A CONCEPTUAL-MODEL FOR NUTRIENT AVAILABILITY IN THE MINERAL SOIL-ROOTSYSTEM

We propose a conceptual model based on our results from rhizospheric s tudies of a Norway spruce stand growing on a nutrient poor podzol in S outhwest Sweden. We assume that dynamic linkages exist between three s oil fractions: bulk soil, rhizosphere (Rhizo) and soil root interface (SRI). The soil fractions were characterized by organic matter content , electrical conductivity, pH, and soluble and exchangeable cations. A nalyses showed great differences among the three soil fractions, espec ially the properties of the SRI. Cation exchange capacity and base sat uration were higher in the rhizosphere and SRI than in the bulk soil. We attribute this to accumulation of organic matter (OM) in the rhizos phere and SRI. Moreover, the rhizosphere and SRI fractions had lower p H and higher titratable acidity than the bulk soil. Any possible negat ive effects of Al to the roots could be offset by accumulated organic matter and base cations (BC). The calcium-aluminum balance followed a consistent trend: bulk < rhizo < SRI. The results suggest that soil ar ound the roots exhibits a different chemical composition than that of the root-free (bulk) soil, indicating more favorable conditions for ro ots. We suggest that trees growing on nutrient-poor acid soils invest their energy around roots to create a favorable microenvironment for b oth roots and microorganisms. Our results suggest that existing models which attempt to connect tree growth to soil acidification need modif ication. Such modification would include horizontal variation (bulk so il, rhizo and SRI) besides the vertical ones normally emphasized. It i s possible that the conceptual model may enable a better understanding and description of naturally existing relationships between soil and plants under normal and stressed conditions.