Simulated climate change reduced the capacity of lichen-dominated biocrusts to act as carbon sinks in two semi-arid Mediterranean ecosystems

The importance of biological soil crusts (biocrusts) for the biogeochemistry of drylands
is widely recognized. However, there are significant gaps in our knowledge about how
climate change will affect these organisms and the processes depending on them. We
conducted a manipulative full factorial experiment in two representative dryland
ecosystems from central (Aranjuez) and southeastern (Sorbas) Spain to evaluate how
precipitation, temperature and biocrust cover affected the assimilation and net C
balance of biocrusts. Chlorophyll fluorescence, net photosynthesis and dark respiration
were measured in situ bimonthly during a year. We also conducted daily cycle
measurements of net photosynthesis in winter and at the end of spring. In Sorbas,
warming reduced the fixation of atmospheric C in biocrust-dominated microsites
throughout the year. In Aranjuez, there was an interaction between the three factors
evaluated; during winter, net photosynthesis was significantly greater in high biocrust
cover plots under natural conditions and in the rainfall exclusion treatment. During the
daily surveys, rainfall exclusion and warming reduced C fixation in Sorbas and in
Aranjuez respectively. The effects of the treatments evaluated varied with the rainfall
and non-rainfall water inputs (NRWIs) registered before the measurements. Our results
suggest that changes in NRWI regimes as consequence of warming could have a
greater impact on the C balance of biocrusts than changes in rainfall amounts. They
also indicate that climate change may reduce the photosynthetic ability of lichens, with a consequent reduction of their dominance in biocrust communities at the mid to long term. This could reduce the ability of dryland ecosystems to fix atmospheric C.