Masters Degrees (Agronomy)

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Browsing Masters Degrees (Agronomy) by browse.metadata.advisor "Jacobs, Karin"

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    Agroecosystem diversification for sustainability : the effects of crop rotation on soil microbial diversity, fertility and yield
    (Stellenbosch : Stellenbosch University, 2016-03) Venter, Zander Samuel; Hawkins, Heidi-Jayne; Jacobs, Karin; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Agricultural intensification can involve the simplification of agroecosystems to crop monocultures requiring agrochemicals to maintain soil fertility and control pests. This can have negative impacts on the ecosystem services flowing to and from agroecosystems and thus, more sustainable management practices are necessary. Broader ecological theories propose that ecosystem biodiversity is important for ecosystem function. Within agriculture, increasing aboveground diversity through crop rotation, a component of conservation agriculture, can enhance the ecosystem services supporting an increase in cash-crop yield. The extent to which the belowground microbial diversity can be manipulated through crop rotations and may facilitate the yield increase is unclear. This thesis aimed at investigating (1) the relationship between above and belowground diversity in crop rotation systems, and (2) the relationship between diversity in (1) and agroecosystem function in terms of soil fertility, including nitrogen fertility, and crop yield. A quantitative review of the literature using a meta-analysis of 27 studies from around the world found that soils under a higher diversity of crops in rotation produced higher microbial richness (+15.11%) and diversity (+3.36%) scores. This effect was significantly influenced by the type of microbial analysis method used, the length of the study trial, and the percentage annual ground cover. There was a high between-study heterogeneity and no correlation to soil nitrogen fertility. A field study at Langgewens Experimental Farm (Western Cape Department of Agriculture’s 19-year wheat-legume crop rotation trial in the Swartland Local Municipality of South Africa) added to the meta-analysis. Community level physiological profiling (CLPP) and automated rRNA intergenic spacer analysis (ARISA) were used as measures of functional and genetic microbial diversity, respectively. Increasing crop diversity through rotations of wheat with medic (Wm) or a combination of medic and clover (Wmc) resulted in greater wheat plant stem length and N concentrations when compared to wheat monoculture (WW). This effect seemed to be less linked with microbial diversity per se than with the Rhizobium species present because both microbial analyses found no differences in soil microbial activity, richness or diversity with increasing crop diversity. The lack of relationship between above and belowground diversity is likely due to other abiotic drivers of microbial community structure such as P availability, Na and K excess, and pH, all of which correlated to microbial activity and functional richness in our study. The role that microbial diversity plays in the agroecosystem diversity-function relationship remains complex as revealed by the lack of correlation between functional and genetic diversity scores. However, the relation between crop diversity and functional components including wheat yield and soil N followed a hump-shaped curve. The results of this thesis suggest that soil biodiversity and function are decoupled in agroecosystems. This provides support for the hypothesis that ecosystem function may be a product of either specific productive species (selection effect), or the facilitative interaction of multiple species (complementarity effect). Further investigation into the role of specific functional microbial groups in the yield increase of crop rotation systems using next-generation sequencing is required.