Relevance of cryptogamic communities in Earth system processes under the impact of global change

Cryptogamic communities (CC) are assemblages of photoautotrophic non-vascular organisms, such as cyanobacteria, algae, lichens, and bryophytes, being accompanied by heterotrophic bacteria, microfungi, and archaea. They colonize almost all substrates on Earth, including soil, rocks, and vegetation. CC are particularly relevant in drylands, where they cover vast regions as biological soil crusts (biocrusts) colonizing the uppermost centimeters of the soil. Biocrusts perform highly relevant ecosystem processes, as they fertilize the soil by fixing carbon and nitrogen, balance water cycling, influence plant germination and growth, and effectively stabilize dryland soils.

Knowledge of the composition and distribution patterns of CC across spatial scales serves as a prerequisite to investigate their functional roles in regional and global Earth system processes. Thus, together with my research team, I investigated the microbial composition of biocrust types, defined by the visible photoautotrophic component (cyanobacteria, lichens, bryophytes), and discovered that these were characterized by distinct microbial communities that in turn impacted physiological biocrust functioning. For biocrust mapping, we established a deep learning-based classification technique, which, besides identification, also allows neighborhood and growth analyses. For biocrust identification at the regional scale, I developed one of the first high-resolution remote sensing algorithms utilizing hyperspectral imagery.

During my research, I investigated several functional roles of CC. In an international research team, we identified biocrusts as key nitrogen (N) fixers with an annual fixation of ~11.5 Tg, corresponding to ~18% of the overall N fixation in natural biomes. During N cycling in CC after fixation, we were the first to show major releases of reactive gases NO and HONO. These emissions were linked to precipitation, but heterogeneous microscale mechanisms limited process understanding. To address this, we now developed a controlled in-situ setup and applied a mechanistic model to clarify the underlying processes.

Applying a modeling approach, we developed a first global biocrust map, revealing that biocrusts cover ~18 * 10⁶ km², corresponding to about 1/3 of the global dryland area. Combining this map with data on the soil-stabilizing role of biocrusts, we assessed biocrust relevance in global dust cycling. Our results revealed that biocrusts reduce dryland dust emission and cycling by 60%, thus preventing the release of ~0.7 Pg of dust per year. This study was the first assessing global biocrust relevance and including this in Earth system models.

Although biocrusts thrive under extreme environmental conditions, our studies univocally demonstrate their high sensitivity towards global change. Utilizing our deep learning-method on a 16-year biocrust monitoring project on the Colorado Plateau, we observed a decrease in biocrust coverage by ~40%, with lichens and bryophytes reacting particularly sensitive to extended drought events. Similarly, biocrust coverage on the global scale will decrease by 16-39% until the year 2070 according to our mapping and modeling approach.

In summary, my research reveals that CC play key roles in biogeochemical processes but are also vulnerable by global change, which need to be considered both in ecosystem management and Earth system models to fully reflect their role and effectively meet future challenges of the Anthropocene.