Health and Sustainable Agriculture
Editor: Christine Jakobsson
Sustainable Agriculture
1
Combatting Soil Degradation
All organisms on earth are permanently exposed to ionis- ing radiation coming from natural and artificial sources.
Natural radionuclides (NRN) come to the planet without anthropogenic activities. They are generated in the at- mosphere under the influence of cosmic radiation (14C,
3H, 22Na etc.) and are found in rocks (radioactive long-life isotopes 40K, 87Rb and members of two radioactive se- ries originating from 238U and 232Th). It is mainly 238U and
232Th that form background radiation in ecosystems. The level of radiation is not the same in different places on the globe and depends on the concentration of radionuclides in the Earth’s crust. Concentrations of natural radionu- clides in soil usually correlate to the radionuclide con- centration in bedrock. Concentrations of radionuclides (ultra microelement) in plants are linearly related to the radionuclide concentration of the soil.
The natural background levels of an agroecosystem can be increased if phosphorus fertilisers are applied.
Concentrations of natural radionuclides (NRN) in phos- phorus fertilisers can be similar to mean concentrations in soil (25 Bq kg-1), or ten-fold higher (Tables 36.1 and 36.2) (Drichko et al., 2008).
Production, transportation, storage and application of such fertilisers cause an additional exposure dose for humans. Therefore it is necessary to set limit values for NRN concentrations in fertilisers.
The strictest normative value in Russia has been set for exposure to fertiliser dust and this still remains today.
However, during the past 20 years numerous regulations were revised and finally the normative rate of NRN in phosphorus fertilisers was set to:
C(U) + 1.5C(Th) ≤ 4.0 kBq kg-1
where C(U) and C(Th) are the concentrations of 238U (226Ra) and 232Th (228Th) in radioactive equilibrium with the other isotopes of the U and Th series. This limit value has been included into the valid normative documents on radiological safety in Russia (Anon., 1999).
The content of artificial radionuclides in soil depends on the number of nuclear weapons tests and on the quality of work at atomic industries. With nuclear weapon test- ing the majority of radioactive substances go to the strat- osphere (10-50 km from the Earth’s surface). They stay there for a very long time, slowly descending and dispers- ing everywhere on the Earth’s surface. Radioactive fallout contains hundreds of different radionuclides. The most significant in soil contamination are 14C, 137Cs, 95Zr, 90Sr.
The nuclear fuel cycle, part of which are nuclear pow- er stations, is also accompanied by emissions of radioac- tive substances to the environment. The highest amounts of radionuclides reach the environment after accidents at nuclear power stations. In this case, most important from the agroecological point of view are 137Cs and 90Sr. These radionuclides are chemical analogies of the macro-ele- ments K and Ca respectively and they actively migrate in
Contamination of Agricultural Soils with Radionuclides
Marina Efremova
St Petersburg State Agrarian University, St Petersburg, Russia
Alexandra Izosimova
Agro-Physical Research Institute, Pushkin, Russia
36
the food chain. If soil is contaminated with 137Cs and 90Sr, the quality of agricultural products may be degraded for a long time, because the half-life of these radionuclides is 30 years. 137Cs and 90Sr transfer from soil to plants is dependent on the physical-chemical properties of the ra- dionuclides and the soil, climate factors and genetic pe- culiarities of the plants.
Cs belongs to a group of alkaline elements and has the oxidation state I, which means that it does not form in- soluble compounds naturally. Sr belongs to a group of alkaline-earth elements. It is a metal and has oxidation state II.
The mobility of radionuclides in the soil and their avail- ability to plants depend on the following soil properties:
• Mineralogical composition and soil texture.
• pH.
• Organic matter content.
• Cation composition of soil solution.
• Ca and K concentrations.
The fate of artificial radionuclides in soil is determined by general sorption processes. Radionuclides take part in the processes of ion exchange, physical adsorption, co-precip-
itation, etc. However Klechkovsky (Cigna and Durante, 2005) indicated that sorption of radionuclides is specific, e.g. takes place under conditions of very low concentra- tions of sorbed substance. Due to extremely low concen- trations, the behaviour of artificial radionuclides depends considerably on natural isotopic and non-isotopic carriers – stable nuclides of the element or its chemical analogues.
Changes in concentration of a macroelement (K, Ca) in the soil can influence the distribution of a radionuclide (137Cs,
90Sr), while changes in concentration of a radionuclide do not influence the distribution of the macroelement in the soil. Indeed, even in very severe radiological situations, the mass concentration of radionuclides in soil is very low and equal to about 0.4-1.0 g km-2.
