Heavy metal status and physicochemical properties of agricultural soil amended by short term applicaton of animal manure

A survey was conducted to estimate short term heavy metal build up in arable soils amended with animal manure. Soil samples were collected from parts of the Michael Okpara University of Agriculture farm amended with animal manure and analyzed for heavy metal concentration and physicochemical properties. Zinc was the most abundant mineral with a mean concentration of 112.34 mg/kg, while Cd had the lowest mean concentration of 2.16 mg/kg. Lead and Chromium had the mean concentrations of 14.90 and 16.96 mg/kg, respectively. Arsenic was, however, not detected. Calcium was the most abundant exchangeable base followed by Mg, K and Na. Exchangeable acidity ranged from 1.60 to 2.40 while available P ranged from 5.40 to 67.81. Physicochemical properties of the samples showed that the soils were loamy with 77-91 % sand, 6.00 -11.9 % clay and 2.89-13.91 % silt. Soil pH in water ranged from 5.16 to 7.19. The application of animal manure affected physicochemical properties and caused heavy metal enrichment of the agricultural soils studied.


INTRODUCTION
As a result of the high cost of inorganic fertilizers, small-scale farmers now apply animal manures, sewage sludge and municipal waste to improve soil fertility.Amendment of agricultural soil with domestic refuse and animal manure provides a cheap means of enriching the soil with plant nutrients and organic matter.However, this may lead to elevated levels of heavy metals in soils and possible risk of eutrophication.Though some of these heavy metals are useful to plants and animals when they are present at trace levels or in the required proportion, they may, however, be dangerous when they accumulate in large quantities (Tuzen, 2003).Kidd et al., (2007) have reported that metal behaviour in soils and plant uptake are dependent on the nature of the metal, soil physicochemical properties and plant species.It is pertinent that for soil to fulfill its function in agricultural production and as a habitat for numerous beneficial microorganisms, heavy metal accumulation has to be minimized to a level that is not deleterious to the agro-ecosystem.Thus longterm accumulation of heavy metals in agricultural soils may cause serious (ecological) problems if tolerance levels are exceeded.
The soil acts a long-term sink for heavy metals, some of which have long residence times, depending on the element and soil characteristics (Alloway, 1995).More often heavy metals input to agricultural soils may come from inorganic fertilizer application.Other major sources of heavy metal input to agricultural soils include atmospheric deposition, sewage sludge, agro-chemicals, livestock manures, irrigation water, industrial wastes and compost (Nicholson et al., 1999;Alloway, 1995;Zhou et al., 2005).
Waste-amended soils have been reported to have high organic matter content, which affects the degree of aggregation of soil particles, reduces bulk density and increases porosity of soils (Anikwe and Nwobodo, 2002).Continuous application of animal manures and municipal wastes on soils may increase heavy metal concentration in soils (Conway and Pretly, 1991;Anikwe and Nwobodo, 2002).Thus, large scale use of these secondary raw material fertilizers should be handled with caution.
The environmental safety of utilizing livestock and poultry manure for intensive farming is now attracting great attention because the manures often contain high concentration of heavy metals and organic pollutants.Kidd et al., (2007) reported that metals derived from waste biomass are generally organically bound and less available for plant uptake than the more mobile salt impurities found in commercial fertilizers.On the other hand, Conway and Pretly (1991) argue that soils on which poultry and pig manures are continuously applied at high rates accumulate heavy metals, thus jeopardizing the good functioning of soil, contaminating crops and posing human health risks.In the same vein, Cang et al., (2004) have suggested that the application of organic fertilizers largely derived from bio-wastes should be strongly regulated to forestall unwarranted heavy metal enrichment of arable soils.The monitoring of heavy metals in the agro-ecosystem is very important since metals do not only bioaccumulate but also biomagnify from one trophic level to the other.The aim of this study is to assess the short-term effect of arable soil amendment with animal manure on soil physicochemical properties and heavy metal content.

Study area
The study area is located within Michael Okpara University of Agriculture, Umudike, Nigeria.It is part of the University's farmland where animal manure was applied to improve soil fertility.Umudike is about 10 km south-east of Umuahia, capital of Abia State of Nigeria and is located between latitudes 05° and 05° 25'N and longitudes 07 o and 07 05'E.

