Impact of different nitrogen additions on microbes and exopolysaccharides excretion in cyanobacterial biocrusts

Abstract Background Recently, it has been found that nitrogen (N) deposition could strongly affect the spatial pattern of biocrusts by reducing their cover. However, as the key cementing materials in the formation and stabilization of biocrusts, little has been known about the response of exopolysaccharides (EPS) to N deposition. Method Three N sources nitrate nitrogen (NN), ammonia nitrogen (AN), urea (UN) with three gradients (2 mg/g, 4 mg/g, 8 mg/g) were set to evaluate the effects of N addition on the microbial community and EPS excretion in laboratory. Results Our results showed the addition of AN and UN (2-4 mg/g) strongly decreased the cyanobacterial biomass, indicated by chlorophyll-a and16s rDNA gene copy-number. The results also suggested that although medium and high NN (4-8 mg/g) inhibited the growth of Microcoleus vaginatus , they promoted Cyanobacteria growth. Additionally, high-throughput sequencing results showed N addition increased the α-biodiversity of biocrusts, and microbial community shifted from more Cyanobacteria to more Proteobacteria and Actinobacteria, especially driven by AN and UN. Notably, EPS in biocrusts was significantly reduced after high-N addition, and the co-reduction of cyanobacterial biomass and EPS would undoubtedly affect the stabilization and persistence of early-stage biocrusts. Meanwhile, the reduced proportion of Rhamnose and Fucose in EPSs may further reduce the adhesion of EPS to soil. Conclusion These findings improve our understanding of biocrusts' responses to N deposition. Considering the importance of cyanobacteria and EPS in biocrusts, cyanobacterial biocrust coverage may face more serious challenges, in the context of continuous increasing N deposition in drylands.