Intercambios de CO2 entre atmósfera y ecosistemas kársticos: Aplicabilidad de las técnicas comúnmente empleadas

La cuantificación del balance anual de carbono en los distintos ecosistemas terrestres así como la identificación de los distintos procesos que intervienen, adquiere mayor importancia con la entrada en vigor en febrero de 2005 del Protocolo de Kyoto, acordado en 1997. En este estudio se pretende abordar dicha temática, cuantificando e identificando los procesos que intervienen en el balance anual de carbono en un ecosistema semiárido de sustrato carbonatado ubicado en la Sierra de Gádor (Almería) en el paraje conocido como El Llano de los Juanes. Con tal fin se realizó un seguimiento del comportamiento de este ecosistema durante aproximadamente tres años (mayo de 2004 hasta septiembre de 2006). Las herramientas empleadas han sido: la técnica eddy covariance, que estima el balance anual de carbono y vapor de agua mediante la cuantificación de flujos turbulentos; las cámaras de flujo de CO2 del suelo, que nos proporcionará información puntual sobre las emisiones de CO2 del suelo, y la técnica de isótopos estables que se emplea para la identificación de los procesos que intervienen en el balance de carbono. La técnica eddy covariance es una de las herramientas más usadas para la cuantificación del balance anual de carbono de un ecosistema (Baldocchi et al., 2001). La aplicación de esta poderosa herramienta requiere unos conocimientos micrometeorológicos previos y una continua búsqueda de nuevos avances en metodologías que resulten efectivas y aplicables (Aubinet et al., 2000). Como paso previo a cualquier interpretación de los resultados de flujos de CO2 obtenidos con esta técnica, es necesario estudiar la idoneidad de la ubicación de la torre de medida dentro del ecosistema. Con ese objetivo se estudia si los flujos de CO2 medidos con esta técnica proceden de un área representativa del ecosistema y si sus dimensiones no superan la extensión de superficie homogénea que rodea a nuestra torre (fetch). Entre todos los modelos disponibles en la bibliografía para análisis del área de medida (footprint) del sistema eddy covariance (EC) se ha seleccionado el modelo FSAM (Flux-Source Area model) (Schmid 1994) por su simplicidad y fácil aplicación. Además de este análisis se han realizado otros estudios para determinar la fiabilidad de los datos obtenido con el sistema EC. Así se ha realizado el análisis del cierre del balance de energía obtenido en nuestro ecosistema, con el fin de estudiar la fiabilidad de las medidas de los flujos turbulentos de calor sensible (H) y latente (LE) realizadas con nuestro sistema EC. Por otro lado se comparó el flujo de CO2 obtenido con el sistema EC con los valores de flujo de CO2 del suelo medidos con el sistema de cámaras LI-8100. Con todos estos análisis hemos verificado la idoneidad del lugar seleccionado para la instalación del sistema y la fiabilidad de los flujos turbulentos medidos en El Llano de los Juanes. Dada la complejidad de técnica EC, es importante tener en cuenta que aún en la actualidad existen numerosas incertidumbres relativas al cálculo del balance anual de carbono y a la aplicación de esta técnica (Dragoni et al., 2007). Por esta razón, cada vez son más numerosos los artículos enfocados a la detección de problemas relacionados con esta técnica y sus posibles soluciones (Baldocchi et al., 2000; Feigenwinter et al., 2004; Burba et al., 2006; Liu et al., 2006a). En este trabajo destacamos la detección de un nuevo problema asociado con una fuente de sobrestimación de la asimilación de carbono por parte del ecosistema. El origen del problema está en los problemas de mantenimiento del analizador de gases en el infrarrojo de trayectoria abierta, que se emplea para medir la densidad promedio de CO2 así como sus fluctuaciones, que son necesarias para el cálculo del flujo de CO2. La exposición de este problema, así como su solución ya ha sido publicada y se recoge en este trabajo (Serrano-Ortiz et al., 2008). Una vez estudiada la fiabilidad de los flujos