Soil microbial functionning affected by rhizobial inoculation and arabic gum production in mature Acacia senegal plantation

In a previous study, the rhizobial inoculation of mature trees of A. senegal has resulted in an increase of the production of arabic gum. Since the inoculation of selected strains of rhizobia could both affect plant nutrition and the functions of endogenous microbial communities, this study was aimed at describing some of the different soil biotic parameters (respirometry, total microbial biomass, ammonification and nitrification activities) related to soil fertility. A ten-year old A. senegal plantation of eighty trees located in Rollo (Department of Dahra, Senegal) was selected for this study. Half of the trees had been inoculated with a mixture of selected rhizobia strains at the beginning of the rainy season (July 2002). Soil samples were collected beneath 4 inoculated trees and 4 non-inoculated trees. These trees were selected in regard to their gum production. Soil were sampled along a North-South transect from 0-25, 25-50, 50-75 and 75-100 cm soil layers at a distance from the tree trunk of 0m, 1m, and 3m, respectively. We regrouped together the soil samples corresponding to the same depth and distance to the trunk and there were kept at their field humidity pending analyses. Biological activities (respirometry, total microbial biomass, ammonification and nitrification activities) were usually high in the areas where roots are largely present (in the top depth of the soil, 0-25 and 25-50 cm, and close around the tree). Soil total microbial biomass of inoculated trees was significantly higher than that measured in non-inoculated trees. This was particularly evident in the 0-50 cm depth layer. By contrast, any other relationship between soil parameters and the inoculation could be concluded from our results. For inoculated tree treatment, our results indicated a positive relationship between gum production and ammonium produced (in the 0-50 cm soil layer) under laboratory conditions. While a positive relationship between nitrification and the production of gum of non-inoculated trees was observed. In conclusion, our results suggest that the nitrogen activities in soils of trees may be different if they are inoculated or not with rhizobia. We will have to confirm this assumption by further studies but all these results will be useful for allowing a sustainable arabic gum production in drylands. (Texte intégral