BSCs are communities of living organisms on the soil surface in arid and semi-arid ecosystems, comprising up to 70 % of the surface in some areas. Although BSCs may represent the main resource for soil organic carbon in arid and semi-arid ecosystems, their role as a carbon source or sink has been little studied. The EU-funded project BIOSOC (Contribution of biological soil crusts to the carbon balance in drylands) investigated the role of BSCs at different stages of development in the carbon balance of arid and semi-arid zones. Researchers monitored in situ CO2 fluxes in BSCs at different successional stages in a representative catchment and related them to key environmental factors controlling the CO2 exchange rates. Researchers measured air temperature and relative humidity just above the biocrusts as well as soil moisture, photosynthetically active radiation and rainfall. They also mapped the distribution of different BSC types at different successional stages using novel techniques adapted for covering large areas. The BSC carbon balance in a small catchment was measured by combining the CO2 flux function from the different crust types with their spatial coverage. Results show that biocrusts behave as important CO2 sinks during certain periods in drylands due to their physiological characteristics. This enables them to be active and to photosynthesise at very low humidity, offering a photosynthetic advantage compared to higher plants. Hence, water availability is a decisive variable activating both photosynthesis and respiration. Findings also revealed the important role of microbial communities in soils below BSCs in controlling CO2 emission fluxes. BIOSOC research will improve knowledge of the carbon balance in Mediterranean ecosystems and the important role that BSCs may have as carbon sinks. The results of this work form the basis for a tool for the regional classification of stages of development of BSCs. This can be used as a soil degradation indicator, as well as for generating a model of the carbon balance in crusted soils in larger Mediterranean areas.
Biological soil crusts, dryland, arid, soil organic carbon, BIOSOC, Mediterranean