Biological soil crusts (biocrusts) form a Bliving skin^ at the soil surface in many low-productivity ecosystems around the world including waterand cold-limited environments, and early-successional seres (Belnap et al. 2003). They may be composed of any configuration of soil surfacedwelling cyanobacteria, eukaryotic algae, lichens, mosses or liverworts, and support assemblages of decomposers and a faunal food web (Belnap et al. 2003). These soil surface communities have global relevance, as it has been recently estimated that they cover about 12% of the terrestrial surface currently (Rodríguez-Caballero et al. 2018). Biocrust communities are perhaps an ideal subject for the journal Plant and Soil, as they are simultaneously plantlike, due to their dominance by photoautotrophs, yet biocrusts are also clearly a physical feature of the soil given that component organisms are enmeshed in, adherent to, or otherwise in direct contact with the soil surface. The activity of the organisms is what engineers the wellaggregated thin layer at the soil surface that we recognize as a biocrust (Belnap et al. 2003). The contributions of biocrusts to ecosystem function has fueled much research interest, initially based on the observation of biocrusts’ soil aggregating and erosion-resisting nature. More recently biocrusts have been identified as a multifunctional, globally-relevant ecosystem element instrumental in: 1. building or otherwise altering soil nutrient stocks through N-fixation (Elbert et al. 2012), dust trapping (Reynolds et al. 2001) and nutrient cycling (Strauss et al. 2012), 2. influencing hydrological properties of soil such as the water balance (Chamizo et al. 2016), and 3. the thermal energy balance of the ecosystem (Coradeau et al. 2016; Rutherford et al. 2017). Biocrust science was sparse before the 1970s, as evidenced by few publications and little recognition of the concept; however, a series of key events in the biocrust research community has brought biocrusts from a niche interest into the mainstream. First, the initial publication in 2001 of an edited volume (Belnap and Lange 2003) served to bring together disparate threads floristic, biogeographical, physiological, functional, applied of biocrust research through an ecological lens. One effect of this publication was to demonstrate that biocrusts were a global phenomenon, and hasten the internationalization of the field from a handful of Plant Soil https://doi.org/10.1007/s11104-018-3735-1

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Biocrusts: the living skin of the earth

Nitrogen pools in soil covered by biological soil crusts of different successional stages in a temperate desert in Central Asia

Arid and semiarid ecosystems make up approximately 41% of Earth’s terrestrial surface and are suggested to regulate the trend and interannual variability of the global terrestrial carbon (C) sink. Biological soil crusts (biocrusts) are common dryland soil surface communities of bryophytes, lichens, and/or cyanobacteria that bind the soil surface together and that may play an important role in regulating the climatic sensitivity of the dryland C cycle. Major uncertainties exist in our understanding of the interacting effects of changing temperature and moisture on CO2 uptake (photosynthesis) and loss (respiration) from biocrust and sub-crust soil, particularly as related to biocrust successional state. Here, we used a mesocosm approach to assess how biocrust successional states related to climate treatments. We subjected bare soil (Bare), early successional lightly pigmented cyanobacterial biocrust (Early), and late successional darkly pigmented moss-lichen biocrust (Late) to either ambient or + 5°C above ambient soil temperature for 84 days. Under ambient temperatures, Late biocrust mesocosms showed frequent net uptake of CO2, whereas Bare soil, Early biocrust, and warmed Late biocrust mesocosms mostly lost CO2 to the atmosphere. The inhibiting effect of warming on CO2 exchange was a result of accelerated drying of biocrust and soil. We used these data to parameterize, via Bayesian methods, a model of ecosystem CO2 fluxes, and evaluated the model with data from an autochamber CO2 system at our field site on the Colorado Plateau in SE Utah. In the context of the field experiment, the data underscore the negative effect of warming on fluxes both biocrust CO2 uptake and loss—which, because biocrusts are a dominant land cover type in this ecosystem, may extend to ecosystem-scale C cycling.

Climatic Sensitivity of Dryland Soil CO2 Fluxes Differs Dramatically with Biological Soil Crust Successional State

Biocrusts mediate a new mechanism for land degradation under a changing climate

Karst rocky desertification (KRD), a land degradation form which is widespread but unique to karst ecosystems, has become an ecological disaster in southwest China Biocrusts play crucial roles in many ecological processes of the degraded ecosystems However, little is known about the effects of biocrusts on soil properties and soil microbial communities in the progression of KRD We sampled soil beneath moss biocrusts and bare soil in four grades of KRD (none, light, moderate, and severe) to compare soil nutrients, soil microbial diversity, community composition, structure, and networks across the range of KRD progression Moss biocrusts had a positive effect on all soil nutrients and buffered the negative effects of KRD progression compared to bare soil Moss biocrusts significantly increased soil microbial richness but had little contribution to diversity and community composition Both soil bacterial and fungal communities were significantly correlated with total and available phosphorus, total potassium, soil temperature, slope, and altitude Soil bacterial and fungal communities showed different sensitivities and strategies in face of environmental degradation in KRD-affected ecosystems Moss biocrust restoration could be used as a supplementary method in promoting ecological restoration in areas undergoing KRD due to their positive effects on soil nutrients and soil microbial richness Our findings filled a knowledge gap pertaining to the microbial ecology of biocrust in regions experiencing KRD

Moss biocrusts buffer the negative effects of karst rocky desertification on soil properties and soil microbial richness

Biological soil crusts (biocrusts) play numerous crucial roles in drylands, which comprise over 40% of Earth’s terrestrial surface. Among these key contributions is the fixation of atmospheric nitrogen. Yet, relatively little is known about the N2 fixation capabilities of different lichen species that are found in late successional biocrust communities across drylands globally. In order to improve our species-specific understanding of biocrust lichen N2 fixation, we collected biocrusts dominated by four common species of lichens – Collema spp., Gyalolechia desertorum, Psora decipiens, and Squamarina lentigera – that represent a range of lichen families and morphological types. Nitrogenase activity of the biocrust community dominated by these lichens was evaluated using the acetylene reduction assay. Additionally, biocrust community composition was assessed using the point-intercept method along transects at varied distances from exposed bedrock. As expected, Collema spp.-dominated biocrusts had the highest rates of nitrogenase activity, with rates up to seven times larger than those of the other three target species. Nitrogen concentrations and carbon:nitrogen ratios of lichen tissue differed among lichen species. However, when the composite biocrust profile was assessed (i.e., biocrust tissue, microbial cells, and mineral soil to a 2 cm depth) these among-species differences in total nitrogen disappeared. Community composition changed according to distance from exposed bedrock, with a higher diversity of lichens closer to the bedrock. Multiple drivers, including climate and land use change, affect biocrust community composition and species-specific functional information, even within a group such as late successional biocrusts, could help in forecasting the potential effects of global change on N2 fixation, and consequently, soil fertility in drylands.

Species-specific nitrogenase activity in lichen-dominated biological soil crusts from the Colorado Plateau, USA

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Species-specific nitrogenase activity in lichen-dominated biological soil crusts from the Colorado Plateau, USA