Soil Fertility Status under Different Tree Cropping System in a Southwestern Zone of Nigeria

Tree cropping has been known to bring about changes in edaphic component among other components of the ecosystem through their interactions with the soil and soil faunas. Premised on this, this study assessed the effects of sole cropping of teak and intercropping of cocoa and kola on the soil fertility status. The study was carried out using stratified-randomed sampling technique for the study plots in all the sampling sites. Three sampling sites consisting of four (4)-4oo m2 sampling plots each were established in which vegetation and some soil parameters were assessed. Results analysis showed that the synergistic interaction of leaves decomposition of cocoa and kola improved the organic matter content of the soil under the cocoa/kola site. Considerable improvement in soil fertility was enjoyed in the cocoa/kola site due to the large girth sizes and basal area of trees present in the cocoa/kola site while soil under the sole cropping of teak was impoverished. The degradation effects was due to the high rate of nutrient uptake of the teak, organic matter content was high in the forest site (9.12%) and cocoa/kola site (7.34) while the least was in the teak site (3.04%). A very strong correlation existed between organic matter content and some vegetation parameters.


Introduction
Trees are known to bring about changes in edaphic, micro-climatic, floral, faunal and other components of the eco-system through bio-recycling of mineral elements, environmental modifications (including thermal and moisture regime) and changes in floral and faunal composition etc (Shukla, 2009).Trees can improve the nutrient balance of soil by reducing unproductive nutrient losses from erosion and leaching and by increasing nutrient inputs through nitrogen fixation and increase biological activities by providing biomass and suitable microclimate (Schroth and Sinclair, 2003).Fisher (1995) outlined five ways that a tree can ameliorate soil conditions at a given site.These include Nfixation, surface soil nutrient enrichment through litterfall and root turnover, increase in soil organic matter (SOM) through additional litter and root inputs, changes in above-and belowground microclimate (moisture and temperature extremes, aeration, etc.), and through the increase in organism activity within the rhizosphere of perennial roots.Okigbo (1983) found tree cropping plantations to be ecologically sound production enterprise in the humid tropics while Singh et al. (1990) stated that tree farming systems are usually sustainable, profitable and environmentally sound.The important components of the agroforestry that yield certain environmental benefits such as reduction of pressure on forest, efficient recycling of nutrients, reduction in nutrient leaching and soil erosion (Sunita and Uma, 1993).About 20-25% of the total living biomass of the tree is in roots and there is a constant addition of organic matter to the soil through dead and decaying roots.Sunita and Uma (1993) reported that trees showed preferential enrichment of the soil in term of Ca, Mg, Na, P and N.
The potential role of trees in reducing run-off and erosion is well appreciated and removal of vegetation cover results in an increase in bulk density.There has been reports of reclamation of wastelands by growing trees over them, Sisagh et al. (1990) reported that alkali soils can be improved by growing Prosopis juliflora trees while Onim et al. (1990) also noted that improvement of the existing cropping systems through incorporation of tree legumes could assist in alleviating low soil fertility.Triadiati et al. (2011) and there have been reports of reclamation of wastelands by growing trees over them; leguminous trees like Gliricidia sepium and Leucaena leucocephala have been noted by Schroth and Sinclair (2003) to increase the availability of nitrogen through biological nitrogen fixation.Kamanga et al. (2000) opined in his research on tree-based cropping systems in Malawi that tree legume-based cropping systems improves soil fertility over the traditional cropping systems and this is explained in the deep capture system of trees (Ramun, 2000).Boley et al. (2009) avert that established plantations like rubber and citrus can modify microsite conditions regardless of the tree species origin and this was collaborated

Vegetation study
All woody plants of girth breast height of 7 cm and height of 1.5 cm were enumerated according to Adesina (1989).Plant samples were identified/or confirmed in the Herbarium of the Department of Botany, Obafemi Awolowo University, Ile-Ife.The following vegetation parameters were considered: • tree density; • tree height; • tree girth; • basal area; • volume of wood.

Statistical analysis
One-Way Analysis of Variance was used to determine variability among the study sites and Duncan Multiple Range Tests was adopted for validation of the ANOVA (SPSS 14.0, 2005).Pearson correlation was used to determine relationship between organic matter content and the other edaphic properties and also between organic matter content and vegetation parameters.

