Content of macronutrients in oat (Avena sativa L.) after remediation of soil polluted with cobalt

Methodological approach

The research was based on a pot experiment conducted in a greenhouse at the University of Warmia and Mazury in Olsztyn (north-eastern Poland). Polyethylene pots, each holding 9 kg of soil, were used to set up the trials. The experiment was conducted with three replications, on loamy sand with the following shares of fractions: sand (> 0.05 mm) 73.9%, silt (0.02–0.05 mm) 24.1% and clay (< 0.002 mm) 2.0%. The soil had the following characteristics: pH in 1 M KCl—5.05, hydrolytic acidity—28.40 mmol(+)/kg, total exchangeable bases—46.50 mmol(+)/kg, cation exchange capacity—74.90 mmol(+)/kg, base saturation of the sorption complex—68.08%, content of organic carbon—12.15 g/kg, content of available forms of phosphorus—40.99 mg/kg, potassium—46.29 mg/kg and magnesium—116.21 mg/kg, content of total cobalt—9.98 mg/kg of soil. The soil was polluted with increasing doses of cobalt: 0, 20, 40, 80, 160, 320 mg/kg of soil (cobalt chloride) and mixed with substances such as granulated bovine manure, clay, charcoal, zeolite (in an amount equal 2% of soil mass per pot) and calcium oxide (in a dose corresponding to 1 hydrolytic acidity). Source of origin of the tested substances was as follows: granulated bovine manure (granulation diameter from 4 to 5 mm) from the Fertigo Company, clay (0.002 mm fraction) from the subsoil layer (30–60 cm deep into the soil) in Tomaszkowo, near Olsztyn, Poland (53° 42′ 50.73″ N, 20° 26′ 02.41″ E), charcoal—hardwood species of European origin from Dancook Company, zeolite (clinoptilolite type)—originating from Central Mexico’s zeolite deposits (Villa de Reyes, San Luis Potosi State) from EREM Company, calcium oxide—Kujawit type fertilizer from ANKOM Kielce Company. The chemical composition of these substances was as follows: granulated bovine manure: Co—0.529 mg/kg, P—21.80 g/kg, K—16.30 g/kg, Na—0.379 g/kg, Ca—28.61 g/kg, Mg—6.020 g/kg; clay: Co—2.956 mg/kg, P—1.088 g/kg, K—1.290 g/kg, Na—0.161 g/kg, Ca—14.48 g/kg, Mg—0.525 g/kg; charcoal: Co—0.200 mg/kg, P—0.260 g/kg, K—0.766 g/kg, Na—0.172 g/kg, Ca—15.01 g/kg, Mg—0.477 g/kg; zeolite: Co—0.313 mg/kg, P—1.836 g/kg, K—5.435 g/kg, Na—0.215 g/kg, Ca—15.85 g/kg, Mg—0.553 g/kg; calcium oxide: Co—1.632 mg/kg, P—0.088 g/kg, K—0.436 g/kg, Na—0.073 g/kg, Ca—351.87 g/kg, Mg—0.901 g/kg. The choice of the above cobalt doses was made in accordance with permissible concentrations of this metal as set in the Regulation of the Minister of the Environment, of 9 September 2002, on the soil quality standards and the earth quality standards (the currently binding legal act is the Regulation of the Minister of the Environment of 1 September 2016 on the assessment of the land surface contamination). Doses of soil amendments were determined based on their beneficial effect on soil physicochemical properties. In addition, each pot was once treated with 100 mg N (NH₄NO₃), 35 mg P (KH₂PO₄), 100 mg K (KH₂PO₄ + KCl), 50 mg Mg (MgSO₄·7H₂O), 0.33 mg B (H₃BO₃), 5 mg Mn (MnCl₂·4H₂O) and 5 mg Mo ((NH₄)₆Mo₇O₂₄·4H₂O)/kg of soil. The doses of cobalt, soil amendments and fertilising nutrients were applied to soil at the onset of the experiment. Afterwards, each pot was sown with oat (Avena sativa L.) of the variety Zuch, at a plant density of 15 plants per pot. Oat was harvested after 78 days, at full ripeness stage, and the harvested plants were separated into grain, straw and roots.

