Impact of Different Land-Use Systems on Soil Physicochemical Properties and Macrofauna Abundance in the Humid Tropics of Cameroon

Department of Soil, Water and Atmosphere, Institute of Agricultural Research for Development (IRAD), Ekona, PMB 25 Buea, South-West Region, Cameroon Entomology-Nematology Unit, Institute of Agricultural Research for Development (IRAD), Ekona, PMB 25 Buea, South-West Region, Cameroon Department of Soil, Water and Atmosphere, Institute of Agricultural Research for Development (IRAD), Nkolbisson, P.O. Box 2123, Yaounde, Centre Region, Cameroon Biotechnology Unit, Institute of Agricultural Research for Development (IRAD), Ekona, PMB 25 Buea, South-West Region, Cameroon Rubber Programme, Institute of Agricultural Research for Development (IRAD), Ekona, PMB 25 Buea, South-West Region, Cameroon


Introduction
Environmental degradation caused by inappropriate land use is a worldwide problem that has attracted attention in sustainable agricultural production systems [1].
e productivity and sustainability of soil depends on dynamic equilibrium among its physical, chemical, and biological properties [2].ese properties are continuously influenced by land uses.According to Di et al. [3], agricultural management practices can largely influence the quality of the soil which in turn is intrinsically linked to the sustainability of agroecosystem functions and productivity.erefore, successful agriculture requires the sustainable use of soil resources as soil could easily lose its quality and quantity within a short period of time [4].Soil health maintenance is essential for sustained food productivity, the decomposition of wastes, storage of heat, sequestration of carbon, and the exchange of gases.Since 1945, it is estimated that 38% of the cultivated areas in the world have been degraded.Annually, approximately 24 billion tons of topsoil is lost.
is is equivalent to about 9.6 million hectares of land.erefore, soil degradation and/or changes in soil quality that result from wind and water erosion, salinization, losses of organic matter and nutrients, or soil compaction are of great concern in every agricultural region in the world [5].In an attempt to reverse the trend of declining soil quality, recent studies have focused on identifying suitable soil management practices.However, soil quality cannot be directly measured, and soil quality information is usually deduced from observed or modelled soil physical, chemical, or biological attributes [6,7].Within the context of agricultural production, Karlen et al. [8] attributed high soil quality to be equivalent to longterm high productivity and the system resiliency without significant soil or environmental degradation [3].Larson and Pierce [9] outlined five soil functions that may be used as criteria for judging the soil quality: to hold and release water to plants, streams, and subsoil; to hold and release nutrients and other chemicals; to promote and sustain root growth; to respond to management and resist degradation; and to maintain suitable soil biotic habitats.Macrofauna found in soil and surface litter are known to play a central role in soil processes such as nutrient cycling, organic matter decomposition, and improvement of physical attributes such as aggregation, porosity, and water infiltration [10,11].ese small organisms, such as insects and other invertebrates, play a vital role in the production and maintenance of healthy soils, and therefore are key elements in the development of sustainable agriculture and forestry [12].
Human activities frequently cause the degradation of soil environment which leads to reduction in the number of animal and plant communities, where species able to bear stress predominate and rare taxa decrease in abundance or disappear [13].With continuous cultivation, physical properties and productivity of many soils commonly decline due to decrease in organic matter content and soil pH [14].Changes in soil properties caused by cultivation and management and their consequences to soil productivity have generated significant research concern for many years [15].Information on the effects of land management on soil physicochemical properties and upon soil invertebrate communities under different land-use systems in the humid tropics of Cameroon is scanty and thus makes it difficult to recommend or adopt appropriate soil conservation and management practices.
e objective of this study was therefore to evaluate the land-use effects on selected soil physicochemical properties and macrofauna abundance.

Study Site and Soil.
For the purpose of this study, five different land-use systems were selected in Fako Division in the Southwest region of Cameroon known for intensive agricultural production. is area is characterized by two main seasons: the rainy season from mid-March to mid-November and the dry season from mid-November to mid-March [16].It has a temperature ranging between 20 °C and 28 °C and annual rainfall of about 3000-5000 mm. e area is composed of undulating high and low lands with many rocks and gravels due to volcanic eruptions.e land-use types include secondary forest (SF), banana plantation (BP), oil palm plantation (PP), and sugarcane plantation (SP), all located in Buea subdivision and rubber plantation (RP) in Muyuka subdivision.e exact locations of the fields and soil types are presented in Table 1.e soils in BP, PP, SP, and SF belong to the Mussaka series and RP to the Ekona series.Soils in these areas are formed on older mudflows and have a well-developed argillic horizon.ey belong to one and the same continuum, with properties which are changing gradually [17].

