0272-B3

Nutrient Dynamics of a Watershed in a Bench Mark Site in the Guinea Savannah Agro-ecological Zone of Nigeria

Olaleye A.O. ¹ , Ogunkunle, A.O. ² , Akinyemi, J.O. ¹ , Alabi, M.O. ³ , and Fatumbi, A.O. ²

Abstract

A preliminary investigation was carried-out within a benchmark site located in the Guinea savannah agro-ecological zone of Nigeria which occupies about 15.96% of the total land area in the West Africa sub-region. The site is located between latitude 9 ^◦ 03'N and longitude 6 ^◦ 00'E. Five perforated piezometers (1.50m) each were randomly inserted into three wetland soils. Soil solutions within each of the piezometers were sampled monthly using a fabricated suction pump between July and December of 1993 and 1995 respectively. The main objective of the investigation was to evaluate the monthly and yearly concentrations of water soluble K, Ca, Mg, Na Fe, Mn, Zn, Cu, and NH₃-N in this water shed.

The soils of the benchmark site varied between Typic Tropqauept (Eutric Gleysol) and Aeric Tropaquept (Dystric Fluvisol) using USDA Soil Taxonomy and FAO-UNESCO classification systems. The soil chemical properties of the entire pedons varied widely. The pH and EC ranged between 3.1 - 5.3 and 0.1-0.2 ds/m. The organic carbon, total N and base saturation ranged between 2.0-21 g kg ^-1 , 0.30-1.0 g kg ^-1 and between 39.5-83.60% respectively. The exchangeable cations (K, Ca, Mg & Na) ranged between 0.85-2.70, 0.10-0.84 & 0.10-0.23 c mol kg ^-1 . The cation exchange capacity (CEC) ranged between 1.90-11.39 cmol kg ^-1 and was dominated by exchangeable Ca. The available P (Bray-1-P) ranged between 1.90-16.30 mg kg ^-1 , while Fe, and Zn concentrations were between 0.10-43.80 as well as between trace and 0.05 mg kg ^-1 .

The results of the analysis of variance (ANOVA) using general linear model (GLM) of Statistical Analysis Systems (SAS) showed that between the two seasons (or years) of samplings, there were no statistically significant differences between the concentrations of the water soluble Ca, Mg, K, Na, Mn, Fe and NH₃-N except Cu and Zn that were statistically significant (p<0.05). However, across months, statistically significant differences (p<0.05) were observed between the concentrations of the following nutrients: Na, Cu, Mn, Fe and NH₃-N. Highest concentrations of the afore-mentioned nutrients were recorded in the month of August which coincided with the period of high agricultural activity when wetland rice is being fertilized.

Introduction

Water pollution is a major concern to many people and materials that causes this includes: pesticides, sediments, organic materials, oils metals and fertilizers. Agricultural and related practices as well as industrial and domestic effluents have been identified as a major contributor to pollution of water in the developing countries and especially sub-Saharan Africa (SSA). There is dearth of comprehensive scientific data on the quality assessment of ground and surface waters in Nigeria and SSA. The development of meaningful sustainable environmental protection strategies depends on the availability of such reliable scientific data.

Wetland ecosystems are considered by many authorities to be among the most threatened of all environmental resources (Olaleye, 1998). The total land area occupied by the Wetlands (coastal plains, inland basins, river flood plains, and inland valleys) in West Africa was estimated to be 3912 km ² (Hekstra and Andriesse, 1983). Of these, the wetland in the Guinea savanna occupies 33.10% (or 1295 km ² ). In Nigeria and elsewhere in the West Africa sub-region, during the 20 ^th century, the rate of loss of wetland has generally known to be very high and most of these physical losses have been due to the conversion of wetlands to other land uses such as industrial, agricultural, and residential uses (Olaleye, 1998).

Nigeria lies on the southern coast of West Africa between 2° and 15° E longitude and 5° and 15° N latitude. The country offers an array of environments, ranging from the belt of mangrove swamps and tropical rainforest in the lower elevations along the coast, to the open woodland and savanna on the low plateau. Much of the southern half of the country is characterized by a bimodal rainy season and annual rainfall pattern exceeding 1300 mm, while the northern half of the country has annual rainfall levels less than 1100 mm.

