Effects of Fallow Genealogical Cycles on the Build-up of Nutrients in Soils of the Cross River Rainforest, South-Southern Nigeria

Effects of Fallow Genealogical Cycles on the Build-up of Nutrients in Soils of the Cross River Rainforest, South-Southern Nigeria
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  84 Effects of Fallow Genealogical Cycles on the Build-up of Nutrients in Soils of the Cross River Rainforest, South-Southern Nigeria * Offiong, R ! " and #wara, ! # $ DOI:  http://dx.doi.org/10.4314/ejesm.v4i4.10 Received December 15 th  2011; accepted January 4 th  2012 Abstract The study examined the effect of fallow generational cycles on the buildup of nutrients in the soil. Fallow sequence of 1 st   , 2 nd   , 3 rd   , 4 th  and  th  generations were studied. The quadrat approach of sampling was employed to collect soil samples !surface and subsurface" from fi#e plots of 1$m x 1$m across the fi#e  fallow generational cycles. %esult showed that the mean proportions of organic matter !&'", total nitrogen !T(", cation exchange capacity !)*)" and a#ailable phosphorus !+#. " increased substantially in the 1 st   and 2 nd   generation of fallows probably due to reduced cropping frequency. The )+ result further re#ealed that topsoil a#ailable phosphorus, topsoil exchangeable sodium and topsoil exchangeable  potassium constituted the most significant soil properties that progressi#ely increased across the fallow generations. +s usual, nutrients in the fallow generations were confined to the topsoil. The increase in soil nutrients was attributed to the increased in tree si-e, #egetation co#er and adequate ground co#er which helped to conser#e and buildup essential nutrients in the soil by minimi-ing the loss of nutrients through soil erosion. The study suggested that for efficient conser#ation practices, fallow land mostly the 1 st   generation fallow which was fast approaching climax should be considered for reser#e. For this to be  feasible, alternati#e means of li#elihoods should be pro#ided to reduce the wanton destruction, concentration and dependence on forest resources for sustenance. Key words: Fallow Genealogy, Vegetation Development, Soil, Physical Property, Chemical property,  Disturbance frequency Introduction The destruction of forest for agricultural (mostly food crop cultivation) and non-agricultural (residential, road construction and industrial expansion) purposes usually brings about tremendous changes in soil and vegetation components as well as results in the development of different fallow vegetations. These changes are apparently influenced by the frequency and pattern of disturbance. The destruction of vegetation mostly through shifting cultivation leads to regeneration of fallow vegetation with diverse tree and shrub species (Smith and Scherr, 2002). Fallow according to Styger and Fernandes (2005) refers to a resting period for disturbed agricultural land between two cropping cycles during which soil fertility is restored. Fallows have more roles than just fertility (Styger et al.,  1999), as they offer economic prospects with regard to their carbon sink function, nutrient restoration; provide cash income in times of immediate need and help to balance food supply. Furthermore, fallows produce wood, fibers, and medicinal plants for households and can serve as pastures for livestock. For resource-poor farmers with constraints on their labour, inputs, and access to new techniques, fallows are economically often a good option for optimizing agricultural production, especially when non-crop products can be harvested (Kupfer et al.,  2006). Fallows or forest regeneration or regrowth as the case may be, is a common terminology in ecological studies as it pertains to the re-establishment of forest after a period of disturbance (food crop cultivation). On this note, majority of the studies in the literature (Nye and Greenland, 1960; Aweto, 1981b; Agboola, 1994; Fernandes et al.,  1997; Brand and Pfund, 1998; Feldpausch et al.,  2004; Addo-Fordjour et al.,  2009) considered fallows based principally on succession time or number of years a piece of land is deliberately allowed to fallow, to replenish its loss nutrient. The principal focus of majority of these studies is to characterize soil or vegetation components and in most cases both components (Aweto, 1981c; Hughes et al.,  1999; Feng et al.,  2007) to understand the changes that have taken   Ethiopian Journal of Environmental Studies and Management Vol. 4 No.4 2011 1  Dept. of Geography & Environmental Studies, University of Calabar, Nigeria 2  Dept. of Geography, University of Ibadan, Nigeria Corresponding email:  raphyxx@yahoo.com    85 place in relation to fallow periods. However, the number of times