2.0                                         LITERATURE REVIEW
Bush burning has been a common phenomenon throughout the history of the world. Fire has been a constant companion of humans; it has been used to our own benefit and we are also threatened by it. Fire has been used to reduce thick bush to improve the environment for hunting wildlife and to create openings for their crops. Fire has also been seen as a damaging phenomenon in the environment and should never be tolerated in the forest or field.
The effect of bush burning on soil and soil biota depends on certain factors especially slope. The possibility of physical damage increases as the slope increases. Bush burning has been found to be a popular occurrence in areas characterized by torrential rainfall, strong wind and hot solar in Nigeria. Fire has a depressive and stimulatory effect on the vegetation depending on the type of species. Burning is also believed to get rid of the insects and micro-organisms on the surface and sub- surface of the soil. Basically, soil suffers a heat shock and receives a layer of ash; as a consequence, many soil properties can virtually be influenced by fire.

Many authors have questioned the act of fire for reasons of; the amount of recyclable nutrients that was lost to the atmosphere by volatilization the magnitude of heat generated by the fire and it negative effects on soil and the destruction of the surface organic matter layer and its effects on the greenhouse gas emission (Badia and Marti, 2003). Effect of bush burning and prescribed fire on soil is very complex. It affect soil organic matter, macro and micro nutrients, physical properties of the soil like texture, colour, pH, bulk density as well as soil biota. The impact of fire on forest soil depends on various factors such as intensity of fire, fuel load and soil moisture (Ulery and Graham, 1993)

2.1.1 Soil texture
Soil texture is the amount of sand, silt and clay and organic matter in the soil. Burning leads to coarse texture, especially at temperature exceeding 600°C. However they are not usually affected by fire unless they are subjected to high temperatures at the mineral soil surface (A horizon). Due to burning, soil texture affects how well nutrients and water are retained in the soil. The most sensitive textural fraction is clay, which begins changing at soil temperature of about 400°C when clay hydration and lattice structure begin to collapse.
Clays and organic soils hold nutrients and water better than sandy soils. As water drains from sandy soils, it often carries nutrients along with it. This condition is called leaching which can be increased by bush burning. At temperature of 600°C to 800°C, the complete destruction of internal clay structure can occur (Neary et al., 2008; Ulery and Graham, 1993). Bush burning had been identified as a contributor to soil structural degradation and due to these results to less of plant nutrients.
Blackened layers and sand- sized aggregates formed in the surface soils during burning affect the particle- size distribution and result in coarser textures due to a greater proportion of sand. Effect of fire on soil texture could be due to the irregular pattern of fire severities which led to different types of textural classes at different depth (Mermut et al., 1997).

2.1.2 Soil colour
Many physical properties of soil can be affected by bush burning. The effects are mainly because of burn severity (Ketterings and Bigham, 2000). Soil colour is most noticeable altered in severely burned soil under concentrated fuel in comparison to slightly to moderately burned soil (Ulery and Graham, 1993). The ground is covered by a layer of black or grey ash stays until plant re-colonization modifies the radiation of light and, the temperature regimes of the soil. At higher temperature reddening of soil matrix, redder hue appears due to Fe-oxides transformation and higher values to nearly complete removal of organic matter (Ulery and Graham, 1993; Certini, 2005) while in low to moderate fire ground is covered by a layer of black or grey ash, because of their refractory, charred materials affect the soil colour for a long time (Schmidt et al, 1999).
Post burn colour can represent indicators of fire severity. In this regard, in iron-rich soils, the light color hues become more yellow as values and color purity decreased with short-term heating at 300-600°C and that at 600°C reddening until after 45minutes of exposure. (Ketterings and Bigham, 2000).

2.1.3 Infiltration rate
Significant reductions of soil infiltration rate in areas burned annually were reported in Missouri Ozark forest using an Austin infiltration tube, reported significant reduction of infiltration rates on burned soils. Infiltration rate was recorded with the Diebold infiltration test in burned vegetation; the report showed that destroying the surface mulch by bush burning, regardless of the season substantially decreased water intake. Infiltration rate is the time-rate at which water will move into the soil; this may also be defined as the flux passing through the soil surface and flowing in through the profile.
Beaton (1959) recorded decreasing infiltration rates with the lapse of time after fire in the vegetation zone of southern interior of British Columbia, and that the exposure of the soil surface to the climatic elements was one of the causative factors.                             