Vegetation study
Tectona grandis was the only woody tree in site a; two woody species were found in site B, i.e. cocoa and kola trees while ten woody species were encountered in site C (Tab. 1).The dominant woody species in the sites in term of density are: -site A:-Tectona grandis; -site B:-Theobroma cacao and Cola nitida; -site C:-Dalium guineensis, Treculia Africana and Vocanga africana (Tab.1).
Site A has the highest total tree density of 5,475 ha -1 .The trend of the tree density showed site A>site C>site B. Total tree basal area was highest in site C followed by site A and site B. Celtis zenkeris (21% of total basal area) and Dalium guineensis (24% of total basal area) accounted for the large value of basal area in site C. Tectona grandis (4.55 m 2 ha -1 ) in site A, Theobroma cacao (1.32 m 2 ha -1 ) in site B (Tab. 1).Site C had the highest total volume of wood; 5 times that of site A, 30 times that of site B. Dalium guineensis and Celtis zenkeris (67.71 m 3 ha -1 and 74.88 m 3 ha -1 respectively) contributed largely to the total volume of wood in site C.
Site C had the highest site mean girth of 0.68 m; the girth sizes of Triplichiton scleroxylon, Celtis zenkeris and Albizia adiantifolia largely accounted for the high site girth size (Tab.2).The teak site (site A) had mean girth size of 0.48 m while the cocoa/kola had 0.29 m.
Analysis of variance carried out showed significant variations in the vegetation parameters across the sites by Tanaka et al. (2009) in his study of soil characteristics under cash crop farming in Malaysia that soil properties in rubber farms were similar to those in the secondary forest.The objective of this study was to assess the soil fertility status under different tree cropping systems.

The study area
The study was conducted between 2003 and 2005 in a part of Osun State, Southwestern region of Nigeria.The study area consists of three sites designated as A, B and C; site A is an established teak plantation (Tectonia grandis) of 30 years in Ila Local Government and it is under selective harvesting; site B is a 35 year-old cocoa plantation (Theobroma cacao) intercropped with kola trees (Cola nitida) without specific gapping in Boripe Local Government while site C is a relatively intact matured forest serving as the control located at Ifedayo Local Government.The area lies between the longitude 04° 00' E and latitude 05 o 55' N. The climate of the study area is humid sub-equitorial type that is characterized by distinct dry and rainy season (Aweto, 1995).

Sampling procedures
In each of the sampling sites, four sampling plots of 400 m 2 were demarcated by the use of measuring tape in each study sites and a total of sixteen sampling plots of dimension 400 m 2 were used for the study.

Soil study
Three soil samples were randomly collected at a depth of 0-15 cm using a soil auger in each of the sampling plot and a total of 48 soil sample were used for the analysis.The soil samples were air-dried and gently crushed in a ceramic mortar and sieved through a 2 mm sieve for chemical analysis.The following determinations were carried out on the soil samples: • bulk density was determined by the core method (Blake, 1965); • soil moisture content (SMC) according to Shingsby and Cook (1986); • soil pH determined potentiometrically in 0.1 M calcium chloride using soil a soil solution ratio of 1:2 (Peech, 1965); • soil organic content by the chromic acid digestion method (Walkley and Black, 1934); • nitrate-nitrogen by the Kjeldahl method (Black, 1965); • available phosphorus using Bray No.1 method (Bray and Kurtz, 1945).studied (Tab.2).As expected, site C, (forest site) showed highest values of tree height, girth, basal area and volume of wood.

Soil study
The mean values of all soil properties measured in the forest site (Site C) were higher than those in the other sites (Teak and Cocoa/kola sites) Tab. 3. Bulk density was lowest in site C while it was highest in the teak site showing soil compaction.There was no significance in pH values between teak and cocoa/kola site using DMRT at p<0.05.It is also evident from Tab. 3 that the nutrient elements under teak (site A) are most affected; these elements are available phosphorus, moisture content and organic matter.
The correlation coefficients between organic matter content and vegetation matrices showed positive correlations in all the sites except tree density in teak site which is negative correlations (Tab.4).Soil elements interrelationship in the study sites showed organic matter content exhibits strong positive correlation with moisture content, available phosphorus, pH and nitrate-nitrogen in all the three sites while the strongest correlation exists between organic matter content and moisture content (r=+0.99)(Tab.4).plained the positive correlations between organic matter content and the other edaphic elements assessed (Tab.4).
The role of organic matter content in the build up of soil nutrients appears crucial in all vegetation ecosystems and the higher level of organic matter with subsequent high levels of other soil nutrients under cocoa/kola (Tab.4) is related to the nature of organic matter found under it.It seems that organic matter materials, especially leaves emanating from kola so decomposed faster than that emanating from cocoa because cocoa leaves are lignified making cocoa leaves decompose very slowly (Ekanade, 1990).So the synergistic relationship in which the rapid decomposition of kola leaves brings about higher rate of decomposition of cocoa leaves as well and thereby helps in the restoration of fertility to soils through the process of mimicking the natural systems that have constant inputs of organic carbon via litter deposition of varying quality.This is evident in the positive correlation between organic matter content and tree height, girth and basal area of trees in the study (Tab.4) which conformed to the discovery of Sharma et al. (2009) in his study of the relationship between vegetation and soil parameters in Himalaya region.
The large girth sizes and vegetation cover via basal area in the matured forest and the cocoa/kola site led to the increase in nutrient and organic matter content of those sites as against the teak site.Great girth sizes and basal areas increases litter production which has a protective capacity on soil against accelerated organic matter destruction and nutrient losses through leaching and erosion (Aweto, 1995).The observed high significant contrast in the soil properties between the teak site and the cocoa/kola site is evidently resultant of the differences in the vegetation parameters (girth size, basal area, volume of wood) which culminated to the differences in the nutrient returns of the vegetation units (Shukla, 2009).