Description of Land Use.
e secondary forest was surrounded by oil palm plantation, cocoa farm, and other mixed cropping farms consisting of plantains, cassava, and cocoyams.e banana plantation is owned by the Cameroon Development Corporation (CDC) and was established from 2004 to 2007.Several agronomic/agricultural practices are employed, and these include periodic application of agrochemicals (insecticides, nematicides, fungicides, herbicides, molluscicides, and different kinds of fertilizers and lime), leaf and sucker pruning as well as the use of understorey irrigation system.Field residues (pseudostems, premature fruits, and leaves) and postharvest residue (penducles) are left to rot in the field.e oil palm plantation is also owned by CDC with oil palms that were planted since the year 1985.Field residues (harvested fronds and male inflorescence) are left to rot in the farm.Unlike the banana plantation, the farm was under fallow for 6 months.e sugarcane plantation belongs to a smallholder farmer and was established in 1986.Agrochemicals were never used, and pruned leaves were the only field residues found in the farm.e rubber plantation is cultivated by the CDC, and the farm was established in 1984.e only farm residues were litter falls.

Soil Sampling, Processing, and Physicochemical Analysis.
In all the sites selected, for soil sampling, the aforementioned crops were grown as the sole crops in the different land-use systems.
e factors considered before choosing the sites were all the land-use systems were of the same soil type (volcanic soil), have similar climatic conditions, and operate within the same soil-forming factors.

Physicochemical Properties.
In October 2016, undisturbed soil samples were collected in triplicate from each land use, per field per depth at 0-5 cm, 5-10 cm, 10-15 cm, and 15-20 cm with a 100 cm 3 cylinder for bulk density studies.A soil auger was used to collect composite soil samples for the study of pH, particle size distribution, and organic matter contents.Bulk density was determined according to Blake and Hartge [18].e values for the 0-20 cm soil layer were obtained from the mean of 0-5 cm, 5-10 cm, 10-15 cm, and 15-20 cm. e organic matter contents were determined by the loss on the ignition method and the soil particle size distribution for each sample by the pipette method according to Gee and Bauder [19], from which the soil textural classes were obtained.Soil pH (1 : 2.5 w/v soil/water ratio) was determined using a pH meter.

Macrofauna Sampling and Data Collection.
e humid forest zone has a large and diverse soil-associated fauna that strongly influence soil properties and development.Data on the invertebrate fauna composition on the different land-use systems were collected above ground and within soil/litter.

Above-Ground Data Collection.
Ten different positions were selected randomly on the different sites, and a sweeping net was used to collect the organisms [20] found on the low vegetation (about 0.5 m above soil) found under the main vegetation.e sweep net was swept on the low vegetation, and the organisms captured inside the net were prevented from escaping by folding the opening of the sweep net immediately after sweeping.
e different species of organisms captured were counted in situ according to the different species and recorded in a field hand book. is process was replicated three times on each selected position.

Litter/Below-Ground Sampling and Data Collection.
is was done by random selection of different positions on the different land-use systems.A machete was used to till the upper surface of the soil at an area of 30 cm × 30 cm and a depth of 2 cm.e bulk of the tilled soil (together with the litters) was collected with hand covered by hand gloves and then placed inside labelled plastic bags.e openings of the plastic bags were sealed by tying it with a cord to prevent the soil organisms from escaping.is process was replicated ten times.e plastic bags containing the soil were transported to the laboratory of the Entomology/Nematology unit of the Institute of Agricultural Research for Development (IRAD) for counting and classification of the captured organisms.Samples for the macrofauna were taken from the same place where sampling for soil physicochemical analysis took place.

