taken to the field sites and left in thesoil for 9 months. This duration of exposure was considered to mimic the situation of leaf loss by deciduoustrees in September–October in Chernobyl, and the almost
complete disappearance of such leaves by mid-summer

13

Oecologia

the following year, unless radiation reduces or prevents
decomposition.
We made nylon mesh bags sized ca. 15 × 20 cm withmesh size being 0.2 cm for depositing leaf litter in thefield. This mesh size will exclude some large-sized macrodecomposer groups, and thus this constitutes a potential
limitation of the study. Bags were filled with ca. 18 g leaf
litter, range 10–31 g, before being closed with glue andplaced in the leaf litter in a forest. We filled litter bagswith litter in a cottage in the field. Since the mass of litterremaining at the end of the experiment was the response
variable and the mass of litter at the beginning of the experiment was one of several predictor variables, the potential
impact of differences in the amount of litter was controlled
statistically in the analyses. Since all bags were deposited
randomly, there was no reason to expect any systematic
bias in our findings. Each bag was numbered with a unique
identification number for later identification. We took greatcare to deposit all bags in a container before transport tothe field, and likewise we took care not to lose any material from the bags during deposition. Although pine needles
may be expected to more readily be lost from bags, thiswas not the case because needles were paired and hence
got stuck in the mesh. Thus, needles were more difficult
to remove from the bags than leaf litter at the end of theexperiment. Bags were arranged in random order on blue
plastic strings for ease of later detection, because wildboar Sus scrofa or other large animals may disturb single
bags and such bags may hence be difficult or impossible toretrieve. In addition the coordinates were recorded with a
Global Positioning System (GPS) and a blue plastic band
tied to the nearest tree. Slightly more than a quarter of all
bag contents was placed in a fine nylon mesh bag with themesh being <0.5 mm to prevent invertebrates from gaining
access to the leaf litter (e.g., Lousier and Parkinson 1976).
The number of bags for each species reflected the availability of leaves of the different species at the uncontaminatedsites near Kiev where the leaves were originally collected.
At each site we attempted to find the most and the least
contaminated locations at a maximum distance of 100 m
ensuring that we had at least two high-radiation and two
low-radiation locations within a site. We removed the vegetation, deposited the bags in the litter layer of the forest andcovered the bags with vegetation so that litter bags wereless visible and thus less likely to be removed by humans
or animals. All bags were located in the shade under trees.
When depositing litter bags we simultaneously recorded
the pH using a pH meter (Sharp PH51) and soil moistureusing a Delta-T Thetaprobe and HH2 moisture data logging
system by inserting the instruments into the leaf litter layer
three to five times. The mean estimate for a location wasused in the subsequent analyses. The 572 litter bags werelocated during June 2008 after having been left in forests

13

for 9 months. We only retrieved 560 bags, because 12 werelost due to activity of a tractor and a wild boar S. scrofa.
An additional five bags were partly damaged resulting ina final sample of 555 bags. After collection of the bags inJune 2008 leaf material was air dried at 60 °C for 48 h in a
Fisher Isotemp oven and stored in sealed plastic containers
containing Drierite silica gel prior to weighing on a Sartorius balance to the nearest 0.01 g. The number of different
kinds of litter bags is reported in Electronic Supplementary
Material Table S1, while Table S2 reports mean pH and soil
moisture for low- and high-radiation locations within the
20 study sites (dividing locations into low- and high-radiation locations at the median).
Measuring thickness of the forest floor
During 28–31 May 2013 we returned to the exact same
locations where the litter bags were deposited during September 2007 by using the GPS coordinates. At each ofthese locations we made four measurements of the thickness of the forest floor with a ruler to the nearest millimeterwithin a distance of 1 m. At each site we also recorded thepresence or the absence of stands of pines in the exact spot
where measurements were made because pine needles atleast sometimes take a particularly long time to decompose,and because pines are particularly susceptible to the effectsof background radiation (Arkhipov et al. 1994; Mousseau
et al. 2013).
Measuring background radiation levels
We measured radiation levels in the field at ground levelat each location where a litter bag was deposited using ahand-held dosimeter (Inspector; SE International, Summertown, TN). We measured levels two to three times at eachof these locations and averaged the measurements for eachlocation. These data were correlated with data from governmental measurements at ground level published by Shestopalov (1996), estimated as the mid-point of the ranges in
the published maps. These analyses showed a high degreeof consistency between the two methods (Møller andMousseau 2007). Radiation levels vary by several orders ofmagnitude at a scale of 1 km due to heterogeneity in deposition of radioactive material since the Chernobyl accident(Fig. 1; Shestopalov 1996).
Statistical analyses
Initial leaf litter mass, final leaf litter mass, and radiationlevel were log10 transformed to normalize the data. We
developed a full statistical model for all litter bags to assess
the relationship between final leaf litter mass (response
variable) and initial leaf litter mass, presence or absence ofOecologia
Table 1  Summary statistics for soil and decomposition variables forthe study of decomposition at Chernobyl
Variable

