Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux

It has long been suspected that lichens increase the rate of physical and chemical weathering of rock surfaces, thus playing a role in biogeochemical cycles. However, the relative weathering flux of elements from lichen-colonized rock versus bare rock has been minimally studied; previous attempts to quantify the effect of lichen-cover on weathering have focused disproportionately on evidence from altered weathering rinds on basalt. Here, in a field experiment on hornblende granite in New Jersey (USA), we measured the cumulative waterborne net efflux of five elements over 31 days and six rain events, from multiple constructed miniature watersheds consisting of either lichen-covered or exposed bare rock. On average, lichen-covered watersheds showed approximately double the silicon flux, and three times the calcium and magnesium flux compared to bare-rock. Additionally, efflux of these elements was higher in lichen-covered watersheds across all six rain events. This suggests that lichens do indeed promote increased chemical weathering compared to bare rock, thus likely increasing sequestration of atmospheric CO2 under equal conditions of atmospheric pCO2, temperature, and rainfall. It was also observed that lichen-covered watersheds showed a 50% reduction in iron flux, and had a greater ratio of calcium and magnesium to silicon flux compared to bare-rock watersheds. The possible causes of these patterns are discussed.