Species Composition And Nitrogen Dynamics Of Biological Soil Crusts In The Mu Us Desert

Nitrogen (N) deficiency severely constrains net primary productivity of desert ecosystems. Biological soil crusts (hereafter BSCs) distribute widely in desert ecosystems, serving as the major N source. Due to changes of top soil properties and microbe communities, BSCs strongly impact N dynamics of top soil, and hence influencing nutrient supply as well as vegetation growth.and development. Therefore, BSCs have theory and practice significance for soil improvement and vegetation rehabilitation in desert ecosytems. From now on, researches in this field are too limited to provide us systematic knowledeges, which restricts understanding of N dynamics in deserts and combating desertification.Given this, we studied community structure, N fixing, mineral N accumulation and N lose of various types of BSCs which widespread in the Mu Us Desert, by field surveys, indoor stimulated experiments and biochemical experiments. Our results show as follows:(1) Diazotrophic community structure makes an important contribution to nitrogenase activity (hereafter NA) of BSCs. From lower stage to higher stage of BSCs, richness and abundance of diazotrophs as well as nitrogenase activity all show a unimodel curve. The diazotrophs, especially cyanobacteria, contribute signifcantly to nitrogenase activity (90.9%). And the contribution varied as temperature and moisture change. The more suitable the moisture content and temperature are, the more contribution the cyanobacteria make. The explanation of cyanobacteria species for NA can reach 96.9% at the optimum conditions. Regression analysis suggests several cyanobacteria species which are sensitive to all environmental conditions, especially temperature, and have highly relative abundance. Cyanobacteria crusts have the highest value estimated N inputs,14.43±1.18 mg N2 m-2 a-1, being meaningful to desert management.(2) From lower stage to higher stage of BSCs, it is increasingly beneficial to N mineralization and mineral N accumulation by soil properties improvement and increasing abundance of microbe. Mineral N of BSCs is positively correlated to water content and temperature, total organic carbon, total N, available N and fractal dimension, abundance of bacteria, actinobacillus and fungi, but show negative correlations with C/N and pH. Mineral N content reach maximum in June-August, given to the most suitable conditions of moisture and temperature as well as the highest soil nutrient contents.(3) From lower stage to higher stage, improvement of soil structure and decreasing alkalinity efficiently inhibit N gas loss. Increases of fractal dimension and porosity of BSCs inhibit N gas loss, and decreases of C/N and pH also inhibit N loss.Our research shows that BSCs have great effects on N dynamics in desert ecosystems. BSCs generally promote available N nutrient accumulation from lower stage to higher stage, providing reference of theory and practice to biological measures of combating desertification. N dynamics in BSCs are profoundly influenced by microbe and cryptogam, whose community structure show great correlations with N fixation function and mineral N accumulation. To clarify biological mechanism of N dymamics of BSCs, further study will employ new biotechnology to detect soil microbial functional groups.