THE EFFECT OF SOIL PROPERTIES ON YIELDING AND CADMIUM ACCUMULATION IN SOME CROP PLANTS

Florian Gambuś, Jerzy Wieczorek
Department of Agricultural Chemistry, Agricultural University of Cracow, Poland

ABSTRACT

In Poland, soils contaminated with cadmium constitute about 1.6 % of arable lands whereas in the œlšskie province they make up 21.5 % of agricultural lands. Because of the fact that the metal is counted among the most dangerous environmental poisons, research was undertaken to determine soil properties most impacting plant growth and their ability of cadmium accumulation.

The studies were conducted as a pot experiment on 15 soils diversified as to their properties, which might affect solubility of cadmium contained in them and its bioavailability. Oats, rape and maize were cultivated in each of the soils.

Sensitivity of the tested plant species to soil requirements and ability for cadmium accumulation by plants were decreasing in the following order: rape > maize > oats. From among the tested soil properties acidification most affected the uptake and accumulation of cadmium in rape, whereas soil organic carbon concentration affected it to a lesser degree. Cadmium uptake by rape was increasing with the growth of these soil properties. Oats and rape developed mechanisms alleviating the results of soil properties affecting cadmium adsorption from the substratum.

Key words: pot experiment, soil properties, cadmium availability, oats, rape, maize.

INTRODUCTION

Beside lead and mercury, cadmium is counted among the most dangerous environmental poisons. Its toxicity reveals itself as plant growth inhibition, reduction of root system and chloroses. Physiological effect of cadmium excess involves disorders in photosynthesis, transpiration, transformations of nitrogen compounds, changes of cell membrane permeability and DNA structure [3, 9]. Soils apparently polluted with cadmium constitute about 1.6 % of the arable areas in Poland. However, in the southern part the hazard is far more serious, as in the opolskie province such soils constitute 2.1, in malopolskie 7.5 and in œlšskie province 21.5 % of arable lands [13].

Hitherto conducted studies on crop plant sensitivity to heavy metals in the substratum have been conducted under various conditions – in aquacultures, on artificial substrates and on soil materials, as pot and field experiments with various metals and various concentrations in the substratum [5, 11]. The conclusions reached, although different at times, point to great diversification of metal accumulation ability in individual plant species and cultivars and in tolerance of heavy metals absorbed by plants. On the other hand, total heavy metal content in soil usually cannot be a direct indicator of their bioavailability, which is mostly determined by sorption-desorption processes occurring on the boundary between solid and liquid soil phase [4, 12]. General trend and intensity of these processes depend on many different soil properties, from among which soil reaction, organic substance content, soil adsorbing capacity and forms of metal in soil are most often mentioned [3].

The goal of the studies was to determine soil properties, which specifically influence the growth and ability of cadmium accumulation by plants. Three crop species often present in a crop rotation: oats, rape and maize were tested in a pot experiment.

MATERIALS AND METHODS

The research was conducted on 15 soils diversified as to their properties which might affect solubility of their cadmium and its bioavailability (Table 1). Total cadmium content in these soils ranged between 0.23 and 1.37 mg·kg^-1. They represented sandy, loamy and silty soils collected from the top layer (0-20 cm) of arable soils situated in the vicinity of Krakow. The content of floatable particles (< 0.02 mm in diameter) in these soils ranged between 15 and 49 % and organic carbon content between 3.5 and 34.6 g·kg^-1 d.m. of soil. Considering soil reaction, one soil was very acid, two were acid, seven were slightly acid and five were neutral.

Phytoavailability of cadmium contained in soil was tested in a pot experiment comprising 4 pots with each soil. The pots were filled with 2 kg of previously dried soil sifted through a mesh of 0.5 cm in diameter. Identical NPKMg fertilisation was used for the whole experiment. It comprised 0.4 g N, 0.1 g P, 0.4 g K and 0.05 g Mg per pot, applied as water solutions of chemically pure salts. Oats (Avena sativa L.), Farys c.v. was sown in the first year and harvested after 80 days of vegetation, in the second year spring rape (Brassica napus ssp. oleifera), Star c.v. was sown and harvested at the flowering phase after 56 days of vegetation and maize (Zea mays), KLG 2210 F₁ c.v. and harvested after 72 days of vegetation. Dry mass yield and the plant cadmium concentrations were determined after harvest.

Soil reaction (pH) was assessed by a potentiometer, hydrolytic acidity (Hh) and base cation exchange capacity (BEC) by Kappen method. Cation exchange capacity (CEC) was computed from these values. Organic carbon content was determined by Tiurin method though its oxidation with potassium chromate(VI), and soil texture was assayed by aerometric Boyoucose-Cassagrande method modified by Prószynski. Total cadmium content in soil was determined after soil sample solution in a mixture (3:1) of concentrated nitric(V) and chlorous(VII) acids similarly as in plant material ash solutions, in a Philips PU 9100X atomic absorption spectrophotometer or in ICP-AES JY 238 ULTRACE atomic emission spectrophotometer. Limits of determination for cadmium (mean + 3 SD) for the applied apparatuses were respectively: 11.6 and 0.196 µg·dm^-3. The accuracy of soil analysis methods was verified on the basis of certified reference material GSS-8 (GBW 07408 – State Meteorological Bureau, Beijing, China).

RESULTS AND DISCUSSION

Sensitivity of tested plant species concerning their soil requirements may be ranked as follows:

rape > maize > oats.

