Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, dif fering from that of the bulk soil found only a few mm from the root. The ai m of this study was to investigate adaptation mechanisms of herbs growing i n acid soils through studying their rhizosphere chemistry in a greenhouse e xperiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 i n the soil solution) in the greenhouse. The concentrations of NO3-, SO42-, phosphates, Ca2+, Mg2+, Mn2+, K+, Na+, NH4+ and pH were analysed in soil so lutions obtained by centrifugation. The general pattern found was a depleti on of nutrients in the rhizosphere compared with their concentrations in th e bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appe ared to be caused by plant uptake of NO3- (r(2)=0.88). The ion concentratio ns in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher gro wth rates showed a higher degree of ion depletion (except for Na+, SO42-) i n the rhizosphere, there were also species specific responses. A field stud y of five herbs at five oak forest sites in Southern Sweden (Scania) was al so carried out. In addition to the soil solution concentrations, the loss o n ignition (LOI) and the concentrations of 0.1 M BaCl2 extractable K+, Mg2, Mn2+, Ca-2+, and Al ions were measured. The amount of soil solution Al wa s determined as free ionic (quickly reacting) Al. For all species and sites , the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measu red in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO3- and quickl y reacting Al. The lower concentrations of quickly reacting Al in the rhizo sphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were on ly small and mostly non-significant. Plant-soil interactions and the abilit y of plants to utilise heterogeneity of the soil appear to be more importan t for plant growth in acid soils than recognised heretofore. Rhizosphere st udies provide an important means of understanding plant strategies in acid soils.