Transfer of ¹³⁷Cs into Maize (Zea mays L.) at Different Growth Stages in the Area Contaminated by Chernobyl Fallout

1. Introduction

One of the most serious pathways of agricultural land contamination is atmospheric fallout following accidents at radiation-related facilities. Typically, the contamination covers large areas and is very long-term, which creates difficulties in the use and rehabilitation of affected croplands. In particular, after the Chernobyl NPP accident in 1986, radioactive contamination of soil with the main dose-contributing radionuclide ¹³⁷Cs in excess of 37 kBq m² was recorded in Europe over an area of about 215,900 km², of which 60,000 km² was in the European part of Russia alone, including >23,000 km² of agricultural land [1]. A revision of radioecological state of these arable lands after 30 years (which corresponds to a half-life of ¹³⁷Cs with T_1/2 30.17 years) showed that the area of cropland remaining radioactively contaminated had decreased by only 32-47% [2], depending on the landscape and cropping features in different regions, with the major part of the radionuclide still concentrated within the arable horizon [3,4]. Nowadays, 40 years after the Chernobyl accident, the spread of ¹³⁷Cs along the food chain "agricultural soils–crops–(farm animals)–humans" still poses certain risks to public health, but at the same time, well-detectable quantities of the radionuclide make it possible to clarify migration patterns in natural conditions of post-Chernobyl territories.

Cereals are usually the reference for other agricultural crops, as they form the basis of a field rotation under different bioclimatic conditions and zones. Moreover, maize, wheat and rice are considered by FAO to be among the world's essential human food staples and key elements of global food security, accounting for ≈40% of calories and protein in food consumption [5,6]. Maize (Zea mays L.) is a leading cereal crop widely cultivated in various regions and countries over an area of ≈200 million hectares, with the highest productivity (≈6 000 kg ha^-1) among cereals and annual global world production ≈1.1 billion tons [6,7]. Belonging to Graminea family, maize differs from other cereals in the type of photosynthesis C4, high rates of metabolism and biomass growth and specific architecture of roots and shoots, which distinguishes it as a separate agro-industrial group. It is a multipurpose crop, used mainly as feed and food, and also partly for non-food purposes [6]. In the human diet fresh, boiled, frozen, or canned kernels, maize flour and meal, as well as processed maize products (corn flakes, etc.) are valued for their high starch content, essential mineral elements and B vitamins [8].

However, the nutritional value of maize for humans and livestock is only realised if food and feed are environmentally safe for consumption. Maize is considered to be sensitive to abiotic stresses, including metal toxicity [9]. In turn, contamination of food products with heavy metals and radionuclides can negatively impact human health and is a serious agricultural problem in many countries [10]. Generalized IAEA analysis of ¹³⁷Cs transfer to maize grain, stems, and shoots in radioactively contaminated territories, conducted for temperate, subtropical and tropical climate zones, shows that the intensity of biological migration of the radionuclide in the “soil – maize” system is somewhat higher than for cereals in general, and, besides, increases significantly in conditions of hot arid climate [11,12]. It appears that quantitative assessment of the intensity of ¹³⁷Cs accumulation in maize based on the predictor coefficient (transfer factor, TF) is extremely important in theoretical and applied senses. Although the TF is not a complete constant even for a fixed “soil-plant” pair, it is currently the main tool for estimating the intensity of ¹³⁷Cs root uptake according to the phylogenetic model of radionuclide bioavailability [13,14,15].

The question arises, to what extent does the accuracy of TF estimation depend on the stage of a crop phenological development at which the accumulation of ¹³⁷Cs in biomass is accounted for? There is not much information that sheds light on this problem, especially in the context of field research on radioactively contaminated land. However, there are reasons to consider seasonal variations in the intensity of root uptake of ¹³⁷Cs as a significant factor in TF variability [16,17,18]. Seasonal changes in ¹³⁷Cs activity concentrations in leaves were observed for macrophytes of Glyboke Lake in the Chornobyl Exclusion Zone [18], as well as in coniferous and deciduous broad-leaf trees in the radioactive contamination zone around the Fukushima-1 NPP [19]. A decrease in TF was shown as the fruiting bodies of the Boletus edulis grew [20]. In field model experiments, significant variability in TF was shown depending on the growth stage of the agricultural crop, and in the case of cereals (wheat, brown rice), the trends in ¹³⁷Cs accumulation in straw and grain had different temporal fluctuations [21]. In contrast, in another pot experiment, a synchronous increase in TF values was found in both brown rice grain and its straw as plants grew, and the trend was maintained for root uptake of ¹³⁷Cs from soils with different properties [22]. At the same time, some field studies did not reveal any regular changes in ¹³⁷Cs activity concentration in plants of Ericaceae and Juncaceae families, but revealed seasonal trends for a member of the family Cyperaceae depending on ontogenesis stage [23]. Thus, there is no common understanding of the patterns of changes in ¹³⁷Cs activity concentration in crop organs at different stages of their growth and assessment of the significance of seasonal changes in TF for predicting the intensity of transport of radionuclides from soil to plant.