turbulentos medidos con la técnica EC y resueltos los problemas detectados, estamos en condiciones de analizar los resultados obtenidos. En este trabajo estudiamos la variación estacional e interanual del flujo diurno de CO2 (FC) y la evapotranspiración (ET) en El Llano de los Juanes. Esta variabilidad, es consecuencia de una adaptación a las condiciones ambientales extremas a las que se ve sometido el ecosistema. Analizando la influencia de algunas de las variables medioambientales sobre los intercambios de carbono diurnos, observamos la importancia de la distribución de las lluvias a lo largo del año en la determinación del ciclo fenológico de las plantas y la productividad del ecosistema (Serrano-Ortiz et al., 2007). Además, las campañas de medida mensuales del flujo de CO2 en el suelo, realizadas con el sistema de cámaras LI-8100 nos han permitido estudiar las variables de las que depende la variabilidad estacional de las emisiones de CO2 del suelo. Llegando a la conclusión de que el factor que condiciona la respiración del suelo en nuestro ecosistema semiárido, durante la época de crecimiento biológico, es el contenido de agua en el suelo. Además, para estudiar más a fondo el comportamiento del flujo de CO2 en nuestro ecosistema, hemos recurrido a modelos no lineales ecofisiológicos que relacionan estos intercambios de CO2 con variables medioambientales (Falge et al., 2001b). Estos modelos suponen que el flujo de CO2 medido procede única y exclusivamente de procesos biológicos (fotosíntesis y respiración durante el día y respiración durante la noche). La aplicación de estos modelos a nuestro ecosistema nos confirma que durante la época de crecimiento, sin carencia de agua en el suelo, los modelos ecofisiológicos pueden usarse para describir el comportamiento del flujo de CO2. Sin embargo, estos modelos no son extrapolables a otras épocas del año (Serrano-Ortiz et al., 2007). En épocas de senescencia se detectan emisiones de CO2 a la atmósfera, que no dependen de variables ambientales que condicionan la respiración (temperatura) y por tanto no pueden modelizarse atendiendo a modelos ecofisiológicos. La hipótesis que formulamos en este trabajo es que estas emisiones son consecuencia de fenómenos de ventilación de los macroporos del sistema carbonatado, que parecen estar correlacionadas con la velocidad del viento. La dependencia de estas emisiones con el viento ha sido verificada con el sistema de cámaras LI-8100. En esta línea, estudios de fenómenos de desgasificación de cuevas e intercambios turbulentos de CO2 con la atmósfera, llevados a cabo en la Cueva de Altamira (Cuezva 2008), avalan la teoría de existencia de flujos de CO2 de procedencia geoquímica en ecosistemas localizados sobre sustrato karstico. Para estudiar la procedencia de los flujos de CO2 medidos con nuestro sistema EC en El Llano de los Juanes, hemos usado la técnica de isótopos estables. El análisis de la huella isotópica nos permite discriminar los procesos que intervienen en el intercambio de CO2 entre la atmósfera y nuestro ecosistema, dado que el CO2 de procedencia geoquímica tendrá una huella isotópica distinta al CO2 al procedente de la respiración. Tras analizar isotópicamante el CO2 medido durante épocas de crecimiento biológico y épocas de senescencia observamos que existen diferencias entre la huella isotópica del carbono en ambas estaciones. No obstante, parece oportuno seguir profundizando en la investigación con el fin de confirmar de modo concluyente la contribución de flujos abióticos de CO2 en el ecosistema de estudio. Finalmente, tras casi tres años de medidas, estamos en condiciones de estimar el balance anual de carbono en El Llano de los Juanes (Sierra de Gádor). Nuestro ecosistema actúa como sumidero de carbono asimilando aproximadamente 25 g C m-2 por año. Además vemos cómo la distribución de las lluvias condiciona la funcionalidad de nuestro ecosistema y cómo la duración del periodo de sequía determina la duración de las emisiones de CO2 de procedencia geoquímica, por ventilación del suelo.Tesis Univ. Granada

