Mineral nutrient mobilization by plants from rock: influence of rock type and arbuscular mycorrhiza

Rock colonization by plant roots and their associated microbiota is one of the major drivers of mineral weathering, nutrient cycling, soil formation and ecosystem stability. Yet the mechanisms of bio-uptake of lithogenic elements from rocks with differential nutrient availabilities and limitations are yet to be established. Here we present results from a mesocosm experiment that examined lithogenic element dissolution and uptake (P, K, Ca, Mg, Mn, Fe, Na, Ti, Al and Si) in Bouteloua dactyloides (buffalo grass) grown on four different granular porous media (basalt, rhyolite, granite and schist) comprised of primary mineral assemblages as influenced by arbuscular mycorrhiza (AM; Rhizophagus irregularis). Our results demonstrated that nutrient mobilization (chemical denudation + plant uptake) in such oligotrophic systems is governed by nutrient supply in the parent material, nutrient availability in pore water solution, and plant physiology. Overall, total major lithogenic element mobilization in planted columns (with and without AM) exceeded abiotic controls in all substrates. Differences in total mobilization among substrates occurred as follows: Fe, Na, Ti and Al reached high values in planted treatments in basalt, P and Mn in rhyolite, Ca and K in granite and K in schist, suggesting enhanced dissolution of primary minerals in the presence of plants. Element biomass enrichment of Mn, Fe, Ti and Al appeared to be higher in basalt than the rest of the substrates; however, high Al availability limited Ca and Mg uptake and plant growth in this rock media. Presence of mycorrhiza enhanced shoot biomass in rhyolite due to increased P uptake, and increased concentrations and total uptake of lithogenic elements in plants in all rocks but granite. As expected, AM significantly increased plant root concentrations of P, K, Ca, Mn, Fe, Ti, Al in basalt, and Mn shoot concentrations in rhyolite, as well as root total uptake of K, Ca, Mg, Mn, Fe, Na, Ti, Al and Si in basalt. At the same time, AM decreased Ca, Ti and Al concentrations in shoots grown in rhyolite, a possible protection mechanism against Al toxicity. The importance of AM in nutrient uptake is also reinforced by positive correlations between AM infection rate and P, Ca and Mn total uptake across all substrates. Moreover, total mobilization of Ca, Mg and Mn in rhyolite, was significantly higher in the AM versus non-AM treatment, contrary to K, Ca, Mg, Na and Si in schist. Our work demonstrates how mineral weathering and associated nutrient release is promoted by plant processes, further enhanced by plant associated with symbiotic AM, and yet more pronounced in basalt and rhyolite compared to granite and schist.