Mean SECV

Radiation (μSv/h)pH
Moisture (%)
Initial leaf litter mass (g)

20.26
5.24
10.16
17.81

262.52 0.09–240.25
23.87
3.1–7.8
39.38
3.1–21.0
19.51 10.00–31.20

Final leaf litter mass (g)Proportional loss relative toinitial mass

4.86 0.10
0.273 0.005

47.47
21.07

0.26–13.95
0.02–0.88

Thickness of forest floor (cm)6.81 0.43

53.53

2.0–16.0

2.26
0.05
0.17
0.15

Range

Sample size was 555 leaf litter bags. Proportional loss relative to initial mass represents the proportion of the mass of the litter that is lostfrom the initial mass of the litter
CV Coefficient of variationa fine mesh bag, tree species litter, pH, soil moisture andradiation nested within site (predictors), as implemented
in the statistical software JMP (SAS 2012). We nested
the radiation variable within the categorical variable Sites
because we were mainly interested in quantifying the effectof radiation on decomposition at each of the sites. Contrasts
within sites were important because differences in decomposition related to radiation among sites could be due to
confounding variables rather than radiation. This model
included all the predictors that we had a priori biologicalreasons to include. The objective of this global model wasto test whether mass loss was related to background radiation within study sites.
Second, we also tested this objective in a more conservative statistical model with the mean amount of remaining

leaf litter for each of the 20 study sites being the response
variable and mean background radiation level being thepredictor variable. This model was used to test whether
mean mass loss was related to mean background radiationlevel across the 20 different sites.
Third, an even more conservative statistical model wasused to test the same hypothesis based on pairs of spatiallyneighboring study sites ranked with respect to radiation.
We related the difference between two neighboring sites
in mean loss of leaf litter to the difference in mean background radiation between these two neighboring sites. The
justification for this analysis is that it controls for effects ofother potentially confounding variables because neighboring sites differing in level of background radiation generally are similar in terms of microclimate, soil type, soil pH,
soil moisture, vegetation and other variables of potential
significance.
For the thickness of the forest floor we developed amodel with thickness of the forest floor as the response
variable and background radiation, proportional massloss from the litter and presence or absence of pine trees
as predictor variables. We predicted a positive effectof background radiation on the thickness of the forest
floor if radiation had a negative impact on decomposers.
In contrast, we predicted a negative effect of mass loss
from litter bags on thickness of the forest floor because
greater mass loss should imply less litter. Finally, we
predicted a positive effect of presence of pine stands inthe areas surrounding the area where we measured thethickness of the forest floor because pine litter at least
sometimes takes longer to decompose than litter fromdeciduous trees.

