Abstract: Selected soil properties and concentrations of aerial pollutant gases of a pip pen environment were investigated at the on-set of rains in 2005. A free survey technique was used in the study. Results revealed that soils are highly weathered (very low silt-clay ratios) and differed greatly at epipedons. Carbon monoxide (CO) had a good relationship (R2 = 0.54; p<0.05) with clay content and soil pH. Similar result was recorded when these soil properties were regressed to methane (CH4).These results present clay content and pH as good predictors of gas emission status in a livestock environment (R2 = 0.51) at p<0.05. Aerial pollutant gases concentrations were relatively low but there is need to study the temporal viabilities of these gases.
INTRODUCTION
Livestock production in the tropics is gradually changing from extensive and semi-intensive to intensive husbandry systems. The relative success of commercial pig and poultry production in the tropics (FAO, 2000) has made them very attractive in the tropics, especially with the rising unemployment. Livestock farmers remove swine wastes daily (Okoli et al., 2006) and dispose them on soils surrounding pig pens, without consideration of the environmental impact. Soils receiving these wastes respond differently to them. Differences were found in the rate of emission of ammonia, methane and dinitrogen oxide in two soils affected by pig manure (Velthol et al., 2005). These differences could be due to variability in composition of swine dung as affected by dietary protein and cellulose (Karr et al., 2006).
Concentration of aerial pollutant gases is highly related to edaphic properties. Chantigny et al. (2004) reported significant relationships between ammonia (NH3) volatilization and selected soil properties following application of anaerobically digested pig slurry. Soil pH and nitrification activity (Sommer and Sherlock, 1996) affect volatilization of gases. Soil pH and carbonate content could stimulate volatilization (Sommer and Hutchings, 2001) while presence of plants reduce NH3 volatilization (Morvan et al., 1997). These dump sites scarcely permit vegetal growth. In addition, when animal manure is left in the open as most farmers do, it may lose its potassium and phosphorus but much of its nitrogen and varying amounts of other nutrients through leaching and volatilization (Matsumoto et al., 1997). It is reported that effects of animal wastes on soil are felt immediately after application (Gordon et al., 2001) but prolonged dumping of these wastes may have profound effects on the pedogenesis of affected soils.
Characterization is helpful in the appraisal of soil productivity. The dearth of pedological information on soil of most developing countries is a major constraint to agricultural production and food security. Studies on the characterization and classification of soils of the study area are scanty (Chukwu and Asawalam, 2001; Chukwu and Chinaka, 2001) and non-existent in assessment and prediction of concentrations of aerial pollutant gases. It is hypothesized that as pig dung is dumped on soils, their responses and consequent losses into the atmosphere vary. Based on the above, the study aimed at characterizing these soil while estimating concentrations of aerial pollutants in the study locations.
MATERIALS AND METHODS
Study Area
The study was conducted in Umuiahia Agricultural zone, southeastern Nigeria
in 2005. The site lies between latitudes 5°301 and 5°391N
and longitudes 7°271 and 7° 331 E and covers about
545 km2. Coastal Plain Sands (Benin formation) is the major geological
material from which soil are derived. The area is located within the humid tropics
with an annual rainfall ranging from 2250 to 2500 mm and annual temperature
range of 27-29°C. Rainforest vegetation predominates although it has been
altered by anthropogenic activities. Both arable and livestock agriculture are
practiced in the study area. Open dump waste disposal system is common in the
smallholder livestock industry.
Field Sampling
A free survey guided by intensity of livestock activity aided the selection
of 3 sampling locations, namely Amuzukwu, Ubani and Ossah. Field studies were
conducted at the on-set of 2005 rainy season. Although livestock are allowed
to roam about with little supplementation from kitchen wastes (Okoli et al.,
2003), farmers rear pigs in pens, resulting to disposal of wastes around pig
pens. Pig pens are have concrete floor and farmer practice daily removal of
dung, which eventually end on soils surrounding these pens. In each sampling
location, a pig pen environment was chosen for the purpose of this study based
on the intensity of husbandry and flock size. On each pig pen environment, a
soil profile pit was dug, described and sampled according to the procedure of
FAO (1998). Munsel colour chart was used in soil colour determination (moist
status) in the field. A roller tape was used to determine depth of pedons. These
soil samples were air-dried, crushed and sieved using 2 mm sieve before laboratory
determinations.
Laboratory Determinations
Particle Size Distribution (PSD) was determined by hydrometer method according
to the procedure of Gee and Or (2002). Silt Clay Ratio (SCR) was calculated
as the value of silt divided by the clay equivalent in the PSD determination.
