Effects of soil physicochemical properties and genetic characteristics on distribution of acacia senegal (L) willd. varieties in the dryland areas of Kenya

Acacia senegal is a dryland multipurpose tree species highly valued for gum Arabic production, agroforestry and desertification control. The aim of this study was to investigate the edaphic and genetic factors that affect distribution of the three Kenyan indigenous varieties (A. senegal var. senegal, var. kerensis and var. leiorhaehis) for the purpose of conservation of genetic resources and improvement of smallholder livelihoods in the drylands of Kenya. In the first part of research, inter-simple sequence repeat (ISSR) and chloroplast micro satellites (cpSSR) markers were used to study genetic diversity among seven Kenyan populations of A. senegal (Kajiado, Magadi, Kibwezi, Ntumburi, Ngarendare, Daaba and Kulamawe) embracing the three putative varieties. In the second part, soil physicochemical properties were assessed by collecting soil samples under the canopies of the three varieties taking into account distance from the trunks (0, 1 and 2 m) at a depth of 0 - 25 em and comparing with the soils from the open canopies. The third part estimated the potential of the three varieties to fix nitrogen in their natural ecosystems using 15N natural abundance method using two reference species namely; Balanites aegyptiaea and Commiphora africana. The study was conducted in the arid and semi arid lands of Kenya. The two molecular markers detected similar levels of Nei's gene diversity (HISSR= 0.211, HcpSSR= 0.212) among the A. senegal populations. Analysis of molecular variance (AMOV A) detected significant genetic variations within and among populations (P<O.OOl and P<O.Ol for ISSR and cpSSR, respectively), whereby the seven sites were differentiated into two regions (north and south). There were significant differences in soil 'Qa,,:{sicocaemica\. 'Q'to'Qerties aro.o~ the tmee varieties (f<O.05 and P<O.Ol). Soil nutrients under the canopies were higher than in the open canopies mainly due to the effects of litter accumulation. The estimated nitrogen fixed (%Ndfa) values for the three varieties ranged from 18.20 - 32.21% with A. senegal var. senegal showing the highest values. The mean nitrogen content in leaves ranged from 2.46 - 4.0% and were higher than those of the adjacent non-fixing reference species. The results indicate that there is genetic diversity and variation among and within the three indigenous varieties of A. senegal in Kenya, which can provide raw materials for tree improvement programmes. The three A. senegal varieties have beneficial effects on soil fertility improvement and this would most likely enhance herbage productivity both in quality and quantity in the Kenyan drylands. Domestication and improvement will enhance conservation and sustainable utilization of the species, improve productivity and quality of gum and hence directly empower the drylands communities who are key collectors of gum and other forest products.