Changes in Nutrient Contents of Soil Across Different Land Uses in a Forest Reserve

In order to assess the changes in soil chemical properties’ resulting from conversion of forest to other agricultural land uses a study was conducted in Okomu Forest Reserve (Nigeria). Five soil samples collected from top and sub soils of marked points within the undisturbed forest, farmland, cocoa plantation, oil palm plantation and rubber plantation were analyzed for pH, organic carbon, total nitrogen, available phosphorus, exchangeable acidity, sodium, potassium, calcium and magnesium. The results showed variations in all chemical parameters across the land use types with soil depths (top and sub soils). Soil pH ranged from 4.88 (in the cocoa plantation subsoil) to 6.75 (in the forest top soil). Soils (top and sub soils) of the rubber plantation, forest and cocoa plantation had the highest stock of soil organic carbon (SOC), N and exchangeable Na respectively. Available P was highest in the top soil of the rubber plantation and sub soil of the farmland. Exchangeable Ca and Mg were highest in the top soil of the forest and sub soil of the rubber plantation. Exchangeable K and C:N was highest in the farmland top soil and cocoa plantation sub soil. Relationship among the chemical parameters varied with soil depth. Conversion of forest and cocoa plantation resulted in the highest decline (55.78% and 44.40%) in soil N. There is the need to regulate the conversion of natural forests to agricultural lands and plantations as conserve the nutrient cycling processes.


Introduction
Land use changes have remarkable effects on the dynamics of soil properties (Ozgoz et al., 2013).A change from forest cover to cultivated land may hinder addition of litter that enhances nutrient content of soils (Ozgoz et al., 2013), increase rates of erosion (Biro et al., 2013), loss of soil organic matter and nutrient (Saha and Kukal, 2015), and accelerate rate of soil degradation (Barua and Haque, 2013).This process, in turn, leads to a decline in soil fertility and loss of biological activity and diversity (Yao et al., 2010).Vegetation cover is, therefore, a key indicator of soil degradation as plants play a role in the control of soil erosion (Cedar, 1998;Keesstra et al., 2009;Kropfl et al., 2013;Tejada and Benitez, 2014;Bochet, 2015).
Indeed, changes in land cover have significant effect on the amount and diversity of biomass returned to the soil, which also disrupt the richness of nutrient restored to the soil.It is perhaps a known fact that soil erosion intensity and amount of nutrient element loss varies depending on the vegetation type at a particular place and time.This is so because, the rate of nutrient element loss in both dissolved and sediment bound forms will depend on the ability of vegetation canopy to effectively intercept the direct impact of raindrops that strike the soil surface (Iwara, 2011).If the canopy is not dense enough or well developed, little nutrients return to the soil as well as large quantities of nutrient will be removed from the soil surface during periods of heavy rainstorm when the soil is saturated.
Different studies have examined the effects of land use/cover change on physico-chemical properties of the soil (Lal, 1996;Bossuyt et al., 1999;Eneji et al., 2003;Evrendileka et al., 2004;Emadi et al., 2008;Agoume and Birang, 2009;Gol, 2009;Offiong et al., 2009).These studies nevertheless characterized changes in soil properties in relation to emerging land use/cover change in their respective ecosystems.The conversion of Forest Reserve to other land-uses in recent times has caused many complex changes in the forest ecosystem whose impact raises diverse ecological problems (Henrik et al., 2010).Increasing population coupled with urban poverty has exacerbated the migration of people into rural areas especially villages closer or inside the Forest Reserves in Nigeria (Adeofun and Akinsanmi, 1997).This has led to the de-reservation of substantial parts of most of the Forest Reserves in Nigeria (such as Okomu, Omo, Oluwa, and Olokemeji forests) for the establishment of monoculture plantations, while encroachment by landless farmers for agricultural activities has further impoverished many (Ola-Adams, 1996,