Effects of herbaceous covers and mineral fertilizers on the nutrient stocks and fluxes in a Mediterranean olive grove

The preservation of nutrient capital, soil fertility, and carbon (C) sequestration capacity in Mediterranean olive groves requires evaluation of agricultural practices beyond short-term productivity. We aim to contribute with a mechanistic understanding on the effects that the preservation of herbaceous cover and the use of chemical fertilizers have on the performance of olive trees and on the biogeochemical cycles of the agroecosystem. We compared nutrient fluxes and aboveground leafy stocks in an olive grove that had been organically managed for more than 60 years, in a treatment in which the annual spontaneous herbaceous cover was maintained (H), and after two years of shift to conventional management treatments in which the growth of herbaceous vegetation was avoided by the use of herbicides (NH), and where exclusion of the herbaceous cover is also combined with the supply of mineral fertilizers (NHF). Maintenance of herbaceous vegetation in H contributed to the retention of a high aboveground capital of C and nutrients, particularly nitrogen, (N), phosphorus (P) and potassium (K) that were about 2.9, 3.9 and 7.4 times greater than in NH, respectively. The permanence of herbaceous cover stimulated olive tree leaf litter decomposition rates by about 86 % and increased nutrient release. However, the H treatment led to a 37 % decrease in olive yield and lowered olive foliar N and P content as negative short-term effects. The addition of fertilizers (N, P, K, and Mg) in mineral and solid form in NHF resulted inefficient to improve olive tree nutritional status and olive production, and decelerated olive tree litter decomposition rates by 21 % and nutrient release. The nutrient retention in organic forms in the fast-growing species of herbaceous covers and the progressive nutrient release as litter decomposes may contribute to regulate and better adapt nutrient availability to the nutrient requirements of olive trees.European CommissionMinistry of Economy, Innovation, Science and Employment of the Junta de Andalucia (Andalucia Talent Hub Program, Marie Sklodowska-Curie actions, COFUND) 291780European Union Horizon 2020 research and innovation program under the Marie Sklodowska-Curie fellowship "StoiCa" 750252Spanish GovernmentERDF funds-"ELEMENTAL" CGL2017-83538-C3-1-R ERDF funds-"INTEGRATYON3" PID2020-117825GB-C21Serra Hunter fellowship Programme from the regional Government of CataloniaJuan de la Cierva postdoctoral contracts - MCIN/AEI FJC2018-038192-I IJC2020-045630-IEuropean Union NextGenerationEU/PRTRMCIN/AEI MdM-2017-0714HIPATIAUAL postdoctoral fellowship - University of Almeri

Cave ventilation is influenced by variations in the CO2-dependent virtual temperature

Dynamics and drivers of ventilation in caves are of growing interest for different fields of science. Accumulated CO2 in caves can be exchanged with the atmosphere, modifying the internal CO2 content, affecting stalagmite growth rates, deteriorating rupestrian paintings or creating new minerals. Current estimates of cave ventilation neglect the role of high CO2 concentrations in determining air density – approximated via the virtual temperature (Tv) –, affecting buoyancy and therefore the release or storage of CO2. Here we try to improve knowledge and understanding of cave ventilation through the use of Tv in CO2-rich air to explain buoyancy for different values of temperature (T) and CO2 content. Also, we show differences between T and Tv for 14 different experimental sites in the vadose zone, demonstrating the importance of using the correct definition of Tv to determine air buoyancy in caves. The calculation of Tv (including CO2 effects) is currently available via internet using an excel template, requiring the input of CO2 (%), air temperature (ºC) and relative humidity (%).This research was funded by the Andalusian regional government project GEOCARBO (P08- RNM-3721) and GLOCHARID, including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects CarboredII (CGL2010-22193-C04-02), SOILPROF (CGL2011- 15276-E) and CARBORAD (CGL2011-27493), as well as the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 244122

The temporary effect of weed-cover maintenance on transpiration and carbon assimilation of olive trees

The maintenance of spontaneous weed cover is a conservation practice used in olive groves. Herbaceous plants in alleys between the trees can increase the capacity of this agroecosystem to remove carbon. However, the influence of this practice on carbon assimilation at the leaf scale has not yet been studied in olive trees. Also, the presence of other species competing with olive trees for soil water has the potential to modify the water use efficiency, a key parameter in a climate change context. In this study, leaf-scale net carbon assimilation (Aleaf), transpiration (Eleaf) and water use efficiency as the ratio Aleaf/Eleaf(WUEleaf) were quantified in olive grove divided by two different treatments: (1) a weed-free (WF) ecosystem in which weed growth was inhibited by applying herbicide; and (2) a weed-covered (WC) ecosystem in which spontaneous herbaceous plants were kept and then mechanically mowed and left on the ground. A portable leaf photosynthesis system was used to measure olive leaf fluxes for both treatments, and likewise for the ecosystem scale via two eddy covariance towers assessing gross primary production (GPPeco), evapotranspiration (ETeco), and water use efficiency (WUEeco). We found that the average Aleaf was 24% higher in the WF treatment while GPPeco decreased 32% compared to WC treatment. However, Aleaf was significantly different between treatments only during weed growth: January-May (Aleaf-WF = 7.6±3.7 μmol CO2m−2s−1; Aleaf-WC = 5.1±3.1 μmol CO2m−2s−1) while Aleaf was similar between the two treatments after mowing. Mowed weeds decreased Tsoil and VPD, and these changes were accompanied by a decrease in Eleaf in olive trees. Therefore, this led to WUEleaf-WF>WUEleaf-WC when the weeds were growing and the opposite after mowing. Thus, although the presence of spontaneous weeds increased the annual ecosystem C uptake in the olive orchard, both Aleaf and seasonal fluctuations in WUEleaf were reduced with weed maintenance

