Endomycorrhizae in Miocene paleosols: Implications in biotite weathering and accumulation of dolomite in plant roots (SW Madrid Basin, Spain)

Abstract

This work provides evidence of arbuscular mycorrhizae (or endomycorrizhae) and their role in mineral weathering and soil carbonate production from Miocene paleosols. Formation of the paleosols-bearing mycorrhizae took place in marginal lacustrine and distal-fan deposits in the Madrid Basin. Endomycorrizhae fossils are preserved as carbonate in biotite-filled rhizoliths. Fossilized mycorrhizae are morphologically identical to those of living counterparts and consist of two linked parts. The extraradicular segment surrounding the root is represented by a calcitic mycelium comprising spores and two types of hyphae that are seen to attach and to corrode the mineral grains. The intraradicular part is made of a network of branched filaments, hyphal coils, arbuscules and likely vesicles that are mixed with the cortical root cells of the plants fossilized as dolomicrosparite aggregates. The preferential formation of the dolomite aggregates on biotite grains in rhizoliths is interpreted to reflect a focused interest of the plants on this mineral and indicates the mineral may have been used as a source of plant nutrients. The close association of the carbonate with the silicate further suggests that the precipitation of dolomite in the root cells and the weathering of the biotite resulted from interrelated processes. Main mechanisms to produce mineral dissolution include organic acids and chelate secretions by the plant roots and fungi which would remove nutrients. These results confirm that carbonate paleosols provide an excellent reservoir of information on ecological interactions and biogeochemical cycling, and expand the range of biogenic processes and their resultant products involved in the formation of carbonate soils. This approach has broad applications given the abundance of carbonate paleosols and the endomycorrhiza record dating back 400 million years.