Table 2  Final leaf litter mass in relation to radiation level (nested within sites), site, initial leaf litter mass, tree species and presence or absenceof a fine mesh bag
Variable

Sum of squares

df

F

P

Radiation(Site)
Site
Initial leaf litter mass
Tree species
Least square means SE

0.235
0.504
1.062
0.415

17
16
1
3

2.13
4.85
163.5921.30

0.0054<0.0001<0.0001<0.0001

Pine
Oak
Maple
Birch
0.194

3.144
3.135
3.119
3.079
1

0.044
0.044
0.044
0.044
29.87

<0.0001

Fine mesh bag
No fine mesh bag

3.147
3.093

0.044
0.043

3.342

515

Fine mesh bag
Least square means SE

Error

Slope (SE)0.784 (0.061)

The full model had the statistics F = 18.99, df = 39, 515, r2 = 0.59, P < 0.0001

13

Oecologia

Results
Mass loss from litter
Summary statistics for the different soil and litter mass loss
variables and thickness of the forest floor are reported inTable 1. Radiation was the variable with the largest coefficient of variation due to the choice of sites varying greatly
in background radiation level. pH was on average 5.2 withmost values being acidic. The mean proportional loss relative to initial leaf mass was 27 %.
A full model based on all litter bags explained 59 % of
the variance (Table 2). The model fitted the data as shownby the non-significant lack of fit (F = 0.99, df = 505, 9,
P = 0.57). There was a significant effect of radiation nested
within sites, with an additional effect of site (Table 2).
Unsurprisingly, initial leaf litter mass also accounted fora significant effect with larger amounts being present at
the end of the experiment when a larger amount was present at the start of the experiment. Tree species differed inrate of litter mass loss, ranked from slowest to quickest as
pine, oak, maple and birch (Table 2). A Tukey post hoc test
revealed that birch differed significantly (P < 0.05) fromthe three other species, while there were no other significant differences. The effect of fine mesh on litter mass loss
was significant implying that access to the leaves by large
invertebrates increased the rate of litter mass loss (Table 2).
Two separate analyses for the litter bags with and withoutfine mesh net similar to the statistical model in Table 2 produced a significant effect of radiation in the absence of finemesh (F = 2.53, df = 1, 365, P = 0.0008), but produced
no significant effect of radiation in its presence (F = 1.13,
df = 1, 118, P = 0.34). There was no additional significant effect of pH (F = 3.58, df = 1, 514, P = 0.06) or soil
moisture (F = 0.46, df = 1, 514, P = 0.50). A model that
included a non-linear effect by inclusion of a quadratic
radiation term did not provide a significant effect for thatvariable (F = 0.00, df = 1, 514, P = 0.97). Therefore, there
was no evidence of a radiation threshold or a non-linear
effect. There was no statistically significant interaction
between radiation and tree species when this interaction
was entered into the model in Table 2 (F = 0.62, df = 3,
512, P = 0.61). This implies that radiation did not have a
different effect on mass loss in the four different tree species. Likewise, there was no statistically significant interaction between radiation and presence or absence of finemesh when this interaction was entered into the model inTable 2 (F = 0.04, df = 1, 512, P = 0.85). This implies thatthe effect of radiation on mass loss was not significantly
different in the absence or the presence of fine mesh andhence soil invertebrates. Thus, the effects of radiation on
mass loss were independent of tree species and presence orabsence of fine mesh.

13

Fig. 2  Decomposition estimated as the proportion of initial leaf litter
mass decomposed in relation to measured background radiation level(μSv/h) for the study sites. Statistics for the model are reported

