The role of fish ponds in the nutrient dynamics of mixed farming systems
Muendo, P.N. (2006). The role of fish ponds in the nutrient dynamics of mixed farming systems. PhD Thesis. Universiteit Wageningen: Wageningen. ISBN 90-8504-459-6. 120 pp.
Rapid population growth in developing countries has resulted in increased demand for food, leading to increased pressure to extend land under cultivation and to intensify food production. Because most of the arable land has already been utilized, further intensification of agricultural production has involved conversion of marginal lands such as forest reserves, communal grazing lands and fragile areas such as river banks and steep hill slopes. Intensified crop production on marginal lands enhances the risk for soil degradation, like soil fertility decline. In many situations, inorganic fertilizers are not available or are too expensive, and efficient utilization of organ~residues such as crop residues and manures is constrained by a temporal mismatch between availability and application. Diversification of farming activities potentially increases nutrient efficiencies. One promising additional activity is aquaculture and the development of integrated aquaculture - agriculture (IAA) systems. Although integrated aquaculture - agriculture farming systems have been developed and practiced in some parts of Asia, they have not been widely adopted. In many developing countries, especially in Africa, aquaculture itself is still poorly developed. Aquaculture is considered separately from agriculture and its benefits are measured in terms of fish production, ignoring its role in nutrient cycling through integrated farming systems. Yet, the majority of nutrients entering ponds, including fertilizers, feeds and nutrients contained in inflows from channels or run-off from watersheds, accumulate in the sediment. These nutrients are a potential nutrient source for terrestrial agriculture. This thesis explored the use of fish ponds as nutrient traps (besides fish production) to increase the nutrient use efficiency in mixed farming systems. Focus was put on (i) nutrient utilization efficiency of agricultural by-products such as crop residues and animal manure in aquaculture production, and(ii) in quantitative aspects of sediment and nutrient accumulation in aquaculture ponds, and pond sediments' potential as a fertilizer in landbased agriculture In chapter 2, aquaculture components were ideotyped for existing agricultural farming systems and benefits from resultant ideotyped integrated aquaculture - agriculture (IAA) farming systems were evaluated and quantified. The results showed that integration of an aquaculture component in agricultural farming systems provided the opportunity to recycle eroded nutrients. In addition, aquaculture provided an opportunity to utilize nutrients from agricultural by-products otherwise lost through leaching during storage. As the majority of nutrients added to ponds accumulate in the sediment, nutrients are stored for later use. In different agro-ecological zones of the Kenyan highlands this practice reduced soil fertility decline by 23 - 35%, increased agricultural production by 2 - 26% and raised the overall farm food productionby 22 - 70%.The results indicatedthat there may be more benefits from pond sediment utilization than fish production alone and also demonstrated that integration of aquaculture is not a threat to agricultural production. The nutrient storage capacity of ponds and the linked increase in productivity largely compensates for the small loss in land surface for crop production. In chapter 3, the nutrient utilization efficiency of agricultural by-products in fish ponds was investigated and compared to that of supplemental foods. Using multivariate analyses, the trophic pathways in organically fertilized and feed driven semi-intensive culture environments were explored. By ANOVA models, water quality, sediment quality and tilapia growth and yields in the two environments were also compared. In both environments, a phytoplankton based food web dominated, and fish nutrition in both environments was mainly based on natural foods. Extrapolated fish yield data indicated that with equal nutrient i
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