Agriculture/ Agriculture, pedology and agricultural chemistry

academician Eleshev R.E., candidate of agricultural sciences Malimbaeva A.D., Ph.D. Shibikeyeva A.M.

Kazakh national agrarian university, Kazakhstan

HEAVY METALS CONTENT AND CHANGES IN CONTINUOUS AND SYSTEMATIC APPLICATION OF PHOSPHORUS FERTILIZERS IN AN INTENSIVE VEGETABLE ROTATION

Abstract The article presents three year results of the research on continuous and systematic application of mineral fertilizers on the degree of contamination of irrigated foothill dark chestnut soil by total and mobile forms of heavy metals.

Keywords: mineral fertilizers, vegetable rotation, late cabbage, dark chestnut soil, heavy metals, the TLV (threshold limit value).

Introduction

Mineral fertilizers are the main factor in increasing the productivity of agricultural crops including vegetables. Industrial mineral pomaces have been being the basis of mineral nutrition of plants providing a quick supply of easily digestible forms of nutrients. At the same time while fertilizers being applied, soil along with nutrients also receives the harmful substances in the form of heavy metals and other toxic elements (such as lead, cadmium, fluoro, chloro, nitrates and others.) As a result, contamination of soil by toxic substances happen associated with nutrient elements of fertilizers. As in vegetable production annual applying of mineral fertilizers for crops amounts to an average of 300-500 kg / ha in physical weight, about the same amount is ballast; it is easy to calculate the amount of harmful substances the soil receives.

Considering the importance of ecological purity of the soil and products, we studied the effect of applied mineral fertilizers on the degree of contamination of foothill dark chestnut soil by heavy metals.

Materials and methods The research was conducted at the permanent study area of the Kazakh Research Institute of potato and vegetable growing in the conditions of 4-pole intensive vegetable crop rotation on the irrigated foothill dark chestnut soil.

Crop rotation is conducted in 4-pole intensive vegetable crop rotation, pledged in 1992 on a dark chestnut soil: 1. White cabbage (late), 2. Cucumber, 3. Tomato, 4. Root vegetables (carrot, table beet).

Crop rotation was deployed over space and time. The crop studied in the rotation is late cabbage, Snow White cultivar. The area of the test plot was 67,2 m² (4,2 m х 16 m), the replication was quadruplicated.

The scheme of fertilizer application under the cabbage is following: 1. Control (no fertilizer); 2. N₆₀P₃₀K₃₀ (single dose); 3. N₁₂₀P₆₀K₆₀ (double dose); 4. N₁₈₀P₉₀K₉₀ (triple dose).

Types of fertilizers that are made in intensive vegetable crop rotation: ammonium saltpeter (34% of active ingredient), double superphosphate (40% of active ingredient), potassium chloride (60% of active ingredient). All types of fertilizers were applied in early spring 1,5-2 months before planting seedlings of late cabbage.

Analyses of the soil for heavy metals were performed on the devices and equipment of V.V. Dokuchaev Soil Science Institute (Moscow) in the laboratory of "Chemistry and physical chemistry of soils" under the guidance of the candidate of biological sciences Rogova O.B. The method is intended for acid decomposition of soil in autoclaves under the action of microwave radiation with temperature control for subsequent determination of metal by spectroscopic methods on atomic emission spectrometry with microwave plasma 4100 MP-AES Agilent.

Results of the research By late cabbage grown in intensive vegetable crop rotation, generally in 22 years (for the period from 1992 to 2014) the following number of industrial pomaces (kg / ha) was made with single, double and triple norms of mineral fertilizers in physical weight: urea (46% of an active ingredient of nutrient nitrogen) with a single – 1572, double - 3144 and triple 4717 kg / ha; double superphosphate (46% an active ingredient of phosphorus) with a single dose – 1575, double - 2700 and triple 3825 kg; potassium chloride (60% of an active ingredient of potassium) respectively 600, 1100 and 1600 kg / ha. In total for this period the number of NPK made was: single norm -3747, double norms - 6900 and triple norms - 10142 kg / ha of fertilizers ballast.

