Environ. Sci. Technol. 1999, 33, 882-886
Mulching as a Countermeasure for Crop Contamination within the 30 km Zone of Chernobyl Nuclear Power Plant TERESA SAURAS YERA* AND RAMON VALLEJO Departament Biologia Vegetal, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain JOANA TENT AND GEMMA RAURET Departament Quı´mica Analı´tica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain NICOLAI OMELYANENKO AND YURI IVANOV Ukrainian Institute of Agricultural Radiology, 7, Mashinostritelej St, vil. Chabany, Kiev, Ukraine
The effect of mulch soil cover on crop contamination by 137Cs was studied within the 30 km zone of Chernobyl Nuclear Power Plant. Experiments were performed with oats (Avena sativa) over a three year period. In 1992 soil surface was covered by a plastic net. In 1993 two straw mulch treatments were applied at a dose rate of 200 g m-2 using 137Cs contaminated and clean straw, respectively. A similar mulch treatment was applied in 1994, and two mulch doses of clean straw were tested (200 and 50 g m-2). Protection of the soil with a plastic net significantly increased crop yield and reduced crop contamination (by about 30%). When clean straw was used as a mulch layer, a significant decrease of about 30-40% in 137Cs activity concentration was observed. Mulching with 137Cs contaminated straw did not reduce crop contamination, probably due to an increase in soil available 137Cs released from the contaminated mulch. Mulching has been shown to be an effective treatment both for reducing 137Cs plant contamination and improving crop yield. Therefore, it can be considered as a potential countermeasure in a postaccident situation.
Introduction Plant contamination by soil particle resuspension has been demonstrated, while rainsplash and wind have been identified as the dominant transport mechanisms in the process (1-4). Bare soil exposed to rain and wind erosion during the intercropping periods of annual crops is particularly susceptible to resuspension. Raindrops detach particles from the soil surface which splash onto plant surfaces, while the wind may carry soil particles into the atmosphere and cause long-range resuspension, as occurred in decontaminated areas in the Chernobyl zone which were quickly recontaminated by resuspension of radionuclides from surrounding contaminated areas (5). Mechanical disturbances by pedes* Corresponding author phone: 34-93-402 14 62; fax: 34-93-411 28 42; e-mail:
[email protected]. 882
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 6, 1999
TABLE 1. Soil Characteristics (21) soil type 137Cs surface deposition (1986) (kBq m-2) pH (KCl) organic matter (%) base saturation (%) cation exchange capacity (cmolc kg-1) exchangeable K (cmolc kg-1) exchangeable Ca (cmolc kg-1)
soddy-podzolic 1100-2500 4.5-5.0 1.1-1.4 50-53 3.7 0.11 1.1
trians or vehicles, conventional agricultural practices such as ploughing or harvesting, and grazing of livestock also enhance soil resuspension (6-8). Dreicer et al. (9) showed that a significant fraction of superficial contamination of plants can be attributed to rainsplash mechanisms. They also found that rainsplash contamination was mainly due to the silt-clay fraction and was dependent on rainfall characteristics. Other studies have shown that in field conditions the soil-to-plant concentration ratios for contaminants can be from one to several orders of magnitude higher than in the greenhouse where rainsplash did not occur (10-12), which demonstrated the importance of resuspension in plant contamination. The resuspension of Chernobyl-contaminated soil particles by wind and anthropogenic activities has recently been investigated (13). However, few studies have considered the rainsplash effect in the Chernobyl area, and there are few data on the radionuclides deposited on plant surfaces (14). Mulching has been reported to reduce rainsplash, soil erosion, topsoil-temperature fluctuation, and soil water evaporation (15). Mulching also increases water infiltration into the soil (16) and nutrient uptake by the crop (17). Covering of crops with mulch, usually in combination with minimum or no-tillage practices, is becoming an increasingly common for soil and water conservation, especially in dryland farming. These agricultural practices spread after the 1970s and are predicted to increase in the future (18-20). These farming conservation practices have not been reported in the Chernobyl area, either as a common cropping system or as a countermeasure after the NPP accident. This study seeks to determine whether covering the soil surface contributes to a reduction in the 137Cs transfer to cereal crops and to test the effectiveness of mulching as a countermeasure to decrease 137Cs plant contamination within the 30 km zone of Chernobyl some years after the accident.
Material and Methods Area of Study. This study was carried out in Kopachy (Ukraine). The experimental area is 5 km south of the Chernobyl nuclear power plant and is contaminated with fuel and condensed particles (21). Table 1 shows the main soil characteristics including 137Cs activity concentration. The climate is continental, with a mean annual temperature of 6.5 °C and mean annual precipitation of 589 mm. Experimental Design. Three experiments were carried out successively in 1992, 1993, and 1994. In each experiment the crop selected was oats (Avena sativa), and the customary local agricultural practices, including ploughing at a depth of 20 cm, fertilizing, and liming, were followed. The dose of fertilizers applied was (N:P2O5:K2O) 60:60:60, and the fertilizer composition was ammonium nitrate (34% N), double superphosphate (46% P2O5), potassium chloride (60% K2O), and dolomite powder (CaO 60-65%, MgO 35-40%). Seed density was 15 g m-2, and the different treatments were applied immediately after seeding. For every treatment, four plots of 10.1021/es980569o CCC: $18.00
1999 American Chemical Society Published on Web 02/04/1999
TABLE 2. Cropping Schedule for Each Year and Air Temperature, Precipitation, and Potential Evapotranspiration (PET) during the Study Periods and in the Previous Ten-Year Period year cropping period air temp (°C)
precipitation (mm)
PET (mm)
a
1992 1993 1994 10 yr av 1992 1993 1994 10 yr av 1992 1993 1994 10 yr av 1992 1993 1994
April
May 5th
13th (s)a 22nd (s)a 7.0 6.6 7.8 10.7 41 22 31 28 38 35 45 58
June
(s)a
(f)a
5th (f)a 14.2 13.0 16.3 13.0 47 30 46 56 90 84 106 81
July
17.3 17.8 15.9 15.7 61 40 71 81 112 120 106 101
8th 28th (h)a 8th (f)a 19.3 19.2 17.2 20.4 76 22 155 41 127 131 116 133
August 10th (h)a 12th (h)a 18.2 21.8 16.4 18.7 62 31 37 44 108 137 101 111
(s) seeding date; (f) flowering date; and (h) harvesting date.
7 × 5 m2 were used as replicates following a Latin square design. The seeding, flowering, and harvesting schedule is presented in Table 2. Air temperature and precipitation were recorded every year during the cropping period. Experiments To Evaluate the Effect of Covering the Soil on Plant Contamination in 1992. Oats were grown in protected and control plots. In the protected plots, the soil surface was covered by a plastic net with the aim of avoiding rainsplash. The plastic net consisted of a shading cloth with 80% light extinction efficiency and between 1 and 3 mm mesh size. The plots were encircled by a 1.25 m-high plastic fence to reduce lateral contamination. This fence was installed on the outer 5 cm of the protected surface and consisted of a shading cloth of a 60% light extinction efficiency. Control plots were left uncovered. Experiments To Evaluate the Use of Mulching as a Countermeasure for Plant Contamination by Soil. In 1993 two straw mulch treatments were applied at a dose of 200 g m-2 and compared to unmulched controls. The two mulch treatments differed in their level of 137Cs contamination, one mulch was highly contaminated at the time of application (1.5 kBq kg-1), and the other was considered uncontaminated (
