Assessment of Impact of Land Uses and Some Soil Chemical Properties on the Spatial Distribution of Soil Bacterial in Southwest Nigeria

Introduction

Land use changes significantly impact the spatial distribution of microorganisms in various ecosystems. Microorganisms play a crucial role in maintaining ecological balance, nutrient cycling, and overall soil health. As anthropogenic activities alter land use, the structure, function, and diversity of microbial communities are also affected, leading to changes in ecosystem processes [1]. Understanding these effects is essential for sustainable land management practices.

Microorganisms, including bacteria, fungi, archaea, and viruses, are fundamental to soil health and ecosystem functionality. They participate in key processes such as organic matter decomposition, nutrient mineralization, nitrogen fixation, and the suppression of soil-borne diseases [2]. The diversity and activity of microbial communities are influenced by various factors, including soil properties, climate, vegetation, and land management practices. Consequently, any change in land use can have profound implications for the microbial ecology of soils.

In recent decades, there has been a growing awareness of the need to understand how land use changes impact soil microbial communities. This is particularly important in regions undergoing rapid transformation due to human activities. Land use changes, such as deforestation, urbanization, and agricultural expansion, can lead to habitat fragmentation, loss of biodiversity, and alterations in soil structure and chemistry [3]. These changes, in turn, influence the composition, diversity, and function of soil microbial communities, with potential consequences for ecosystem services and agricultural productivity.

Irele and Okitipupa Local Government Areas (LGAs) in Ondo State, Nigeria, have experienced considerable changes in land use over the past decades. These changes, driven by agricultural expansion, urbanization, and deforestation, have had significant impacts on the local environment and biodiversity. The region’s reliance on agriculture, particularly the cultivation of cash crops like cocoa and oil palm, has led to extensive land conversion and habitat alteration [4].

Agricultural expansion in Irele and Okitipupa LGAs has primarily been driven by the need to increase food production and economic development. However, this has often come at the expense of natural ecosystems. The conversion of forests and natural grasslands to agricultural fields alters the physical and chemical properties of soils, which in turn affects the microbial communities that inhabit them [5] (Ayangbenro and Babalola, 2021). Tillage, use of chemical fertilizers, pesticides, and monoculture practices disrupt the soil structure, reduce organic matter content, and create an environment that favors certain microbial taxa over others. This can lead to a decline in microbial diversity and changes in the functional capabilities of the soil microbiome.

Urbanization is another significant driver of land use change in Irele and Okitipupa LGAs. The expansion of urban areas involves the construction of buildings, roads, and other infrastructure, which leads to the sealing of soil surfaces and the disruption of natural habitats. Urban soils are often characterized by high levels of pollutants, compaction, and reduced organic matter, all of which negatively impact microbial communities [6]. The introduction of exotic plant species in urban landscapes can also alter the composition of soil microbial communities by changing the types and amounts of organic matter inputs.

Deforestation, driven by logging and the need for land for agriculture and urban development, has led to the loss of forests in Irele and Okitipupa LGAs. Forest soils typically harbor diverse and complex microbial communities due to the high organic matter content and the presence of various plant species [7]. The removal of forest cover reduces litter input, alters soil moisture regimes, and increases soil erosion, all of which affect the microbial communities. Deforestation often leads to the dominance of opportunistic and fast-growing microbial species that are less efficient in nutrient cycling and organic matter decomposition compared to the original forest microbial communities. This research is however to evaluate the significance of the impact of land use changes on the distribution of bacteria communities in the study area.

  Study Area Description

Geography and Topography

The study focuses on Okitipupa and Irele Local Government Areas (LGAs) in Ondo State, Nigeria. This region is characterized by a tropical climate with specific topographical features. The terrain in this area is primarily undulating, with gentle slopes that influence both land use and soil properties. The topographic map will be utilized to identify and delineate the terrain features, including slopes and land use types, which are crucial for understanding the spatial distribution of microorganisms.

Climate

The climate of Okitipupa and Irele LGAs are representatives of the hot and humid tropic climate typical of southwestern Nigeria. This region experiences seasonal rainfall, high temperatures, and high humidity. The total annual rainfall often exceeds 2000 mm, with a narrow range of soil temperatures between 27°C and 28°C (Esu, 2014). The area undergoes two distinct seasons: the rainy season, dominated by the southwest trade winds bringing moisture from the Atlantic Ocean between March and November, and the dry season, influenced by the northeast trade winds from December to February, known locally as harmattan. The Inter-Tropical Discontinuity (ITD) controls these seasonal variations, affecting other climatic parameters such as temperature, humidity, and wind patterns. The growing season in this region spans approximately 240 to 300 days annually [8].