The Coefficient Of Linear Extensibility (COLE) was estimated as the difference
between the moist length and dry length of a clod to its dry length. Mathematically,
it is given as follows:
| COLE | = | (Lm-Ld)/Ld…1 (SSS, 2003) |
| where Lm | = | Length of clod when moist |
| Ld | = | Length of clod when dry. |
Soil pH was estimated electrometrically using 1:2.5 soil-solution ratio (Hendershot et al., 1993) Exchangeable cations were estimated by Inductively Couple Plasma Atomic Emission Spectrometer (ICP-AES) (Integra XMP, GBC, Arlington Heights, IL). Effective Cation Exchange Capacity (ECEC) was obtained by summation of Total Exchangeable Acidity (TEA) and Total Exchangeable Bases (TEB). Base Saturation (BS) was computed as a sum of exchangeable basic cations divided by Cation Exchange Capacity (CEC). The CEC was determined by percolating 2.5 g soil with 100 mL of 1 M ammonium chloride for about 4 h. Before percolating the soil sample, samples were soaked with extraction solution overnight. Total carbon was measured by elemental analysis (CNS 2000, Leco Ltd. Monchengladbach, Germany) as CO2 via infrared detection after dry combustion at 1250° in duplicate. Two detection limits, namely 0.1 and 0.09 g kg-1 were used for Organic Carbon (OC) and nitrogen, respectively. Carbon-nitrogen and calcium magnesium ratios were obtained by calculations.
Estimation of Pollutant Gases
Methane (CH4), ammonia (NH3), carbon monoxide (CO),
sulphur oxide (SO2) and nitrous oxide (NO2) were determined
in three intensively managed pig farms at Amuzukwu, Ubani and Ossah in Umuahia,
Southeastern Nigeria in April 2005. Readings were taken at 10.00 am and 1.00
pm from 7-18th April, 2005 according to the procedure of Wathes et al.
(1997), in which representative samples were taken. Concentrations of pollutant
gases were estimated in milligram per kilogram (mg kg-1) while Lower
Emissible Limit (LEL) was used for CH4, using the handheld personal
gas detector (Crowcon Instruments Ltd, England). The gas detector employs a
catalytic bead sensor for NH3, CO, SO2 and NO2.
Gas detectors were held about 1 m above dump litter level and readings were
recorded within seconds. All analyses were calibrated for zero and spanned before
and after reading. Pig environment temperatures were estimated a nd stood at
26.2-26.5°C (Amuzukwu), 26.3-26.7°C (Ubani) and 26.1-26.5 (Ossah) during
samples period.
Soil Classification
Field and analytical data were used in classifying soils using the USDA.
Soil Taxonomy (Soil Survey Staff, 2003). Results of soil classification were
correlated with FAO/UNESCO legends (FAO, 1998).
Date Analysis
Soil data were subjected to regression analysis using SAS computer software
(SAS, 2000).
RESULTS
Soil Properties
Soil are deep with crumb structure at surface horizons. Sandy textures predominate
the site except in the profile pit at Ubani, where sub-surface horizons are
clay. With the exception of subsurface horizons of Ubani (72-170 cm), soil are
well-drained. Colour of soil ranged from dark brownish colouration at surface
horizons to red colours except in Ubani soils which are characteristically grayish
with mottles at deeper horizons (Table 1).
Soils are highly weathered, having Silt-Clay Ratio (SCR) less than 1.2 (SCR = 0.1-0.7) and decreased with depth (Table 2). Clay content of s oil increased with depth in all the locations while Ubani had high values of silt (10-15%) (Table 2) Results are consistent with the findings of Akamigbo (1999) in soils of Bende, Uzuakoli and Item in the same geographical area.
Soil are moderately acidic with soils of Ubani showing least acidity throughout the profile pit. Total Exchangeable Based (TEB) were higher than Total Exchangeable Acidity (TEA), leading to higher values of base saturation in the area (Table 3). Effective Cation Exchange Capacity (ECEC) was generally very low (ECEC<4.31 cmol kg-1) when compared with the rankings of AIRDA (1994) which described soils having cation exchange capacity less than 6.0 cmol kg-1 as very low. Organic carbon distribution varied with location and decreased with depth in all the pedons. Total Nitrogen (TN) followed the same trend both spatially and vertically in soil profile pits. Higher values of carbon-nitrogen ratio were recorded in Amuzukwu, which agrees with the results of a study conducted by Nwokocha et al. (2003).
Exchangeable magnesium dominated exchangeable calcium in Amuzukwu and Ubani soils. Generally, low values of calcium magnesium ratio (<1.8) were obtained and this is in line with the findings of Oti (2002).
| Table 1: | Morphological characteristics of study site |
| SL = Sandy Loam, SCL = Sandy Clay Loam, C = Clay, Cr = Crumb, sbk = sub angular blocky, m = medium 1 = weak, 2 = modest, f = fine, YR = Yellowish Red, WD = Well Drained, PD = Poorly Drained | |
| Table 2: | Physical properties of studied soils |
| SCR = Silt Clay Ratio, COLE = Coefficient of Linear Extensibility | |
| Table 3: | Soil chemical properties in the study site |
| TEB = Total Exchangeable Bases, TEA = Total Exchangeable Acidity, ECEC = Effective Cation Exchange Capacity BS = Bases Saturation, OC = Organic Carbon, TN = Total Nitrogen, C/N = carbon-nitrogen ratio, Ca-Mg = calcium-magnesium ratio | |
| Table 4: | Classification of studied soils |
| Table 5: | Concentration of aerial pollutant gases in studied pens |
| Table 6: | Prediction equations for concentrations of aerial pollutant gases concentrations in livestock environment (surface horizon) |
| *: Significant at p<0.05, NS = Not Significant | |
Soils were classified as shown in Table 4, with variations resulting from mainly COLE (Table 2) and BS (Table 3) values. The represent highly weathered soils of the humid tropics.