Counting of the Below-Ground Organisms.
Each replicate of soil/litter samples carried to the laboratory was laid out separately on a plastic tray.Using an iron rod and a touch light, the soil samples were spread on a plastic tray and the organisms found within were placed on labelled small vials containing 70% alcohol.Each group of organisms was counted and then classified to at least the level of phylum, class, and order, although some were classified up to family or species level.

Statistical Analysis.
One-way analysis of variance (ANOVA) was used to compare the effects of different landuse systems on soil physicochemical properties.Separation of the means of the soil properties was performed using the Tukey HSD test (p < 0.05).Correlations between macrofauna abundance and some selected soil physicochemical properties were determined by Pearson's correlation coefficient.For all the statistical analyses, JMP 5 statistical software was used.e sand content recorded in sugarcane plantation was highest in the 0-20 cm layer followed by that of oil palm plantation, banana plantation, rubber plantation, and secondary forest whereas the silt content was found highest in the secondary forest followed by sugarcane plantation, rubber plantation, banana plantation, and then palm plantation.Generally, there were significant differences in the soil particle size distributions in the different land-use systems with the soil texture for oil palm plantation classified as clay soil, while banana plantation and rubber plantation ranged from clay to silty clay soils and sugarcane plantation and secondary forest ranged from silt loam to silty clay loam soils (Table 2).

Soil pH.
e soil pH varied from 4.90-5.68 in the different land-use systems (Table 2) with banana plantation having the highest soil pH and oil palm the lowest.is high pH of banana plantation might be attributed to the periodic application of lime (dolomite) in the field.With the exception of oil palm plantation, no significant difference was observed among the land-use systems.However, oil palms grow well in the pH range of 4.3-6.5.It was also reported that the most suitable soil texture for oil palm plantation is sandy clay and silty clay [21].e soil pHs could be categorized as very strongly acidic for soils in oil palm plantation, strongly acidic for sugarcane plantation and secondary forest, and moderately acidic for rubber and banana plantations [22].e majority of tropical soils are acidic.Acidic soils (pH < 5.5) are widespread, especially in humid regions; they cover 30% of the world's total land area and 60% of the total area in the tropics [23].e agricultural solution to acidic soils is therefore the application of lime to achieve pH favourable for most plants (5.5-7.5).Applied and Environmental Soil Science 3

Soil Bulk Density (BD).
e bulk density at 0-20 cm in all land uses varied from 0.91-1.21g/cm 3 with the trend of secondary forest < oil palm plantation < banana plantation < rubber plantation < sugarcane plantation with the mean values of 0.91, 1.06, 1.13, 1.16, and 1.21 g/cm 3 , respectively.
e results showed that bulk density varied signi cantly (P ≤ 0.05) among the land-use systems (Figure 1) except between sugarcane plantation and rubber plantation and between banana plantation and oil palm plantation.e highest bulk density was found in the soil under the sugarcane plantation followed by rubber and banana plantations.In contrast, the lowest bulk density was observed in the secondary forest followed by the palm plantation.e high bulk density of the croplands compared to the secondary forest might be as a result of intensive agricultural practices for croplands [24]. is is also consistent with the result of Oguike and Mbagwu [14].Increase in bulk density as a result of conversion of forest to cultivated land is a re ection of the extent of soil degradation and has been demonstrated by many researchers [25].High bulk density is an indicator of low soil porosity and soil compaction.It may cause restrictions to root growth and poor movement of air and water through the soil [26].

Soil Organic Matter (OM).
Soil organic matter for all land-use types from 0-20 cm depth ranges from a minimum in rubber plantation (14.32%) followed by sugarcane plantation (15.18%), banana plantation (17.45%), and oil palm plantation (18.10%) to a maximum in secondary forest (20.37%).e high OM in the forestland might be as a result of tree leaves, stems, barks, owers, logs, and fruits.In addition, microorganisms, animals, and roots contribute to the increase of OM [27].
ere were signi cant di erences (P ≤ 0.05) among the OM content of the land-use types except between oil palm and banana plantations and between sugarcane and rubber plantations (Figure 2).e high OM content also recorded in PP and BP might be as a result of plant residues that are abundantly available and which are returned to the soil in these environments.Salehi et al. [28] reported that the e ects of trees on soil properties occur mostly due to the increase of organic matter and the release of nutrients from it.Although the OM contents of soils under RP and SP were lower than those of SF, PP, and BP, the values were also high because the leaf litters of these plants are left to rot in the elds.