The aim of the present study was to determine the intensity of root uptake and to quantify ¹³⁷Cs fluxes into maize at different growth stages of plants. The study was conducted to elucidate: (i) the possibility of intra-seasonal dynamics of ¹³⁷Cs transfer to maize at different stages of plant ontogenesis; (ii) ¹³⁷Cs intra-planta bioaccumulation patterns with consideration of the radionuclide distribution over plant organs as they differentiate and develop; (iii) the specific of ¹³⁷Cs root uptake by maize compared to the total flux of mineral nutrients and potassium (by ⁴⁰K) into plant; (iv) the value of TF in “arable clayey soil – maize” system of semi-arid ambient in the remote period after accidental radionuclide fallout; (v) food hygienic characteristics of maize products from the long-term radioactively contaminated area.

It is assumed that the solution of these questions can supplement the general understanding of the processes of root uptake and biological migration of ¹³⁷Cs in agrosystems located in the areas of major radioactive fallout.

2. Materials and Methods

2.1. Study Area, Soil and Crop Characteristics

Field observations under seasonal variability in ¹³⁷Cs root uptake characteristics by maize have been conducted in 2021 in the pronounced area of Chernobyl fallout formed in the southern part of the Tula region of Russia (≈650 km north-east of the Chernobyl nuclear power plant) – the so-called “Plavsk radioactive hotspot” (PRH). In 1986, the PRH lands were contaminated by ¹³⁷Cs at a level of 185-555 kBq m^-2, which was 5-15 times higher than the state standard [1]. At present, the farmlands are still significantly polluted with ¹³⁷Cs, exceeding the permissible level of its accumulation in the soil by 4-6 times [4,24]. An old ploughed horizon (Aop) is ubiquitous in the territory, indicating deep tillage to a depth of 30 cm after the Chernobyl fallout as a countermeasure to reduce the root uptake of ¹³⁷Cs by crops [4]. Soils of the PRH area are mainly presented by arable Luvic Chernozems with high humus content (6.8±0.2% of C_org), neutral pH_H2O (6.6±0.1), bulk density 1.1-1.2 g cm^-3, K_tot 2.0±0.1%, K_ex 235±54 mg kg^-1, clay loamy texture (clay 42.0%, silt 57.8%, sand 0.2%), and mineral composition enriched in clay minerals of the illite and montmorillonite groups [4,25]. Soils with such properties have a pronounced ability to selectively sorb and firmly fix ¹³⁷Cs in the interlayer space of clay minerals [26,27], thereby reducing its bioavailability for plants, but at the same time retaining the radionuclide in the surface root zone for a long time.

The experimental plot was situated in the central part of the PRH area (Figure 1). It was located on the non-erodible middle part of the interfluve slope, with a gradient of 2^o and an absolute altitude of 237 MSL. In 2021, the place in the crop rotation on this plot was occupied by Pioneer maize grain hybrid P7515 (170 by FAO classification – early maturing), which is characterized by drought tolerance, disease tolerance and high yield potential. In the autumn preceding the maize sowing, the chernozem soil was ploughed to a depth of 20 cm (Ap’’ layer) and 60 kg ha^-1 of P and K fertilizers (in terms of the active ingredient) were applied. In spring, when sowing at the end of the second decade of May, the soil was cultivated to a depth of 6-8 cm (Ap’ layer) and 144 kg ha^-1 N fertilizer was applied. The seeding rate was 70-80 thousand units ha^-1, with a row spacing of 70-75 cm, seeds were placed 15-20 cm apart in the row.

2.2. Sampling and Processing

Simultaneous soil and crop sampling in the maize agrosystem was carried out monthly from 19 June to 18 September 2021 at approximately equal intervals. Weather conditions in the study area in 2021 did not differ much from the long-term rates, with average air temperature of +16.8 °C (+1 – +34.1 °C) and precipitation of 311 mm (63 days with precipitation, mostly in May and September, and 60 days without precipitation, mostly in July and August) during the growing season, according to the Plavsk weather station.

The sampling dates corresponded to the following stages of phenological development of maize on the international BBCH-scale: June – leaf development (codes 14-15, 4-5 true leaves unfolded); July – stem elongation (codes 32-34, 3–4 nodes detectable); August – development of fruit (codes 71-73, beginning of kernels development and early milk); September – ripening (codes 87-89, physiological maturity and fully ripe) [28]. The listed stages of maize growth are hereafter referred to as S1-S4, respectively.