Aerial Pollution
The aerial distribution of pollutant gases shows that CO and CH4
had highest mean values of 2.6±0.56 mg kg-1 and 1.7±0.0
LEL, respectively in the immediate environment of pig pens (Table
5). Ammonia volatilized at pig pen environments at Amuzukwu (1.9 mg kg-1)
and Ossah (1.1 mg kg-1) while 0.0±0.0 mg kg-1 was
recorded at Ubani. With the exception of CO (R2 = 0.54) and CH4
(R2 = 0.51), there was non-significant relationship between pollutant
gas emissions and selected soil properties (clay content and pH) as indicated
in Table 6.
DISCUSSION
Moderate crumb structures of the surface horizons indicate some influence of pig dung in aggregating peds. It was observed that soil away from the dump site were weak and granular and this makes them vulnerable to soil degradation. This structural modification may have influenced loss of pollutant gases from the pedosphere to the atmosphere since crumb aggregates create wider pores that allow un-impeded volatilization. This effect is surficial as Chantigny et al. (2004) reported that rapid NH3 volatilization occurred at 0.5 to 2.0 cm soil layer. At deeper horizons, especially in Ubani soils, mottles occurred, indicating imperfect drainage and possible increased reduction reactions. Under this condition, nitrous oxides in the pedosphere are reduced to more environmentally benign nitrogen gas (Clough et al., 2004) and this increases the potential for gaseous loss, although emission depend on carbon supply (Arah and Smith, 1990) antecedent soil moisture (Dendooven et al., 1996) liming (Clough et al., 2003) and soil temperature (Holtan-Hartwig et al., 2002).
Eluviation and consequent illuviation in all the pedons possibly resulted to high clay values (argillation) in the deeper horizons. In addition to this, low silt-clay ratio values suggest pronounced weathering. The implication of this is that movement of pollutant aerial gases is obstructed when in contact with such argillic (Bt) horizons leading to increased evolution of gases towards the epipedal horizons to the atmosphere. Dominance of magnesium over calcium resulting to less than 1.0 Ca-Mg ratio is a further indication of the highly weathered nature of soils and implies that soil of the site are dominated by low activity clays, such as kaolinite. Dominance of soils by low activity clays portends low soil productivity potential. In terms of gaseous emission, it suggests low volatilization due o lower clay activity in Amuzukwu and Ubani soils while Ossah soil with Ca-Mg ratios higher than 1.0 have greater potential of gaseous loss.
The present results revealed that aerial pollutants gases concentration are relatively low during the month of April (on-start of rains).This could be due to reducing temperature in the month of April when the moisture-laden southwesterly winds predominate in the area. Although strong positive relationships between temperature and some gaseous elements like N have been established (Sierra,1997), factors other than temperature may have immensely contributed to low values of pollutant gases (Piatek and Allen, 1999). However, increased emissions are expected when temperatures rise (Nyborg et al., 1997). But, it is also possible that slope position (Izaurralde et al., 2004) and plant type (Lemke et al., 2002) influenced changes in aerial pollution of studied gases. The figures obtained 1.70.±00 Lel (CH4), 1.0±0.0 mg kg-1 (NH3), 2.6±0.56 mg kg-1 (CO), 0.08±0.0 mg kg-1 (SO2) and 0.2±0.0 mg kg-1 (NO2) are much lower than the current exposure limits recommended for animal welfare in Europe or the mean values of 12.3 (winter)-24.2 (summer) mg kg-1 obtained by Wathes et al. (1997) in poultry houses in the United Kingdom. These low values could be attributed to livestock feeding systems (Msangi and Kavana, 2001) and vegetation (Morvan et al., 1997; Lemke et al., 2002), low pH (Sommer and Hutchings, 2000) could account for the low content of pollutant gases, especially as it affects ammonia content in the study. There are abundance of secondary forests surrounding these pig pens whose oxygen output may have masked gaseous pollutants in the air. In addition to vegetation (Morvan et al., 1997) Again, scavengers and farm animal on extensive system feed from these pig pen dump sites containing a mixture of dung and discarded feeds. These activities enhanced air quality of studied sites. However, clay content and soil pH had good relationships with CO and CH4 emission, suggesting that the use of these parameters a s predictors of aerial pollutant gases concentration in livestock environment having highly weathered soils.
CONCLUSION
The study revealed that soils of the study site are highly weathered but differed in characteristics, especially at the surface horizons. Activities of man through pig husbandry and consequent disposal of pig wastes around pens altered some surficial edaphic properties. There were relatively low concentrations of the aerial pollutant gases in the livestock environment. However, there is need for more studies on temporal variability of thee pollutants in the study area, involving dry, wet and transitional seasons.