Hot-Moments of Soil CO2 Efflux in a Water-Limited Grassland

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems

Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates

Comparing biosphere–atmosphere carbon exchange across monsoon (warm-season rainfall) and Mediterranean (cool-season rainfall) regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of the different precipitation seasonality. Considerable crossover behavior occurred between the two rainfall regimes. As expected in these usually water-limited ecosystems, precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature and/or light also played an important role in regulating GEP and NEE at all sites. If significant rainfall occurred in the winter at the Arizona sites, their behavior was characteristically Mediterranean whereby the carbon flux responses were delayed till springtime. Likewise, the Spanish Mediterranean sites showed immediate pulse-like responses to rainfall events in non-winter periods. The observed site differences were likely due to differences in vegetation, soils, and climatology. Together, these results support a more unified conceptual model for which processes governing carbon cycling in semiarid ecosystems need not differ between warm-season and cool-season rainfall regimes.This paper is the result of a fellowship funded by the OECD Co-operative Research Programme: Biological Resource Management for Sustainable Agricultural Systems to R.L. Scott. This paper has been supported in part by the Andalusian regional government project GEOCARBO and GLOCHARID (P08-RNM-3721), European Union Funds (ERDF and ESF), the Spanish flux-tower network CARBORED-ES (Science Ministry project CGL2010-22193-C04-02), and the European Commission collaborative project GHG Europe (FP7/2007-2013; grant agreement 244122)

Deep CO 2 soil inhalation / exhalation induced by synoptic pressure changes and atmospheric tides in a carbonated semiarid steppe

Knowledge of all the mechanisms and processes involved in soil CO2 emissions is essential to close the global carbon cycle. Apart from molecular diffusion, the main physical component of such CO2 exchange is soil ventilation. Advective CO2 transport, through soil or snow, has been correlated with the wind speed, friction velocity or pressure (p). Here we examine variations in subterranean CO2 molar fractions (χc) over two years within a vertical profile (1.5 m) in a semiarid ecosystem, as influenced by short-timescale p changes. Analyses to determine the factors involved in the variations in subterranean χc were differentiated between the growing period and the dry period. In both periods it was found that variations in deep χc (0.5–1.5 m) were due predominantly to static p variations and not to wind or biological influences. Within a few hours, the deep χc can vary by fourfold, showing a pattern with two cycles per day, due to p oscillations caused by atmospheric tides. By contrast, shallow χc (0.15 m) generally has one cycle per day as influenced by biological factors like soil water content and temperature in both periods, while the wind was an important factor in shallow χc variations only during the dry period. Evidence of emissions was registered in the atmospheric boundary layer by eddy covariance during synoptic pressure changes when subterranean CO2 was released; days with rising barometric pressure – when air accumulated belowground, including soil-respired CO2 – showed greater ecosystem uptake than days with falling pressure. Future assessments of the net ecosystem carbon balance should not rely exclusively on Fick's law to calculate soil CO2 effluxes from profile data.This research was funded by the Andalusian regional government project GEOCARBO (P08-RNM-3721) and GLOCHARID, including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects Carbored-II (CGL2010-22193-C04-02), SOILPROF (CGL2011-15276-E) and CARBORAD (CGL2011-27493), as well as the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 244122

