Graduate Thesis Or Dissertation


Activity, composition and structure of soil microbial communities in Savannahshrubs of agroecosystems in semi-arid Senegal Public Deposited

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  • Piliostigma reticulatum and Guiera senegalensis are two native shrubs that coexist with row crops in parkland systems of the Sahel of Africa. Although permanently green all year around in soil depleted in nutrients and dry for a nine-month period, these shrubs have been largely overlooked. Conventional management of these shrubs involves coppicing and burning of aboveground biomass each spring to prepare for the summer cropping season. Previous research has shown these shubs can provide high amount of biomass carbon at landscape levels and that they can, through their roots, move water from the wet subsoil to the dry surface soil at night when photosynthesis stops (hydraulic redistribution). However, the influence of these shrubs on the soil microbial communities and their role in biogeochemical processes is largely unknown. This dissertation reports studies that have been carried out to investigate the impact of these shrubs on the soil microbial communities. The rhizosphere effect of these two dominant shrubs was investigated during both the rainy season and dry season by studying the soil microbial structure, composition and activity. This was done by: 1) profiling microbial communities through phospholipids fatty acid (PLFA) analysis and denaturing gradient gel electrophoresis (DGGE); 2) assaying enzymes (acid phosphatase, β-glucosidase, cellulase, chitinase, urease); and 3) measuring microbial biomass carbon (MBC) and mineral nitrogen dynamics. Decomposition studies were conducted to determine the potential of nonthermal management of shrub residues. This was done by determining the influence of shrub canopy, macrofauna, and residue type on shrub litter decomposition and microbial dynamics under field and laboratory conditions. Microbial communities were more diverse, more active and had greater biomass in shrub rhizospheres. The rhizosphere communities during the dry season were similar to the rhizosphere and bulk communities during the wet season. This suggests that shrub rhizosphere provide root exudates and/or water via hydraulic redistribution that supports microbial communities during the dry season. PLFA and enzyme activities were highly correlated and were more sensitive than DGGE in distinguishing the communities temporally and spatially. PLFA profiling showed that the rhizosphere effect was dominated by fungi and Gram-positive bacteria communities, and stimulated acid phosphatase and β-glucosidase activities. Macrofauna access to shrub residue resulted in higher decomposition rates with 70 to 90% loss of mass after 210 days. There was an increase of MBC and enzyme activities when macrofauna were allowed, and beneath shrub canopy as opposed to outside of the canopy. In a laboratory incubation study, the influence of shrub rhizosphere/canopy on soil was greater than the residue effect on the activity of soil microbial communities. Decomposition of residues showed that leaf litter had a greater impact on soil microbial communities and enzyme activities than did stem materials or stem/leaf mixes. Cellulase and β-glucosidase were highly correlated with the fungal markers and Gram-positive bacteria markers. In conclusion, the results showed that Piliostigma reticulatum and Guiera senegalensis are stimulating microbial activity and communities even in the dry season after six or more months without rainfall. Besides C inputs through litter fall, root turnover and exudates, this suggests that shrubs maintain moisture levels in the dry season for microorganisms by performing hydraulic redistribution of water from wet subsoils to dry surface soils at night through a passive water potential gradient. Consequently, shrubs are maintaining soil health and can drive biogeochemical processes year round which has not been previously recognized. These results provide a foundation to actively manage these shrubs to conserve Sahelian landscapes and to optimize agricultural productivity.
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