Graduation date: 2007
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.