Soils fertility under teak plantation
The deteriorated status of the soils under the teak site as evident in the low organic matter content (3.04%), available phosphorus (34.38 ppm) and nitrate-nitrogen (46.12 ppm) could be related to the frequent fire that is occasionally used to clear the teak plantation.When fire is applied in the dry season which is the period of higher litter accumulation, the subsequent decomposition and mineralization of the litter at later time is not possible since the litter has been burnt and this reduce organic matter input.The degradation effects of the soil properties under the exotic trees can also be related to the total rate of uptake of nu-

Discussion
Soil fertility under matured forest and cocoa/kola farm Soils under matured forest and cocoa/kola farm in the study area rated high in soil nutrients and this is due to the fact that they enjoys minimized anthropic disturbance and the soils are under more or less permanent shelter from rain-wash and sun heat.All these factors are known to be important in ensuring and sustaining high soil fertility in forest and forest-simulation and this was corroborated by Ratnayake et al. (2011) in his work on the effect of cultivation on organic carbon content in soils.Shukla (2009) noted that nutrients accumulated in leaves return to the ecosystem through litter fall and this is the most important component of the forest ecosystem; it is a major pathway of nutrient and energy transfer.Also, Bowen et al. (1988) puts that plant residues or litter has multi-beneficial effects which include maintenance of good soil physical conditions as well as soil organic matter, provision of nutrients and stimulation of biological activity as well as moderately acidity in soil.Nutrient uptake, their retention and release are the three components of nutrient dynamics in forest ecosystem (Shukla, 2009).The nutrient cycling in forest ecosystem comprises of cyclic circulation of nutrients between forest soils and plants through the process of nutrient uptake, retention and restitution.All these contributed to the good soil status in the matured forest and cocoa/kola sites as evident in their organic matter content values (9.12% and 7.34% respectively).
The intercropping of cocoa/kola produces a better soil quality than the teak site.The cocoa/kola site is better with respect to all the edaphic properties (bulk density, organic matter content, available phosphorus and nitrate-nitrogen).Ekanade (1990) opined that mixing of litter components produces synergistic interactions that are beneficial to soil.The improved soil fertility under cocoa/kola could probably be attributed to the high level of organic matter content maintained under the trees through the accumulation of litters.Bernhard-Reversat and Loumeto (2002) affirmed that the most relevant parameter for soil organic matter binding-up could be the amount of standing litter on the soil which integrates litterfall and decomposition and also essential in the soil aggregate stability (Mbah et al., 2007;Emadi et al., 2008).There is a known fact that when there is accumulation of soil organic matter; being a storehouse and source of important plant nutrients, there is usually a "build up" of nutrients in the topsoil (Young, 2000) as found in the values of soil parameters.It also ex- Bulk density was highest in teak (1.62 g/cm 3 ) and this could be a result of the frequent trampling of soil by loggers after burning (Penuel and Drew, 2004).
In a homogenous plant community, the stage of development of the plant communities affects both the nutrient uptake and nutrient return of the vegetation which could cause differentiation of soil properties under various plant communities as encountered under the teak.Ekanade (1991) stated that nutrient immobilization in the tissue of those rapidly growing exotic communities is not matched by the rate of nutrient returns to the soil through the fall and mineralization.Nutrient dynamics is very important for the understanding of ecosystem functioning and ecological status and in teak sole cropping system, more uptake and less returns to the soil leads to deterioration of soil as shown in the result of the soil parameters under teak.

Conclusions
The vegetation-soil relationship in this study has shown the importance of vegetation cover to soil fertility.High basal area and girth size accounted for the high fertility status of the soil under the matured forest and the cocoa/ kola sites.Sole cropping of trees especially exotic types is not encouraged because of the rate of nutrient uptake with fewer returns to the soil (Pandey and Brown, 2000).
It is highly recommended that sole teak establishment should not be used in land management because of its degradation effect on soil fertility.
Tab. 1. Biomass characteristics of the woody species in the study sites Values in the same column with different subscripts are significantly different according to Duncan Multiple Range test at 5% probability level Boley et al. (2009) coefficient between organic matter content and other assessed parameters trients from the soil, hardwoods like teak and sal, are reported byShukla (2009)to return through litter fall only 8 to 10% of the total potassium and phosphorus absorbed by them and deplete the soil considerably, hence the low fertility status of soils under teak.Boley et al. (2009)also affirmed that there is significantly lower concentration of organic matter under teak in his research in Costa Rica.