Annual and seasonal dynamic of carbon sequestration in a Patagonian steppe

Abstract

Arid and semiarid ecosystems comprise approximately 40 % of the global terrestrial surface and play an important role in the carbon-climate system. However, despite their large geographic extension in South America, they are largely under-represented in studies of ecosystem carbon fluxes. Eddy-covariance measurements of net ecosystem carbon exchange (NEE) were carried out in a Patagonian steppe co-dominated by grasses and shrubs in southern Argentina, for four years, including relatively dry and wet years. We evaluated the seasonal and annual variation of NEE, gross primary productivity (GPP) and ecosystem respiration (R_eco), and their environmental controls. This ecosystem exhibits large seasonal fluctuations in global radiation (Rg), air temperature, vapor pressure deficit, soil moisture and leaf area index (LAI). This steppe was a net carbon sink with a mean annual cumulative NEE of -249 g C m⁻² yr⁻¹. The strength as carbon sink was higher (-283.9 g C m⁻² yr⁻¹) in the wet year. NEE was tightly coupled to GPP with a marked peak at the middle of the growing season when LAI achieved its maximum value. R_eco exhibited less fluctuation throughout time than GPP. A random forest machine-learning analysis indicated that Rg, LAI and deep soil moisture are the main drivers contributing to variability in daily and monthly cumulative NEE, GPP and R_eco during the growing season and across the entire study period. Results suggest that deep-rooted shrubs contribute substantially to C fluxes during the growing season (low precipitation and high VPD period) by acceding to deep and more stable water sources which allow extending the period of net carbon capture. This study helps to understand how this ecosystem functions and to predict how this steppe will respond to future climate. In addition, this research highlights the need for conservation of this ecosystem due to its large extension and carbon sequestration capacity.