At each observation period, 3 points were selected on a homogeneous reference site of ≈625 m² area within the experimental plot, in which soil and vegetation characteristics were recorded in combination with each other. Aerial part of the biomass was taken from a fixed area of 1x1 m and manually clipped to at a height of 2 cm from the ground surface to minimise external contamination of the shoots. As a rule, a sample of the above-ground part of the biomass, taken from the recording area, was made up of 14-16 maize plants. The roots remaining in the soil were taken from the same area to a depth of the main rooting zone (Ap’+Ap’’+Aop horizons, ≈30 cm). Plant height and root length were determined at the time of sampling. In addition, large samples of both above- and belowground parts of plants (10-14 kg) were collected over the experimental plot along Z-shaped routes in order to separate various organs and fractions of maize biomass in sufficient quantities and ensure the accuracy of subsequent γ-spectrometric analysis. At the S1 growth stage, plants were divided into above- and below-ground parts only; as maize developed and its vegetative and generative organs formed, the biomass fractionation was increasingly detailed. At growth stages S3 and S4 the following individual biomass fractions were collected: stems, leaves, cob husks, cob kernels, cob stalk, tassels; root necks, brace roots (5-10 mm), medium size seminal and lateral roots (1-3 mm), and fine roots (< 1 mm).

Soil sampling was carried out in a stratified random manner in 3 replicates [29]. Soil core samples were taken to a depth of 30 cm in 10 cm increments using an 8 cm diameter steel cylindrical sampler. Each time, the depths of the soil horizons within the strata sampled were described.

After transfer to the laboratory, aboveground fractions of biomass were weighed for the fresh biomass determination, washed under running water. The maize roots were repeatedly and thoroughly washed from the soil particles under a head of water on a system of screens with a mesh diameter of 1 mm and 0.25 mm until the washing water was completely clear. The thinnest roots that passed through the sieve 0.25 mm were collected quantitatively in a pan, washed by multiple decantation, and then combined with the biomass of fine roots. The fractions of maize roots were then air dried and weighed to determine the fresh biomass. Finally, plant samples were dried at 75^oC for 24-48 h, weighed for the dry biomass determination and then ground in a special mill.

Soil samples were dried at room temperature, weighed, ground using an agate ball mill and sieved to particles ≤1 mm. Soil moisture and bulk density were recorded during sample processing.

2.3. Laboratory Analyses

Determination of the γ-emitting radionuclides – technogenic ¹³⁷Cs and naturally occurred ⁴⁰K – in soil and crop was carried out using Canberra γ-spectrometer (USA) with ultrapure germanium (HPGe) semiconductor detector. Analysis of ¹³⁷Cs was of direct and primary interest for this study; the content of ⁴⁰K in arable chernozem and maize’s organs was determined for comparison in seasonal trends of root uptake of both radionuclides. The detection efficiency of ¹³⁷Cs (661.5 keV) was 2.5%; the detection efficiency of ⁴⁰K (1460.8 keV) – 1.2%. The analytical error of γ-spectrometry did not exceed 2%.

Supplementary soil and maize properties (soil moisture and bulk density, dry matter and ash content in biomass fractions) were determined in 3–4 replicates by conventional techniques.

2.4. Calculations and Statistics

To estimate the seasonal variations in intensity of ¹³⁷Cs root uptake from radioactively contaminated arable chernozem by maize, the values of TF was calculated as (1):

TF_tot = A_plant / A_soil,

(1)

where A_plant was the activity concentration of ¹³⁷Cs in total biomass or maize or in its fraction (Bq kg^-1), and A_soil was the weighted average activity concentration of ¹³⁷Cs in rooting zone 30 cm deep (Bq kg^-1).

The process of ¹³⁷Cs transfer from roots to shoots in maize was evaluated by the translocation coefficient (TLC) calculated as (2):

TLC = A_{aerial parts} / A_roots,

(2)

where A_{aerial parts} and A_roots were activity concentrations of ¹³⁷Cs in the respective above- or belowground biomass fractions (Bq kg^-1).

Potential phytoremediation ability (PPA) was defined as (3):

PPA = (A_plant ˣ M_plant) / (A_soil ˣ M_soil) ˣ 100 (%),

(3)

where M_plant and M_soil were dry mass of maize and rooting zone of soil, respectively, normalised per 1 m².

The obtained data were analysed using the Microsoft Excel 2003 (Microsoft Co., USA) and Statistica 8.0 (Statsoft, USA) software packages using basic descriptive statistics and one-way analysis of variance (ANOVA) with Tukey's posterior test. Factor analysis was carried out based on the principal component analysis (PCA) method with a selection of 2 significant load factors according to the scree test with an explanation of 88% of the variance. Statistical significance was assumed at α < 0.05.

3. Results

3.1. Current Characteristics of ¹³⁷Cs Accumulation in Arable Chernozem

Assessment of the current level of ¹³⁷Cs accumulation in the soil of the maize agrosystem in the PRH area revealed an average of 4.6 times the permissible level of 37 kBq m^-2, up to a maximum of 6.6 times (Table 1).