Classi cation of Organisms Collected.
Of the total of 1579 individual organisms collected from both the sweep netting (810 individuals) and soil/litter extraction (769 individuals) (Figure 3), twenty-one (21) species were identi ed/classi ed up to class or order level while three (03) were unidenti ed (Table 3).e total numbers of individual organisms were highest in the forest and in the oil palm plantation and lowest in the sugarcane plantation.With respect to the di erent collection methods, highest number of individual organisms was recorded from soil-litter extracts in the forest.From sweep netting, the highest number of individual organisms captured was from the palm plantation (PP) (Figure 3). is may be due to the abundance and diversity of oral resources in the fallow oil palm plantation.Applied and Environmental Soil Science e total number of species recorded from the different land-use systems varied based on the collection methods used (Figure 4).From sweep netting, the highest number of species captured was from the sugarcane plantation (SP) and lowest from the forest (SF), while from the soil-litter extraction, the highest number of species was from forest and lowest from sugarcane plantation.Rossi and Blanchart [11] mentioned that tropical forests have higher densities of soil macrofauna compared to cultivated lands.
is high number of soil macrofauna for the forestland might be as a result of its high organic matter content.
is argument was strengthened by the fact that forestlands have a greater diversity and availability of food substrate for the soil fauna, fueled by high cycling of leaves and twigs in the forest litter [29].According to Neeher [30], plant directly a ects soil biota by generating inputs of organic matter above-and below-ground and indirectly by the physical e ects of shading, soil protection, and uptake of water and nutrients by roots.In the forest, there is usually intense cycling of ne roots in the surface layer, which contribute as a nutrient substrate for soil fauna, thus favoring their multiplication.Rossi and Blanchart [11] also showed that macroinvertebrate communities also respond to environmental disturbance induced by land-use management.

Proportion of the Organisms
Collected.First, the 24 species recorded were from three animal phyla: arthropoda, annelida, and mollusca.Majority of these species were arthropods (88%) and then mollusks and annelids (8% and 4%, respectively).Of this arthropod, the majority (81%) were from the class Insecta, 9% for Arachnida, and 5% each for Chilopoda and Diplopoda.
e identi ed insects were from 10 orders, with most from the class Insecta (Table 3).Menedez and Cabrera-Davila [31] found out that of the over 7000 epigeous organisms collected from two di erent land-use systems, the predominant species were mainly insects particularly coleopterans and hymenopterans.All these show the importance of arthropods and most especially insects as biotic components in terrestrial ecosystems such as the ones in this research.Several reports have also shown the leading abundance of insects in natural and agroecosystems as well as their ecological role [11,[31][32][33].

Potential Ecological Role of the Organisms Collected.
Generally, all organisms found in an ecosystem have ecological niches which include what they do in such a system.In this research, organisms recorded were also linked to certain ecological role in the land-use systems as a result of interactions within themselves or with other biotic and abiotic factors in the ecosystems.For the organisms in Table 3, the potential positive ecological role included predation, parasitoidism, improvement/maintenance of soil fertility, and pollination while the negative role is that some are pests.e proportion of these roles is shown in Figure 5, with pest having the highest (41%), followed by predation and soil fertility improvement/maintenance (both 21% each), pollination and unidenti ed roles (each 7%), and parasitoidism (3%).Ponde et al. [33] showed that most animal and microbial groups are negatively a ected by agricultural intensi cation.

