Light, as the necessary condition for the carbon fixation of cryptogams, is the driving energy of the earth’s temperature change. Either seasonal or diurnal carbon exchange at global scale influenced by temperature is actually driven by solar irradiation. Since photosynthesis of most plants mainly occurs in daytime with abundant sunlight, previous studies paid more attention to the effects of temperature and water rather than light5, 18, 47, thus the influence of light on carbon exchange of biocrusts is also poorly known.

NRW accumulation occurs in the period from nightfall to early morning, with variable temperature, Led Bulb Light intensity and NRW amount, and this is an ideal natural opportunity to study the interactions between biocrusts carbon exchange and water, temperature and light. Therefore in this study, we selected 4 types of biocrusts that are highly dominated by variant cryptogams and possess different carbon assimilation mechanisms, namely 2 cyanobacteria crusts (AC1 and AC2), 1 lichen crust with cyanobacteria as photobiont (LC1), the other with green algae as photobiont (LC2) and 1 moss crust (MC). In order to obtain intrinsic potentiality and adaptation patterns of biocrustal carbon exchange responding to temperature, light and water, various light intensities and temperature conditions that were similar to the nights with abundant NRW, as well as two moisture gradients of saturated water (SW) and equivalent NRW (E-NRW) were firstly designed. The respiration rate (R), net photosynthesis rate (Pn), gross photosynthesis (Pg), and light compensation points (LCPs) of different biocrusts were measured, and the pattern of CO2 exchange were synchronously analyzed by multivariate nonlinear regression. The ratios of respiration to gross photosynthesis (R/Pg), which could reflect the biological differences of various biocrust types utilizing water, were also explored. Then, by in situ mesocosm experiments, CO2 exchanges of all chosen biocrusts were analyzed in NRW abundant seasons, the crucial factors and the patterns of CO2 exchange were determined by structural equation models, and the influencing degrees were quantifiably perceived by path coefficients. Finally, the community changes of biocrusts under the background of global climate change were predicted, and the suggestions for subsequent relative studies were proposed.