In individual sampling points the activity concentrations of ¹³⁷Cs in the surface 30-cm layer of arable chernozem varied from ≈330 to 750 Bq kg^-1 in spite of their insignificant distance from each other, which indicated a micro-focal distribution of the radionuclide in the soil cover. Such microfocal spatial heterogeneity of soil contamination with ¹³⁷Cs, initially formed during the atmospheric Chernobyl precipitation into terrestrial ecosystems, persists on agricultural lands of PRH area even 35–40 years after the accident with a coefficient of variation (C_v) of 20%. Similar patterns of spatial variability in ¹³⁷Cs accumulation levels in arable soils of the PRP area were noted in earlier observations by Golosov et al. in 1999 [30], in the modern study of Zhidkin et al. [24], and have also been recorded in remote croplands of Central Europe following the Chernobyl fallout [31,32]. Apparently, it may be possible to recognize the persistence of certain microspatial heterogeneity in ¹³⁷Cs distribution in the post-Chernobyl area’s croplands as a characteristic feature of contamination in the areas of major radioactive fallout.

The vertical distribution of ¹³⁷Cs throughout the Ap^/ (0-5 cm) – Ap^// (5-21 cm) – Aop (21-30 cm) horizons was more uniform than spatial: the maximum difference in activity concentrations between the mean values for the layers 0-10, 10-20, and 20-30 cm did not exceed 120 Bq kg^-1, i.e. less than 25% (in August) (Figure 2). The greatest uncertainty in the levels of ¹³⁷Cs content was observed in the layer 20-30 cm – both due to incomplete homogenization of the material during deep remedial ploughing in 1986 and due to the possible inclusion into this layer of underlying A horizon, which has an insignificant radionuclide content. In general, the distribution of ¹³⁷Cs activity concentrations and inventories in the stratified core of the plough and old plough horizons was rather uniform, without any seasonal dynamics of radionuclide redistribution.

On average, ¹³⁷Cs inventories were distributed almost equally in the 0-10, 10-20, and 20-30 cm layers: 35, 35, and 30%, respectively. Homogenization of ¹³⁷Cs content in plough layers is expected and often assumed a priori by many researchers, although there are few studies that empirically confirming this assumption. For example, in experiments with simulated deposition of ¹³⁷Cs on soil and its subsequent manual ploughing, it was shown that the radionuclide content within the surface 20-cm layer levelled off after 3 treatment cycles [33]. Also uniform distribution of ¹³⁷Cs within the Ap horizon treated by a mouldboard tillage technique was found 7 years after the Fukushima Daiichi accident [34]. The obtained data confirm that after deep rehabilitation ploughing and subsequent several decades of shallower agroturbation, the vertical distribution of ¹³⁷Cs within stratified plough horizons (Ap^/ – Ap^// – Aop) can be considered uniform, which is important to take into account when developing a soil sampling and monitoring schemes in radioactively contaminated agricultural landscapes.

The absence of seasonal changes in the distribution pattern of ¹³⁷Cs in plough horizon is largely explained and indirectly confirms the concept of an extremely strong fixation of radiocaesium in clay chernozems, which determines the minor desorption of the radionuclide from soil exchange complex during atmospheric precipitation. The actual absence of an increase in ¹³⁷Cs mobility during rainfall, including intensive or prolonged, was also noted in a number of field observations in different ecological conditions [3,35,36,37]. Such data indicate a low importance of processes of downward water migration of ¹³⁷Cs in chernozem-like soils during the periods of seasonal precipitation.

3.2. Root Uptake of ¹³⁷Cs by Maize at Different Growth Stages and Intra-Planta Distribution of ¹³⁷Cs

Unlike in soil, ¹³⁷Cs activity concentrations in the maize biomass varied significantly throughout the crop ontogenesis from leaf development to ripening (Table 2). The ¹³⁷Cs content in total biomass increased between growth stages S1 and S2 during leaf development and stem elongation, when the crop was at the peak of physiological activity, but then gradually declined by ≈2 times at the S3 and S4 stages during the periods of appearance and development of generative organs. However, it should be noted that in the maize agrosystem in the PRH area, seasonal fluctuations in ¹³⁷Cs content in total biomass were only evident at the trend level without confirmation of their statistical significance. Similar results with uneven accumulation of ¹³⁷Cs by plants during the vegetation period, but without certain patterns were obtained in field observations for the above- and belowground parts of berry plants and medicinal herbs, as well as for higher vascular macrophytes in areas of the Ukrainian Polesye affected by Chernobyl fallout [18,38] or for leaves and branches of konara oak (Quercus serrata) in the Fukushima region [39].