A more conservative statistical model that related themean mass of leaf litter remaining at each study site to themean level of radiation accounted for 55 % of the variance[Fig. 2; F = 21.94, df = 1, 18, r2 = 0.54, P = 0.0002, slope(SE) = −0.070 (0.015)]. The difference in litter mass loss
between the site with the highest and the lowest radiationlevel estimated from the predicted values according to theregression line was 40 %.
Mean reduction in leaf litter mass was 0.249(SE  = 0.010) for the ten high-radiation sites and 0.276(0.012) for the ten low-radiation sites. This difference of
11 % was statistically significant (paired t-test, t = 2.73,
df = 9, P = 0.023). In fact the difference in reduction inleaf litter mass between neighboring sites was negatively
related to the difference in radiation level between neighboring sites [F = 11.55, df = 1, 8, r2 = 0.59, P = 0.0094,
slope (SE) = −0.021 (0.006)]. Therefore, the conclusionthat radiation reduced the rate of litter mass loss remained
across all three statistical tests.
Thickness of forest floor
A model of thickness of the forest floor in relation to
background radiation level, proportional mass loss fromlitter bags, and presence or absence of pine stands fitted the data. These three factors contributed to explaining variation in thickness of the litter layer (Table 3).
The thickness of the forest floor increased with increasing level of background radiation (Fig. 3a). In addition,
there was a significant effect of proportional loss ofmass from litter bags (Fig. 3b), implying that the forest
floor was thicker in sites where proportional loss was
reduced. Finally, there was a tendency for an increase inthickness of the forest floor in sites with stands of pines(Table 3).

Oecologia
Table 3  Model of the relationship between thickness of the forest floor and background radiation, proportional loss of litter mass, and presenceof absence of stands of pine trees
Sum of squares

F

df

P

Estimate

SE

Intercept
Background radiation
Proportional loss
Presence of pine trees

1.080
0.521
0.051

78.32383.68440.359
3.954

1
1
1
1

<0.0001<0.0001<0.0001
0.051

2.483
0.127
−3.352
0.027

0.281
0.014
0.543
0.014

Error

0.890

69

The overall model had the statistics F = 66.45, df = 3, 69, r2 = 0.74, P < 0.0001

Fig. 3  Thickness of the forest floor (cm) within study sites in relation
to a background radiation level at the study sites (μSv/h) and b proportional loss of litter mass at the study sites. Statistics for the partial
effects for the model are reported

Discussion
The main findings of this study were that decomposition asreflected by mass loss of leaf litter decreased with increasing level of background radiation in forest sites around
Chernobyl resulting in a reduction by 40 % across a radiation gradient differing in radiation level by more than a factor 2,600. Potentially confounding variables such as site
effects, effects of invertebrates, pH or soil moisture did

not change the conclusions concerning effects of radiation
on litter mass loss. We reached similar conclusions whenanalyzing individual litter bags within sites, mean valuesof litter mass loss for sites, or differences in litter massloss between pairs of neighboring sites differing in levelof radiation. This shows that our findings were robust andindependent of the spatial scale of analysis. Finally, massloss from litter bags had consequences for litter mass loss
and accumulation of organic matter in the field because theforest floor was thicker in sites with higher levels of background radiation, lower proportional mass loss from litterbags and sites with stands of pine trees.
We estimated the impact of radiation on litter mass loss
by comparing the rate at the highest radiation level relative to the rate at the lowest background radiation level.
The decrease in rate of litter mass loss was by 40 % in themost contaminated sites compared to the cleanest sites.
While each of our bags with leaf litter only had leaves from
one tree species, patterns of leaf litter mass loss, changes
in nutrient concentration, and decomposer abundance andactivity depend on whether leaves are decaying in mixtures
or on their own (Gartner and Cardon 2004). Therefore,
future studies based on mixed leaf litter may provide resultsthat are more similar to the situation in natural forests.
A quarter of our litter bags were enclosed in fine nylon
mesh to prevent access by large soil invertebrates. While
there is clear evidence of the soil fauna being perturbed
by radioactive contamination from Chernobyl (Krivolutski
et al. 1999; Victorov 1993; Krivolutski and Pokarzhevsky
1992; Maksimova 2005), the information on soil microbes
is rudimentary at best. Ragon et al. (2011) showed similarabundance and diversity of bacteria in biofilms on building
surfaces in Chernobyl and elsewhere in Europe, although
the rate of mutation accumulation was increased in themost contaminated areas in Chernobyl. Zymenko et al.
(1995) and Romanovskaya et al. (1998) reported reduced
abundance of saprophytic bacteria in more contaminatedsoils. We assumed that both microorganisms and soil invertebrates have access to litter bags with coarse mesh, while
only microorganisms have access to litter bags with finemesh. We found a significant difference of <1 % in litter13