Study area
Okomu Forest Reserve is located in Ovia South-West Local Government Area of Edo state and occupied an area of 1082 km 2 within the lowland forest of Nigeria.It is lies geographically between longitudes 5°N to 5°30' E and latitudes 6°N to 6°10'N precisely between the River Osse and Siluko to the East and West respectively and about 40 km from Benin City, the state capital.The reserve derived its name from River Okomu from the Benin word 'Akomu' meaning unity.The reserve was named Okomu during the colonial constitution of the reserve.
Okomu area has well marked rainy and dry seasons.The dry season which lasts from November to March is usually accompanied by northeast trade wind that causes harmattan (Azeez et al., 2010).It receives a mean annual rainfall above 2500 mm per annum (Akinsorotan et al., 2011) falling mainly between March and October with the highest rainfall occurring in June, July and September.The mean monthly temperature is 30 °C.The relative humidity is high and usually not less than 65% during the afternoons in any month of the year.

Collection of soil samples
Five different land use types were identified within Okomu Forest Reserve to include oil palm plantation, cocoa plantation, rubber plantation, secondary forest and cultivated cropland/farmland and each is 0.5 ha in size.Five sampling points were marked out within each land use type and georeferenced using Garmin eTrex X GPS device (Table 1).Soil samples were collected from marked points from 0-15 cm (top soil) and 15-30 cm (sub soil) depths using a Stainless Dutch soil Auger.The soil samples were packed in labelled polythene bags, air dried in the laboratory and passed through 2 mm sieve prior to chemical analysis.

Chemical analysis
Soil pH was determined in 1.0 N KCl employing a 1:1 soil/solution mixture (Thomas, 1996) and read with a digital pH meter after equilibration.Soil organic carbon was determined by the Walkley-Black wet oxidation method (Nelson and Sommers, 1996).Soil nitrogen was determined by the macro Kjeldahl method of digestion, distillation and back titration (Bremmer, 1996).Available phosphorus was determined by the Bray-P1 method (Kuo, 1996).Ogunleye et al., 2004).The structural alteration of the potential forest reserves has not been without cost and has resulted in a mosaic of land-uses within the forest reserves.Both forest litters and soil microbes constitute an important resource that makes forests fertile for arable farming in the tropics (Akachukwu, 2006) and the fertility status of a soil is acknowledged to be a reflection of the soil properties and soil texture characteristics (Michel et al., 2010).The change in forest cover to other forms of land cover such as plantation and grassland results in the tremendous modification of canopy cover, thereby making the area affected susceptible to soil erosion; this affects the stock of soil organic carbon (SOC).The conversion of forest ecosystem to other forms of land cover may decrease the stock of SOC due to changes in soil moisture and temperature regimes, and succession of plant species with differences in quantity and quality of biomass returned to the soil (Offiong and Iwara, 2012).
The conversion of forest land to tree plantations and food crops plots has been shown to adversely affect soil carbon and nitrogen levels (Michel et al., 2010).There are also strong indications that the conversion of tropical forests into land for agriculture or plantation might have negative impacts on soil properties and the carbon budget (Hertel et al., 2009).Hence, a selection of appropriate ecological indicators with well-established sensitivity to disturbances will assist in assessing the extent of these impacts with a view to addressing the associated ecological problems.
Consequently, soil physical and chemical properties have been proposed as suitable indicators for assessing the effect of land-use changes and management (Janzen et al., 1992;Bremer et al., 1994;Alvarez and Alvarez, 2000).This approach has been used extensively by several authors to monitor land-cover and land-use change patterns (Schroth et al., 2002;Airiohuodion, 2003;Faboya, 2010;Michel et al., 2010).Similarly, a lot of studies have been carried out on the soil physiochemical and biological changes (Schroth et al., 2002;Tchienkoua and Zech, 2004;Walker and Desanker, 2004)  , , = − × Exchangeable acidity was determined by titration using 1.0 M KCl for extraction (Bertsch and Bloom, 1996).Soil exchangeable cations were extracted using 1.0 N ammonium acetate, NH4OAc buffered to pH 7.0 (Thomas, 1982).The concentrations of Ca, Mg, K and Na in the filtrate were then determined by spectrophotometry.Na and K concentrations was determined using a flame photometer while Ca and Mg was determined using Atomic Absorption Spectrophotometer (AAS) (Buck Scientific 2.0).