The trends in seasonal changes in ¹³⁷Cs activity concentrations in the organs of maize are more clearly expressed, if to consider the shoots and roots separately. Although the overall trend of seasonal variations in ¹³⁷Cs activity concentrations in aerial biomass was similar to that of total biomass (which explains by the absolute dominance of aboveground parts in the general organ structure), it is in the aboveground part of maize that the change in ¹³⁷Cs activity concentrations during growth was expressed most clearly and statistically confirmed. The maximum concentrations of ¹³⁷Cs activity concentrations were recorded in July at stage S2 (stem elongation) and minimum – in September at stage S4 (ripening). The difference between average values was as high as 2.5. This would seem to confirm the concept of "dilution effect" of ¹³⁷Cs activity concentrations as plant biomass increases, which has been proposed by several authors [18,19,26,39,40]. But, in fact, the process was more complicated: a significant reduction in ¹³⁷Cs activity concentrations occurred only after the beginning of cobs development (S3), most probably not owing to further increase in biomass, but due to the characteristic low content of the radionuclide in maize kernels. Thus, the overall picture of seasonal changes in the ¹³⁷Cs content in maize is apparently associated to a greater extent with changes in the structure of the biomass at different stages of growth, which are superimposed by seasonal fluctuations in the levels of radiocaesium accumulation in individual plant organs.

In vegetative organs, an increase in ¹³⁷Cs activity concentrations was generally observed during the period of intensive growth of vegetative organs in June and July (S1-S2), the lowest values were observed in August upon entering the generative phase (S3). A similar pattern of seasonal rise and fall in ¹³⁷Cs activity concentrations in green biomass seems to be characteristic of annual plants with an intermittent vegetation cycle, including agricultural crops [3,40,41], regardless of the current year's weather conditions (air temperature or rainfall) [38]. But then in September at maturity and fully ripe (S4) the radionuclide content in the stems and leaves of maize increased slightly again. Thus, in terms of "dilution effect”, the dynamics of ¹³⁷Cs content in maize shoots during the growing season can be explained only partially for selected stages of plant growth. There was also no evidence of a reduction in the ¹³⁷Cs content in stems and leaves as they aged, owing to the mechanism of translocation into young leaves noted by some authors for woody species [19,43,44,45].

For the maize’s generative organs – cobs and tassels, no statistically significant fluctuations in the ¹³⁷Cs accumulation level were found from the stage S3 of the beginning of the kernel’s formation to the stage S4 of full crop maturity.

Variability in the average ¹³⁷Cs activity concentrations in the belowground biomass of maize was also weakly expressed. Only in the last period of observations at the end of the growing season (S4) ¹³⁷Cs activity concentrations decreased with statistical significance, primarily due to a reduction in metabolic activity and gradual die-back of the middle and especially fine roots.

In general, when comparing seasonal fluctuations of ¹³⁷Cs activity concentrations in the above- and belowground parts of maize, relatively independent seasonal trends could be observed. They represented fluctuations in the characteristic levels of the radionuclide accumulation in individual plant organs and tissues, which in turn differed markedly in ¹³⁷Cs activity concentrations throughout the entire growing time. Finally, before the harvest in September, ¹³⁷Cs activity concentrations in aerial parts of maize decreased in the following order: leaves > tassels, cob husks > cob stalks, stems > kernels, with the maximum difference in values being ≈10-fold. The phenomenon of effective protection of the grain of the Graminea family plants against the accumulation of radioactive elements is well known and has been confirmed in many previous studies [3,4,22,40,46,47,48]. There is also a more general hypothesis that vegetative organs protect maize kernels from negative abiotic factors, in particular, heat stress [49]. However, the differentiation of ¹³⁷Cs accumulation inside the cob revealed in the present study (at the stage of ripeness S4 the ratios of activity concentrations in cob were: kernel: stalk: husks = 1: 3: 4), as well as 4 times higher radiocaesium content in maize’s male generative organs (tassels) compared to the female ones (cobs) are rather new and interesting facts.

In the belowground part of maize by the end of the growing season, the following distribution of ¹³⁷Cs activity concentrations was observed: fine roots > medium roots > brace roots, root necks. The difference in the accumulation of the radionuclide between the fine roots and the root neck in August (S3) and September (S4) was 7–13 times. It is characteristic that the highest ¹³⁷Cs activity concentrations were associated with fine roots, which have the greatest absorption capacity and supply plants with nutrients from the soil. It can be supposed that at S1 and S2 stages of phenological development, fine roots of maize were also characterized by significantly higher rates of ¹³⁷Cs accumulation than medium ones, but the difficulty of collecting representative samples during the juvenile period did not allow differentiating the belowground biomass by root size for these stages. At growth stages S3 and S4 after the emergence of brace roots, they were characterized by parameters of ¹³⁷Cs accumulation relatively similar to those for the root necks and stems of maize.

The ability of some plant species roots to act as a biological barrier for ¹³⁷Cs has been confirmed in several field observations and model experiments [3,4,47,50,51], and it is reasonable to assume that the rhizofiltration effect of ecotoxicants is achieved precisely due to the fine roots, which are the first and most actively interacting with the soil. The accumulation of ¹³⁷Cs in roots with difficult subsequent transfer to the aboveground part may be determined by the "avoidance strategy" used by plants to protect stress-sensitive photosynthetic tissues from the accumulation of pollutants [52].