Oecologia

mass loss between bags with and without fine mesh. Since
we did not find a significant interaction between presenceor absence of fine mesh and background radiation, we conclude that a given level of radiation had an equally negative
impact on litter bags with and without mesh.
We found differences in the rate of litter mass loss
among tree species with greater loss in birch than in thethree other species. Such differences between conifers anddeciduous trees are well described in the literature (e.g.,
Berg and Ekbohm 1991; Gillon et al. 1994; Taylor and Parkinson 1988). These differences in litter mass loss among
tree species are as expected. Surprisingly, we did not find asignificant interaction between tree species and backgroundradiation on level of mass loss from litter. This finding suggests that all species were equally impacted by backgroundradiation, and it further suggests that it is the decomposers rather than the litter that are affected by the effects ofbackground radiation. An experiment that consisted of litterbags with both clean and contaminated litter would allow
us to determine whether contamination effects on litter
quality affect rates of decomposition.
While mass loss from litter bags is only one of several
components of decomposition, the ultimate test of an effectof background radiation on decomposition is through theeffects on the thickness of the forest floor. We made sucha test in 2013. The experiment on mass loss from litterbags was made in 2007–2008, so there is a temporal delay
between the two parts of this study. We see no obvious reason why this should be a cause of consistent bias, and it
is our clear impression based on extensive field work in
1991, 1995 and 2000–2013 that the thickness of the forest floor is consistently positively related to backgroundradiation every year. Here we have shown that the thicknessof the forest floor increased with the level of backgroundradiation.
We have conducted research in Chernobyl since 1991
and have noticed a significant accumulation of litter overtime. This accumulation of litter is demonstrated here to
be significantly positively related to background radiation,negatively related to the proportional loss of litter massfrom the litter bags, and to be greater in the presence ofpine stands. Accumulation of litter may have consequences
for the risk of fire because accumulation of dead plant matter for 27 years implies an accumulation of fuel and hence
an elevated risk of fire followed by an increased risk ofsubsequent redistribution of radionuclides. Obviously, factors other than litter accumulation will impact the risk offire. These include the probability of fire ignition, which
will have been affected by increased summer temperaturesin recent decades, reduced levels of precipitation, the large
spatial scale of the areas with accumulation of litter, thereduced levels of defense against insect pests and the resultant rate of tree mortality (A. P. Møller et al., unpublished13data). Several studies have suggested non-negligible risksof a major Chernobyl fire (e.g., Kashparov et al. 2000;
Yoschenko et al. 2006a, b), and our study provides important information on factors that relate to this risk. The accumulation of litter over time may also have consequences forthe mineralization of organic matter in the forest floor andhence primary productivity, suggesting that growth rates oftrees and other plants may be suppressed in the presenceof poor levels of decomposition caused by elevated levelsof background radiation and could in part explain reduced
growth rates of pine trees in radioactive regions of Chernobyl (Mousseau et al. 2013).
In conclusion, we have shown severely depressed levels of litter mass loss in the most contaminated forest
areas around Chernobyl, with reductions of 40 % relative
to background levels recorded in uncontaminated, control
areas. These findings suggesting a linear dose–responseof decomposition in relation to background radiation
were independent of potentially confounding variables
such as pH, soil moisture, study site and method of analysis. These results have a number of implications for
management of contaminated areas, risk of fire and redistribution of radionuclides in forest ecosystems around
Chernobyl.
Acknowledgments We are grateful for logistic help during our visits to Ukraine from O. Bondarenko, I. Chizhevsky, A. Erhardt and A.
Litvinchuk. We received funding from the University of South Carolina School of the Environment, Bill Murray and the Samuel Freeman Charitable Trust, NATO and the Fulbright Program to conduct
our research.

