Divergence in physiological responses between cyanobacterial and lichen crusts to a gradient of simulated nitrogen deposition

Nitrogen (N) deposition in arid lands is known to be increasing. However, N deposition gradients have unclear effects on physiological characteristics of biocrusts. This study tested if physiological characteristics are stimulated by low levels of N deposition and reduced by high levels of N deposition. We simulated N deposition at various rates to plots of cyanobacterial and lichen biocrusts in the Gurbantunggut Desert and measured indicators of growth and stress. In cyanobacterial crusts, most evidence suggests that biomass and growth are unaffected by lower levels of N but suppressed at the highest level. Biomass and growth of lichen crusts were less sensitive to N addition but, in the case of actual photochemical efficiency, were also suppressed at the highest N addition rate. Most osmotic adjustment substances of cyanobacterial and lichen crusts did not show significant responses after N addition. In the two crusts, minor increase in antioxidative enzyme activity was found in some N addition rates but no similar trends between the crust types were observed. Physiological performance of cyanobacterial crusts was more sensitive to high levels of N addition than that of lichen crusts. Increasing N deposition might greatly affect cyanobacterial crusts before lichen crusts.