DEVELOPING BIOGEOCHEMICAL TRACERS OF APATITE WEATHERING BY ECTOMYCORRHIZAL FUNGI

Chronic acid deposition has depleted Ca from many New England forest soils, and intensive harvesting may reduce P available to future rotations. Thin glacial till soils contain trace amounts of apatite, a calcium phosphate mineral which may be an important source of these nutrients to forest ecosystems. The extent to which ectomycorrhizal fungi enhance the weathering rate of primary minerals in soil remains poorly quantified, in part due to tracers which are subject to within-plant fractionation. Rare earth elements (REEs) and Pb isotope ratios show potential for revealing differences in soil apatite weathering rates across parent materials and management treatments.

To test the utility of these tracers, we grew birch seedlings under controlled P-limited conditions, supplemented with mesh bags containing granite chips. Our experimental design included nonmycorrhizal (NM) and ectomycorrhizal cultures (Cortinarius or Leccinum). Resulting mycorrhizal roots and leachates of granite chips were analyzed for these tracers.

REE concentrations in roots were greatly elevated in treatments with granite relative to those without granite, demonstrating uptake of apatite dissolution products. Roots with different mycorrhizal fungi accumulated similar concentrations of REEs and were generally elevated compared to the NM cultures. Leaches of granite chips grown in contact with mycorrhizal hyphae show elevated REE concentrations and significantly radiogenic Pb isotope signatures relative to bulk rock, also indicating apatite dissolution. These results are consistent with analyses of sporocarps collected at the Bartlett Experimental Forest, in which Cortinarius Pb isotope ratios were more radiogenic than those of other ectomycorrhizal sporocarps. Taken together, the experimental and field results imply that the coupled approach of REE and Pb isotopic values afford a means to assess differences in the contribution of mycorrhizal weathering to ecosystem nutrient budgets.