RELEASE OF HEAVY METALS FROM NON CALCIC BROWN

SOIL AMENDED WITH DIFFERENT ORGANIC MANURES

K.A. Nandasena and N.P.U. Nilantha

Department of Soil Science, Faculty of Agriculture,

University of Peradeniya, Sri Lanka

ABSTRACT

Almost all fertilizers and manures contain a series of unwanted impurities, including useful essential trace elements as well as noxious toxic elements or heavy metals such as cadmium (Cd), chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni), lead (Pb) and manganese (Mn). Generally the organic manures such as animal wastes, municipal sludge and compost contain fairly high amounts of heavy metals. An application of those organic wastes introduces large amounts of heavy metals to the soil. This may adversely affect the environment as those trace metals accumulate in plant and then enter to the food chain. Farmers who cultivate crops in non calcic brown (NCB) soil in eastern province also use large quantities of organic manure to improve the soil fertility and nutrient supply. However, the release and accumulation of heavy metals in this NCB soil by the application of organic manures have not been studied. Therefore, the present study was conducted to observe the heavy metal release from NCB soil after amended with composts and poultry manure.

Three types of organic manure viz. compost from crop residue (CR), compost from town waste (TW) and poultry manure (PM) were amended separately to NCB soil at the rates of 20 t/ha and 40 t/ha. Soil with organic manures were packed in glass leaching column and incubated at room temperature. The mineralized elements were leached out using 0.01 M Calcium Chloride at 1, 7, 12, 28, 42 and 70 days after incubation. Leachates were analyzed for Mn, Zn and Cd using Atomic Absorption Spectrophotometer.

Amounts of heavy metals mineralized and released were decreased with time. Treatment of 40 t/ha have released high amounts of Mn, Zn and Cd than 20 t/ha treatment. TW compost had released high amount of heavy metals when compared to CR compost and PM. For example 40 t/ha amended TW compost released 8.87 ppm of Mn, 2.4 ppm of Zn and 0.73 ppm of Cd during the 70 days of incubation. Next highest release was given by PM. The rate of heavy metal release from all organic manures showed a consistency. The rate of release was ranged from 0.01 to 0.02 ppm/day for all organic manures. Hence, organic manure especially TW compost should be used in agriculture with caution and concerning the environment.

INTRODUCTION

All fertilizers contain a series of unwanted impurities, including useful essential trace elements as well as noxious toxic elements or heavy metals such as cadmium (Cd), chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni), lead (Pb) and manganese (Mn)(Kim, 1994). Generally the organic manures such as animal wastes, municipal sludge and compost contain fairly high amounts of heavy metals (Narval et.al., 1992). An application of those organic wastes introduces large amounts of heavy metals into the soil (Zhu and Alva, 1993). This may adversely affect the environment as those trace metals accumulate in plant and then enter to the food chain. It was also reported that in certain areas especially upcountry of Sri Lanka, farmers apply heavy loads of organic manure making the soil and the shallow ground water contaminated with heavy metals and plant nutrients (Nandasena, 2003; Wijewardena et.al., 2001). Farmers who cultivate crops in non calcic brown (NCB) soil in eastern province also use large quantities of organic manure to improve the soil fertility and nutrient supply. However, the release and accumulation of heavy metals in NCB soil by the application of organic manures have not been studied. Therefore, the present laboratory incubation study was conducted to observe the heavy metal release from NCB soil amended with composts and poultry manure.

MATERIALS AND METHODS

Three types of organic manure viz. compost made from crop residues (CR), compost made from town waste (TW) and poultry manure (PM) were used in this study. These three manures were incorporated to the NCB soil collected from Ampara at the rates of 20 t/ha and 40 t/ha. There were six treatment combinations with three replicates. Soils mixed with organic manures according to the treatment combinations were packed in glass leaching columns and incubated at room temperature in the dark. The mineralized fractions of elements were leached out with 0.01 M Calcium Chloride solution at 1, 7, 14, 28, 42 and 70 days after the incubation. Leachates were analyzed for Mn, Zn and Cd using Atomic Absorption Spectrophotometer. The cumulative mineralized Mn, Zn and Cd from different treatment combinations were compared using Duncan multiple range test. Mineralization rates for three elements were also calculated using the curve fitting technique.

RESULTS AND DISCUSSION

Soil used in this study is sandy textured NCB soil having pH of 6, CEC of 12 meq/100 g, organic matter% of 0.52. The available Mn, Zn and Cd contents of initial soil sample were 0.5, 0.2 and 0.06 ppm respectively. According to the results, amounts of heavy metals released have been decreased with time. The treatments of 40 t/ha have released high amounts of Mn, Zn and Cd than 20 t/ha treatment in all three organic manures. When compared to CR compost and PM, TW compost had released high amount of heavy metals. A 100 g of NCB soil amended with TW compost at the rate of 40 t/ha released 8.87 ppm of Mn, 2.4 ppm of Zn and 0.73 ppm of Cd during 70 days of incubation. Next highest release of heavy metals was given by soil amended with PM at the rate of 40 t/ha. CR compost (both 20 and 40 t/ha) released lowest amounts of heavy metals during the 70 day incubation with few exceptions.

