A conceptual model for nutrient availability in the mineral soil-root system

We propose a conceptual model based on our results from rhizospheric studies of a Norway spruce stand growing on a nutrient poor podzol in Southwest Sweden. We assume that dynamic linkages exist between three soil 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. Analyses showed great differences among the three soil fractions, especially the properties of the SRI. Cation exchange capacity and base saturation were higher in the rhizosphere and SRI than in the bulk soil. We attribute this to accumulation of organic matter (OM) in the rhizosphere and SRI. Moreover, the rhizosphere and SRI fractions had lower pH and higher titratable acidity than the bulk soil. Any possible negative 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 around the roots exhibits a different chemical composition than that of the root-free (bulk) soil, indicating more favorable conditions for roots. We suggest that trees growing on nutrient-poor acid soils invest their energy around roots to create a favorable microenvironment for both roots and microorganisms. Our results suggest that existing models which attempt to connect tree growth to soil acidification need modification. Such modification would include horizontal variation (bulk soil, rhizo and SRI) besides the vertical ones normally emphasized. It is 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.