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

Background Recently, it has been found that nitrogen (N) deposition strongly affects the coverage of biocrusts. However, we know little about the response of exopolysaccharides (EPSs), the key cementing material in the formation and stability of biocrusts, to N deposition. Method Three N-sources including nitrate, ammonia and urea were added to biocrusts at three rates (2 mg/g, 4 mg/g, 8 mg/g) to evaluate the effect of N additions on the growth of biocrusts and the abundance of EPS. Results Our results showed 2 mg/g of nitrate–N had no obvious effect on the cyanobacterial biomass, while 4 and 8 mg/g of nitrate–N inhibited the growth of Microcoleus vaginatus, the dominant cyanobacterium in biocrusts, but promoted other cyanobacteria growth. Ammonia-N and urea-N strongly decreased the cyanobacterial biomass, indicated by chlorophyll-a and 16 s rRNA gene copy-numbers. On the whole, N additions had a positive impact on the α-biodiversity of biocrusts. However, Ammonia-N and urea-N shifted the bacterial communities from more Cyanobacteria to more Proteobacteria and Actinobacteria. Notably, lesser-N (2 mg/g) promoted the excretion of EPSs, while greater-N (8 mg/g) had the opposite effect, and the total proportion of rhamnose and fucose in EPSs decreased in all treatment groups. Conclusion N additions (except 2 mg/g of nitrate–N) reduced cyanobacterial biomass and affected the bacterial communities in biocrusts, which would obstruct the development and succession of biocrusts. Meanwhile, the simultaneous reductions of the EPSs contents and proportion of rhamnose and fucose in EPSs may further reduce stability and persistence of cyanobacterial biocrusts, after N additions.