Relative Abundance of Key Species.
e key species were selected based on their potential role as environmental bioindicators and as pests, and these include earthworms, ants, beetles/weevils, snails, and spiders.For species captured in the soil litter, with the exception of relative abundance of snails (mainly Limicolaria species) and earthworms that were highest in the banana and palm plantations, respectively, the highest numbers of ants were from the forest (Table 4).Contrary to the case of snails, the total numbers of the other organisms were lowest in the banana plantations.
is might be due to many di erent pesticides (including WHO class 1a pesticides such as terbufos and oxamyl) that have been or are being applied frequently in the banana plantations in the country [34].Mongyeh et al. [35] and Okolle et al. [36] have reported the high impact of terbufos on mortality of banana borer weevils, ants, and earthworms.
ese chemicals therefore create an unfavourable environment, repelling and killing or a ecting the reproductive capacity of especially the soildwelling organisms.Of recent, snails (Limicolaria spp) have become serious problem on banana fruits in the plantations of Cameroon especially during the rainy season.Unlike the distribution cases of soil-litter organisms, the abundance of these organisms captured from sweep nets varied with respect to the di erent land-use systems (Table 4).Earthworms were not caught while the number of snails collected was few.Highest numbers of beetles and spiders were caught in the rubber plantation.Ants were highest in the SF and lowest  Applied and Environmental Soil Science in the SP while spiders were highest in the RP but lowest in the BP.As for beetles/weevils, they were highest in the RP and lowest in the BP while fruit ies were highest in the BP.An interesting result was that mosquitoes (especially the tiger mosquitoes) were really high in the banana and rubber plantations.Irrigation water that usually settles on the banana stumps might be encouraging oviposition of these tiger mosquitoes.

Correlation between Soil Properties and Macrofauna
Abundance.Correlation matrix of 10 variables representing various soil physicochemical properties and microfauna number under di erent land-use systems revealed signicant correlation in 6 variable pairs out of fty-ve pairs (Table 5). is result con rmed the strong signi cant negative correlation between OM and BD (r −0.9653).e decrease of soil organic matter (OM) by the conversion of the forestland into cultivated elds probably caused the higher bulk density in the cultivated soils [37].Hajabbasi et al. [38] reported that deforestation and subsequent tillage practices resulted in nearly 20% increase in bulk density and 50% decrease in soil organic matter for a soil depth of 0-30 cm over 20 years in central Zagros.ere was also a strong signi cant negative correlation between bulk density and ants population (r −0.8828).Cerdà and Jurgensen [39] in their work on the in uence of ants on soil and water losses from an orange orchard in eastern Spain found that in anta ected soil, there was a reduction of soil bulk density, an increase in soil organic matter, and an increase in macropore   Applied and Environmental Soil Science flow as compared to soil without ant activity.Soil pH was negatively correlated with earthworms abundance (r � −0.9072), indicating that at higher pH, there is a decrease in earthworm number.Similar negative correlation was obtained by Karmegam and Daniel [40].

Conclusion
is study evaluated some selected physicochemical properties of soils and macrofauna abundance under different land-use systems (i.e., secondary forest and oil palm, banana, rubber, and sugarcane plantations) operating within the same soil forming factors and climatic conditions.Several soil physicochemical properties varied significantly among land uses.e results revealed that organic matter content might be as a result of quantity and type of plant residues that are returned to the soil in these environments.e number and diversity of soil/litter fauna were influenced by the type of soil management practiced in the different landuse systems.Negative significant correlations were observed between pH and number of earthworms and between ants and bulk density.Like the case of the banana plantation, excessive use of pesticides such as organophosphates has detrimental effects on ground dwelling organisms such as coleopterans, ants, and earthworms.erefore, encouraging farmers to return agricultural residues (farm or processed residues) to rot in their fields could replenish degraded soil quality parameters for sustainable agricultural production and productivity in the study area.

Figure 1 :
Figure 1: E ect of land-use change on soil bulk density (BD).

Figure 2 :Figure 3 :
Figure 2: E ect of land use on soil organic matter.

Figure 4 :
Figure 4: Total number of species captured from sweep nets and soil-litter extractions in the di erent land-use systems.

Figure 5 :
Figure 5: Proportion of potential ecological role of the organisms recorded.

Table 1 :
Description of sampling sites used for the study.

Table 2 :
pH, sand, silt, and clay properties in di erent land-use systems.

Table 3 :
Phylum, class, and potential role of species recorded from sweep nets and soil litter from di erent land-use systems in Fako Division, SW region, Cameroon.

Table 4 :
Relative abundance (%) of key species collected from sweep net and soil litter on di erent land-use systems.

Table 5 :
Pearson's correlation matrix of some selected soil physicochemical properties and macrofauna abundance.Correlation coe cient is signi cant at the 0.01 level.* Correlation coe cient is signi cant at the 0.05 level.