Carbon balance and determinant processes in semiarid ecosystems of southeast Spain

Abstract

Climate change represents the most important global challenge of the 21st century and global warming is one of the backbones of the climate change paradigm, which is greatly caused by the increase of atmospheric carbon dioxide (CO2). Fossil fuel combustion, industrial activities and land use change have provoked the rise of atmospheric CO2 molar fraction since the pre-Industrial Era to the present, from ~280 ppm to 406 ppm. However, the CO2 growth rate within the atmosphere is half than expected owing to the mitigation role of biosphere, where terrestrial and aquatic ecosystems globally act as carbon sinks. In this regard, terrestrial ecosystems absorb a higher amount of atmospheric CO2 compared to the ocean and also present a higher inter-annual variability that greatly drives the atmospheric CO2 patterns. Therefore, it is necessary to deeply understand the global carbon (C) cycle in order to improve terrestrial C models that will allow us to identify the feedback mechanisms between terrestrial biosphere and global change and also to design proper mitigation and adaptation policies. Nevertheless, besides their great global extension and their demonstrated influence in global C budget, drylands are underrepresented in global long-term monitoring infrastructures designated to directly measure biosphere-atmosphere CO2 exchange. Additionally, the functioning of these water-limited ecosystems differs from the better-known temperate ecosystems, where main limiting factors are light and temperature instead of water. Thus, the principal aim of this thesis has been to investigate the relevant processes involved in the C balance of drylands and especially, of semiarid ecosystems, which correspond to the drylands’ subtype with the largest global extension. Concretely, this thesis is spatially framed in two experimental sites, Amoladeras and Balsa Blanca, located in the Cabo de Gata-Níjar Natural Park (Almeria), in southeast Spain, the most arid region of Europe where ecosystems are subjected to high aridity conditions and degradation processes. For that, we have used the eddy covariance technique, a standardized methodology used worldwide, and other additional methodologies, such as flux chambers and soil CO2 probes, that also allow the measurement of CO2 exchange between terrestrial ecosystems and the atmosphere.