Effects of rainfall manipulations on carbon exchange of cyanobacteria and moss-dominated biological soil crusts

Biological soil crusts (biocrusts) are a vital biotic component of dryland ecosystems that impact global nitrogen (N) and carbon (C) cycling. Water availability is the main controller of physiological function of biocrusts in these ecosystems, yet few studies have compared the carbon balance of different biocrusts responding to different water treatments. In this study, we explored the effects of water availability on carbon exchanges of different biocrusts by measuring in situ net CO₂ fluxes in cyanobacteria and moss-dominated biocrusts under four water treatments (2 mm, 5 mm, 10 mm, and 20 mm) in a fenced grassland on the Loess Plateau, China. Our results indicate that water availability played a vital role in carbon balance of the biocrusts. Under higher water additions, both cyanobacteria and moss-dominated biocrusts fixed carbon but lower water additions did not always lead to net carbon gains, especially for the moss-dominated crusts. Importantly, our data indicate the existence of water thresholds for net carbon fixation of both cyanobacteria and moss-dominated biocrusts, below which carbon loss occurs. This implies that higher water availability would benefit moss growth, while lower water availability results in suitable circumstance for the growth of cyanobacteria.