Rey, Ana

Recent studies have highlighted the need to consider geological carbon sources when estimating the net ecosystem carbon balance (NECB) of terrestrial ecosystems located in areas potentially affected by geofluid circulation. We propose a new methodology using physical parameters of the atmospheric boundary layer to quantify the CO2 coming from deep ground origin in a steppe ecosystem located in the SE of Spain. Then, we compared published NECB estimates at the site with seasonal patterns of soil CO2 efflux and biological activity measured by satellite images over a 2-year period (2007/2008). The alpha grass ecosystem was a net carbon source (93.8 and 145.1 g C m-2 year-1, in 2007 and 2008, respectively), particularly as a result of large amounts of carbon released over the dry period that were not related to biological activity. While the highest ecosystem CO2 emission rates were measured over the dry period (reaching up to 15 lmol m-2 s-1), soil CO2 efflux rates (ca. 0.5 lmol m-2 s-1) and plant productivity were minimal during this period. After using a linear relationship between NECB and wind speed for different stability conditions and wind sectors, we estimated the geological flux FGEO (217.9 and 244.0 g C m-2 in 2007 and 2008, respectively) and subtracted it from the NECB to obtain the biological flux FBIO (-124.0 and -98.9 g C m-2 in 2007 and 2008, respectively). We then partitioned FBIO into gross primary productivity and ecosystem respiration and proved that, after removing FGEO, ecosystem respiration and soil CO2 efflux followed similar seasonal patterns. The annual contribution of the geological component to NECB was 49.6 and 46.7 % for the year 2007 and 2008, respectively. Therefore, it is clear that geological carbon sources should be quantified in those ecosystems located in areas with potential natural emission of geological gases to the surface.

At a semiarid steppe site located in the SE of Spain, relatively large CO2 emissions were measured that could not be attributed to the ecosystem activity alone. Since the study site was located in a tectonically active area, it was hypothesized that a part of the measured CO2 was of geologic origin. This investigation included a survey of soil CO2 efflux, together with carbon isotope analyses of the CO2 in the soil atmosphere, soil CO2 efflux (i.e., Keeling plots), groundwater and local thermal springs. These measurements confirmed the hypothesis of degassing from geologic sources. In areas with local faults and ancient volcanic structures, soil CO2 efflux rates were significantly higher (i.e., up to 6.3 and 1.4 mmol CO2 m 2 s 1) than measurements in a comparable site that was some distance from fault sites (means of 1.0 and 0.43 mmol CO2 m 2 s 1 in March and June, respectively). The CO2 concentration in the soil atmosphere at the eddy covariance site reached 0.14% v/v at 0.70 m soil depth with a 13C-enriched isotopic composition (d13C from 10.2‰ to 16.6‰), consistent with the isotopic composition of the soil CO2 efflux estimated by Keeling plots (i.e., 16.6‰). 13C-enriched CO2 also occurred in local aquifers, and there was evidence of degassing from deep crust and mantle at regional scale by the helium isotopic ratio in spring waters located about 30 km (R/Ra: 0.12) and 200 km (R/Ra: 0.95) NW of the eddy covariance site. This study highlights the importance of considering CO2 sources of geologic origin when assessing the net ecosystem carbon balance of sites that may possibly be affected by circulation of such CO2-rich fluids.

Despite the advance in our understanding of the carbon exchange between terrestrial ecosystems and the atmosphere, semiarid ecosystems have been poorly investigated and little is known about their role in the global carbon balance. We used eddy covariance measurements to determine the exchange of CO2 between a semiarid steppe and the atmosphere over 3 years. The vegetation is a perennial grassland of Stipa tenacissima L. located in the SE of Spain. We examined diurnal, seasonal and interannual variations in the net ecosystem carbon balance (NECB) in relation to biophysical variables. Cumulative NECB was a net source of 65.7, 143.6 and 92.1 g C mˉ2 yrˉ1 for the 3 years studied, respectively. We separated the year into two distinctive periods: dry period and growing season. The ecosystem was a net source of CO2 to the atmosphere, particularly during the dry period when large CO2 positive fluxes of up to 15 μmol mˉ2 sˉ1 were observed in concomitance with large wind speeds. Over the growing season, the ecosystem was a slight sink or neutral with maximum rates of -2.3 μmol mˉ2 sˉ1. Rainfall events caused large fluxes of CO2 to the atmosphere and determined the length of the growing season. In this season, photosynthetic photon flux density controlled day-time NECB just below 1000 μmol mˉ2 sˉ1. The analyses of the diurnal and seasonal data and preliminary geological and gas-geochemical evaluations, including C isotopic analyses, suggest that the CO2 released was not only biogenic but most likely included a component of geothermal origin, presumably related to deep fluids occurring in the area. These results highlight the importance of considering geological carbon sources, as well as the need to carefully interpret the results of eddy covariance partitioning techniques when applied in geologically active areas potentially affected by CO2-rich geofluid circulation.