Table 1 – The amount of heavy metals penetrated in the foothill dark chestnut soil with fertilizers

Types of experiments	Applied fertilizers in physical weight, kg / ha		Cu	Zn	Pb	Cd
Control		0	0	0	0	0
N1P1K1	N	1572	1,5	0,8	0,4	0,5
	P	1575	2,4	45,3	51,0	4,1
	K	600	2,0	0,6	0,7	0,2
	Total	3747	6,0	47,0	52,1	4,2
N2P2K2	N	3144	3,2	1,5	0,8	1,0
	P	2700	42,0	78,0	88,0	7,0
	K	1100	3,0	1,2	1,3	0,2
	Total	6900	48,0	804,0	90,0	8,2
N 3P3K3	N	4717	4,8	2,4	1,2	1,5
	P	3825	59,0	110,0	124,0	10,0
	K	1600	5,3	1,7	1,7	0,34
	Total	10142	69,0	114,2	127,0	12,6

With these amounts of mineral fertilizers soil received the following total amount of heavy metals (g / ha): cadmium (Cd) with a single norm - 4.2, double -8.2 and a triple - 12.6; Lead (Pb) - 52,1; 90.0 and 127.0; Zinc (Zn) - 47.0; 804.0 and 114.2, as well as copper (Cu) - 6,0; 48.0 and 69.0 g / ha.

The values obtained have appeared rather small, even with the high doses of fertilizers being used. Based on these values, it can be concluded that the entry of heavy metals in the soil by mineral pomaces are so low that fertilizers can not constitute a danger as a source of soil pollution with heavy metals.

It was also calculated the share of each of the fertilizers in the entry of heavy metals in the soil. These data indicate that both content and concentration of heavy metals are phosphate fertilizers which ones on average from 2-3 to 8-31 times higher compared to nitrogen and potassium fertilizers. The share of phosphate fertilizers is 93,5-94,7%. The entry of heavy metals into the soil by nitrogen and potassium fertilizers is rather small: 2.9-3.1 and 2.4-3.6%.

Observation of nature of heavy metals accumulation in contionuous and systematic application of fertilizers under conditions of stationary experience in 4-pole intensive vegetable crop rotation on dark chestnut soil was not carried out in the year of initiation of experiment. These studies were conducted by several researchers [Aitbayev etc.]

Chemical determination of total forms of heavy metals in 2012-2014 revealed that their content in dark chestnut soil was as following: Cd - 2,1, Pb - 32,0, Zn - 78,5, Cu - 35,0, in the background section not exposed to anthropogenic impact (Table 2).

The content of total forms of heavy metals increased in the variants fertilized with different doses, and the higher the rate of fertilizer, the higher was the amount of the total forms of heavy metals studied. For example, the cadmium content increased from 3,2 to 3,7; lead from 31,0 to 38,4; zinc from 76,0 to 82,7; Cu from 37,5 to 38,0 mg / kg of soil. In control variant the content was as following: Cd - 2,5; Pb - 30,4; Zn – 73,2 and Cu - 30,3 mg / kg of soil.

When comparing the background section to the control variant which wasn’t fertilized, it was also noted the difference in the content of heavy metals in soil. This was especially true for Zn the amount of which decreased by 5.2 mg, so did Cu by 1.7, Pb by 1.6 and Cd by 0.42 in the dark chestnut soil.

Reduced Zn as well as a slight decrease of other investigated elements in the control variant of the experiment can be explained by annual cultivation of vegetable crops in the rotation and the alienation of elements with the economic part of the product, while in the background section all the vegetation remains in place, and while mineralization of organic matter the elements that exist its structure enter the soil and again become available to plants.

Table 2 – The content of total and mobile forms of heavy metals in the foothill dark chestnut soil under intensive vegetable crop rotation of cabbage, mg / kg, 0-30 cm

Types of experiments	Cd	Pb	Zn	Cu
	total forms
Background section	2,1	32,0	78,5	35,0
Control (without fertilizers)	1,68	30,4	73,2	33,3
N60P30K30	3,2	31,0	76,0	37,5
N120P60K60	3,6	35,7	80,1	36,0
N180P90K90	3,7	38,4	82,7	38,0
TLV	3	100	300	100
	mobile forms
Background section	0,31 7	1,72 19	0,55 143	1,57 22
Control (without fertilizers)	0,33 8	1,60 19	0,45 163	1,55 23
N60P30K30	0,60 5	1,87 17	0,65 117	1,56 20
N120P60K60	0,58 6	1,68 21	0,61 131	1,58 21
N180P90K90	0,64 6	2,28 17	0,57 145	1,67 23
TLV	1	60	60	50

The increase in total forms of Zn in the fertilized variants, especially where higher doses of phosphorus fertilizers are made, can be explained by the transition of Zn into an inaccessible form and being fixed in the solid phase of the soil. Despite these differences dark chestnut soil in the matter of Zn existence are classified as with low content.