An important applied aspect of the obtained data on ¹³⁷Cs activity concentrations in different fractions of the maize biomass was the assessment of the full compliance of the food products obtained in the fields of PRH with the sanitary-hygienic requirements of the Russian Federation, which correspond to 60 Bq kg^-1 dry weight [53]. In turn, maize stems and leaves, which were characterised by a relatively higher ¹³⁷Cs content, also met the requirements of the state veterinary regulations for cattle feed, amounting to ≤ 80 Bq kg^-1 dry weight (were in force until 2016) [54].

4. Discussion

4.1. Comparative Analysis of ¹³⁷Cs Activity Concentrations and Basic Growth and Development Parameters of Maize at Different Growth Stages

During the observation period, the maize plants have undergone significant changes in growth and phenological development indicators. The height of the aerial part of the crop increased 6-fold and its biomass more than 300-fold; in the belowground biomass, the same parameters changed by a factor of 3 and 14, respectively (Table 3). The greatest rate of maize shoot's linear increment was observed in June-July during the period of leaf unfolding between growth stages S1 and S2 (≈5 cm per day on average), while the increase in aboveground biomass was most pronounced in August-September during the fruiting and ripening between growth stages S3 and S4 (≈66 g m^-2 per day on average). Maize roots were most located in the Ah’+Ap’’ horizons, forming a rhizosphere space about 20 cm deep by July, while belowground biomass increment continued from July to August (≈7 g m^-2 per day on average), changing to a decrease in biomass by September when some of the roots died off. Nevertheless, the total biomass of maize increased steadily over the whole observation period, with its dynamics were strongly influenced by the growth dynamics of the aerial parts of the crop (correlation coefficient r² = 0.99).

In addition to changes in biometric traits of maize, the biomass structure changed significantly during the growing season, especially when generative organs appeared and developed. The ratio of above- to belowground parts of maize biomass also changed: if in June at growth stage S1 the roots dominated the shoots at a ratio of 3:2, then from July until the end of the growing season at stages S2-S4 the belowground biomass was in proportion to the aboveground as 1:5 – 1:6. These structural changes largely determined the seasonal variability in dry matter content. Thus, increased growth of vegetative mass of maize in July due to the intensification of photosynthesis was accompanied by a 1.5-2-fold decrease in dry matter content both in the shoots and in the roots, but after the cob formation this indicator significantly increased until September (twice as much as the initial estimate for the total biomass).

The seasonal variability of ash content was different and revealed a gradual decrease in percentage of mineral nutrients in the above- and belowground organs (≈3 and ≈2 times, respectively), which corresponded to a downward trend in transpiration rates from seedling to ripeness [55]. Zhang et al. [56] considered that the prevalence of root biomass at an early stage of plant ontogenesis allows maize roots to absorb nutrients better, which contributes to the successful formation of shoot organs, while the reduced intensity of root growth in the later stages of crop development contributes to formation of dry matter content in the shoots. The content of potassium in maize (according to ⁴⁰K) changed in close correlation with ash content (r²=0.75), reflecting its essential contribution to the total flux of nutrients from soils to plants.

When seasonal fluctuations of the levels of ¹³⁷Cs accumulation in maize biomass were compared with variations in the productivity shoots and roots, the dry matter content or ⁴⁰K activity concentration, no co-directional or opposite pairwise correlations were found. There was only a moderate positive correlation between ¹³⁷Cs activity concentrations and ash content in total biomass of maize (r²=0.57), most likely testifying to passive involvement of the radionuclide in metabolism during the periods of intensive root uptake of nutrients.

In turn, multi-factor analysis with the ordination of features along axes 1 and 2 of the principal components confirmed the specificity of ¹³⁷Cs accumulation in maize, which was reflected in the opposition between its content and biomass productivity, as well as the overall consumption of nutrients during the growth season (Figure 3a), explaining 88% of the variance. The characteristics of total and aboveground biomass of maize shifted along the first ordinal axis closely related to biomass growth and dry matter formation as it passed through ontogenetic stages; while belowground biomass characteristics remained relatively stable and separated from the varying characteristics of aboveground biomass along the second ordinal axis (Figure 3b). Similar changes in the structure of maize biomass at the stages of ontogenesis were in good agreement with the above-described seasonal dynamics of ¹³⁷Cs activity concentrations in the total, above- and belowground biomass.