Clay minerals with layered structures (such as montmo- rillonite and hydrous micas) considerably influence the stability of artificial radionuclides in soil. Minerals of this group determine exchangeable (on the surface) and non-ex- changeable (in interlayer space) sorption of 137Cs. Sorption of 90Sr is mainly exchangeable and therefore this element is more mobile in the soil-plant system than 137Cs.
Non-exchangeable fixation of 137Cs in the interlayer space of clay minerals determines the long-term presence of this element in the root habitable zone, while its bio- logical availability generally decreases. If there is K in the system, the sorption of 137Cs may be decreased because macroelements displace radionuclides from exchange sites (Wauters et al., 1994; Nisbet et al., 2000). However rather high K concentrations are needed for the decreas-
Deposit, mine Number of samples
238U(226Ra) [Bq kg-1]
232Th(228T) [Bq kg-1] Apatite
Kola 4 26 70
Seligdar 1 100 1,030
Belaya Zima 2 80 610
Phosphorite
Kingisepp 9 190 40
Podmoskovnoe 1 410 17
Bryansk 4 140 11
Verhnekamskij 3 360 18
Gornaya Shoriya 2 440 35
Aktyubinsk 1 390 30
Sejbinskoe 1 410 6
Belkinskoe* 2 1,480 <1
Oshurkovskoe 1 19 20
Telekskoe* 1 430 22
Fertiliser 238U(226Ra) [Bq kg-1]
232Th(228Th) [Bq kg-1]
Phosphorite concentrate 460 30
Superphospate 24 44
Triple superphosphate 130 48
Phosphate slag 30 11
Ammophos 4 40
Nitroammophos 4 33
Nitrophos 330 26
Nitroammophoska 4 19
Nitrophoska 9 19
Foreign fertilisers 400-4,000 15-440 Table 36.2. Average weighted NRN concentrations in selected Russian phosphorus fertilisers.
Table 36.1. NRN concentrations in raw phosphate materials in Russia.
*Concentrated product after flotation
Combatting Soil Degradation
ing coefficient of 137Cs distribution in the solid phase-soil solution system. The influence of K is not significant if its concentration in the soil solution is 0.4 g l-1, but the effect is 2-3 times stronger if the K concentration is 4.0 g l-1. Therefore, when K concentration in soil is very high, an increase in availability of Cs for plants may be observed.
However K fertilisers (within the range of agronomically reasonable doses) decrease Cs uptake 3-5-fold more in comparison with the control, because K competes with Cs for the sorption sites on the root surface (Drichko and Tsvetkova, 1990).
Competition between the chemical analogues Ca and Sr is also rather distinct in the soil-plant system. Liming decreases Sr accumulation in agricultural products 3-20- fold depending on biological characteristics of the plants and soil properties. Increasing pH, induced by lime, de- creases the mobility of radionuclides. Thus neutralisa- tion of soil acidification decreases accumulation of 137Cs in harvested crops 2-4-fold (Aleksakhin and Korneeva, 1992). Yudintseva et al. (1980) observed an antagonism between Ca2+ and Mg2+ in lime and pollutant cations in soil solution. The application of lime decreased plant uptake of radionuclides belonging to the Periodic Table groups I and II, although the solubility of their hydroxides is very high.
Organic fertilisers should be applied to decrease accu- mulation of 137Cs in the food chain. Organic fertilisers in- crease the sorption capability of the soil and this measure is most effective on light soils. For example, application of organic fertilisers to soddy podsolic sandy loam soil decreased Cs uptake by peas 3-fold, while organic fertilis- ers applied to the soddy podsolic light loam soil decreased Cs uptake 1.5-fold. However, radionuclides are weakly sorbed by organic matter and therefore their transfer from peat soils is more intense than from mineral soils (Drichko et al., 1996).
Low quantities of Sr are very well co-precipitated with Ca phosphates, which is why application of phosphorus fertilisers on soils contaminated by 90Sr decreases Sr up- take by plants 5-8-fold (Firsakova et al., 2002). Nitrogen fertilisers should be applied carefully, as higher doses give higher Cs and Sr concentrations in plants. The selection of crops that accumulate relatively low amounts of radionu- clides is also a very important measure for the manage- ment of contaminated territories. Cereals in this case are
more preferable for cultivation than vegetables and root crops. Cultivation of technical crops and seed breeding are the safest activities on contaminated lands.
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