Biodiversity of Soil Macrofauna Functional Groups And Their Effects On Soil Structure, As Related To Agricultural Management Practices Across Agroecological Zones Of Sub - Saharan Africa.
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Soil macrofauna, especially earthworms and termites are important components of the soil ecosystem and, as ecosystem engineers, they influence formation and maintenance of the soil structure and regulate soil processes. Despite advances made in understanding the links between soil macrofauna and agricultural productivity, this component of biodiversity is still very much a “black box”. In this thesis, I proposed to link soil biodiversity to soil functioning through the diversity of the soil structures produced by ‘ecosystem engineers’ like earthworms and termites, i.e. organisms, which physically modify and create habitats for other soil organisms and plant roots. This study aimed at providing an understanding of the link between soil macrofauna and crop management practices on soil aggregation and soil organic matter (SOM) dynamics as this is key to the improvement and the management of infertile or degrading soils. The methodological approach used in this study involved assessment of: 1. How agricultural management affects earthworm and termite diversity across sub-humid to semi-arid tropical zones. 2. The influence of soil macrofauna on soil aggregation and SOM dynamics in agro-ecosystems of sub-Saharan Africa as influenced by management practices. 3. How management practices (e.g. tillage and use of organic inputs) influence macrofauna-induced biogenic structures in East and West African soils. 4. Disclosing farmers’ knowledge and perception on the roles of termites in Western Kenya. In chapter 2, I examined how agricultural management affects earthworm and termite diversity across sub-humid to semi-arid tropical zones. This study, conducted in 12 long-term agricultural field trials of Eastern and Western Africa, provides new insights on diversity of earthworms and termites in SSA, since it is the first time that a study like this is done on this scale. In each trial, treatments with high and low soil organic C were chosen to represent contrasts in long-term soil management effects, including tillage intensity, organic matter and nutrient management and crop rotations. High soil C was considered to reflect relatively favorable conditions, and low soil C less favourable conditions for soil macrofauna. For each trial, a fallow representing a relatively undisturbed reference was also sampled. I have shown that earthworm and termite diversity and abundance were low in fallow, high-C and low-C agricultural treatments in 12 long-term trial fields across the sub-humid to semi-arid tropical zones in Eastern and Western Africa. This is in contrast to most typical native or undisturbed forest ecosystems of the tropical zones. Environmental variables contributed 42% and 25% of variation observed in earthworm and termite taxonomic abundance, respectively. Earthworm and termite taxa were less abundant in the relatively cooler, wetter and more clayey sites characteristic of Eastern Africa, compared to the warmer, drier and more sandy sites in West Africa. Continuous crop production has significant negative effects on earthworm-, but little effect on termite diversity, as compared to long-term fallow, and agricultural management resulting in high soil C increases earthworm and termite diversity as compared to low-C soil. I conclude that fewer species of earthworms and termites are favored under agricultural management that leads to lower soil C. Results indicate that soil disturbance that goes with continuous crop production is more detrimental to earthworms than to termites as compared to fallow. In chapter 3, a broad regional study was conducted to examine how management intensity affects soil macrofauna, and how macrofauna in turn influence soil aggregation in agro-ecosystems of sub-Saharan Africa. My study has shown that macrofauna, especially earthworms, and to a smaller extent termites, are important drivers of stable soil aggregation, in conjunction with climate, soil organic C content and soil texture in sub-Saharan agroecosystems. However, the beneficial impact of earthworms and termites on soil aggregation is reduced with increasing management intensity and associated soil disturbance due to cultivation. I suggest that this knowledge is important in designing agricultural management systems aimed at increasing long-term soil fertility in sub-Saharan Africa. In chapter 4, a long-term trial at Kabete, Kenya was selected to examine in detail the interactive effects of organic and inorganic fertilizerson soil macrofauna diversity and soil aggregation and SOM dynamics in arable cropping systems. Differently managed arable systems were compared to a long-term green fallow system representing a relatively undisturbed reference. Application of manure in combination with fertilizer significantly improved aggregate stability and C and N stabilization in arable soil. Furthermore, manure-fertilizer application enhanced earthworm diversity and biomass. Significant correlations between the amount and C and N contents of aggregate fractions and whole soil C and N were observed with earthworm parameters (Shannon diversity index, abundance and biomass), but not with termite parameters. Factor and regression analyses showed that earthworms had a profound effect on aggregation, C and N stabilization in whole soil and in aggregate fractions, whereas contributions of termites were limited. Therefore, my results indicate that long-term application of manure in combination with fertilizer result in higher earthworm Shannon diversity and biomass, which leads to improved soil aggregation and enhanced C and N stabilization within this more stable soil structure. These practices therefore result in the dual benefits of improving soil physical and chemical properties. In contrast, no significant improvements in soil aggregation and C and N stabilization were found when organic inputs were applied in the form of maize stover as compared to the no-input control, irrespective of fertilizer addition. Under the conditions studied, earthworms were more important drivers of aggregate stability and C and N stabilization in aggregate fractions, but termites less so. In chapter 5, a micromorphological approach was used to describe and quantify macrofauna-induced biogenic structures in undisturbed soil samples (i.e. thin sections) from long-term field experiments in East and West Africa. Management systems differing in tillage intensity and with or without organic amendments (manure/crop residue) were compared. My study has shown the soil management practices tillage type and addition of organic inputs influence soil fauna activities with a significant impact on soil structure and hence soil physical properties. Among the management practices assessed across two agroecological zones, fallowing, conservation tillage plus residue application (in East Africa) and hand-hoeing plus manure (in West Africa) enhanced biogenic soil structure formation, resulting in a well developed soil structure and a continuous pore system through many faunal channels. By contrast, intensive tillage and absence of organic inputs resulted in soil with less biogenic soil structural features and was, therefore, prone to slaking. Chapter 6 describes farmers’ knowledge on the occurrence and behavior of termites, their perception of the importance of termites in their cropping systems and the management of termite activities in their farm fields in Nyabeda, Western Kenya. Being the main actors in environmental conservation or degradation, farmers’ knowledge and perception can enrich scientific understanding of the ecology and sustainable management of termites under different agroecological conditions. My research has shown that farmers in Nyabeda were aware of the existence of termites, their activities and nesting habits and had local names for termites that they frequently encountered. Geographic location explained 23% of the variance in farmers’ perception and management of termites, whereas socio-economic variables explained only 5%. Ninety percent of the farmers perceived termites as pests and maize was rated as the most susceptible crop to termite attack, especially during the flowering/tasseling stage and in wet periods. More than 88% of the farmers used control measures against termites, further indicating a lack of awareness or appreciation of the beneficial effects often ascribed to termites with respect to soil properties in crop production. There is an urgent need for more research to assess the trade-offs between positive and negative impacts of termites on crop yields, as well as to get an understanding of the effects of different termite control strategies used by farmers on agroecosystem functions.