Effect of the shrub Piliostigma reticulatum on the biological functioning of a soil used for the cultivation of millet
Thesis defended on 28 April 2014 at the University of Cheick Anta Diop (UCAD)
In sudano-sahelian regions, shrubs growing in the area are often used by the local population, including for agriculture. By incorporating these shrubs within cropping systems, it is possible to intensify ecological processes to improve primary productivity (principle of ecological intensification). This study was based on the hypothesis that the local shrub Piliostigma reticulatum can be used to improve biofunctioning in soils used for the cultivation of millet, based on the biological interactions (microorganisms and nematofauna) involved, in particular, in the supply of nutrients.
The study focused on the microbial communities (abundance, activities, and functional and genetic diversity), nematode communities (functional guilds) and the trophic network developed by these communities within the soil. Four types of samples were taken from the top soil horizon (0-10 cm) from an existing experimental site (Nioro-du Rip, Senegal): i) bare soil (control, C), ii) millet cultivated as sole crop (M, millet), iii) the shrubs (P, Piliostigma reticulatum) and iv) association of millet and shrubs with management of residues of coppicing the shrubs in the form of mulch (M+P). An experiment in controlled conditions determined the effect of adding P. residues on a population of phytoparasitic nematodes of the Helicotylenchus dihystera species.
Below the shrubs, there were higher concentrations of elements (in particular mineral N) (P vs. C), there was a significant increase in microorganism catabolic activity (measured using MicroResp™, in particular related to trehalose, also known for its role in closing stomata) and there was a change in the structure of the microorganism and nematode communities. In the soil used for the cultivation of millet / shrub association with mulch (M+P), there were higher mineral N concentrations and an increase in certain enzymatic activities (urease, arylsulfatase and deshydrogenase) and catabolic activity of protocatechic acid (phenolic acid). The results also showed a change in the structure and diversity of the total bacterial community whereas, for fungi, only the structure was affected by the use of P. reticulatum in the system cultivated. The structure of nematode assemblages was affected with a significant increase in bacterivorous nematodes and a significant reduction in phytoparasitic nematodes, in particular Hoplolaimidae. The study in controlled conditions confirmed the nematicidal effect of residues of P. reticulatum on Helicotylenchus dihystera. The analysis of the trophic networks based on nematofauna showed that the M+P treatment increased the level of enrichment (indicator of the availability of nutrient elements) and the complexity of the trophic chain.
Incorporating P. reticulatum in the cropping system affects the soil trophic network (more bacterivorous nematodes and fewer phytoparasitic nematodes) and leads to an intensification of the ecological processes involved in the nitrogen cycle and possibly to the inhibition of nitrification.