Abstract

This study investigated the effects of three land use management systems on the status of total and mineral-associated and particulate soil organic carbon content. The study was carried out in selected cocoa farms established within Idanre Forest Reserve, Ondo State, Nigeria. Based on the number of non-cocoa trees (shade trees) per unit area, the farms were classified into dense cocoa agroforest, sparse cocoa agroforest, and pure cocoa plantation. 3 sample plots of 25m x 25m were mapped out alternate to each other, and soil samples were collected at 5 different points along the diagonal of each plot at depths 0-15cm, 15-30cm and 30-45cm using a 3cm diameter soil auger. The results shows that surface soil (0-15cm) total organic carbon (TOC) content was highest under the sparse cocoa agroforest (5.64%) and lowest in the dense cocoa agroforest (2.86%). It was noted that sparse cocoa agroforest had potential for increasing soil organic carbon of an appreciable concentration due to the presence of few large non-cocoa trees which contributed immensely to the soil carbon stock especially through litter fall. The decreasing order for the % TOC is given as Sparse cocoa agroforest > pure cocoa plantation > dense cocoa agroforest. The result of the analysis shows a significant difference (P<0.05) in the %Particulate organic carbon (POC) and %Mineral-associated organic carbon (MOC) stored in the soil by the different land use management systems and at the different depths considered. Carbon content of MOC were higher than the POC in all land use management systems. The study concluded that soil organic carbon stored decreased with increasing soil depth across land use and sparse cocoa agroforestry with few large trees has the potentials of storing organic carbon of appreciable concentration.

1. Introduction

During the last few decades, there is growing evidence that increasing greenhouse gases (GHGs) concentrations are mainly responsible for global warming and associated climatic changes ¹. There is a consensus that the climate on earth is changing and this has led to a series of impacts on the environment and human society ² and affecting the sustainability of various ecosystems and well-beings of human beings. Among the various GHGs, carbon dioxide (CO₂) accounted for 76% of total GHGs emission, whereas methane (CH₄), nitrous oxide (N₂O), and the remaining trace gases accounted for 16%, 6%, and 2%, respectively ^{3, 4}. Furthermore, the residence time of CO₂ is very long about 100 years ^{5, 6}. Sequestering CO₂ from the atmosphere through natural process is cost-effective, environmentally friendly and helpful to achieve food security by improving soil fertility ⁷.

Reports shows that different organic matter pools may show different susceptibility to land use and management. Thus, it appears that the extent to which land use change influences soil organic matter dynamics can be best evaluated by separating organic matter into fractions ^{8, 9, 10}. However, whether the soils will act as a sink or source of CO₂ is highly dependent on the land use, soil properties) ¹¹, and biophysical factors, including climate ¹². According to ¹³ land use management are among the most important determinants of soil organic carbon stocks.

Agroforestry systems have a higher potential to sequester Carbon and have become recognized as an integrated approach to sustainable land use. For instance, the Carbon storage by a Cocoa Agroforestry System (CAS) is a function of the level of management of such plantation. ¹⁴, pointed out that carbon can be stored in agroforestry systems for centuries following the carbon cycle (organic matter decomposition and absorption by plants) if they are managed sustainably. Different carbon pools are present in a CAS; those of cocoa and associated trees remain largely high compared to those of root, herbaceous plants, and litter ¹⁵.

Several studies have reported a decrease in particulate organic matter pool as a result of land use change ^{16, 17, 18}, while others did not find any significant change in particulate organic matter resulting from changed land use ^{19, 20, 21}. Although land use effect on soil organic matter has been studied extensively, study on the effects of associated shade trees in a cocoa agroforestry system on soil carbon storage is still rare. Also, there is a growing body of evidence that land use systems can also affect the organic carbon dynamics in sub-surface soil ^{22, 23, 24}. Thus, this study was conducted to evaluate the potential of different Cocoa Agroforestry Systems (CAS) on the particulate organic matter and mineral-associated organic matter pools of both surface and sub-surface soils.

2. Materials and Methods

The study was conducted in Idanre Forest Reserve which is located in Idanre Local Government Area of Ondo State. Idanre Forest Reserve covers an area of 540.45 Km² and is bordered by Akure-Ofosu Forest Reserve and Ala Forest Reserve ²⁵. The forest reserve is situated between latitude 6°45’0” and 06°58’32” N and longitude 04° 59’15” E and 05°12’4” in the lowland forest zone in Ondo state (Figure 1). Idanre forest reserve is a lowland rainforest at altitudes reaching mostly between 10-400 meters above sea level. The mean annual temperature is between 25°C - 26°C while the minimum and maximum temperatures are 19°C and 33°C respectively. Annual precipitation is between 1200mm - 1800mm ²⁵. The study area consists of three (3) selected sites of different land-use types differentiated by the density of shade trees (non-cocoa trees) per unit area and were classified into dense cocoa agroforest (>15 non-cocoa trees per plot), sparse cocoa agroforest (≤15 non-cocoa trees per plot), and pure cocoa plantation (zero non-cocoa tree).