2.1.4 Bulk density
Bulk density is the mass of a unit volume of a dry soil. It is a method of expressing soil weigh. Bulk density of soil increased due to loosening of the soil bringing about more quantities of sandy soil as a result of fire (Boerner et al., 2009; Certini 2005). Bulk density increase because of disintegration of aggregates and clogging of voids by the ash and dispersed clay minerals as a consequence, soil porosity and permeability decreases (Certini, 2005).Also water holding capacity of the soil decrease (Boix fayos 1997; Boyer and Miller 1994). Bulk density increases with ash depth (Cerdia and Doerr, 2008). Bulk density is expressed in g/cm³ in a formula as given in equation 1: 
Bulk density =             volume of soil (g)
                                  Weight of soil (cm³)

2.1.5 Total porosity
The total porosity of the soil is the fraction of the soil space occupied by water and air. The amount of this pore space is largely determined by the arrangement of the soil particles. Severe bush burning leads to reduction in total porosity of the soil. When soil structure is destroyed by fire, it affects total porosity and pore size distribution in the surface horizons of the soil (Debano et al., 1998). These changes in organic matter causes loss of macro pores in the soil surface reducing infiltration rate (Schmidt et al. 1999).
The formula for total porosity is

 TP =    1 – Bd           x  100
                    Pd                   1

Where TP is total porosity, Bd is bulk density and Pd is particle density.
2.1.6 Moisture content
The impact that bush burning has on soil depends on the intensity of fire and how long it burns (DeBano and Conral, 1978). Bush burning can cause the soil to lose its ability to absorb and retain water. After a fire, the top layer of soil may become water repellent and this cause rain to run off the surface soil without absorbing into the ground (Imeson et al., 1992). The more intense the fire, the greater the chances of moisture evaporation from the soil due to loss of vegetation cover.

2.1.7 Soil temperature
  Soil temperature is the degree of coldness or hotness of the soil. Burning exposes the soil to the sun. This lack of shade raises the temperature of the soil (Doerr et al., 2009). The fire also caused the soil to become water repellent and this soil will be even warmer due to the lack of moisture in the soil. Depending on the ecosystem, this warmer soil may hinder seed germination. Microbes present in the soil if not killed by burning, may not survive in the warmer soil due to hot environment of the soil caused by burning. Many plants depend on soil microbes and, although they may grow, will not thrive if microbes are absent from the soil due to high temperature caused by bush burning.

2.1.8 Soil aggregate and particle size
Burning has also been shown to affect soil aggregate and particle size distribution. Fire – heating of soil reduced the colloidality of the soil and increased the coarse fraction, due partly to the formation of stable aggregates. Fusion of the clay particles, occurring with temperature over 400°C was found by Serrasolsas and Khanna (1995) to decrease the clay content due to the formation of  particles.

2.2.1 Soil pH
Soil pH is the negative log to base 10 of the hydrogen ion concentration. The soil reaction is determined by the activities of H+ and OH‾ ions in the soil (Nnoke, 2001). Soil pH generally increased after burning (Tufekcioglu et al., 2010; Aref et al., 2011; Boerner et al., 2009). During the combustion process, several previously bond nutrients are released in their elemental or radical forms. Certain positive ions collectively called cations are stable at typical combustion temperatures, and remain on site after burning in the form of ash or uncombusted hydrocarbon. If in the ash form they are subsequently leached into the soil where they exchange with H+, the resulting increase in H+ in solution lowers the pH. A lower pH typically increases the nutrient cycling of various elements critical for plant growth. Ash deposited after a fire is composed mostly of salts. If exchange sites are available, these salts can effectively increase soil pH by capturing the salt cations as they leach through the soil profile. Soil pH was found to be increased by the soil heating as a result of organic acids denaturation (Khanna et al., 1994). However significant increases occur only at high temperature, in coincidence of the complete combustion of fuel and the consequent release of bases .The capacity of ash to neutralize acidity is well correlated with the sum of the concentration of K, Ca, Na and Mg in the ash itself (Ulery et al, 1993). The topsoil pH could increase as much as three units after burning. This will be due to the production of K and Na oxides, hydroxides and carbonates.

2.2.2 Organic matter  
The most intuitive change soil experience during burning is loss of organic matter (Certini, 2005). The organic horizon is critical component of ecosystem sustainability in that it provides a protective soil cover that mitigates erosion, aids in regulating soil temperature, provides habitat and substrates for soil biota and can be major source of readily mineralizable nutrients (Neary et al, 1999).
The effect of fire on soil organic matter is highly variable from total destruction of soil organic matter to partially scorching depending on fire severity, dryness of the surface organic matter and fire type (Neary et al., 1999; Gonza’lez-Perez et al., 2004). The effect of fire on organic matter is highly dependent on the type and intensity of the fire.

