Faculty of Agriculture, Engineering and Natural Sciences

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Browsing Faculty of Agriculture, Engineering and Natural Sciences by Subject "Acacia mellifera"

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    The influence of Acacia Mellifera on soil fertility, herbage quality and composition and sandy soils in camel-thorn savannas of Namibia
    (2013) Nzehengwa, James M.
    Acacia mellifera is viewed negatively due to its invasive nature to the extent that farmers tend to remove it from their farms without really considering that it may have beneficial effects. Thus the study looked at the role it plays on soil fertility, herbage quality, and botanical composition with reference to N-fixation. Soil and grass samples were collected at Corsica Resettlement farm at 6 distances within three sub-habitats from bases of five A. mellifera trees outwards. The nutrient concentrations of soil (Total N, % of Organic, Ca, K, Cu) and that of herbaceous plants (N, Ca, P, K, Cu) were found to be significantly higher (p<0.01) under tree crown zone than other sub-habitats and decreased along a distance gradient from A. mellifera tree outwards. This is attributed to biological N fixation, shading, litter, ‘nutrient pump’ and decomposition of roots and nodules. Soil Mg and P and plant Mg, Mn and Zn concentration did not significantly differ among sub-habitats. Amounts of plant N, Ca, K increased with amounts of soil N, Ca, K. Herbaceous species composition varied significantly among sub-habitats, with at most 85% similarity between areas under trees and those outside the tree crowns. Perennial grasses (Stipagrostis uniplumis and Stipagrostis ciliata) were commonly found in all clusters, while forbs only occurred under the tree crown zone, leading to the observed differences. The study confirms that A. mellifera enriches the soil fertility and improves herbage quality through N-fixation, but this process is also interactively linked to the dynamics of other nutrients.
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    Overlap in soil water sources of savanna woody seedlings and grasses
    (John Wiley & Sons, 2012) Kambatuku, Jack R.; Cramer, M.D.; Ward, D.
    Seasonal availability of water is a key controlling factor in semi-arid savanna vegetation structure, function and interactions. Understanding of woody plant interactions with grasses in savannas has long been underpinned by Walter's two-layered niche differentiation hypothesis that postulates that grasses and trees source water from different depths. The Walter hypothesis persists in the literature, despite contrary evidence and a lack of quantitative empirical tests of the theory. We conducted a greenhouse experiment to determine the following: (1) whether tree seedlings and grasses obtain water from different depths on rocky and sandy soils; (2) whether interspecific competition affected tissue water content of clipped grasses; and (3) the influence of repeated grass clipping on soil moisture. Grass competition significantly reduced tree seedling rooting depth on both rocky and sandy substrates. Trees had significantly longer roots on rocky substrates than on sandy substrates for all combinations (trees only, trees with unclipped grasses and trees with clipped grasses). Results indicated a three-tier soil moisture depletion pattern, with a top layer (15 cm) exclusively exploited by grasses, an intermediate zone (25–35 cm) utilised by both grass and tree seedling roots and deeper subsoil exclusively tapped by tree seedling roots. Our results are consistent with Walter's hypothesis, but we distinguished between three rather than two layers of tree and grass root interactions in acquiring soil moisture