Vegetation and Land Use

The study area is a forest zone featuring a mix of secondary forest, farm fallows, and arable farmland. Approximately 15% of the land is dedicated to arable farming, primarily growing cassava and guinea corn, along with scattered oil palm trees and cocoa plantations. The secondary forest occupies about two-thirds of the area, characterized by a mix of woody trees and shrubs. The land use types present significant implications for soil microbial communities due to differences in vegetation cover, organic matter input, and land management practices.

Methods

Site Survey and Mapping

To establish a comprehensive understanding of the terrain and land use, existing perimeter and topographic maps will be reviewed alongside new data collected from field surveys. The dominant slope and physical features will be assessed through visual inspections and traverses using transects, footpaths, and trunk roads. A topographic contour map will be used as the base map, and features such as land use types and man-made structures will be marked. Geographic Positioning System (GPS) technology will record the exact coordinates for these features [8].

Soil Survey and Mapping

A preliminary survey of the study area will be conducted using cadastral maps, satellite imagery, and topographical sheets to locate field boundaries and identify any discrepancies. If these materials are unavailable, a reconnaissance survey will establish reference transects. Five transects of varying lengths will be set up across the study area. Soil sampling points will be determined using GPS and a Dutch soil auger. Soil samples will be collected from depths of 0-30 cm and 30-60 cm, with physical characteristics recorded for each sample (0-30 cm and 30-60 cm depths).

Preparation of Point Map

Coordinates of soil sample sites will be recorded using a GARMIN GPS72H receiver. ArcGIS 10 software will be employed to prepare point maps displaying the locations of soil samples. The spatial and non-spatial databases will be integrated to generate spatial distribution maps, using MS Excel and ArcMap to link data points and visualize results [9].

Experimental Design

The experiment was conducted in a randomized complete block design (RCBD), 

Figure 2: Plot layout and sampling technique adopted for each land use system

Soil Sample Collection and Sampling Procedure

In the study sites, thirty sampling points that were 100m apart were marked in a grid-mesh construction using GPS markings in each of the Mesh. The points fell within five land uses were identified: forestland, Oil palm plantation, watershed, farmland, and residential area.

Systematic strip sampling was employed in laying out the plots. Three transects were cut 100 m apart in each of the study sites and three sample plots of 20m × 20m in dimension were laid in alternate positions along each transect at 50m intervals. Soil core samples were collected at a depth of 0-30cm in each of the selected sites and at three points along the diagonal of each plot using a 3cm diameter soil auger. The auger samples were composited separately by bulking samples collected from the same plot and depth, air-dried at room temperature, and sieved through a 2mm sieve in preparation for analysis after removing coarse fragments and roots. The composite soil samples were taken to the laboratory for analysis.

 Soil Particle Analysis

Soil particle size analysis will be conducted to determine sand, silt, and clay content using the hydrometer method. A 30-g sample of oven-dry soil will be treated with Calgon solution, stirred, and transferred to a sedimentation cylinder. Hydrometer readings will be taken after 40 seconds and 2 hours to measure the sand, silt, and clay fractions, with calculations performed to determine the percentage of each component [9].

Soil Chemical Properties Determination

Soil pH will be measured in a 1:2 (w) soil-to-water ratio using a calibrated pH meter. Soil organic carbon (SOC) concentrations will be determined using the Walkley-Black wet oxidation method, with potassium dichromate and concentrated sulfuric acid. The mixture will be boiled, and excess reagents will be titrated to determine the organic carbon content. Organic matter will be computed by multiplying organic carbon values by a Van Bemmelen factor of 1.724. Total nitrogen (N) will be measured using the micro-Kjeldahl method, while available phosphorus (P) will be determined by the Bray method. Exchangeable bases (Ca, Mg, Na, and K) will be extracted with neutral ammonium acetate, and their concentrations will be determined by titration and flame photometry.