a piece of land is cultivated in the past 70 years has not attracted research attention. The reason for this is not far-fetched, which perhaps may be attributed to the increase in global population and pressure on available land. This paper therefore attempts to invoke provoking contentions as well as introduce a new thought of research on fallow management and its effects on soil nutrient restoration. Fallow genealogy as used in this context refers to the number of times a piece of land is cultivated in the past seventy years. In this regards, a piece of land that has been cultivated once in the past 70 years, is termed 1 st  generation fallow, one that is cultivated twice is termed 2 nd  generation fallow and so on. Though, these types of fallow genealogies/generations are not easy to come by in some parts of the tropics, except in areas where the forest vegetation has not been wantonly exploited, probably due to low population and availability of large expanse of forested land. Soil fertility restoration for ecosystem productivity and sustainability is perhaps the rationale for deliberately allowing disturbed hectare of land to regain its loss nutrient. Low soil fertility is increasingly recognized as a fundamental biophysical cause for declining food security among small-farm households in sub-Saharan Africa (Sanchez et al.,  1997; Mafongoya et al.,  2005), therefore exploring areas with immense forest resources and minimal disturbance will go a long way to improve food security as well as serve as conservative spots for biodiversity preservation. The implications include reduction in on-site land degradation (control of soil erosion and nutrient loss among others) and off-site environmental degradation (eutrophication of water bodies, sedimentation of water channels among others). Since, the generation of fallows identified in this study was cultivated once, twice or thrice as the case may be in the past 70 years, net loss of nutrients from the topsoil is completely minimized, due to the increase in litter fall. Styger and Fernandes (2005) noted that in the early stages of a fallow when biomass is increasing and nutrient uptake is rapid, there may actually be a net loss of nutrients from the topsoil. They further argue that it is only later in the fallow development, when litter fall greatly exceeds the increase of nutrient uptake into biomass, that the amount of nutrients in the topsoil may be increased and restored. This perhaps is obvious as the decrease in the frequency of disturbance (continuous land cultivation for food crop cultivation) speeds up the recovery capacity of a once disturbed ecosystem, thereby triggering nutrient fluxes. Statement of problem Globally, forest destruction especially for the cultivation of food crop is usually accompanied by unprecedented changes in soil and vegetation components. The loss in forest vegetation/cover makes the soil susceptible to soil erosion, thereby facilitating soil and nutrient loss. The continuous loss in soil nutrient due to the loss in vegetal cover affects the productivity capacity of the soil to sustain increased food production. However, the destruction of forest though accelerates soil erosion problems, but at the long-run leads to the development of different successional and generational vegetation depending on the fallow age and the number of times a piece of land is cultivated. The gradual change in fallow time brings about drastic development of forest regrowth with inherent effect on the buildup of nutrient through the accumulation of biomass which in situ decomposes to add nutrient to the soil. The development of forest also affords the soil adequate cover and protection from harsh climatic conditions mostly the erosive force of rainstorm. Indeed, the growth in vegetation over time helps to improve the soil structure as well as ensure the soil eco-balance and nutrient fluxes. This perhaps is only feasible when a once disturbed ecosystem or piece of land is allowed to fallow, during which the soil regains its fertility. Previous studies as noted above considered fallows based principally on succession time or number of years a piece of land is deliberately allowed to fallow. The principal focus of these studies is to characterize soil or vegetation components and in most cases both components to understand the changes that have taken place in relation to fallow periods. But, the number of times a piece of land is cultivated in the past 70 years has not attracted research attention. This paper therefore evaluates the dynamics and build-up of nutrients in different generations of fallows. The paper is timely as the quest for soil fertility restoration is on the front burner with regards to Ethiopian Journal of Environmental Studies and Management Vol. 4 No.4 2011  86 the changing trends in the global climate and food security. Furthermore, this study provides baseline information on fallow genealogical cycle and its resultant effects on the soil.  