Abstract
On million-year timescales, carbonate rock weathering exerts no net effect on atmospheric CO2 concentration. However, on timescales of decades-to-centuries, it can contribute to sequestration of anthropogenic CO2 and increase land-ocean alkalinity flux, counteracting ocean acidification. Historical evidence indicates this flux is sensitive to land use change, and recent experimental evidence suggests that trees and their associated soil microbial communities are major drivers of continental mineral weathering. Here, we review key physical and chemical mechanisms by which the symbiotic mycorrhizal fungi of forest tree roots potentially enhance carbonate rock weathering. Evidence from our ongoing field study at the UK's national pinetum confirms increased weathering of carbonate rocks by a wide range of gymnosperm and angiosperm tree species that form arbuscular (AM) or ectomycorrhizal (EM) fungal partnerships. We demonstrate that calcite-containing rock grains under EM tree species weather significantly faster than those under AM trees, an effect linked to greater soil acidification by EM trees. Weathering and corresponding alkalinity export are likely to increase with rising atmospheric CO2 and associated climate change. Our analyses suggest that strategic planting of fast-growing EM angiosperm taxa on calcite- and dolomite-rich terrain might accelerate the transient sink for atmospheric CO2 and slow rates of ocean acidification.
Highlights
Fossil fuel burning and deforestation have increased the global atmospheric CO2 concentration by ca. 40% over the past century, an increase of approximately 120 ppm (IPCC 2014)
The breakdown of silicate and carbonate rocks are generally regarded as slow processes, recent observational evidence (Gislason et al 2009) and regional-scale modelling (Beaulieu et al 2010, 2012; Goll et al 2014) indicate that CO2-driven climate change may impact on weathering over timescales as short as decades
This review shows that there are plausible physical and chemical mechanisms involving mycorrhizal fungi that could accelerate carbonate rock weathering within forest soils
Summary
Fossil fuel burning and deforestation have increased the global atmospheric CO2 concentration by ca. 40% over the past century, an increase of approximately 120 ppm (IPCC 2014). Whether these species achieve tolerance through increasing mycorrhizosphere acidification and carbonate weathering, is uncertain Since forests and their associated mycorrhizal fungi are key drivers of mineral weathering (Landeweert et al 2001; Taylor et al 2009), any changes in tree growth and associated soil communities are likely to result in changes to weathering rates, this is poorly documented and should be an area for future forestry research. This latter assumption was made as in the field soil CO2, carbonic acid and organic acid exudates all contribute to the overall, acidic, soil pH and inclusion of an additional k2[H2CO3]s may result in an overestimation of theoretical weathering rate In this study, these soil acidifying agents have not been separated, as their relative importance is likely to vary seasonally and in response to climatic conditions, changing soil hydrology and biological activity. Petrological examinations show chalk, dolomite, marble and limestone have a (a) −6
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