Microbiological Analysis

Enumeration of heterotrophic bacteria and fungi was carried out by the pour plating technique. This was done by inoculating 0.1 ml tenfold serially diluted samples onto nutrient agar (Bacteria). The mineral salt media [10] contains the following composition in grams per erper liter of distilled water NaCl 10g, MgSO₄. 7H₂O, 0.42g, KCl, 0.29g, K2 HPO4, 1.2g, KH₂ PO₄, 0.83g, NaNO₂, 0.42g, Agar –Agar, 15g, PH 7.2 and 2ml of petrol/diesel. The inoculated nutrient Agar plates were incubated at 370 C for 24 hours while the potato dextrose Agar plates were incubated at room temperature for 3-5 days.

Estimation of number of bacteria: No. of bacterial cells /1gm dry soil = No. of colonies ×inverted dilution Dry weight of 1gm soil sample

The unit of measurement here (CFU) is the colony-forming unit. Where the colony may be the yields of the growth and multiplication of a single cell or more.

Data Analysis

Soil and bacteria data were compiled using Excel and analyzed with SPSS version 23. Descriptive statistics, including means and standard deviations, were calculated for soil bacteria. Analysis of variance (ANOVA) was performed to identify significant differences between various land uses. Duncan’s New Multiple Range Test was used for post-hoc comparisons at a 95% confidence level.

3.0. RESULTS AND DISCUSSION

3.1. RESULTS

Soil Chemical Properties and Land Uses

pH of the study area was slightly acidic across all the land uses in Irele, it ranged from 5.83 to the least value of 4.92 in forestland and oil palm plantation land uses respectively, similar trend was also observed in Okitipupa, with the highest value of 5.23 to the least value of 4.95 in forestland and oil palm plantation respectively, however, there was no significant difference among the values in the two locations across the land uses Tables 1 and 2.

Forestland recorded the highest total organic carbon with values of 3.36 and 3.03 g/kg-1 while the farmland has the lowest of 1.23 and 1.90 g/kg-1 in Irele and Okitipupa respectively, Organic matter (OM) followed a similar trend across all the land uses in the two locations. In the same vein, forestland recorded significantly the highest total N of 0.23 kg-1 while farmland recorded the lowest value of 0.12 kg-1. Total P followed the same trend as the total N, the value of total P was statistically higher in the forestland with the value of 10.99 kg-1 and 9.79 kg-1 in Irele and Okitipupa while the lowest values of 7.83 kg-1 and 7.78 kg-1 were recorded in the residential area and farmland in Irele and Okitipupa respectively. Cation exchange capacity (CEC) was significantly higher in forestland over the other land uses and ranged between 10.77 gkg-1 to 8.73 gkg-1 in forestland and farmland respectively in Irele local government while it ranged from 11.87 gkg-1 to 9.76 gkg-1 in Okitipupa local government. The exchange acidity (EA) was significantly higher in oil palm plantations than in the forestland and ranged from 2.87 kg-1 to 1.87 kg-1 respectively in the Irele Loca government. It also ranged from 2.47 kg-1 to 1.79 kg-1 in oil palm plantations and forestland in Okitipupa.

Bacteria Distribution across the Land Uses

The distribution of bacteria species is presented in Table 3, some of the identified bacteria are: Zymomonas mobilis, Cellulomonas spp, Micrococcus spp, Bacillus subtilis, Bacillus linchemiformis, S. saprophyticus, salmonella spp, Proteus mirbilis, Streptococcus spp, Enterobacter sppArthrobacter spp, Chromoba. Lividum, Xanthomonas spp, Chromoba. Violaceum, Lactobacillus spp, Clostridium spp, Acinetobacter spp, Pseudo. Aeruginosa, Rhizobium spp, Citrobacter spp, Corynebacterium spp, Peptococcus spp, Flavobacterium spp, Diplococci spp, Alcaligenes spp.