The assessment of the contamination of soil is the subject to different regulations. These are the approximate permissible concentration (APC), the maximum permissible amount (MPC) and the threshold limit value (TLV). In our country such regulations are not designed to assess the degree of contamination of the soil, so we present data on the TLV, which is developed by researchers in different countries.

The TLV for total content of heavy metals in soils for agricultural purposes, developed by the authors from different countries, is within the limits (mg / kg) for Cd - 1-3, Pb - 100-200, Zn - 280-300 and Cu - 100-140 [1].

The content of mobile forms of cadmium, lead, zinc and copper in the fertilized variants with various arising doses of fertilizers increases. For example, in the fertilized variants fertilization increased the content of cadmium by 0.25-0.31; lead by 0,08-0,68; zinc by 0,12-0,2 and copper by 0.01-0.12 mg / kg of soil in dark chestnut soil. On the control the content was respectively 0.33; 1.60; 0.45 and 1.55 mg / kg.

It was also calculated the coefficients of the mobility of heavy metals: the ratio of mobile form of heavy metal to the content of total forms of heavy metals in the soil in mg / kg.

Zinc was characterized by greatest mobility, the mobility coefficient of which achieved 117-145% on fertilized variants and on the control that rate was even 163% of total content higher. The next element of the mobility coefficient was 20-23% copper, on the control which achievedn23%. The third one was lead where its mobility was 17-21% in fertilized variants, whereas 19% on the control. Cadmium was characterized by the least mobility rate of 5-6%, a little higher than 8% on the control. According to the study of mobility of heavy metals in the dark chestnut soil they can be arranged in the following decreasing way: Zn > Cu > Pb > Cd.

The slight increase in the content of heavy metals in soil in the process of mineral fertilization can be explained, on the one hand by the poor mobility of heavy metals in an alkaline medium (pH of the soil solution of dark chestnut soil is 7.3), and on the other hand by its slight accumulation in the soil due to the annual removal of heavy metals by vegetable crops of intensive rotation.

For an objective estimation of soil contamination it is necessary to know not only the total content and the amount of mobile forms, but also the protective functions of soil. Protective capabilities of soil in relation to heavy metals allow completely describe the state of heavy metals in soil of long-term field experiments, as well as the degree of contamination of soil by heavy metals. To estimate the protective capacity of soil in relation to heavy metals it have been calculated for each horizon: the coefficient of protection (C_p):

C_p = 100- С_mobile/ С_total х100%,

where С_mobile - the amount of mobile (acid-soluble) forms, mg/kg of soil, С_total – the total content of heavy metals, mg/kg of soil. The coefficient of protection of the soil C_p shows which part of the chemical element of its total content is in a tightly bound and inaccessible form.

Application of fertilizers in dark chestnut soil leads to a change in the coefficient of protection of cadmium compared to the control variant (Table 3). For example, in the surface soil the use of mineral fertilizers from single to triple norms causes a decline of this indicator for cadmium 81-84, whereas on the control of 87%. For the rest heavy metals as lead, zinc and copper, this figure is almost the same and ranges 94-95% for lead, 99% for zinc and 95-96% for copper.

Table 3 - Indicators of soil contamination with heavy metals at a continuous application of mineral fertilizers in intensive vegetable crop rotation, 0-30 cm

Types of experiment	Coefficient of total contamination				Coefficient of contamination with mobile forms				Coefficient of protection of soil
	Cd	Pb	Zn	Cu	Cd	Pb	Zn	Cu	Cd	Pb	Zn	Cu
Control (without fertilizers)	80	95	93	87	106	93	82	99	87	95	99	96
N60P30K30	152	97	97	107	194	109	110	99	81	94	99	95
N120P60K60	171	112	102	103	188	98	111	101	84	95	99	95
N180P90K90	176	120	105	109	206	132	104	106	83	94	99	96

To estimate contamination of dark chestnut soil with heavy metals the coefficients of contamination of soil have been calculated. Coefficient of total contamination (C_t.c.):

C_t.c. = С_total/ С_backgroundх100%,

where: С_total – total content of the element, mg/kg of soil, С_background – background total content of the element, mg/kg of soil. The coefficient of total contamination C_t.c. shows how modern total content of the chemical element is greater than its natural (background) level in the soil.

Thus, the highest figures of total contamination are observed in the variants with increasing doses of fertilization for cadmium (from 152 to 176%), lead (from 97 to 120%) and copper (from 103 to 109%) in relation to the control group (80 95; 87%), which is a consequence of technogenic pollution of the soil with these elements. This figure is slightly lower for zinc, but it also increases in the cases with fertilizers and indicates the absence of pollution of dark chestnut soil with this element. Thus, it is 97 to 105% for zinc, in relation to the control (93%).

