Biophysical and geochemical processes control antagonistically the soil-atmosphere CO2 exchange during biocrust ecological succession in the Tabernas Desert

Biological soil crusts (biocrusts) have been reported to play a considerable role in the global carbon budget through CO2 uptake by photosynthesis. However, it is still unclear if ecosystems dominated by biocrusts are net carbon sinks. That is mainly because so far, most research have focused on characterizing photosynthesis ex-situ, neglecting the underlying soil component, and particularly the in-situ spatio-temporal variability of soil CO2 fluxes, which can be substantial. Moreover, it is still unknown how those CO2 fluxes evolve during the ecological succession of biocrusts and which are the biophysical and geochemical factors that control them. Therefore, this research aimed to (1) identify those factors and (2) describe and explain the evolution of annual cumulative soil CO2 fluxes over ecological succession in a dryland.To this end, we conducted continuous measurements over 2 years of the topsoil CO2 molar fraction (χs) in association with below- and aboveground microclimatic variables in 21 locations representative of the ecological succession of biocrusts, characterized by 5 stages: (1) physical depositional crust; (2) incipient cyanobacteria; (3) mature cyanobacteria; (4) lichen community dominated by Squamarina lentigera and Diploschistes diacapsis and (5) lichen community of Lepraria isidiata. Those measurements were also conducted under plants (Macrochloa tenacissima, Salsola genistoides, and Lygeum spartum). Using spatio-temporal statistics, an explanatory model of χs dynamics was calibrated on the first year of data and cross-validated to test prediction on the second year. An explanatory model of annual cumulative fluxes was also developed.The biocrust type, soil water content (ϑ) and temperature (Ts) and interactions between those variables explained and predicted efficiently the χs dynamics. Among those factors, the effect of ϑ was preponderant and dependent on Ts and antecedent soil moisture conditions. The magnitude of the ϑ effect tended to increase in late successional stages, producing greater CO2 emissions, most likely as a result of progressive soil organic carbon accumulation resulting in greater substrate availability for microbial respiration, and higher porosity enhancing CO2 diffusion. The calcite content (and potentially indirectly the pH through a buffering effect of CaCO3) also played a role in explaining annual cumulative CO2 fluxes. Those fluxes were particularly mitigated where CaCO3 was abundant, apparently due to a substantial nocturnal uptake of atmospheric CO2 by soil (influx) throughout the study. The cumulative annual influx represented up to 115% of the cumulative annual efflux, generating a net annual carbon uptake by soil in some locations. Influxes have been increasingly reported recently from drylands soils, which are now regarded as potential carbon sinks. Those influxes have been attributed to different abiotic processes which are still debated. In this ecosystem, in the light of our observations, we assume that a geochemical process of CO2 dissolution in soil water followed by CaCO3 dissolution that consumes CO2 might be involved. If this assumption could be verified, this geochemical process consuming CO2 would need to be separated from biocrust photosynthesis and respiration, when measuring soil surface CO2 fluxes, to not overestimate and underestimate respectively the biotic contribution to the global carbon budget.