Methods of laboratory and statistical analyses

Prior to chemical analyses, the plant material was dried in adequately prepared dryers, at a temperature of 60 °C, and subsequently ground in an electric mill. The plant material thus prepared underwent ‘wet’ mineralisation in concentrated 95% sulphuric acid, after which the following determinations were made: total nitrogen by the Kjeldahl’s method (Bremner 1965), phosphorus by the colorimetric method (Ostrowska et al. 1991), and potassium, calcium, magnesium and sodium by atomic absorption spectrometry (Ostrowska et al. 1991). The results were analysed statistically with the help of two-factorial analysis of variance ANOVA and PCA, supported by Statistica software (Dell Inc. 2016).

Nitrogen

In the series without neutralising substances, in treatments with 20, 40 and 80 mg Co/kg of soil, a slight increase in total nitrogen was noted in oat grain, while its rise in straw (by 90%) and roots (by 56%) was significant (Table 1). In soil contaminated with the highest cobalt doses, the growth and development of the test plant was either completely inhibited or very severely retarded, which prohibited an analysis of the plant material with respect to its content of macronutrients. When charcoal, zeolite and calcium oxide had been added to soil, the content of nitrogen decreased in all organs of oat, although the strongest impact was observed to have been produced by calcium oxide on grain, straw and roots of oat. These substances caused a decrease in total nitrogen by 8, 10 and 13%, respectively, to the plant organs mentioned, in comparison to the series without added neutralising substances. The application of manure had the strongest effect on the increase in the amount of nitrogen in oat straw (by 9%) and roots (by 8%).

Phosphorus

In the series without neutralising substances, the content of phosphorus increased by 15% in grain, by 86% in straw and by 19% in roots of oat exposed to the soil contamination with cobalt (Table 2). The application of manure to soil had the strongest impact on an average increase in the content of this macronutrient in grain (by 11%), straw (by 85%) and roots (by 18%). The remaining substances caused a decrease in the content of phosphorus in oat grain and roots, and their influence on root phosphorus content was much stronger than on grain. A decrease in the content of phosphorus in oat roots ranged from 19% (zeolite) to 38% (calcium oxide). Calcium oxide also caused a 12% decrease, while zeolite and clay were responsible for a 11–12% increase in the content of phosphorus in straw.

Potassium

The highest increase in the grain content of potassium (by 21%) was noted in the series without neutralising substances polluted with 40 mg Co/kg of soil, whereas the dose of 160 mg Co/kg of soil resulted in the highest increase in potassium content (by 24%) found in roots of the test plant (Table 3). Reverse correlations were observed regarding straw, but the strongest effect was produced by the dose of 320 mg Co/kg of soil, which resulted in a 32% decrease in the potassium content. Similarly to phosphorus, an average content of potassium in grain, straw and roots of oat grown in soil treated with manure increased by 12, 5 and 16%, respectively. The application of zeolite lowered the content of potassium in all organs of oat while calcium oxide added to soil caused a reduction of potassium in oat grain and straw. Noteworthy, both these substances had a stronger impact on the potassium content in straw than in grain or roots of oat. Moreover, clay and calcium oxide and especially charcoal led to an increase in the potassium content in oat roots. Clay had a contrary effect on the potassium content in oat roots.

Sodium

In treatments without neutralising substances, the most severe impact of cobalt contamination on the sodium content (an increase by 22%) was observed in oat straw under the influence of the dose 80 mg Co/kg of soil (Table 4). Soil remediation with manure and zeolite caused an increase in the sodium content in all analysed oat parts, while the other substances led to the same result but only in oat straw. The effect of the applied substances on the sodium content in straw was much stronger than in grain or roots of oat.

Calcium

The content of calcium in oat grown without neutralising substances added to soil was much higher under the influence of cobalt contamination (by 97% in grain, 28% in straw and 40% in roots of oat) compared to the object without cobalt (Table 5). In the treatments with calcium oxide, higher concentrations of sodium were determined in oat grain and roots, 40% and 14%, respectively. Manure led to a reduction in the content of calcium in all oat organs. Clay, charcoal and zeolite acted analogously with respect to the calcium content in oat straw and root, unlike in grain of oat.

Magnesium

Both increasing doses of cobalt in the series without neutralising substances and the applied substances had little effect on the magnesium content in oat grain, straw and roots (Table 6).