Hypotheses The study hypotheses that:    There are significant variations in the buildup of nutrients across different generation of fallows.    Nutrients in fallow soils increase substantially in relation to the number of times a piece of land is cultivated. Materials and Method Study Area The study was carried out in Oban Group Forest in Akamkpa Local Government Area, Cross River State. Geographically, the area lies between longitude 8 0 060’ and 8 0 50’E and latitude 5 0 00’ and 5 0 57’N. The climate is characterized by a double maxima rainfall beginning in the month of March to August, then August to October, reaching its peak in the month of July and September; the annual rainfall in about 2000-3000mm (Offiong, 2005). The average temperature here is about 27 0 C; relative humidity is between 80%. The tropical maritime and tropical continental air masses are common in the area. The area is characterized by luxuriant evergreen forest vegetation with few patches of secondly forest which constitutes the fallows. The rainforest consists of broad leaf species, amounting to 60-100 species per sq km. Many trees grow to the height of 30m - 60m tall. Many of the trees have large buttresses, smooth balks, columnar holes and thick woody climbers. Several species of birds, mammals, reptiles and insects are found in the area. The soils of the area are mainly loamy-sand and clay-loamy soils (Offiong, 2005). The terrain is undulating flood plain found around the Ikpan River. Sampling procedure and data collection The procedure of data collection began with a reconnaissance survey to the area, during which fallow generations ranging from 1 st , 2 nd , 3 rd , 4 th  and 5 th  were identified and delineated for soil sampling. Identification of fallow generations was done by the local farmers, as they remembered the number of times a particular fallow was cleared and cultivated in the past 70 years. In each generation of fallow, a belt transect of 80m was established, from which soil samples were randomly collected from 5 plots of 10m x 10m out of the 8 laid. In each plot, 5 surface (0-15 cm) and subsurface (15 – 30cm) soil samples were randomly collected with a soil auger and then composited.  Laboratory analysis The soils were put in polythene bags with labels; they were thereafter air-dried and taken to the laboratory for analysis. Soil organic carbon was determined by the method of Walkley and Black (1934); total nitrogen by the Kjeldahl method (Bremner and Mulvaney, 1982) and available phosphorus was determined by the method of Bray and Kurtz (1945). The soils were leached with 1M neutral ammonium acetate to obtain leachates used to determine exchangeable bases and soil cation exchange capacity, while pH value was determined using a glass electrode testronic digital pH meter (Model 511) with a soil: water ratio of 1:2. Soil particle size composition was analysed using the hydrometer method (Bouyoucos, 1926).  Data analysis Data obtained from the procedures above were analysed using descriptive, univariate and multivariate statistical techniques. Descriptive techniques such as tables, simple percentages and averages were used to represent the data for easy comparison; univariate technique such as One-Way ANOVA was performed on the soil properties to determine if significant variation exist among the sampled generation of fallows, while a multivariate technique through the use of principal components analysis (PCA) was performed to reduce the set of soil variables as well as find the most significant soil properties that increased substantially across the fallow generations. Results and Discussion Surface soil properties Table 1 shows information on the physical and chemical properties of surface soils across the fallow generations. The table showed that the concentration of sand was high in the 2 nd  and 4 th  generations with mean values of 82% respectively, while the lowest concentration of sand was obtained in the 1 st  generation with mean value of 71%. Sand proportion in the fallow generations varied significantly (p<0.05). The proportion of Effects of Fallow Genealogical Cycles on the Build-up.............................. Offiong & Iwara EJESM  Vol. 4 No.4 2011  87 silt did not vary (p>0.05), as it was the same across the fallow generations with mean values 7.4% respectively. However, for clay, the highest mean value of 13% was obtained in the 3 rd  and 5 th  generations, while the lowest mean value of 1.4% was obtained in the 1 st  generation