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Rheinischen Friedrich-Wihelm-Universität Bonn (2007)

The contribution of different fauna communities to improved soil health : a case of Zimbabwean soils under conservation agriculture

Nhamo, Nhamo

Titre : The contribution of different fauna communities to improved soil health : a case of Zimbabwean soils under conservation agriculture

Auteur : Nhamo, Nhamo

Université de soutenance : Rheinischen Friedrich-Wihelm-Universität Bonn

Grade : Doktor der Agrarwissenschaften 2007

Résumé
For improved and sustainable soil productivity, the application of soil conserving cropping systems is important. This study characterizes soil fauna as a basis for comparing conservation agriculture and conventional ploughing systems and focuses on soil health of sandy, sandy loam and clayey soils in smallholder farms under sub-humid and semi-arid conditions in Zimbabwe. Fauna contribution to soil health was explored. Beetle larva (Coleoptera), earthworm (Opisthophora), mycorrhiza spore (Glomales), nematode (Tylenchida and Dorylaimida) and termite (Isoptera) densities were measured in conventionally ploughed (CP), direct seeded (DS), ripped (RR) and basin (BA) treatments on farms at Henderson, Chinyanga, Kajengo (all in agro-ecological zone II), Makwara and Zhinya (agro-ecological zone IV). Maize and soyabean were used as test crops. Rates of maize mulch loss were determined for winter and summer seasons. Shannon Wiener indices were used to analyse the changes in species and genera of weeds and nematodes respectively. Weed species responses to treatments and weed management regimes were explored at each site. Perceptions of soil fauna on the farms were sought from farmers in Shamva and Zimuto. High species richness was measured under mulched ripper and direct seeder treatments. Beetle larvae and termite densities increased significantly (P < 0.05) under DS and RR compared to CP across all sites. Nematode densities were not responsive to the mulch and tillage treatments. There was a near balanced ratio between densities of phyto-parasitic and free living nematodes over the two seasons (2006 and 2007). There was an increase in densities with time, though low numbers were measured. Planting soybean after maize increased nematode genera evenness (2.16) and diversity (5.70) on ripped and direct seeded treatments on clayey soils. Low dominance of phyto-parasitic nematode genera suggests low risk of maize infection. Termite activities resulted in an increased gallery construction especially on mulched DS and RR treatments. Gallery material built on the surface of sandy soils contained more clay, soil organic mater and silt compared to adjacent soils, but on clayey soil, the clay fraction of gallery material (3 %) was lower than on the adjacent soil (39 %). The mycorrhiza spore densities in the gallery material were always lower than those in the soil. In the litter bag experiment, the decomposition of mulch material was determined. Lower decomposition rates of surface mulch were measured in winter (8 - 20 %) compared to summer (45-65 %), with highest losses in the semi-arid zone at Makwara. The decomposition constants k calculated from the exponential decay function y = a.e-kx during summer were 10-fold those in winter. The k values were lowest at Henderson, (0.017 and 0.17) and highest at Makwara (0.025 and 0.24) for winter and summer, respectively. Richardia scabra, Galinsoga parviflora and Eleusine indica were common weeds across the five sites. Introducing a late weeding treatment reduced weed densities significantly at Henderson compared to other sites. Participatory rapid appraisal results show that farmers perceived termites as the dominant fauna on the farm and as a pest to maize. Under conventional ploughing systems, complete crop residue removal is practised. Management of maize residues after harvest is mainly meant to secure livestock feed during the dry winter season. On a small scale, mulching was practiced using crop residues and tree litter in vegetable gardens. On farms with Chromic Luvisol, Ferrali-Gleyic Arenosol and Gleyic Luvisols and Brachstegia-Julbernadia dominated miombo savanna vegetation in eastern and southern Africa, in the short term, beetle larvae and termites densities responded to conservation agriculture better than nematodes and earthworms. The increased fauna biomass improves soil health. Macrofauna participates in decomposition, soil turnover and modification. Gallery building by termites, for example, increases surface clay, silt and organic matter contents in sandy soils. Furthermore, termites through their galleries, participate in mycorrhiza spore inoculation of the soil.

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