Data analysis
Data on concentrations of soil pH and nutrients were analysed using GLM model in SAS 9.1.3software for Windows (SAS Institute, 2003).The means were separated using Duncan's Multiple Range test at α = 0.05.Bar graphs for the concentrations of soil nutrients were plotted using Microsoft Excel 2013 (Microsoft Corporation, 2012).Bar graphs for deterioration indices were plotted using Origin 8 Pro for Windows (Origin Lab Corporation, 2007).Deterioration/improvement in soil nutrients in the land uses relative to the undisturbed forest was calculated using the deterioration index (DI) of Ekanade (1991) as adopted by Oyedeji et al. (2016) where: X = mean value of soil nutrient in the forest soil (reference site); Xi = mean value of soil nutrient in the compared site.Note: a positive value indicates deterioration and a negative value indicates improvement.

Concentrations of soil chemical parameters
There were significant differences (P < 0.05) in soil pH, organic carbon, nitrogen, phosphorus, exchangeable acidity and soil cations (Na, K, Ca and Mg) in the different land uses.Significant variations also exist between the levels of the soil parameters in the top soil and sub soil.There were also significant interactions between the land use and soil depth (top and sub soils) for all assessed soil parameters (Table 2).
The soil pH ranged from very strongly acid (4.88) in the sub soil of cocoa plantation to neutral (6.75) in the top soil of Okomu forest.The top soil of cocoa plantation was most acidic (5.05) among the land uses.In all the land uses, pH was significantly higher in the top soil than the sub soil, except in the oil palm plantation (Fig. 1a).Organic carbon was generally highest in the rubber plantation -1.46% and 1.25% respectively in the top soil and sub soil.The least % OC in the top soil and sub soil were 0.80% and 0.73% in the cocoa plantation and farmland respectively.Soil OC was generally higher in the top soil than sub soil except in the cocoa plantation (Fig. 1b).Nitrogen content in the top soil and sub soil was significantly highest in the forest land -48.19 mg kg -1 and 47.72 mg kg -1 respectively.The least N content was in the top soil of the farmland (21.31 mg kg -1 ).The forest and cocoa plantation had higher N concentration in the top soil than the sub soil (Fig. 1c).Carbon-nitrogen ratio was significantly highest in the top soil of the farmland (555.59).The least C:N was in forest sub soil (212.47).The top soils of the land use types had higher C:N than the sub soils, except in the cocoa plantation (Fig. 1d).Available phosphorus concentration was generally highest in the top soil of the rubber plantation (14.20 mg kg -1 ) while the farmland had the highest P in the sub soil (10.12 mg kg -1 ).The least levels of P in the top soil and sub soil were 6.77 mg kg -1 and 5.63 mg kg -1 respectively in the oil palm plantation and forest (Fig. 1e).Exchangeable acidity was significantly highest in the top soil of cocoa plantation (0.16 meq/100 g).The sub soil of the forest had the lowest acidity (0.05 meq/100 g).Acidity was generally higher in the top soil than the sub soil, except in the oil palm plantation where measure of soil acidity was statistically the same (0.10 meq/100 g) in the top soil and sub soil (Fig. 1f).
Sodium had the highest concentrations among the exchangeable cations across the land use types.Exchangeable Na was highest in the top soil (4.48 mg kg -1 ) and sub soil (5.01 mg kg -1 ) of the cocoa plantation.The farmland and rubber plantation had the lowest concentrations in the top soil (3.41 mg kg -1 ) and sub soil (3.09 mg kg -1 ) respectively.Na was generally higher in the sub soil than top soil except in the rubber plantation.Exchangeable K was however highest in the top soil of the farmland (0.87 mg kg -1 ).The cocoa plantation had the highest exchangeable K in the sub soil (0.82 mg kg -1 ) and least exchangeable K in the top soil (0.46 mg kg -1 ).Soil K in the top soil of the forest and farmland were higher than in the respective sub soils.Soil Ca was highest in the top soil of the forest (2.90 mg kg -1 ) and sub soil of the rubber plantation (2.39 mg kg -1 ).The farmland had the least Ca concentrations in the top soil (0.98 mg kg -1 ) and sub soil (1.34 mg kg -1 ).Soil Ca was higher in the top soil than the respective sub soils, except in the farmland.Soil Mg was highest in the top soil of the forest (1.17 mg kg -1 ) and sub soil of the rubber plantation (0.95 mg kg -1 ).The farmland had the least concentration of Mg in the top soil (0.39 mg kg -1 ) and sub soil (0.54 mg kg -1 ).The concentration of Mg in the top soil and sub soil of the rubber plantation were at par (0.95 mg kg -1 ) (Table 3).

