Plagioclase weathering by mycorrhizal plants in a Ca-depleted catchment, inferred from Nd isotope ratios and REE composition

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The neodymium isotope ratios (¹⁴³Nd/¹⁴⁴Nd) and rare earth element compositions of eight plant species in a well-studied Ca-depleted research forest (the Strengbach catchment) were measured. The ¹⁴³Nd/¹⁴⁴Nd ratios distributed between the higher ratio of apatite and the lower ratio of plagioclase. The ratio correlated well with the type of infected fungi. Ectomycorrhizae (EM)-infected plants tended to show lower isotope ratios than Ect-uninfected plants. This is supported by the REE abundance patterns: higher ¹⁴³Nd/¹⁴⁴Nd plants tend to show a more negative Eu anomaly. Based on the reported soil profile, the Nd isotope ratio of the surface soil is considered to be influenced significantly by Nd in plant litter. Combining Nd isotope and Eu anomaly data, the REE cycle in the catchment can largely be understood by mixing three endmembers of Ca-bearing plagioclase, apatite and carbonate in loess. EM-infected plants are estimated to incorporate 50–70% of Nd ultimately from plagioclase, whereas for non EM-infected plants, less than 30% of Nd is from plagioclase, perhaps indirectly from the recycled pool. Comparison of the results with reported mineral soil profiles and REE composition of acid leachates of soil indicates that plagioclase is dissolved by the symbiotic action of ectomycorrhizae, providing a direct evidence of active plant-induced weathering in a natural system. It is likely that plants dissolve plagioclase to compensate for insufficient Ca in soil.