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Biological soil crusts, formed by an association of soil particles with cyanobacteria, lichens, mosses, fungi and bacteria in varying proportions, live in or directly on top of the uppermost soil layer. To evaluate their role in the global carbon cycle, gas exchange measurements were conducted under controlled conditions. Moss-dominated soil crusts were first analyzed as moss tufts on soil, then the mosses were removed and the soil was analyzed separately to obtain the physiological response of both soil and individual moss stems. Net photosynthetic response of moss stems and complete crusts was decreased by insufficient and excess amounts of water, resulting in optimum curves with similar ranges of optimum water content. Light saturation of both sample types occurred at high irradiance, but moss stems reached light compensation and saturation points at lower values. Optimum temperatures of moss stems ranged between 22 and 27A degrees C, whereas complete crusts reached similar net photosynthesis between 7 and 27A degrees C. Under optimum conditions, moss stems reached higher net photosynthesis (4.0 vs. 2.8 mu mol m(-2) s(-1)) and lower dark respiration rates (-0.9 vs. -2.4 mu mol m(-2) s(-1)). Respiration rates of soil without moss stems were high (up to -2.0 mu mol m(-2) s(-1)) causing by far lower absolute values of NP/DR ratios of soil crusts as compared to moss stems. In carbon balances, it therefore has to be clearly distinguished between measurements of soil crust components versus complete crusts. High rates of soil respiration may be caused by leaching of mosses, creating high-nutrient microsites that favor microorganism growth.