Compared with the late 70's, the rate at which wetlands in Nigeria and elsewhere in the West Africa sub-region have been depleted is difficult to be estimated. In addition, little or no investigation has been carried-out to assess the impact of physical degradation on the ground water chemistry in all the agro-ecological zones within the sub-region. The present investigation was therefore carried-out with the sole aim of monitoring the ground water nutrient dynamics in a benchmark site located in the Guinea savanna of Nigeria during two wet seasons of 1993 and 1995 in order to ascertain whether there are variations in the ground water chemistry due to their utilization.

Materials and Methods

This study took place in the Northern Guinea Savanna (Edozhigi village), which is a benchmark site delineated by the Wet Land Utilization Research Project (WURP) in 1983 (Heckstra and Andriesse, 1983). The site located at points Latitude 9°03'N and Longitude 6°00'E. In this site, an area of about 100-m x 60-m was demarcated and mapped using rigid-grid method during October and December 1992 dry season. Detailed methods are presented elsewhere (Olaleye et al., 2001). The climate of the area is under the influence of both the South-West monsoon and the North-East trade winds (Grove, 1970). The rainfall pattern is uni-modal with a peak in August, and the mean annual rainfall is variable ranging from 366 mm in August to 6.15 mm in November. The air temperature ranges between 31°C in August and a maximum of 39.5°C in March. The mean relative humidity reaches a minimum of 40% in February and a maximum of 80% in August. The climatic parameters during this period are presented in Table 1. An estimate of the mean monthly soil temperature is usually obtained by adding one degree centigrade to the mean annual air temperature (Soil Survey Staff,1975). The difference between the coldest and the hottest month is generally not more than 5° C. The monthly solar radiation ranged between 18.6 in January and 23.8 MJ m ^-2 day ^-1 in June with a mean of 21.3 MJ m ^-2 day ^-1 .

The area is underlain by Cretaceous sediments of the Pre-Cambrian Basement Complex and a subsequent marine incursion collectively known as the Nupe sandstones (Russ, 1930). It consists of Cretaceous, feldspathic sandstone and siltstone. The interbedded oolitic ironstones seem to be of sedimentary origin, with iron possibly introduced after deposition. The upper oolitic ironstone may partly have a lateritic origin and may therefore occur at different levels above the sandstone (Adeleye and Dessauvage,1970). The parent material has several weathering cycles (Hekstra et al., 1983). As a consequence, quartz is by far the predominant mineral, while feldspars and mica occur to a smaller extent (Smaling et al., 1985). The majority of the minerals have weathered into kaolinite and sesquioxides. The major soil types within the experimental site varied between Inceptisol and Entisol (Table 2).

The vegetation of the study area can be described as savanna woodland with sparse tree population and continous cover of grasses. The most common trees are Sheabutter (Vitellaria paradoxa Gaertn.f.), and Oil-palm (Elaeis guineensis L.). The grasses are mainly Andopogon sp; Hyparrhenia sp; Schizachyrium sp and Pennisetum spp.

A detailed soil map of the experimental site was charted using a Surface Mapping Systems Software (SURFER version 5.01) (Golden Software (1994), and this is presented in Figure 2 (Olaleye, 1998). Within each of the soil types identified, five Ploy Vinyl Chloride (PVC) pipes (1.2m long, and a diameter (_= 12 cm) perforated (about _ =1 cm) from the bottom up to 1m. The bottoms of each PVC pipes were filled with pre-washed sand and to minimize aeration through the pipes, the perforated parts were wrapped with nylon cloth (Sylla et al., 1994). Each of the PVC pipes were randomly inserted into the soils to a depth of about 80 cm. Water samples were collected monthly using a fabricated suction pump during July to December of 1993 and 1995. These were subsequently stored in 20-mls plastic vials and were later acidified using 0.1N HCl. Prior to adidifying, the pH of the water samples were measured using digital pH meter. The samples were later transported into the laboratory and stored in the refrigerators at a temperature of 4_ C prior to analysis. Data were analysed using General Linear Model (GLM) procedure of the Statistical Analysis Systems (SAS Inst., 1996). The means were separated using Duncan Multiple Range Test (DMRT) at 5%.

Results and Discussion

Benchmark soils

The soils of the benchmark site varied between Typic Tropqauept (Eutric Gleysol) and Aeric Tropaquept (Dystric Fluvisol) using USDA Soil Taxonomy and FAO-UNESCO classification systems (Olaleye et al. 2002). The soil chemical properties of the entire pedons varied widely. The pH and EC ranged between 3.1 - 5.3 and 0.1-0.2 ds/m (Table 1).
The organic carbon, total N and base saturation ranged between 2.0-21 g kg ^-1 , 0.30-1.0 g kg ^-1 and between 39.5-83.60% respectively. The exchangeable cations (K, Ca, Mg & Na) ranged between 0.85-2.70, 0.10-0.84 & 0.10-0.23 c mol kg ^-1 . The cation exchange capacity (CEC) ranged between 1.90-11.39 cmol kg ^-1 and was dominated by exchangeable Ca. The available P (Bray-1-P) ranged between 1.90-16.30 mg kg ^-1 , while Fe and Zn concentrations were between 0.10-43.80 as well as between trace and 0.05 mg kg ^-1 .

