Estimating impacts of lichens and bryophytes on global biogeochemical cycles

Abstract Lichens and bryophytes may significantly affect global biogeochemical cycles by fixation of nitrogen and biotic enhancement of surface weathering rates. Most of the studies suggesting these effects, however, are either conceptual or rely on upscaling of regional estimates to obtain global numbers. Here we use a different method, based on estimates of net carbon uptake, to quantify the impacts of lichens and bryophytes on biogeochemical cycles at the global scale. We focus on three processes, namely, nitrogen fixation, phosphorus uptake, and chemical weathering. Our estimates have the form of potential rates, which means that we quantify the amount of nitrogen and phosphorus needed by the organisms to build up biomass, also accounting for resorption and leaching of nutrients. Subsequently, we use potential phosphorus uptake on bare ground to estimate chemical weathering by the organisms, assuming that they release weathering agents to obtain phosphorus. The predicted requirement for nitrogen ranges from 3.5 to 34 Tgyr−1 and for phosphorus it ranges from 0.46 to 4.6 Tgyr−1. Estimates of chemical weathering are between 0.058 and 1.1 km3 yr−1 of rock. These values seem to have a realistic order of magnitude, and they support the notion that lichens and bryophytes have the potential to play an important role for biogeochemical cycles.