|Other Abstract||Biological soil crusts (BSCs; complex communities of cyanobacteria, algae, bryophytes, lichens and fungi aggregated with soil particles), the same with vascular plants coverage, are a common and widespread characteristic of soils in arid and semiarid climates throughout the world. As a particularly important component of desert ecosystem and landscape, BSCs widely distributed in many arid and semi-arid regions. They affect physical process, hydrology, soil erosion and nutrient cycle, thus affecting the adiacent vascular plants. Despite the importance of biocrusts in controlling water and nutrients, experimental evidence of their effects on vascular plant species is scarce and contradictory. In Gurbantunggut Desert, the largest fixed and semi-fixed desert in China, the previous relative research mainly focused on the seed germination and seeding growth. However, there are still many fields belong to the blank. In the present research, we attempted to experimentally investigate whether and how biocrusts affect desert vascular plant growth and physiological characteristics during different growing periods, nitrogen transformation and plant community structure in the Gurbantunggut Desert, looking forward to provide important scientific basis on the potential vegetation succession direction in this desert ecosystem. Three desert plant species, including Erodium oxyrrhynchum, Alyssum linifolium, and Hyalea pulchella.
The effect of lichen-dominated BSCs on the growth of three desert vascular plants. The results showed that the effects of BSCs on the growth of three desert vascular plants were different during their whole growing period. The biomass and average leaf area of the three species in crusted soils were found to be higher than those in uncrusted soils in the early growth period and lower in the later part of the growing period. S/R of E. oxyrrhynchum and A. linifolium increased in the early growth period and decreased in the later part of the growing period. While BSCs did not significantly affect allometric scaling relationship, and all of the allometric relationship were isometric. The existence of BSCs affected soil moisture and nutrient content. Soil nutrient was the crucial limiting factor for plant growth in the early growing period, especially the available phosphorus and available potassium. However, water, instead of nutrient, was the crucial limiting factor in thelater growing period.
The effect of lichen-dominated BSCs on the physiological characteristics of three desert vascular plants. The physiological response of three desert vascular plants to BSCs were identical. The results showed that leaf water content, photosynthesis(Pn),Stomatal conductance (Gs), transpiration rate (E), water use efficiency (WUE), PS Ⅱ photochemical efficiency(Fv/Fm), PS Ⅱ quantum yield(φPSⅡ)and chlorophyll content of the three species in crusted soils were found to be higher than those in uncrusted soils in the early growth period and lower in the later part of the growing period. Soluble sugar and proline contents and antioxidant enzyme activities were always higher in crusted soils than those in uncrusted soils. In contrast, soluble protein contents were always lower. The factors which affect the growth could also affect physiological process.
The effect of lichen-dominated BSCs on 15N translocation and element uptake. In this experiment, the habitat-typical plant, E. oxyrrhynchum was chosen to evaluate the ecological roles of BSCs on N (including 15N-Glu, 15N-NH4Cl, and 15N-NaNO3) translocation from soil to plant in a temperate desert in China. The results showed that increased distance from the substrate application point led to a reduction in the concentration of δ15N in both E. oxyrrhynchum and in the soil. The concentration and uptake of three types of N isotope in soils and plants were different between crusted soils and bare soil. Soil covered by BSCs also accumulated considerably more 15N-Glu than bare soil; 15N-NH4Cl and 15N-NaNO3 served as key N source and dominated the allocation in bare soil. In two sites, the concentration of dissolved organic N (DON) (15N-Glu) in E. oxyrrhynchum was higher than that of dissolved inorganic N (DIN) (15N-NH4Cl and 15N-NaNO3). Plants of E. oxyrrhynchum growing in BSC covered plots accumulated more δ15N than those growing in bare soil. Our study supports the hypothesis that BSC facilitate 15N allocation in soil and vascular plants in a temperate desert of Northwestern China. BSCs also influenced nutrient uptake by plants, especially promoting uptake of N and K, while did not affect the uptake of P and C. The influence on uptake of other elements(K、Ca、Mg、Na、Cl) was species-specific.
The effect of lichen-dominated BSCs on plant community structure. BSCs may be important in changing desert plant diversity. Vegetation density decreased obviously after the removal of BSCs. The existence of BSCs did not significantly influence plant species. Plant species increased only 1 after the removal of BSCs. The dominant species in BSCs site was E. oxyrrhynchum. However, the density of E. oxyrrhynchum decreased a lot after the removal of BSCs, and Alyssum linifolium became the dominant species. Community biomass and vegetation coverage in BSCs site were significantly higher than those in BSCs-removed site. In contrast, compared with BSCs-removed site, Shannon Wiener diversity index (H '), species richness (R) and vegetation height were significantly lower in BSCs site.|