In the experimental site, the soils varied between Aeric Tropaquept (Eutric Fluvisol) and Typic psamment (Dystric Fluvisol) (Fig. 1). Detailed physico-chemical results of each of the pedons have been presented elsewhere (Olaleye et al. 2001). These results showed that the soil properties varied widely and was typical of the aforementioned properties of the benchmark soils.

Chemical properties of water samples

The pH of the water samples ranged between 5.7 - 6.1. The results of the analysis of variance (ANOVA) using general linear model (GLM) of Statistical Analysis Systems (SAS) showed that between the two seasons of samplings, there were no statistically significant differences between the concentrations of the water soluble Ca, Mg, K, Na, Mn, Fe and NH₃-N except Cu and Zn that were statistically significant (p<0.05) (Fig 2). A closer observation of the figure showed that higher concentrations of the nutrients were recorded in the 1993 cropping season. This difference may be attributed to the different periods of measurements in the watershed. Godoy et al., (2001) reported that the soil solution chemistry of the deciduous and evergreen Northofagus forest was affected by differences in times of measurements. Of all the aforementioned nutrient elements, the harmful one to aquatic and human consumption is the NH₃-N. Its concentrations in both seasons ranged between 1.34 and 1.47 mg l ^-1 . In line with World Health Organisation (WHO) standard, the NH₃-N in the ground water should not exceed 1.50 mg l-1 (WHO, 1984). Higher concentration is known to increase significantly the chlorine demand of water and toxic to aquatic organisms especially fishes. Increasing nitrates, nitrites and ammonia in the groundwater have been associated with the application of fertilizers as well as urbanization. However, across months, statistically significant differences were observed between the concentrations of the following nutrients: Na, Cu, Mn, Fe and NH₃-N. Generally, highest concentrations of the afore-mentioned nutrients seemed to have occurred in the month of August which coincided with the period of high agricultural activity when wetland rice is being fertilized. Subsequently, the various concentrations of these nutrients declined (Fig. 3). Udousoro (1998) reported that in the river and Lagoon waters, the concentrations of major metals were of the following order: Na > Ca > Mg > K and Na > Mg > Ca > K. The concentrations of the trace metals in these medium was reported to have been of the following order: Fe > Zn > Mn > Cu. The aforementioned result was in agreement with the findings in this investigation. The order of occurrence of trace elements is: Fe > Mn > Zn > Cu.

Conclusion

The result showed that across the two years, there were no significant difference in the water soluble concentration of Ca, Mg, Na, K, Mn, Fe and NH₃-N. However, significant differences (P<0.05) were observed in Cu and Zn across the two years. In 1994, the concentration of Cu was 0.11mg.l ^-1 compared with 0.09mg l ^-1 in 1996. The concentration of Na in 1994 was 1.22 mg l ^-1 compared with 1.70 mgl ^-1 in 1995. Comparing all the months the result showed no significant differences in the Ca, K, Mg and Zn concentration.

There were significant differences in Na concentration across the months in this order;
2>5>1>3>6 (17.20> 15.27> 12.34> 11.26> 8.75>6.64). Cu: 5>1> 4> 6> 3> 2 (0.12> 0.12>0.09>0.09>0.07>0.06). Mn: 2> 3> 1> 6> 4> 5 (3.18> 1.37> 0.99> 0.62> 0.50> 0.24). Fe: 2>1> 6> 3> 4> 5: (40> 27.35> 19.05> 14.87> 9.23> 8.65. NH₃-N: 2> 3> 1> 4> 5> 6 (4.01>1.84>1.45>0.52> 0.37>0.24)

References

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¹ Department of Soil Science and Farm Mechanization, College of Agriculture, Ogun State University, PMB 2002, Ago-Iwoye, Ogun State, Nigeria.* Corresponding author: [email protected]., Phone (H): 234-02-8105649.
² Agronomy Department, University of Ibadan, IBADAN, Oyo State, Nigeria.
³ National Cereals Research Institute, Bida, Niger State, Nigeria.