Microbiotic crusts on soil, rock and plants: neglected major players in the global cycles of carbon and nitrogen?

Microbiotic crusts consisting of bacteria, fungi, algae, lichens, and bryophytes colonize most terrestrial surfaces, and they are able to fix carbon and nitrogen from the atmosphere. Here we show that microbiotic crusts are likely to play major roles in the global biogeochemical cycles of carbon and nitrogen, and we suggest that they should be further characterized and taken into account in studies and models of the Earth system and climate. For the global annual net uptake of carbon by microbiotic crusts we present a first estimate of ~3.6 Pg a−1. This uptake corresponds to ~6% of the estimated global net carbon uptake by terrestrial vegetation (net primary production, NPP: ~60 Pg a−1), and it is of the same magnitude as the global annual carbon turnover due to biomass burning. The estimated rate of nitrogen fixation by microbiotic crusts (~45 Tg a−1) amounts to ~40% of the global estimate of biological nitrogen fixation (107 Tg a−1). With regard to Earth system dynamics and global change, the large contribution of microbiotic crusts to nitrogen fixation is likely to be important also for the sequestration of CO2 by terrestrial plants (CO2 fertilization), because the latter is constrained by the availability of fixed nitrogen.