Photosynthesis of Soil-Crust Biota as Dependent on Environmental Factors
Limited productivity of vascular plant communities in semiarid and arid regions generally results in low soil organic carbon content. In these situations, crust biota can be the most important autotrophic contributors of fixed carbon, delivering this carb
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18.1 Introduction Limited productivity of vascular plant communities in semiarid and arid regions generally results in low soil organic carbon content. In these situations, crust biota can be the most important autotrophic contributors of fixed carbon, delivering this carbon to the soil ecosystem by leaching and decaying processes. The photosynthetic carbon assimilation of crustal organisms thus plays an important role by contributing to the humus reservoir of the soil, supporting heterotrophic soil life, and supplying nutrients for the phanerogamous vegetation (see Beymer and Klopatek 1991 and Chaps. 19–21). The microenvironment in which the soil-crust biota are living – the uppermost millimeters of the topsoil of desert and steppe formations – is one of the most extreme habitats where autotrophic organisms are found. They are exposed to high and low temperatures, long-lasting and extensive desiccation, and excessive sun radiation. Soil-crust organisms must have high resistance to extreme temperatures, large water loss, and extensive light stress (see Chap. 1). Their photosynthetic and respiratory activity is restricted to short periods of hydration. Therefore, another requirement for their existence is the capability to begin metabolism as soon as moisture becomes available, and to make use of even very small amounts of water for photosynthetic carbon gain. Only a small subset of species of cyanobacteria, algae, lichens, and bryophytes have developed the necessary adaptations for existence in the soilcrust environment. Evidently, all of them are poikilohydrous in nature. Ecophysiological work which analyzes the special functional features of soil-crust biota existence and their photosynthetic production is rare. We are far from a complete understanding of their characteristics, and the existing literature covers only a minimal number of the different soil-crust types, organisms, geographical regions, and climatic situations. Most of the published work reports on laboratory measurements. Photosynthetic performance of total soil-crust samples, containing a variety of different organisms, has been studied with material from the US (Beymer and Klopatek 1991; Ecological Studies, Vol. 150 J. Belnap and O.L. Lange (eds.) Biological Soil Crusts: Structure, Function, and Management © Springer-Verlag Berlin Heidelberg 2001
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Jeffries et al. 1993a,b; Garcia-Pichel and Belnap 1996), from savannas in Venezuela (San José and Bravo 1991), from sand dunes in The Netherlands (De Winder 1990), and from Israel (Lange et al. 1992). Ecophysiology of different lichen species growing in the soil-crust environment was investigated with material from Israel (Lange et al. 1993), central Europe (Lange et al. 1995), and from the US (Lange et al. 1997a, 1998). Field measurements of CO2 exchange in soil-crust organisms under natural conditions are known from Israel (Lange et al. 1970), Namibia (Lange et al. 1994b), from central European habitats (Lange et al. 1997c; Lange 2000a,b), and from the United States (Phillips and Bel
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