Sample collection
The sampling area was divided into five stations.Three soil samples were randomly collected from three sampling sites in each station and pooled together to obtain a composite sample.Soil samples were taken in depths of approximately 0-15 cm and 15 -30 cm using a soil auger.The soils were air-dried, ground and screened through a 2 mm sieve and transferred to polythene bottles until analysis.

Soil analysis
All determinations were carried out in triplicate on representative soil samples (< 2 mm fraction).Samples of each soil were analyzed for particle size (sand, silt and clay) by hydrometer method (Bouyoucos, 1951;1962).Soil pH was measured in H 2 O using a 1:1 soil: water ratio (Black, 1965).Organic matter was analyzed by the dichromate oxidation method ( Walkley and Black, 1934).Exchangeable cations were extracted with 1 M ammonium acetate (Jackson, 1958).Potassium and sodium in the extracts were determined by flame photometry while Ca and Mg were determined by atomic absorption spectrophotometry (AAS, Bulk Scientific Atomic Absorption/Emission Spectrophotometer 200 A).Available P was determined colorimetrically using the molybdenum blue method (Murphy ands Riley, 1962).Exchangeable acidity was estimated by titration as described by McLean (1965).
For heavy metal analysis soil samples (0.5 g) were placed in 100 ml beakers and digested with 10 ml of concentrated HCl: HNO 3 (3: 1) mixture.The residue was cooled, diluted to 30 ml with deionized water and filtered.Heavy metal concentrations in the prepared samples were determined by atomic absorption spectrophotometry.

Soil physicochemical properties
Some selected physicochemical properties of the soil samples are presented in Table 1.The soils are predominantly sandy loam with clay and silt contents ranging from 6.00 to 11.9% and 2.89 to 13.91%, respectively.The results indicate that the sand fraction of the soil decreased slightly with soil depth in Plots 1, 2 and 5 whereas the reverse was the case in Plots 3 and 4. The sandy nature of the soils makes amendment with animal manure suitable because the high permeability of the soil will allow large quantities of leachates to pass through the soil thereby making crops to absorb nutrients readily.
Organic carbon (OC) and organic matter (OM) contents were higher in the upper layer in all the plots.The higher levels of OM present in the upper layer of all the plots could be attributed to the application of animal manure.An increase in the OM content of soil was found after amendment of agricultural soils with sewage sludge (de las Heras et al., 2005).Kidd et al., (2007) and Anikwe and Nwobodo (2002) also reported that long term application of biosolids on agricultural soils increased soil OM tremendously.Enweozor et al., (1988) classified soil OM content of < 2.0 % as low; 2.1-3.0 % as medium and > 3.1 as high.Following this classification the amended agricultural soil investigated had high OM in the upper layers except Plot 3. Organic matter acts as a reservoir of both essential and non-essential mineral elements for plant growth and development.Thus increased OM content may lead to increased soil fertility.High OM content tends to buffer the soil by preventing excessive pH changes due to the release of exchangeable cations during mineralization of OM (Anikwe and Nwobodo, 2002).
The soil pH in water ranged from 5.16 to 7.19.Soil pH was higher in the amended soil than in the control soil sample (Table 1).Animal manure application increased soil pH and therefore led to a decrease in exchangeable acidity (Table 1 BDL=Below detection limit of pH between 6 and 7.5.The increased pH in the animal manure-amended soil may be a positive productivity indicator in an acidic tropical soil where low pH limits uptake of nutrients (Anikwe and Nwobodo, 2002).
There were remarkable increases in soil organic matter, exchangeable bases and available phosphorus following short-term land application of animal manure.It is therefore believed that these parameters will increase tremendously if animal manure is applied for a long time.

Heavy metal concentration
The concentrations of heavy metals were higher in the amended soils compared to the unamended (control) soil (Table 2).Zinc was the most abundant metal determined with a mean concentration of 112.34 ± 45.02 mgt/kg (range: 65.78 to 159.02 mg/kg).Plots 4 and 5 had the

Table 2 : Levels (mg/kg) of heavy metals in the amended soil Location Concentration of metal (mg/kg)
). Exchangeable acidity was decreased from average value of 2.25 to 1.77 by the application of animal manure.Several studies have shown that the application of biosolids such as animal manure and compost on acid soils increases the soil pH appreciably (Hue and Amien, 1989; Anikwe and Nwobodo, 2002; Ano and Ubochi, 2007; Kidd et al., 2007).According to Isirimah et al., (2003) most plants and soil microorganisms thrive best in soils