Abstract
Trees and their associated rhizosphere organisms play a major role in mineral weathering driving calcium fluxes from the continents to the oceans that ultimately control long-term atmospheric CO2 and climate through the geochemical carbon cycle. Photosynthate allocation to tree roots and their mycorrhizal fungi is hypothesized to fuel the active secretion of protons and organic chelators that enhance calcium dissolution at fungal-mineral interfaces. This was tested using 14CO2 supplied to shoots of Pinus sylvestris ectomycorrhizal with the widespread fungus Paxillus involutus in monoxenic microcosms, revealing preferential allocation by the fungus of plant photoassimilate to weather grains of limestone and silicates each with a combined calcium and magnesium content of over 10 wt.%. Hyphae had acidic surfaces and linear accumulation of weathered calcium with secreted oxalate, increasing significantly in sequence: quartz, granite < basalt, olivine, limestone < gabbro. These findings confirmed the role of mineral-specific oxalate exudation in ectomycorrhizal weathering to dissolve calcium bearing minerals, thus contributing to the geochemical carbon cycle.
Highlights
Of growth-limiting nutrients, in particular phosphorus, from the soil environment thereby enhancing ecosystem productivity[11,12,13]
Weathering of biotite and apatite in microcosms by P. involutus in symbiosis with host trees has revealed that significant chemical and physical alteration of minerals can take place in conditions where the minerals are not immersed in water[22,33,34,35]
The aim of the present study was to test the hypotheses that (i) ectomycorrhizal weathering is driven by fungal allocation of photoassimilate received from a host plant, (ii) the release of calcium from minerals, and its accumulation as a weathering product by a symbiotic fungal partner, is linked to mineral-specific rates of fungal secretion of oxalic acid and the formation of calcium oxalate
Summary
Of growth-limiting nutrients, in particular phosphorus, from the soil environment thereby enhancing ecosystem productivity[11,12,13]. The aim of the present study was to test the hypotheses that (i) ectomycorrhizal weathering is driven by fungal allocation of photoassimilate received from a host plant (in this case, Pinus sylvestris), (ii) the release of calcium from minerals, and its accumulation as a weathering product by a symbiotic fungal partner (in this case, P. involutus), is linked to mineral-specific rates of fungal secretion of oxalic acid and the formation of calcium oxalate To address these hyphotheses, monoxenic microcosms were designed with mineral weathering arenas, containing grains of a range of important carbonate and silicate rocks and minerals (limestone, basalt, gabbro, granite, olivine, microcline) of various calcium and phosphorus concentrations and a control mineral (quartz, 0.01% CaO, 0.0006% P2O5). Quantification of calcium oxalate accumulation on symbiotic ectomycorrhizal fungal hyphae revealed the extent to which calcium mass transfer, resulting from plant-drived fungal weathering is linked to oxalate secretion
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