References
Aerts R (1997) Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship. Oikos
79:439–449
Albers D, Migge S, Schaefer M, Scheu S (2004) Decomposition of
beech leaves (Fagus sylvatica) and spruce needles (Picea abies)
in pure and mixed stands of beech and spruce. Soil Biol Biochem
36:155–164
Arkhipov NP, Kuchma ND, Askbrant S, Pasternak PS, Musica VV(1994) Acute and long-term effects of irradiation on pine (Pinus
silvestris) stands post-Chernobyl. Sci Total Environ 157:383–386
Attiwill PM, Adams MA (1993) Nutrient cycling in forests. New Phytol 124:561–582
Berg B, Ekbohm G (1991) Litter mass-loss rates and decomposition
patterns in some needle and leaf litter types: long-term decomposition in a Scots pine forest. Can J Bot 69:1449–1456
Berg B, Berg MP, Bottner P, Box E, Breymeyer A, Calvo de Anta
R, Coueaux M, Escudero A, Gallardo A, Kratz W, MadeiraM, Mälkönen E, McClaugherty C, Meentemeyer V, Muñoz F,
Piussi P, Remacle J, Virzo de Santo A (1993) Litter mass loss
rates in pine forests of Europe and Eastern United States: somerelationships with climate and litter quality. Biogeochemistry
20:127–159
Brown GG (1995) How do earthworms affect microfloral and faunal
community diversity? Plant Soil 170:209–231

Oecologia
Cornelissen JHC, Perez-Harguindeguy N, Diaz S, Grime JP, Marzano
B, Cabido M, Vendramini F, Cerabolini B (1999) Leaf structure and defence control litter decomposition rate across species
and life forms in regional floras on two continents. New Phytol
143:191–200
Gartner TB, Cardon ZG (2004) Decomposition dynamics in mixedspecies leaf litter. Oikos 104:230–246
Gillon D, Joffre R, Ibrahima A (1994) Initial litter properties anddecay rate: a microcosm experiment on Mediterranean species.
Can J Bot 72:946–954
Gonzalez G, Seastedt TR (2001) Soil fauna and plant litter decomposition in tropical and subtropical forests. Ecology 82:955–964
Howard PJA, Howard DM (1974) Microbial decomposition of threeand shrub leaf litter. Oikos 25:341–352
Howard PJA, Howard DM (1980) Effect of species, source of litter, type of soil, and climate on litter decomposition: microbial
decomposition of three and shrub leaf litter. Oikos 34:115–124
JMP (2012) JMP version 10.0. SAS Institute, Cary, NC
Kalbitz K, Solinger S, Park JH, Michalzik B, Matzner E (2000)
Controls on the dynamics of dissolved organic matter in soils: a
review. Soil Sci 165:277–304
Kashparov VA, Lundina SM, Kadygriba AM, Protsaka VP, Levtchuka
SE, Yoschenkoa VI, Kashpurb VA, Talerko NM (2000) Forest
fires in the territory contaminated as a result of the Chernobyl
accident: radioactive aerosol resuspension and exposure of firefighters. J Environ Radioact 51:281–298
Krivolutski DA (2000) Problems of sustainable development andecological indication in radioactively contaminated areas. Russ J
Ecol 31:233–237
Krivolutski DA, Pokarzhevsky AD (1992) Effect of radioactive fallout
on soil animal populations in the 30 km zone of the Chernobyl
NPP. Sci Total Environ 112:69–77
Krivolutski D, Martushov V, Ryabtsev I (1999) Influence of radioactive contamination on fauna in the area of the Chernobyl NPP
during first years after the accident (1986–1988). Bioindicators ofradioactive contamination. Nauka, Moscow, pp 106–122
Lousier JD, Parkinson D (1976) Litter decomposition in a cool temperate deciduous forest. Can J Bot 54:419–436
Maksimova S (2005) Radiation effects on the populations of soil
invertebrates. In: Brechignac F, Desmet G (eds) Equidosimetry.
Springer, Berlin, pp 155–161
Møller AP, Mousseau TA (2006) Biological consequences of Chernobyl: 20 years after the disaster. Trends Ecol Evol 21:200–207
Møller AP, Mousseau TA (2007) Species richness and abundance ofbirds in relation to radiation at Chernobyl. Biol Lett 3:483–486
Mousseau TA, Welch SM, Chizhevsky I, Bondarenko O, Milinevsky
G, Tedeschi DJ, Bonisoli-Alquati A, Møller AP (2013) Tree rings
reveal extent of exposure to ionizing radiation in Scots pine Pinus
sylvestris. Trees 27:1443–1453
Niedree B, Vereecken H, Burauel P (2012) Effects of low-level radioactive soil contamination and sterilization on the degradation ofradiolabeled wheat straw. J Environ Radioact 109:29–35
Osono T (2007) Ecology of ligninolytic fungi associated with leaf litter decomposition. Ecol Res 22:955–974