In many cases, the amounts of spring rape straw yield harvested from individual soils differed statistically significantly and ranged widely between 5.8 and 22.1 g·pot^-1 (Table 2). The crop yielded poorly not only on the most acidified soils (soils Nos. 2, 3 and 13) but visibly decreased its yields also on light soils with small content of soil humus (soils Nos. 1, 2 and 3). The largest yields of the aboveground parts of spring rape were produced on heavy soils with slightly acid or neutral reaction (soils Nos. 14 and 15). For the other two crop species a decline in the aboveground part yield may be observed only on acidified soils with small content of organic substance (e.g. soil No. 3). A more pronounced effect of disadvantageous soil properties on the amount of harvested grain than straw yield was registered in oats. The observations have been corroborated by coefficients of yield variation computed for crops on individual soils (Table 2). The lowest values of this coefficient, respectively 14.9 and 17.3 %, were determined analysing variability of oats grain and straw yields. Visibly higher value of variation coefficient was noted for yield of maize and especially rape, respectively 29.3 and 33.8 %. The fact testifies that these crops have greater soil requirements than oats.

Computed values of simple correlation coefficients (Table 3) demonstrate that the factors visibly reducing rape yield include: soil pH (r= 0.80), its hydrolytic acidity (r =-68) and to a lesser degree also content of floatable silt particles (dia. < 0.02mm – r=0.54) and total content of cadmium in soil (r=0.51). In case of oats, only pH had statistically significant effect upon its yielding (r=0.62), whereas for maize, despite considerable diversification in this crop yields on individual objects, no significant dependence was registered between the harvested yields of its aerial parts and studied soil properties. The influence of total cadmium content on yield of maize and oat aboveground parts was non-directed (Table 2). The above data confirm information on great rape sensitivity to acid soil reaction [1] and to soil content of heavy metals [4] and more serious soil requirements of this crop in comparison with maize and oats [7].

There was a significant diversification among individual crop species concerning cadmium content. On an average the smallest amounts of this metal were found in oats straw and grain, respectively 0.51 and 0.38 mg·kg^-1, at variation coefficients of cadmium concentration in plant material samples collected from the studied soils oscillating on the level of 30 %. Over twice more cadmium than in oat straw was assessed in the aboveground parts of maize, on an average 1.05 mg·kg^-1 at apparently bigger diversification of this feature (variation coefficient = 42.3 %). Rape accumulated definitely the greatest quantities of cadmium, on average 3.63 mg·kg^-1, at variability among individual objects reaching 153 % (Table 2). Exceeded values of 5 mg Cd·kg^-1, considered harmful for plants, were registered in three samples of rape aerial parts cultivated in acid or very acid soils (soils Nos. 2, 3 and 13) [9]. The above-mentioned results are convergent with results obtained by Gambus [6] in his studies on various plant species ability to accumulate heavy metals. The studies have demonstrated that from among the compared plant species rape reveals the greatest ability for cadmium accumulation, followed by maize, whereas oats had the least.

Presented in Tables 2 and 3 variation coefficients of cadmium content in the aboveground parts of studied plants and simple correlation coefficients between the metal concentrations and soil properties, confirm an apparent effect of some soil properties on this element content only for rape. Cadmium accumulation and absorption by rape depended most on soil hydrolytic acidity (r=0.95) and pH (r =-0.70) and on organic carbon content (r =0.54). Plants grown in most acidified soils (Nos. 2, 3 and 13) contained usually several-fold bigger amounts of this metal as compared to samples collected from neutral soils (Nos. 11 and 15). No statistically significant dependencies among soil substratum properties tested in the experiment and cadmium plant concentrations were registered for oats and maize, which suggests that their tissue concentration is not a simple function of one soil property but a result of complex impact of different soil and plant factors, as earlier suggested by Dudka [2] and Gambus [4].

Multiple linear regression was used to determine simultaneous impact of several soil properties on cadmium accumulation by individual plant species. Basic physico-chemical soil properties were assumed as independent variables for the equation constructed by progressive stepwise regression. The following soil characteristics were considered in the equations describing cadmium content in the aboveground parts of tested plants:

Where: F = content of <0.02 mm fraction.

Determined regression equations confirmed considerable specific differences in the tested plant susceptibility to cadmium accumulation depending on different soil properties. Cadmium content in rape may be predicted with quite high accuracy if we know hydrolytic acidity and pH of the soil in which it grows (R² = 0.79) but it is far more difficult for oats and maize. Cadmium content in oat straw (R² = 0.52) may be determined approximately on the basis of this metal soil concentrations and the content of floatable silt particles, organic carbon and soil pH. An attempt at constructing the analogous equation for the aboveground parts of maize, grown in the same soils, using stepwise method proved unsuccessful. Multiple regression equation presented above which describes the tested metal content in maize was determined by a standard method and is a far more complex model, considering a high number of independent variables, but presents a considerable prognostic value (R² = 0.95).

Relationships quoted above may also testify that oat and maize have developed a mechanism alleviating the results of soil properties impact on cadmium absorption from the substratum. Present investigations have also confirmed a conception formerly presented in other papers [8, 10, 14] that total content of an element in soil cannot provide a standard of its bioavailability. However, it should be emphasised that rape and maize cultivated in soils with highest cadmium contents (soils Nos. 6 and 9) accumulated one of the bigger amounts of this element (Table 2).

CONCLUSIONS

1. Tested crop species differed from one to another with their sensitivity to variable soil conditions. The soil reaction and to a lesser degree small humus content in soil were the main factors reducing the yield of tested plants. Rape proved the crop most demanding as to the soil conditions, maize and oats placed next.

2. From among the studied soil properties soil acidification and to a lesser extent soil organic carbon content had the biggest impact upon cadmium uptake and accumulation. Amount of cadmium absorbed by rape was increasing with increasing value of these features.

3. Oat and rape developed mechanisms alleviating the results of soil properties effect on cadmium absorption from the substratum.

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