According to PCA imaging data, the most impressive result was the distinction between of ¹³⁷Cs from ⁴⁰K, as well as ash content along the second ordinal axis. While the seasonal trend of ⁴⁰K content in maize, as expected, completely coincided with the dynamics of the general accumulation of mineral elements, ¹³⁷Cs showed specific accumulation in biomass, mainly due the suppression of its translocation to from roots to shoots. In the modern literature, the question of the similarity or difference between ¹³⁷Cs and potassium in the processes of plant nutrition is controversial. Due to the similarity of the chemical properties of potassium and ¹³⁷Cs, and due to the main mechanism of Cs⁺ penetration into plant cells through non-specific cation channels common to monovalent ions [3,41,57,58], a number of researchers have drawn analogies between these elements [19,59,60,61]. Others point out that the biogeochemical cycles of ¹³⁷Cs and potassium in the "soil-plant" system are not analogous [41,58,62,63,64,65]. There is also experimental evidence that at a high available potassium content in soil, e.g. ≥1 mmol l^-1 or kg^-1 (in the studied arable chernozems of the PRH area K_ex is 10-18 mmol kg^-1 [25]), root uptake of ¹³⁷Cs actually does not correlate with potassium [66,67]. The data obtained in this study can be considered as intermediate between the two positions: passive involvement of ¹³⁷Cs in maize biomass during periods of intensive consumption of nutrients was detected, but its distribution in the plant organs did not coincide with the distribution of potassium.

A paired analysis of ¹³⁷Cs activity concentrations in the crop biomass and basic growth and phenological development parameters of maize did reveal certain complicated patterns (Figure 4). The conditional share of ¹³⁷Cs to total and aboveground biomass was indeed highest in June (S1) and lowest in September (S4); but the ratio ¹³⁷Cs/DMC was maximum in July (S2) and the minimum in September (S4). The ¹³⁷Cs to ash or ⁴⁰K ratios were highest in July (S3) and September (S4) and lowest in June (S1).

And in the belowground part of maize, similar trends were observed for ¹³⁷Cs/biomass and ¹³⁷Cs/DMC ratios, but relatively smoothed seasonal fluctuations in the ¹³⁷Cs/ahs and ¹³⁷Cs/⁴⁰K ratios, with a maximum in August (S3). Thus, the trigger for the variability of ¹³⁷Cs transfer parameters from soil to maize plants was obviously the ratio of intensity of the main physiological processes – photosynthesis, consumption of nutrients, biosynthesis, transport and decay of substances, emergence of generative organs, etc. – which changes as the crop progresses through the stages of ontogenesis.

4.2. TF and TLC Values at Different Growth Stages of Maize

A valuable indicator of ¹³⁷Cs biological migration is TF, which reflects the intensity of element consumption by roots and can be used as a predictor in a phylogenetic model of radionuclide transfer from soil to plants [14]. The TF values obtained in this study varied during the growing season, reflecting fluctuations in ¹³⁷Cs content in maize and its separate organs (Table 4). While the fluctuations of TF values in aboveground biomass and its fractions showed no statistically significant differences at growth stages, the seasonal course of TF parameters in belowground part and especially in roots showed a marked increase from leaf development stage in June (S1) to cobs development stage in August (S3), followed by a reduction to baseline TF_roots values at maturation (S4). Seasonal variations in TF value in total biomass of maize were "dampened" by the predominance of aerial part in its structure.

The seasonal variations in TF of ¹³⁷Cs for the main consumed products of maize cultivation – kernels (grain) and green matter (stems + leaves + young cobs) for fodder – were also not statistically confirmed. The quantitative TF values obtained for grain as well as stems + leaves of maize grown in the semi-arid PRH area were in reasonable agreement and fell within the range of variation of the average TF values recommended by the IAEA for temperate [11] and arid [12] areas, estimated as 1.2 ^x 10^-2 and 8.0 ^x 10^-4 (grain), and 2.2 ^x 10^-2 and 3.8 x 10^-3 (stems and leaves), respectively. Thus, it can be assumed that for practical purposes of agricultural land-use planning in areas contaminated by condensed radioactive fallout, generalised or empirically determined regional TF values are an acceptable tool for decision making.

The intensity of intra-plant transfer of radionuclides from belowground to aboveground organs is also an important feature of biological ¹³⁷Cs migration, which is characterized by the TLC index. In particular, Zhu et al. [68] and Staunton et al. [69] suggested that differences in the behavior of radionuclides in “soil–plant” systems with different plant species may be determined not by the total root uptake of ¹³⁷Cs, but by the specificity of their transfer from roots to shoots. Rather uUnexpectedly and notably, the TLC values were slightly greater than 1 at the early growth stages of maize (S1 and S2) and less than 1 at the reproductive stages (S3 and S4). So the predominant accumulation of ¹³⁷Cs in the roots of plants from the Gramineae family, identified in a number of studies [4,46,69,70], is probably achieved during the ripening period, including the harvest stage.

4.3. Inventories of ¹³⁷Cs in Maize, Balance of the Radionuclide in “Arable Chernozem–Maize” System, PRA Value

Owing to a steady increase in maize biomass, total ¹³⁷Cs inventories involved in the crop also increased continuously from the initial observation date in June until the plants reached full maturity in September, regardless of seasonal variations in the radionuclide content in the above- or belowground parts (Figure 5). From the developmental stage of 3–5 leaves (S1) to ripening (S4), total ¹³⁷Cs inventories in maize biomass increased from 4 to 280 Bq m^-2, i.e. ≈80-fold, while the biomass itself grew by 115 times. Consequently, in the process of plant growth, ¹³⁷Cs was continuously consumed by the crop even at the late stage of ripeness, and not only redistributed between tissues, as was noted in several studies based on comparative measurement of the radionuclide activity concentrations in young and old leaves [18,40,70].