Deterioration index for soil chemical nutrients in the land use types
The deterioration index (DI) for nutrient elements in Figs.2a-b expresses the relative percentage of the chemical parameters in the land uses to that in the forest soil.A positive DI denotes a deterioration of the measured parameter while a negative DI denotes improvement of the parameter over that in the forest soil.
The result of deterioration index revealed positive DI for OC in the top soil of cocoa and oil palm plantations with 27.27% and 0.91% respectively.There was negative DI (improvement) for OC in the farmland (-7.27%) and rubber plantation (-30.00%).There was deterioration (positive DI) in nitrogen in the four land uses with the farmland accounting for the highest (55.78%) in the top soil.Available P deteriorated in the farmland, cocoa and oil palm plantations with 18.66%, 9.85% and 21.55% respectively.There was marked improvement (negative DI) in available P in the rubber plantation (-64.54%).Exchangeable Na improved in the top soil of the cocoa (-13.99%) and oil palm plantations (-7.38%), while K improved in the farmland (-33.85%) and oil plantation (-9.23%).Exchangeable Ca and Mg showed different levels of deterioration in the different land uses (Fig. 2a).
In the sub soil, there was improvement in OC except in the farmland where a positive deterioration was observed (27.0%).Soil N deteriorated in the four land use types with the cocoa plantation having the highest (44.40%).Available P improved (negative DI) in the sub soil of the land use types.Exchangeable Na also improved in the sub soils except in the rubber plantation where deterioration was observed (23.50%).There was improvement in sub soil exchangeable K across the land use types.Exchangeable Ca and Mg deteriorated in land use types except the rubber plantation (Fig. 2b).