fallow. There was significant variation in silt contents among the generation of fallows (p<0.05). The levels of porosity and moisture content in the soil were high in the 2 nd  and 3 rd  generations with mean values of 60g/kg respectively, and in the 5 th  generation with mean value of 33%, while the lowest values of 56g/kg and 27% were obtained in the 1 st  generation fallows respectively (table 1). There were significant variations in porosity and moisture contents among the generation of fallows (p<0.05). In the same vein, the concentrations of organic carbon (OC) and total nitrogen (TN) varied among the fallow generations (p<0.05) with high mean values of 1.27% and 0.31% in the 1 st  generations; it was closely followed by the 2 nd  and 3 rd  generation of fallows with mean values of 1.16% and 0.28% respectively. The lowest proportions of OC and TN were obtained in the 5 th  generation with mean values of 0.22% respectively. For exchangeable calcium (Ca) and magnesium (Mg), the highest mean values of 3.09meg/100g and 0.41meg/100g were obtained in the 5 th  and 2 th  generations, while the lowest mean values of 2.23meg/100g and 0.33meg/100g were obtained in the 1 st  and 5 th  generation of fallows respectively. The proportions of Ca and Mg varied among the fallow generations (p<0.05). In addition, the levels of exchangeable sodium (Na) and potassium (K) were high in the 3 rd  and 2 nd  generations with mean values of 0.41meg/100g and 0.14meg/100g, while the lowest mean values of 0.07meg/100g and 0.06meg/100g were obtained in the 5 th  generation respectively. There were significant variations in the contents of Na and K among the generation of fallows (p<0.05). The highest concentrations of cation exchange capacity (CEC) and Av. P (available phosphorus) were in the 3 rd  and 1 st  generations with mean values of 5.60meg/100g and 101.28mg/kg -1 , while the lowest mean values of 4.00meg/100g and 1.49mgkg- 1  were obtained in the 1 st  and 4 th  generations respectively. The proportion of CEC in the fallow generations did not vary (p<0.05), while Av. P proportion varied significantly among the generation of fallows (p<0.05). The levels of pH and electrical conductivity were high in the 2 nd  and 3 rd  fallow generations with mean values of 4.7 and 0.47 dsm- 1 , while the lowest mean value for pH was obtained in the 1 st  generation (4.46), and for EC, the lowest mean value of and 0.07 were obtained in the 1 st  and 2 nd  fallow generations respectively (table 1). Subsurface soil properties The levels of physical and chemical properties of subsurface soils in the fallow generations are shown in table 2. The table showed that the concentration of sand was high in the 4 th  generation followed by the 3 rd  and 5th generations with mean values of 79.2% and 75.2% respectively. There was significant variation in the sand content among the generation of fallows (p<0.05). Silt content was high in the 1 st  and 2 nd  generations with mean values of 9.4%, it however reduced to 5.4% in other generations. Silt content varied across the fallows (p<0.05). Clay content did not vary significantly across the fallows (p>0.05), though the 3 rd  and 5 th  generation had relatively higher mean values of 19.4% respectively, followed by the 1 st  and 2 nd  generation with mean value of 17.4% respectively. The proportion of porosity was relatively the same in the 2 nd , 3 rd  and 4 th  generations with mean value of 62 g/kg, and reduced slightly in the 1 st  generation with mean value of 59 g/kg. The content of porosity varied significantly across the fallow generations (p<0.05). The level of soil moisture was high in the 1 st  generation fallowed by the 5 th  generation with mean values of 31.8% and 30.1% respectively, and reduced to 28% in the 4 th  generation. Also, soil moisture content varied significantly across the fallow generations (p<0.05) (table 2). More so, the chemical properties of sub-surface soils (table 2) showed that the highest levels of organic carbon (OC) were obtained in the 3 rd  generation, followed by the 2 nd  generation fallow with mean values of 0.93% and 0.92% respectively, while the lowest mean value of 0.71% was obtained in the 1 st  generation fallow. There was significant variation in the concentration of OC across the fallow generations (p<0.05) The level total nitrogen was the same in the 2 nd , 3 rd , 4 th  and 5 th  fallow generations with mean values of 0.22%, and reduced to 0.17% in the 1 st  generation. Total nitrogen content varied Ethiopian Journal of Environmental Studies and Management Vol. 4 No.4 2011  88 significantly across the fallow generations (p<0.05) Furthermore, the concentration of exchangeable calcium (Ca) and magnesium (Mg) was high in the 2 