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 (χ_s) 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 χ_s 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 (T_s) and interactions between those variables explained and predicted efficiently the χ_s dynamics. Among those factors, the effect of ϑ was preponderant and dependent on T_s 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.