Mineralization and Release of Mn

The amount of Mn released from organic manures during the incubation period is given in Table 1. There was a significant difference between two rates of each manure application on the total mineralized Mn released during the incubation. Release of mineralized Mn from 20 t/ha and 40 t/ha each manure amended soil showed a significant difference irrespective to the type of organic manure. When the rate of manure application increased, Mn release has also been increased.

Low amount of Mn was released from CR compost incorporated soil. TW compost and PM mineralized and released relatively high amount of Mn. Total amounts of Mn released from TW compost and PM were 8.87 and 7.89 ppm respectively. Out of the three heavy metals studied, highest total mineralized metal released was accounted for Mn which ranged from 3.63 to 8.87 ppm for all manures (Table 1). Mineralized Mn from organic manure exists in soil in three oxidation states as Mn2+, Mn3+ and Mn4+ ion species. Plant roots absorb Mn primarily as Mn2+ even though arable soils mainly contain Mn4+ than Mn2+ ions.

Mineralization and release of Zn

Amount of mineralized Zn released from NCB soil amended with organic manure is given in Table 2. This release of Zn was second only to the Mn release in terms of the quantity. In all organic manures, 40 t/ha amendment showed a higher release of Zn compared to 20 t/ha rate at the end of the incubation. Out of the three organic manures, only TW compost has shown a significant difference on release of Zn between 20 t/ha and 40 t/ha rates. Even though, Zn is the most abundant heavy metal that occurs in the soil the adsorption of Zn to NCB soil appeared to be very low (Nandasena and Rathnayake, 2001). Much of the Zn is entering to the soil through the application of town waste and sewage sludge (Verloo and Willaert, 1990). Even though, Zn is considered as an essential micro nutrient for the plant growth, at high concentrations in soil Zn could be phytotoxic reducing the growth of plants or killing them at tissue concentrations that would not be toxic to animals (Lindsay, 1972). The pH of the NCB soil is 6 which is slightly acidic and this acidic nature increases the solubility of Zn and probably occurs mainly as Zn2+ and Zn (OH)+ than Zn (OH) 2 ion species.

Plants take up Zn2+ ions from soil when the Zn enriched organic manures are applied in large quantities. In such situations where, the soil is highly contaminated with Zn, plant leads to high uptake, and can exceed 500 μg Zn per g plant dry matter, a level considered to be toxic (Wild, 1994).

Mineralization and Release of Cd

Total amounts of Cd released from all treatment combinations were very low and ranked third compared to the total release of Zn and Mn. There were significant differences in total amounts of Cd released, among the compost types and the added rates (Table 3). The results indicate that the incorporation of high amount TW compost releases increasing amounts of Cd when compared with other manures. When the TW compost application increased from 20 to 40 t/ha the release of Cd has been increased by twofold from 0.36 to 0.73 ppm. The second highest release of Cd was given by PM. Cd is not an essential plant nutrient and the Cd uptake by plant depends on solubility. Cd is present in soil solution as Cd2+ ion which is highly mobile. Generally, plants take up high amounts of Cd when the soils enriched with it. Therefore, higher uptake is evident when animal slurries and compost made from town waste are applied. The consumption of Cd is considered to be harmful to human.

Rate of Mineralization

Data were best fitted to the exponential model; Y = aebX where, Y = released amount of element for a given time period (ppm), a = initial amount of heavy metals (ppm), X = times in days, and b = mineralization rate constant (ppm/day). The regression coefficients and the mineralization rates for each element are given in Table 4. The rate of heavy metal release from all organic manures showed somewhat consistency. According to the exponential model all compost mixtures were given same mineralization rate as 0.02 ppm/day for both Mn and Zn irrespective to the type and the rate of manure application (Table 4). For Cd, mineralization rate ranged from 0.01 to 0.02 ppm/day. Only, 40 mt treatment of TW and PM given a mineralization rate of 0.02 ppm/day for Cd. Hence, organic manure especially TW compost must be used in agriculture with caution and concerning the environment.

CONCLUSIONS

When increased amounts of compost are added to the soil, release of heavy metal increases accordingly. Town waste and poultry manure released higher amount of Mn, Zn and Cd than crops residues. Release of heavy metals with time decreased perhaps due to the depletion of added organic manure. Mineralization rate of heavy metals was 0.02 ppm/day with few exceptions.
