Dryland soil-atmosphere CO2 exchange associated to microclimate and Geochemistry over a biocrusts succession

Abstract

This thesis arose in response to several gaps in the current understanding and modelling of the carbon cycle: (1) in spite of its importance on ecosystem and global CO2 emissions, the soil-atmosphere CO2 flux (Fc) was still not well constrained, and its feedback with climate change and Fc was still uncertain; (2) biological soil crusts (biocrusts) were believed to play a considerable role in the global carbon budget due to their photosynthetic activity but little was known about the spatio-temporal variability of Fc under those communities of microorganisms that cover drylands soils worldwide; (3) a nocturnal soil CO2 uptake has been increasingly reported in those areas but the involved biogeochemical mechanisms remained unclear; (4) recent evidence suggested that liquid water input via water vapor adsorption (WVA) by soil had been overlooked in water-limited ecosystems, though it might represent an important process to take into account in climate-carbon cycle feedback models. This thesis aimed to contribute to improve the available knowledge required to address those issues. To this end, a semi-permanent experiment was designed in the Tabernas Desert (Southeastern Spain). A network of environmental sensors was installed to monitor continuously the microclimate over an ecological succession of biocrusts, including CO2 and water vapor measurements in the topsoil and atmosphere. Those measurements were coupled to a geochemical characterization of the soil and soil water. To assess the role of geochemistry and in particular soil carbonates in Fc dynamics, it was necessary to obtain accurate measurements of parameters used as inputs in geochemical models. Therefore, Chapter 1 presents a methodological advance to determine accurately the carbonate chemistry in the soil solid and aqueous phase: a new low-cost device was developed to quantify the calcium carbonate content and reactive surface area in solid samples as well as the dissolved inorganic carbon content in water samples. Chapter 2 presents two years of continuous measurements of the topsoil CO2 molar fraction (χc) and pedoclimatic variables, including soil water content (ϑw) and soil temperature (Ts). Those data were used to develop statistical spatio-temporal models of the χc dynamics over the biocrusts succession. We found that soil CO2 emissions were more sensitive to ϑw and Ts in late successional stages, and that a future enhancement of soil CO2 emissions is a likely outcome of global warming at this site. Nevertheless, we also found that calcite played a role in mitigating CO2 emissions through the uptake of CO2 by soil at night. Our measurements suggested that CO2 consumption processes were progressively masked by the increase in biological CO2 production during succession. That is probably why those processes could mainly be detected in early successional stages and more generally in drylands, as they sustain a low biological activity. In Chapter 3, water vapor measurements were added to the dataset of Chapter 2 and analyzed in association with CO2 measurements. Our main findings were (1) the occurrence of WVA fluxes during hot and dry periods, and new insights on their underlying mechanisms; (2) a coupling between water vapor and CO2 fluxes, well predicted by our models; and (3) cumulative soil CO2 uptake increasing with specific surface area in early succession stages, thus mitigating CO2 emissions. During summer drought, as WVA was the main water source, it probably maintained ecosystem processes such as microbial activity and mineral reactions. Therefore, at this stage of the thesis, we suggested that WVA could drive the detected nocturnal CO2 uptake. In Chapter 4, we further explored the underlying mechanisms involved in this uptake. To this end, measurements of CO2 and water vapor were combined to analyses of the composition of the soil solution after simulated rain events and subsequent geochemical and statistical modelling. We found strong evidence for the occurrence of a geochemical mechanism of coupled gypsum dissolution-carbonate precipitation due to a common-ion effect, and proposed a pathway for its implication in the nocturnal soil CO2 uptake. The main factor limiting the process in this dryland was water availability, but our observations supported that nocturnal water vapor adsorption by soil might lift this limitation under drought conditions. We also discussed the role of soil dissolved organic carbon on calcite precipitation, and a possible connection with the nitrogen cycle and biomineralizing microorganisms among biocrusts. We suggest that this natural geochemical process has the potential to constitute an active long-term carbon sink because the Ca involved in CaCO3 precipitation came from an exogenic source. In summary, this thesis contributed to improve the understanding and modelling of the soil-atmosphere CO2 exchange in semiarid biocrusted soils, by identifying the environmental variables and potential biogeochemical processes controlling those fluxes. It especially emphasizes the role of overlooked natural processes able to mitigate CO2 emissions. A general discussion is provided at the end of this thesis, which connects the contents of the different chapters together with the current state of knowledge.