PCA and correlation coefficients

The calculated correlation coefficients (Table 7) and results of the PCA performed (Figs. 1, 2, 3, 4, 5, and 6) confirm the presence of significant correlations between the content of particular macronutrients in the analysed organs of oat. PCA demonstrated changes in the content of macronutrients in oat grain under the influence of the applied cobalt doses and soil amendments (Fig. 1). The first group of macronutrients, which include potassium, phosphorus and sodium, constituted 48.97%, while the second group, composed of nitrogen, calcium and magnesium, corresponded to 33.87% of total correlation of the data set. Vectors of all analysed macronutrients were similar in length, hence justifying the claim that they had a significant contribution to the variance proportion. The strongest positive correlation was noted between sodium and potassium as well as between potassium and phosphorus, while the strongest negative correlation appeared between nitrogen and calcium. The dispersion of cases in the PCA evaluation, illustrated in Fig. 2, shows that the incorporation of manure and calcium oxide into soil had the strongest influence on the content of the analysed macronutrients in oat grain.

Fig. 1

Content of macroelements in the oats grain illustrated with the PCA method. Key: vectors represent analysed variable (content of N, P, K, Na, Ca and Mg) and points show the grain samples with elements

Fig. 2

Effect of amendments on content of macroelements in the oats grain illustrated with the PCA method. Key: points show the grain samples with elements (WA without amendments, M manure, L clay, C charcoal, Z zeolite, CO calcium oxide, 1–0 mg, 2–20 mg, 3–40 mg, 4–80 mg, 5–160 mg, 6–320 mg Co/kg of soil)

Fig. 3

Content of macroelements in the oats straw illustrated with the PCA method. Key: vectors represent analysed variable (content of N, P, K, Na, Ca and Mg) and points show the straw samples with elements

Fig. 4

Effect of amendments on content of macroelements in the straw illustrated with the PCA method. Key: points show the oats straw samples with elements (WA without amendments, M manure, L clay, C charcoal, Z zeolite, CO calcium oxide, 1–0 mg, 2–20 mg, 3–40 mg, 4–80 mg, 5–160 mg, 6–320 mg Co/kg of soil)

Fig. 5

Content of macroelements in the oats roots illustrated with the PCA method. Key: vectors represent analysed variable (content of N, P, K, Na, Ca and Mg) and points show the roots samples with elements

Fig. 6

Effect of amendments on content of macroelements in the oats roots illustrated with the PCA method. Key: points show the roots samples with elements (WA without amendments, M manure, L clay, C charcoal, Z zeolite, CO calcium oxide, 1–0 mg, 2–20 mg, 3–40 mg, 4–80 mg, 5–160 mg, 6–320 mg Co/kg of soil)

As regards oat straw, the first group of macronutrients (nitrogen, magnesium, potassium) constituted 47.88%, while the second group of macronutrients (sodium, phosphorus, calcium) composed 30.83% of total correlation of the data set (Fig. 3). Among vectors of all analysed macronutrients, the potassium vector was the shortest, implicating that this element had the smallest contribution to the variance proportion. Strong positive correlation was found between nitrogen and magnesium as well as phosphorus and sodium, while strong negative correlation appeared between potassium versus magnesium and nitrogen. Similarly to observations on oat grain, the strongest impact on the content of the analysed macronutrients in oat straw was produced by calcium oxide and manure added to soil, which is best illustrated by Fig. 4.

The principal component analysis results plotted in Fig. 5 reveal the content of macronutrients in oat roots after the soil application of increasing doses of cobalt and substances alleviating the effect of this metal. Magnesium, nitrogen and phosphorus represent 46.88% of correlation of the data set, while potassium, calcium and sodium correspond to 27.80%. The vectors of the macronutrients were of similar length, except magnesium and nitrogen, whose vectors were slightly longer than the others. Hence, it can be concluded that the contribution of these two elements to the building of principal components was the most significant. Strong positive correlation was determined between nitrogen and magnesium, and a weaker one between potassium and calcium as well as between phosphorus and sodium. Weak negative correlation was noted between potassium and sodium. The dispersion of PCA cases shown in Fig. 6 suggests that manure, zeolite and calcium oxide had the strongest effect on the content of macronutrients in roots of oat.