At all stages of maize growth, the main pool of ¹³⁷Cs was associated with leaves, where 35-54% of total radionuclide inventories were stored. The subdominant contribution to ¹³⁷Cs inventories at the end of the vegetative phase of plant development was made by stems (40%) and at the end of the reproductive phase by maize cobs (40%). Tassels, which accumulated ≈1% of the total ¹³⁷Cs inventories, served as the minimum significant depot for the accumulation of the radionuclide, corresponding to their share in the total maize biomass.

From the point of view of the possible organization of phytoremediation measures, it is important that in the period from July to September, at growth stages S2–S4, the ¹³⁷Cs inventories in aerial parts exceeded those in the roots by 4–15 times. Thus, most of the radionuclide contained in maize aboveground biomass can potentially be removed with the harvest. However, in the overall balance of ¹³⁷Cs inventories in the system “arable chernozem–maize” the radionuclide removal with the annual harvest was not more than 0.2%, while in the soil more than 99.8% is deposited accordingly (Table 5). Such a distribution of ¹³⁷Cs, apparently, was due to both the physiological discrimination of its root uptake by maize and the very low bioavailability of the radionuclide from neutral clayey soils [15].

In general, the PRA value of 0.2% in an agrosystem of maize cultivated on radioactively contaminated clayey chernozem seemed too low even in comparison with the natural ¹³⁷Cs decay rate, which is 2.3% per year (according to the radiocaesium decay constant). On the other hand, low bioavailability of ¹³⁷Cs for maize determined the hygienic safety of products obtained in the PRH area and directly consumed by humans or transmitted through cattle into human food chains. This makes it possible to confirm that maize belongs to so-called “safe crops” at all stages of its growth, including both baby corn and fully ripe kernels.

5. Conclusions

The Chernobyl accident in 1986 became a trigger of large-scale studies of the behaviour of ¹³⁷Cs in the environment. Yet, the patterns of seasonal fluctuation in the radionuclide transfer from contaminated soils to agricultural crops still remain insufficiently studied, while being related to changes in the most important physiological and metabolic processes during plant phenological stages, they can clarify the specific of biological migration of ¹³⁷Cs.

For maize cultivated in the post-Chernobyl PRH area with a current ¹³⁷Cs content of 176±19 kBq m^-2 (498±57 Bq kg^-1) in arable clayey chernozem, the study showed low TF values in the total biomass from the development stage of 3–5 leaves to the stage of ripening with statistically insignificant seasonal fluctuations (1.5 ^x 10^-2 – 3.6 ^x 10^-2 with geometric mean 2.2 ^x 10^-2). Along with this, reliable even though asynchronous changes during the vegetation season were found for TF values and hence ¹³⁷Cs activity concentrations in the above- and belowground parts of maize. In the aerial parts of maize, maximum ¹³⁷Cs activity concentrations were observed at the end of the vegetative stage in July, which declined to a minimum in August-September with transition to the reproductive stage. Seasonal dynamics of ¹³⁷Cs content in aboveground biomass was associated with leaves and stems, while the generative organs (cobs and tassels) were characterized by only slightly variable levels of the radionuclide accumulation. At the same time, in August, an increase in the ¹³⁷Cs content was observed in maize roots, which was apparently due to the maximum degree of development of fine roots responsible for the ability of rhizofiltration. Therefore, the key to understanding of the ¹³⁷Cs transfer in the “soil-plant” system lies in systematic recording of its accumulation in plant organs, which have characteristic levels of radionuclide accumulation (significantly changing during growth), while changes in the structure of biomass are manifested precisely during the seasonal stages of ontogenesis.

Significant differentiation of ¹³⁷Cs accumulation levels in individual organs of maize revealed in the present study, which were established at juvenile growth stage and maintained during the growing season, as well as specificity of seasonal variability of radiocaesium in plants in comparison with total biomass, dry matter, nutrients and ⁴⁰K might be another reason to pay more attention to study of ¹³⁷Cs root uptake and translocation in ontogenesis. Such investigations seem to be useful for phylogenetic predictions of ¹³⁷Cs transfer to crops, which is necessary for land use decisions on radioactively contaminated agricultural lands.

As regards assessment of the current possibility of commercial maize cultivation on croplands of the PRH area, low bioavailability of the radionuclide from clayey chernozem, as well as pronounced discrimination of its penetration into kernels allow to recognize the proposed approach to actively use ¹³⁷Cs-contaminated sites in agriculture based on the cultivation of “safe crops” as environmentally sound and acceptable.