Systematic strip sampling was employed in laying out plots. Two transects were cut 100m apart in each of the study site and three sampling plots of 25m x 25m in area were laid in alternate positions along each transect at 50m interval. Soil core samples were collected at 5 different points along the diagonal of each plot at three different depths of 0-15cm, 15-30cm and 30-45cm using a 3cm diameter soil auger. Samples from each plot at each depth were bulked separately and the composite soil samples taken to the laboratory and analyzed for total, particulate and mineral-associated soil organic carbon and other physico-chemical soil properties.

The process of soil physical fractionation was based on the methodology proposed by ²⁷. 50 g of ≥2 mm soil sample was transferred into 250 ml sample bottle and 105 ml distilled water added. The soil suspension was then washed through a ≥53 µm sieve and the residue i.e., the soil samples above the sieve were considered Particulate soil organic matter while the filtrate i.e., minute soil samples that pass through the sieve were considered Mineral associated soil organic matter. The particulate (≥ 53µm) and mineral associated (< 53µm) soil fractions were oven-dried at 70°C so as not to destroy carbon contained in the samples. The oven-dried samples were reduced into powder form using mortar and pestle and analyzed for organic carbon content using the wet-oxidation method.

Organic carbon was determined in the bulk soil (2mm), ≥53µm, and <53µm soil fractions as Total Organic Carbon (TOC), Particulate Organic Carbon (POC), and Mineral-associated Organic Carbon (MOC) respectively, using the wet oxidation method of ²⁸.

Data collected were subjected to analysis of variance (ANOVA) to determine statistical differences of soil carbon contents. The statistical analysis was performed with SPSS (V. 21), mean separation was performed with Duncan’s Multiple Range Test (DMRT) at p<0.05.

3. Results and Discussion

The results of the particle size distribution, soil texture and pH are presented in Table 1. The soil texture of the study area was predominantly clay. The soil pH of the pure cocoa and the dense cocoa agroforest ranges from medium to slightly acidic with the value of 5.9 to 6.2 while the pH of the sparse cocoa agroforest is neutral with value ranges from 6.7-7.1.

Shade trees in cocoa agroforestry have been found in several studies to improve not just soil carbon concentrations in the research area, but also soil structure and ²⁹, potentially boosting cocoa root development and reducing surface runoff. Cocoa agroforestry is important because of the tangible and intangible benefits provided by the different compositions of the system which include an additional source of income for the farmers, a microclimate modifier, and support for soil carbon storage ³⁰

The effect of land use on the soil organic carbon of the different cocoa-based agroforestry systems are shown in Table 2. In all the depths considered, sparse cocoa agroforest recorded the highest values of %total organic carbon stored (5.67) while dense cocoa agroforest has the least (2.86). These results corroborate a report by ³¹ that cocoa agroforests store a lot of carbon, hence a viable solution to mitigating climate change and the potential to slow deforestation by lowering the need to destroy forestland for agriculture.

In all land use management systems, the values (Table 3) found for MOC (stable pool) were relatively greater than the POC (labile pool), as reported by ³² and ³³. This suggests that the mineral-associated organic carbon pool accounts for a large fraction of total soil organic carbon, which is likely due to environmental conditions that encourage organic matter decomposition (primarily labile pool) and the stable carbon pool's strong physical and chemical stability. According to ³⁴, irrespective of soil variations, particular organic carbon is an accurate soil quality indicator, and so should be considered when evaluating the quality of various soil management strategies.

The highest % MOC (stable fraction) (6.24) was recorded in the dense cocoa agroforest, followed by that of sparse cocoa agroforest (4.61) while pure cocoa plantation has the least value (2.30). The highest % of POC (labile fraction) (2.34) was recorded in sparse cocoa agroforest followed by dense cocoa agroforest (1.68) and pure cocoa plantation (0.80). The higher % of the different partitions of carbon stored in the sparse and dense cocoa agroforest would be as a result of the presence of non-cocoa trees (shade trees) present which provides a continuous flow of organic material inputs and results in the return of organic carbon to the soil surface.

4. Conclusion

There exist differences across land use management systems in their potentials of storing total, particulate and mineral-associated organic carbon in Idanre Forest Reserve. From the results of this study, it could be observed that sparse cocoa agroforestry has the potentials for storing organic carbon of appreciable concentration due to the presence of though few shade trees per hectare compared to the dense cocoa agroforest but with large trees which enhance the amount of carbon stored in the soil. With the fact that there are different carbon pools in the soil, it should be noted that the stable fraction (MOC) stores carbon for a considerable long period and contributes to the soil’s nutrient holding capacity, which according to this study the dense cocoa agroforest has the highest value. Also, in all the different land use types, soil organic carbon fractions were observed to decrease with depth, and the top layer (0-15cm) recorded the highest concentration of these fractions. This research provides valuable information for decision-makers on the type of cocoa agroforestry system suitable for carbon storage to be embarked on; a cocoa agroforestry system with considerable large shaded trees which contribute to the total litterfall, and enhance carbon sequestration in the soil hence mitigating the effects of climate change.

Acknowledgements

I deeply appreciate Engr. Oreoluwa Adedapo for his financial support in publishing this research work. Cocoa farmers in Idanre Forest Reserve are also appreciated most especially Mr. Olorunmola, for his selfless dedication.

I would be ungrateful if I should forget Dr. Johnson Olusola in the department of Forestry Technology, and Dr. Ademola Ajayi in the department of Horticultural Technology, both of Federal College of Agriculture, Akure, Ondo state for their frequent advice and mentorship.

References