Zymomonas mobilis was higher in oil palm plantation with 23.00 cfug^-1 and 10.00 in Okitipupa and Irele local government respectively and dropped to 0.00 cfug^-1 in farmland and watershed in Okitipupa local government. Cellulomonas spp was in Okitipupa local government with 35.00 cfu in Watershed and 25.00 cfug^-1 in Irele local government also found in watershed land use. It dropped to 0.00 cfug^-1  in both local governments, these values were found in residential areas. As well as farmland in Okitipupa local government. Micrococcus spp has the highest counts of in oil palm plantations in Okitipupa and 20.66 cfu in forest land in Irele local government. Bacillus subtilis counts were higher in forestland with 152.00 cfug^-1 and 132.00 cfu in Okitipupa and Irele respectively. Bacillus linchemiformis counts were higher in forest land with 135.00 cfug^-1 in Okitipupa while in Irele local government, the count was 130.00 cfu also in forestland. S. saprophyticus recorded the highest counts in the forest land over other land uses, this trend follows a similar trend recorded for some identified bacteria species. It also recorded a higher count in Okitipupa. Salmonella spp count was higher in the watershed land uses over others with 30.00 cfug^-1 in Okitipupa and 20.00 in Irele. Enterobacter spp has the highest counts watershed with 62 cfug^-1 and 42 cfu in Okitipupa and Irele local government respectively. While the lowest counts of 37.00 cfu and 27.00 cfu were found in residential area land uses. Xanthomonas spp count was higher in the farmland in both local governments with the highest count found in Okitipupa over that of Irele. Chromoba. Violaceum counts were higher in the oil palm plantation and highest in the farmland in Okitipupa and Irele respectively.

In total, forest land has the highest count of bacteria in both Okitipupa and Irele local governments.  The total counts of bacteria counts were 1410.10 cfug^-1 and 1086.9 cfug^-1 significantly higher in Okitipupa local government than in Irele local government. This was followed by oil palm plantation with 1180.07 cfug^-1 bacteria in Okitipupa and while watershed has 899.99 cfug^-1 in Irele local government.  The residential area recorded the least counts of bacteria in both local governments with 990.21 cfug^-1 and 774.97 cfug^-1 in Okitipupa and Irele local governments respectively.

Discussion

Bacteria distribution and soil chemical properties

The variations in soil bacterial communities are markedly influenced by soil chemical properties which encompass a range of factors, including soil pH, nutrient availability (particularly nitrogen, phosphorus, and organic matter), and cation exchange capacity. These properties directly affect the growth, diversity, and community structure of bacterial populations, which play crucial roles in biochemical processes, nutrient cycling, and overall soil health.

Soil pH is one of the most critical chemical properties affecting bacterial diversity and community composition. Research indicates that pH influences microbial processes significantly, as it governs the solubility and availability of various nutrients essential for bacterial growth [11]; The pH of the soil was slightly acidic which may promote the dominance of acidophilic bacteria such as the one found in the study area.  The abundance of bacteria count in the forest land may be due to moderate to higher N found in this land use, for example, studies have demonstrated that total nitrogen content in soils strongly influences bacterial abundance; high nitrogen levels can enhance the growth of nitrogen-fixing bacteria and other microbial taxa that thrive in nitrogen-rich environments [12 ; 13].. However, low counts of bacteria in the farmland may be due to continuous nitrogenous fertilizer, although, this effect may vary depending on management practices and soil types [13].

Many bacteria are found in the forestland over other land uses due to higher organic carbon content, soil organic carbon (SOC) provides energy sources for bacteria and influences the soil’s physical and chemical properties, such as water retention and aggregate stability [14]. High organic matter content typically correlates with greater bacterial richness and diversity, as diverse organic substrates in the soil foster the growth of various microbial populations [12]. For instance, soils enriched with organic matter through composting or cover cropping practices demonstrate higher microbial abundance and diversity compared to intensively managed agricultural soils with low organic content [15;12]. The lowest bacteria count in the farmland may be due to the low level of soil organic carbon found in the study area.

Cation exchange capacity (CEC) impacts soil nutrient retention and availability, thereby influencing bacterial communities. Soils with high CEC can retain more nutrients, provisionally providing a more favorable environment for diverse bacterial taxa [16]. Research has shown that soils with enhanced nutrient retention capacities often support higher microbial activity levels, whereas soils with low CEC may experience more significant fluctuations in nutrient availability, potentially leading to reduced microbial diversity and dominance by resilient bacterial taxa capable of surviving adverse conditions [17; 16] the conditions found in the forestland that makes bacteria more abundance in the area under forestland. Changes in land use, such as transitioning from natural forests to agricultural systems or urban landscapes, typically lead to alterations in soil chemical profiles, often resulting in reduced microbial diversity due to the uniformity of inputs and management strategies [11; 18].  (Bobuľská et al., 2021; Yin et al., 2021).