To estimate the degree of contamination of soil with mobile forms of heavy metals the coefficients of contamination with mobile forms have been calculated for all the types of experiments: The coefficient of contamination with mobile forms (C_c.m.):

C_c.m.= С_mobile/ С_background х100%,

where С_mobile- the amount of mobile (acid-soluble) form of an element, mg/kg of soil, С_background – background content of mobile form of an element, mg/kg of soil. фоновое содержание подвижной формы элемента, мг/кг почвы. The coefficient of contamination with mobile forms C_c.m. shows how the amount of mobile forms exceeds natural (background) level of the content of this chemical element in the soil.

The highest figures of the coefficient of contamination with mobile forms in dark chestnut soil were also noted for cadmium (from 188 to 206) and lead (from 98 to 132) in the fertilized variants, moreover the increase comes due to arising doses of mineral fertilizers. Zinc and copper also tend to have an increase of the coefficient of contamination with mobile form of heavy metals. Thus, it is from104 to 111% for zinc and 101-106% - for copper. The coefficients of contamination with mobile forms on the control were as following: Cd - 106, Pb - 93, Zn - 82, Cu - 99%.

Thus, the total level of content of mobile forms of heavy metals in irrigated dark chestnut soils with the application of fertilizers is low, and increases in the following sequence: Cd <Zn <Cu <Pb.

Analysis of the data showed that the greatest amount of impurities of heavy metals penetrate into the soil with phosphorus fertilizer, followed by potassium and a small amount heavy metals which penetrate into the soil with nitrogen fertilizers.

Continuous and systematic application of fertilizers more than 20 years does not significantly increase the number of mobile and especially total forms of heavy metals in irrigated dark chestnut soils. A certain increase in the concentrations of heavy metals in the soil does not exceed the threshold limit values.

The coefficients of protection of the soil, the total contamination, the contamination with mobile forms can be used as necessary and sufficient indicators of heavy metals in dark chestnut soil of perennial field experience with various increasing norms of application of mineral fertilizers, especially with the application of phosphorus fertilizers that contain significant amount of toxic elements.

Conclusion Thus, studies have shown that continuous systematic application of mineral fertilizers in scientifically grounded doses does not lead to contamination of dark chestnut soil with total and mobile forms of heavy metals.

The comparison of data of the TLV developed by different researchers in different countries listed above including the results of our research shows that due to continuous and systematic application of mineral fertilizers, even in high doses, the exceeding above the standard values of the TLV of the total and mobile forms on the content of the studied heavy metals in the soil was not observed.

Reference list

1. Kuznetsov A.V. Kontrol tekhnogennogo zagryazneniya pochv i rasteniy //Agrokhimicheskiy vestnik. 1997. №5. - P. 7-9. (in Russian)

2 Vez A. Incidence des engrais sur les col, sur la production vegetale et sur lenvironnement. //Rev. Suisse agr. -1989. - V.l. - P. 41-46.

3 Mineyev V.G. Problema tyazhelykh metallov v sovremennom zemledelii //Tyazhelye metally i radionuklidy v agroekosistemakh. Moscow : Agroekolas, 1994. - P. 5-11. (in Russian)

4 Iliyin V.B. K ekologii promyshlennykh gorodov //Tyazhelye metally i radionuklidy v agroekosistemakh. Moscow : Agroekolas,1994. - P. 42-48. (in Russian)

5 Khaziyev F.Kh. Sistemno ekologicheskiy analiz fermentativnoy aktivnosti pochv. Moscow: Nauka 1982. P. 36-37, 62-73, 177. (in Russian)

6 Mortved J.J. Cadmium leveis in soil and plants from some longterm soil fertility experiments in the USA. //J. Env. Qual. - 1987. - V.16.2. - P. 137-142.

7 Ladonin V.F. Vliyanie kompleksnogo primineniya sredstv khimizatsii na sodrezhaniya tyazhelykh metallov v pochvakh i rasteniyakh // Khimiya v selskom khozyaistve. 1995, №4. - P. 32-35. (in Russian)

8 Mineyev V.G., Alekseyeva A.A., Trishina E.A. Tyazhelye metally I okruzhayuschaya sreda v usloviyakh sovremennoy khimizatsii / Soobschenie 2. – Svinets. //Agrokhimiya. 1982, №9.-P. 126-140. (in Russian)