In situ soil solution chemistry in the rhizosphere of mature Norway spruce (Picea abies [L.] Karst.) trees

Active tree roots influence the soil chemistry in their immediate vicinity, the rhizosphere. For the first time, the extent and stability of in situ concentration gradients of major cations and anions in the soil solution around tree roots were studied in the field. Micro suction cups were installed in an acidic forest soil using a root window in a 145-year-old Norway spruce stand. Samples were collected once a week during the growing season from the rhizosphere and from the bulk soil of a non-mycorrhizal long root and of a mycorrhizal root net. For comparison, micro suction cups were installed proximal and distal to a long root in a rhizotron under controlled conditions (homogenised soil, constant water potential, nutrient solution) at the same site. Small volumes of soil solutions were analysed for NH4+, K+, Na+, Ca2+, Mg2+, Al3+, NO3-, Cl- and SO42- by capillary electrophoresis. Total-Al was measured with ICP-AES micro injection, complexed-Al was calculated as the difference between total-Al and Al3+. pH values were determined with an ion-sensitive field effect transistor (ISFET) sensor. In both set-ups a significant increase of the K+ concentrations was observed in the rhizosphere as compared to the bulk soil solutions of the non-mycorrhizal long roots. The concentrations of Al3+, H+ and NH4+ were lower in the rhizosphere. No gradients were observed for Mg2+ and Ca2+ at the root window. A significant depletion of these ions was found in the rhizosphere of the long root only in the rhizotron. In the case of the mycorrhizal roots, contrasting results were found with significantly higher concentrations of Al3+, Mg2+, H+, SO42- and Cl-, and significantly lower K+ concentrations in the rhizosphere. The extension of the gradients was estimated at about 2.5 to 5 mm for the long roots and 1 mm for the mycorrhizal roots. The observed gradients varied strongly with time. In conclusion, for Norway spruce, the effect of non-mycorrhizal long roots on the rhizosphere chemistry differs from that of mycorrhizal roots. Evaluating nutrient availability and the risk of Al3+ toxicity to tree roots from bulk soil analysis can be misleading.