Bacteria distribution and land use

The higher count of bacteria in the forestland showed that forestland serves as a benchmark for microbial diversity due to its rich organic matter and complex rooting systems that provide diverse habitats for soil bacteria. Research shows that forest ecosystems support higher bacterial diversity compared to agricultural systems, primarily due to the presence of multiple plant species contributing varied organic material to the soil [19]. The distinct microclimatic conditions found within forests, such as stable moisture levels and nutrient availability, promote a rich array of microbial species and functional groups [20] (Enaruvbe et al., 2021). Forests are observed to harbor diverse Proteobacteria and Acidobacteria, which are crucial for organic matter decomposition and nutrient cycling [21]. The undergrowth and litter layer of forests also foster complex interactions among bacterial communities, making them resilient to environmental changes [22].. On the contrary, land-use conversion to Oil palm plantations significantly disrupts these microbial communities. Although oil palm plantations can yield economic benefits, they often replace biodiverse forest ecosystems, leading to decreased microbial richness and altered community structures. Studies indicate a marked decline in bacterial diversity within oil palm plantations compared to adjacent forest ecosystems, primarily due to soil compaction, altered nutrient cycles, and homogenized organic inputs [19; 22].. The cultivation practices involved in oil palm farming, such as the application of fertilizers and pesticides, can further select for specific bacterial taxa, often leading to a reduction in functional diversity that is essential for ecosystem stability and resilience [21;22]. Moreover, oil palm plantations are characterized by a unique set of soil properties influenced by management practices, including pH elevation and increased soil salinity, which in turn shape the bacterial communities inhabiting these environments [21]. For instance, the soil bacterial community in oil palm areas tends to be dominated by genera like Pseudomonas and Rhizobium, which are associated with nutrient cycling under agricultural conditions [23; 24]. This dominance signifies a shift from the complex interactions found in forest soils to simpler, highly managed microbial communities in oil palm systems.

Watersheds present another critical land-use type, as they encompass various land-use patterns affecting soil bacterial distribution. Soil bacteria in watersheds experience fluctuations in moisture and nutrient availability due to runoff and soil erosion from surrounding agricultural practices and urban areas. Research indicates that watershed management practices that promote vegetation cover can enhance soil microbial communities by maintaining moisture levels and reducing nutrient loading, thus promoting a more diverse and functional soil microbiome [25; 26]. The interactions between soil and water are key in determining the resilience of microbial communities, as they are sensitive to changes in hydrology and sediment movement [25]. Farmland is often marked by intensive agricultural practices that can drastically alter soil properties and, consequently, bacterial communities. Soil compaction, the shift in pH, and nutrient excesses from fertilizers are common characteristics of agricultural land use, leading to a decline in bacterial diversity and shifts in community structure [27]. Research indicates that monoculture practices dominate farmland, often resulting in the loss of critical microbial functions needed for organic matter decomposition and nutrient cycling [28]. The high presence of S. saprophyticus and Xanthomonas spp suggests bacterial communities found in this study in farmland are frequently found to be dominated by nitrate-utilizing and phosphorus-solubilizing bacteria, adapting to the nutrient-rich conditions created by fertilizers, yet such adaptation might compromise overall soil health and fertility in the long term [28].. The lowest counts of bacteria in residential areas in this study often embody different land use characteristics, including impervious surfaces and landscaping practices that influence soil properties. Soil disturbance and the introduction of synthetic chemicals can lead to distinctly different microbial populations compared to those found in natural ecosystems or agricultural lands. Recent studies indicate that bacterial communities in urbanized areas may exhibit lower diversity and are sometimes dominated by opportunistic pathogens due to higher nutrient loads such as nitrogen and phosphorus from urban runoff [7]. Furthermore, residential landscaping practices can lead to further alterations in soil properties, impacting soil structure and microbial habitat availability (Irvanto et al., 2022).

 Conclusion

This study provides a comprehensive analysis of bacteria population characteristics across different land use types, revealing patterns that align with findings from similar tropical and urban environments while differing from those in arid, rural, and industrial regions.  The study shows that forestland recorded the highest bacteria counts while the residential area recorded the lowest count of bacteria. The study highlights the influence of environmental conditions, land use practices, and human activities on bacteria communities, with important implications for soil health, plant growth, and public health. Future research should focus on expanding the scope of such studies to include a broader range of land use types and environmental conditions, as well as investigating the long-term trends in microbial populations in response to changing land use and environmental factors.

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