Temporal variations of rhizosphere and bulk soil chemistry in a Douglas fir stand

Rhizosphere soil can play a central role in the maintenance of the soil–plant system and influencing the biogeochemistry of forest ecosystems. However, rhizospheric studies in situ to understand the ecosystem functions are still lacking. Therefore, the objectives of the present study were to examine the differences in the chemical properties such as the pH, organic C, exchangeable base cations (EBC = Mg2+ + Ca2+ + K+), exchangeable acidity and cation exchange capacity (CEC) of three soil fractions (Bulk, B; Rhizosphere, R and Rhizosphere Interface, RI) and the evolution of chemical properties of soil samples collected in March and June from a Douglas fir ecosystem located in the Beaujolais Mounts in France. Most of the variables measured (organic matter, CEC and EBC) increased significantly in the same order (B < R < RI), indicating that the rhizosphere was a favorable interface for tree nutrition. These processes were more pronounced in June than in March, as the temperature and biological activities are normally higher in June than in March. The temporal variations of Al charge in R and B seem to depend on OH−, organic complexation and H+ production by roots and/or the organic matter degradation in the bulk soil. In the rhizosphere, the H+ or OH− production depends on the N cycle. The results of this study and the support of independent mineralogical study using the same soil fractions, and of other field studies on the same site have all pointed at the importance of rhizosphere as an excellent indicator for the understanding of the ecosystem dynamics in both short- and long-term.