Pigeon RF, Odum HT (eds)(1970) A tropical rain forest; a study ofirradiation and ecology at El Verde, Puerto Rico. United States
Atomic Energy Commission, National Technical Information
ServicePouyat RV, Parmelee RW, Carreiro MM (1994) Environmental effects
of forest soil invertebrates and fungal densities in oak stands
along an urban-rural land-use gradient. Pedobiologia 38:385–399
Prescott CE, Zabek LM, Kabzems R (2000) Decomposition ofbroadleaf and needle litter in forests in British Columbia: influence of litter type, forest type, and litter mixtures. Can J For Res
30:1742–1750
Rafferty B, Dawson D, Kliashtorin A (1997) Decomposition in two
pine forests: the mobilisation of 137Cs and K from forest litter.
Soil Biol Biochem 29:1673–1681
Ragon M, Restoux G, Moreira D, Møller AP, López-García P (2011)
Sunlight-exposed biofilm microbial communities are naturally
resistant to Chernobyl ionizing-radiation levels. PLoS One
6(7):e21764
Robinson CH (2002) Controls on decomposition and soil nitrogen
availability at high latitudes. Plant Soil 242:65–81
Romanovskaya VA, Sokolov IG, Rokitko PV, Chernaya NA (1998)
Ecological consequences of radioactive contamination for soil
bacteria in the 10 km Chernobyl zone. Microbiol 67:274–280
Shestopalov VM (1996) Atlas of Chernobyl exclusion zone. Ukrainian Academy of Science, Kiev
Staaf H (1980) Influence of chemical composition, addition of raspberry leaves, and nitrogen supply on decomposition rate anddynamics of nitrogen and phosphorus in beech leaf litter. Oikos
35:55–62
Taylor BR, Parkinson D (1988) Aspen and pine leaf litter decomposition in laboratory microcosms: interactions of temperature andmoisture level. Can J Bot 66:1966–1973
van der Heijden MGA, Bardgett RD, van Straalen NM (2008) The
unseen majority: soil microbes as drivers of plant diversity andproductivity in terrestrial ecosystems. Ecol Lett 11:296–310
Victorov AG (1993) Radio-sensitivity and radio-pathology of earthworms and their use as bio-indication of radioactive territories.
Bioindication and radioactive contamination. Nauka, Moscow, pp
213–217
Yoschenko VI, Kashparov VA, Levchuk SE, Glukhovskiy AS, Khomutinin YV, Protsak VP, Lundin SM, Tschiersch J (2006a) Resuspension and redistribution of radionuclides during grassland andforest fires in the Chernobyl exclusion zone. Part II. Modeling thetransport process. J Environ Radioact 87:260–278
Yoschenko VI, Kashparov VA, Protsak VP, Lundin SM, Levchuk SE,
Kadygib AM, Zvarich SI, Khomutinin YV, Maloshtan IM, Lanshin VP, Kovtun MV, Tschiersch J (2006b) Resuspension andredistribution of radionuclides during grassland and forest fires in
the Chernobyl exclusion zone. Part I. Fire experiments. J Environ
Radioact 86:143–163
Zymenko TG, Chernetsova IB, Mokhova SV (1995) Microbiologic
complex in radioactively contaminated sod-potboil soils. Her Nat
Belarus Acad Sci (Biol) 4:69–72

13