nd  and 1 st  generations with mean values of 2.61meg/100g and 0.43meg/100g respectively, and reduced drastically in the 1 st  and 3 rd  generation of fallows with mean values of 1.91meg/100g and 0.28meg/100g respectively. The proportions of Ca and Mg varied significantly across the fallow generations (p<0.05). Likewise, the proportion of exchangeable sodium (Na) was high in the 3 rd , 4 th  generations with mean values of 0.38meq/100g, and reduced to 0.08meq/100g in the 5 th  generation; while the proportion of exchangeable potassium (K) was high in the 2nd generations with mean values of 0.07meq/100g, and reduced substantially to 0.03meq/100g in the 4 th  generation. The proportions of Na and K varied significantly across the fallow generations (p<0.05). The highest levels of CEC and Av. P were obtained in the 2 nd  and 1 st  generations with mean values of 4.05meg/100g and 13.96mgkg- 1 , while the lowest proportion of Av. P was obtained in the 4 th  generation with a mean value of 0.65mgkg- 1 . The levels of CEC and Av. P in the generation of fallow soils varied significantly (p<0.05). pH level was high in the 4 th  generation and reduced thereafter in the 3 rd  generation with mean values of 4.7 and 4.5 respectively. The level of pH did not vary significantly across the fallow generations (p>0.05). The proportion of electrical conductivity (EC) was extremely low in the 1 st  generation, but increased steadily thereafter. The proportion of EC varied significantly across the fallow generations (p<0.05) (table 2).  Result of PCA PCA was performed for soil properties with high intercorrelations across the fallow generations. Principal loadings (correlation coefficients) and the variances (eigenvalues) for the soil variables were computed. In the 1 st  generation fallow, the PCA procedure using varimax rotation (variable maximization) as well as the Kaiser rule of selecting only components with eigenvalues >1(Gaur and Gaur, 2006) extracted three components. The extracted components accounted for 97.7% of total variance in the srcinal data set. Five soil properties with positive and negative values loaded heavily on component 1, they included subsoil silt content (0.95), subsoil exchangeable sodium (0.92), topsoil sand (-0.92), subsoil exchangeable calcium (0.91) and topsoil exchange acidity (0.91). This component was regarded as measuring subsoil silt content. It accounted for 8.25 of the total eigenvalue loading and 37.7% variance in the linear combination of soil properties. The second component also had five soil properties with positive and negative values that loaded heavily on it, these variables included topsoil exchangeable magnesium (0.99), subsoil porosity (-0.98), subsoil available phosphorus (0.97), topsoil organic carbon content (0.95) and subsoil organic carbon content (0.85). This component exemplified subsoil magnesium content and it accounted for 7.25 total eigenvalue loading and 32.9% variance in soil properties. Component three on the other hand had six soil properties that loaded heavily on it. They included topsoil clay (-0.97), subsoil clay (-0.93), subsoil exchangeable magnesium (-0.86), topsoil available phosphorus (0.85), topsoil exchangeable potassium (0.85) and subsoil pH (-0.85). This component represented clay content and it accounted for 5.96 total eigenvalue loading and 27.1% variance in soil properties. The result above showed that subsoil silt content, subsoil magnesium content and topsoil clay content were the significant soil properties that progressively increased in the first generation fallow. In the 2 nd  generation fallow, three components with eigenvalues >1 were extracted and they accounted for 96.8% of total variance in the srcinal data set. Seven soil properties with high positive and negative values loaded on component 1, they included topsoil moisture content (0.95), topsoil available phosphorus (-0.95), subsoil exchange acidity (-0.921), subsoil available phosphorus (0.87), subsoil moisture content (-0.87), topsoil total nitrogen (0.83) and topsoil organic carbon (0.83). This component measured topsoil moisture and phosphorus contents. It accounted for 8.88 of the total eigenvalue loading and 38.6% variance in the data set. Component two had six soil properties that loaded heavily on it, these variables included topsoil exchangeable magnesium (-0.99), topsoil exchangeable sodium (0.99), topsoil exchange acidity (0.97), subsoil exchangeable potassium (0.97), topsoil cation exchange capacity (0.96) and topsoil porosity (-0.88). This component exemplified topsoil Effects of Fallow Genealogical Cycles on the Build-up.............................. Offiong & Iwara EJESM  Vol. 4 No.4 2011
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