Discussion
Soil pH in the acidic range in the cocoa plantation is in line with earlier report by Ololade et al. (2010) for cocoa plantation in Ile-Oluji, Ondo State.pH of the top soil in this study tend to increase with exchangeable K, Ca, Mg and organic carbon.Cations are known to increase soil pH through alkalization thereby creating a favourable condition for microbes to act on plant litter (Opala et al., 2012).Juo and Manu (1996) found that pH tend to decrease with decrease in soil nutrient stocks.This phenomenon may be related to cation uptake by plant, with subsequent release of H + ions, organic matter decomposition into organic acids,  increased CO2 level through root respiration and nitrification.Conversion of natural forest into monoculture plantations of cocoa and oil palm resulted in a decline (27.3% and 0.9%) in soil organic carbon (SOC).Michel et al. (2010) reported significant reduction in SOC in plantations and mixed crop fields compared to natural forest in Cote D'Ivoire, and attributed this to lack of under storey vegetation associated with agricultural lands.High C:N alone, as in the farmland, does not suggest good health of the soil, as low N, pH and CEC in the site will negatively influence the growth of plants.Lewandowski (2002) concluded that high C:N (low soil N) dictates slow decomposition of accumulated biomass and may trigger nitrogen deficiency in plants.Deciduous nature of some trees species in established monoculture plantations (such as cocoa and rubber) build-up of biomass (Abou Rajab et al., 2016) that support the slightest spark of fire, thus accounting significantly in volatilization loss of labile nutrients such as nitrogen (Certini, 2005).Management of such monoculture plantations often eliminate other species including native legumes that contribute significantly to soil nitrogen stock.Although leguminous plants constitute a significant part of conventional cropping system, leaching and volatilization loss limit the availability of the nutrient (Oyedeji et al., 2016).Kutiel and Naveh (1987) in their study of pine forest soil reported 25% decrease in N following burning.The National Wildlife Coordinating Group (2001) also reported that volatilization loss significantly affected N, and to a lesser extent P. The slow mobility of P in soils may be responsible for its low susceptibility to loss by volatilization.
Improvement in soil Ca in the rubber plantation compared to the undisturbed forest in this study supported by the findings of Singh (1998) who reported higher soil Ca in Tectona grandis plantation.Improvement in Na and K in the sub soils across the land uses supports the notion of McGrath et al. (2001) that soil cation exchange capacity (CEC) increased following conversion of forest to pasture or field crops.According to Blake et al. (1999), such improvement often varies with soil chemistry, texture and depth.Such variations possibly accounted for the inconsistencies in the concentrations of the cations across the land uses and with soil depth.In general, decrease in the CEC is reflected in lower pH and organic matter contents in the soils (Owusu-Bennoah, 1997).The resultant low concentrations of Ca and Mg in the top soil and sub soil of the farmland despite high concentration of both cations in the forest soil underscore the need to conserve the forest.Watters (1971) observed that when forest was cleared for cultivation, there was a slight decrease in the cations exchange capacity at all soil depth.
Assessing the impact of land uses on soil organic carbon alone is grossly inadequate since soil organic matter is not chiefly carbon but contains hydrogen, oxygen, nitrogen,  phosphorus, sodium, potassium, calcium, magnesium and other micronutrient elements (Gebreyesus, 2013;USDA, 2014).Although soil organic matter can be partitioned conveniently into these fractions, the components and amount of the end product is not static but reflect a dynamic equilibrium influenced by soil properties and by the quantity of annual inputs of plant and animal residues to the ecosystem (Bot and Benites, 2005).There is the need to protect forest cover and regulate conversion of forests to other land uses as to conserve the soil and ensure efficient nutrient cycling.Agro-ecological practices such as no till farming (conservation agriculture), cover cropping, green manuring and intercropping nitrogen-fixing legumes that improve soil organic matter and nitrogen will go a long way to improve productivity in forest converted to agricultural lands and plantations.

Fig. 1 .
Fig. 1.(a) pH (b) organic carbon (c) N (d) C:N (e) P and (f) exchangeable acidity, in top and sub soil from five different land use types

Fig. 2 .
Fig. 2. Deterioration index for soil minerals from (a) top soil and (b) sub soil in four land use types in relation to a forest land

Table 1 .
over the humid tropical regions of the world.In spite of the economic importance of plantation forestry and agriculture, previous studies on the effects of different land-use types on soil properties in Nigeria are grossly inadequate, hence this study.The present study assesses variations in soil properties associated with different land-use types within Okomu Forest Reserve, Edo State, Nigeria.Geo-position of soil sampling points within five land use types in Okomu Forest Reserve, Edo State, Nigeria 415 310ft †Distances indicated recorded in feet ( ) are height above sea level.

Table 2 .
Results of analysis of variance and mean squares of soil chemical properties at two soil depths in five different land use types at two soil depths

Table 3 .
Exchangeable cations (Na, K, Ca and Mg in mg kg -1 ) at top and sub soil in five different land use types Means with different letter down the column are significantly different at P<0.05

Table 4 .
Pearson correlation for soil chemical properties in the top soil of five land use types

Table 5 .
Pearson correlation for soil chemical properties in the subsoil of five land use types