2.2.3 Nutrient dynamics
            This discussion of fire effect on soil nutrients dynamics will focus on the soil chemical changes, losses and availability of macro and micro nutrients. These nutrients are most likely to affect site production and vegetation dynamics. They may be lost to the atmosphere, deposited as ash, or remains in incompletely burned vegetation or debris (Boerner, 1982). Research suggests that after burning, soil nutrient decreases but their plant available forms increase (Kutiel and Navel, 1987). Burned soil has lower nitrogen than unburned soils, higher calcium, and nearly unchanged potassium, magnesium, and phosphorus stocks (Neff et al., 2005).

2.2.4 Macro – nutrients
             The immediate effect of fire on soil macro-nutrients is its loss through volatilization because of high temperature (Certini, 2005; Neary et al., 1999). During high intensity fire, temperature reaches to 675°C whereas in moderate and low intensity burning, temperature reaches to 400°C and 250°C respectively (Neary et al., 1999). Nitrogen volatilization during prescribed fire is the dominant mechanism of nitrogen loss from this system (Caldwell et al., 2002). At about 500°C, half of the nitrogen in organic matter can be volatilized (Neary et al., 1999; Knicker, 2007).It has long been controversial in fire ecology whether or not fire significantly alters total soil nitrogen pools (Wan et al., 2001). But it is suggested that burning can increase the nitrogen concentration of the residual material. The most significant short-term effects of bush burning are the increase in the soil solution concentrations and / or leaching of mineral forms of N, S and P (Murphy et al; 2006). However, the total amount of nitrogen decreases (Knight, 1996). Magnesium (Mg), calcium (Ca), and manganese (Mn) are respectively less sensitive in comparison to nitrogen because of high threshold temperature of 1107°C, 1484°C and 1962°C respectively (DeBano, 1990). Phosphorus (P), potassium (K) and sulfur (S) is partially affected in high intensity burning.

2.2.5 Micro – nutrients
 The behavior of micro-nutrients, such as Fe, Mn, Cu, Zn, B, and Mo with respect to fire is not well known because specific studies are lacking (Certini, 2005). The influence of burning on soil micro-nutrient availability is useful to understand its effect on post-fire recovery of soils and plants. Marafa and Chau (1999) reported reduction in the amount of Mn by 14% and Zn by 4% after fire event and there is a short-term and medium-term effect of bush burning on soil micro-nutrient availability.
2.2.6 Exchangeable acidity
Exchangeable acidity is the total amount of the cation exchange capacity (CEC) of a soil that is due to H+ and Al³+ ions. Ca ²+, K+, Mg²+ as exchange capacity consists of the sum of all exchangeable bases and exchangeable acidity; exchangeable acidity together with cation exchange capacity forms an indicator for the availability of plant nutrients. Exchangeable acidity is measured only if the pH value drops under seven (7) because only then does the concentration of exchangeable H+ and Al³+ ions becomes significant. Soil exchangeable acidity does not vary much under different condition. However, changes may be found after land use, burning of vegetation leave behind wood ash which sometimes may serve as liming and liming emphasis might therefore be put on monitoring land use changes. Frequent measurement of soil exchangeable acidity may be conducted if land use changes have been detected in order to evaluate its effect on soil properties.

Heavy metal is defined as any metallic element with a relative density greater than 5g/cm³ and harmful to organisms at low or high concentration. However, at the above density, heavy metals become toxic to living organisms. Alloway (1995) stated that toxicants are chemicals that have harmful effects on humans and environmental health. All toxic chemicals, by definition are hazards that pose a potential risk to humans. Such chemicals include heavy metals such as Pb, As, Al, Hg, Cr, Zn, Cu and organic compounds. Heavy metals are metals such as Hg, Pb, Cd and Ni (Cunningham and Saigo, 1999). They are highly toxic. Levels in the parts per million (ppm), so little that you can’t see or taste them and can be fatal. Because metals are highly persistent, they accumulate in food chain, accumulate in soil and have accumulative effects in humans. Also, the nutrient elements exist in two conditions namely; complex and insoluble compounds and simple more soluble forms (Nwite, 2011). Moreover, some such as Co, Cr, Cu, Mn, Mo, P and Zn are essential in small but critical concentrations in soil for the normal healthy growth of plants and animals. They are toxic at high concentrations (Alloway, 1995).
The total metal content of a soil is the result of inputs of metals from several sources; parent material, atmosphere deposition, fertilizers, agrochemicals, organic wastes and other inorganic pollutants minus losses in metals removed in crop material, leaching and volatilization.
 This can be expressed in the following form:
 M total = (Mp + Ma + Mf + Mac + Mow + MIP) – (MCr + Mi) where M is Heavy metals, P denotes parent material, a denote atmospheric deposition, F is fertilizer, ac denotes agrochemicals, Ow is organic wastes, Ip is other pollutants, Cr is crop removal and I denote losses by leaching and volatilization (Alloway, 1995).
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