Analyzing the major drivers of NEE in a Mediterranean alpine shrubland

Two years of continuous measurements of net ecosystem exchange (NEE) using the eddy covariance technique were made over a Mediterranean alpine shrubland. This ecosystem was found to be a net source of CO2 (+ 52 ± 7 g C m−2 and + 48 ± 7 g C m−2 for 2007 and 2008) during the two-year study period. To understand the reasons underlying this net release of CO2 into the atmosphere, we analysed the drivers of seasonal variability in NEE over these two years. We observed that the soil water availability – driven by the precipitation pattern – and the photosynthetic photon flux density (PPFD) are the key factors for understanding both the carbon sequestration potential and the duration of the photosynthetic period during the growing season. Finally, the effects of the self-heating correction to CO2 and H2O fluxes measured with the open-path infrared gas analyser were evaluated. Applying the correction turned the annual CO2 budget in 2007 from a sink (− 135 ± 7 g C m−2) to a source (+ 52 ± 7 g C m−2). The magnitude of this change is larger than reported previously and is shown to be due to the low air density and cold temperatures at this high elevation study site.This research was supported by the regional government Junta de Andalucía, project BACAEMÁ (RNM-332) and the Spanish National flux tower network CARBORED-ES (CGL2006-14195-C02-01/CLI)

Modelling actual evapotranspiration using a two source energy balance model with Sentinel imagery in herbaceous-free and herbaceous-cover Mediterranean olive orchards

To the European Space Agency for the imagery of the Sentinel Missions and its open access. Special thanks to Radoslaw Guzinski for share and make accessible (https://github.com/radosuav/pyDMS) the implemented software for the used sharpening process (likewise to Hector Nieto for the implemented TSEB-PT, https://github.com/hectornieto/pyTSEB).To the Group of Castillo de Canena for the use of their farm as an experimental site and their people for continuous cooperation. We also give special thanks to Andrew S. Kowalski for his advice and suggestions. We would like also to express our gratitude to the anonymous reviewers for their comments and suggestions that enhanced this work. This work was supported by the Spanish Ministry of Science and Innovation through project CGL2017-83538-C3-1-R (ELEMENTAL) and PID2020-117825GB-C21 (INTEGRATYON3) Including European Union ERDF funds [grant number PRE2018-085638]. Funding for open access charge: Universidad de Granada/CBUA.Precipitation deficit and more extreme drought and precipitation events are expected to increase in the Mediterranean region due to global warming. A great part of this region is covered by olive orchards, representing 97.5% of the world’s olive agricultural area. Thus, the adaptation of olive cultivation demands climate-smart management, such as the optimization of water use efficiency, since evapotranspiration is one of the most important components of the water balance. The novelty of this work is the combination of the remote sensing data fusion and the Two Source Energy Balance (TSEB) model (through Sentinel-2 and Sentinel-3 imagery) to estimate the actual daily evapotranspiration (ETd), at high spatial (20 m) and temporal (daily) resolution, in an olive orchard under two management regimes: herbaceous free (HF) and herbaceous-cover (HC); along a three years period, based on the hypothesis that TSEB is still able to track and estimate the evapotranspiration over more complex canopies. The study was carried out from 2016 to 2019 in an olive orchard in the South of Spain, where the flux estimates were validated and assessed by in situ eddy covariance (EC) measurements. The results show better agreement in HC for net radiation (Rn) and the soil heat flux (G), but similar for both surfaces regarding the sensible (H) and latent (λE) heat fluxes, as well as ETd. On both surfaces greater differences obtained at higher H, and the magnitude of overestimation of λE and ETd were influenced by the EC energy imbalance. By contrast, G was overestimated with HC probably influenced by herbs, and equally underestimated for HF surfaces. The obtained results are in agreement with similar studies in tree crop orchards, and show the consistency of the used methodology and its usefulness for some farming activities, even on the more heterogeneous surface.Spanish Government CGL2017-83538-C3-1-R PID2020-117825GB-C21European Commission PRE2018-085638Universidad de Granada/CBU

Winds induce CO2 exchange with the atmosphere and vadose zone transport in a karstic ecosystem

Research on the subterranean CO dynamics has focused individually on either surface soils or bedrock cavities, neglecting the interaction of both systems as a whole. In this regard, the vadose zone contains CO-enriched air (ca. 5% by volume) in the first meters, and its exchange with the atmosphere can represent from 10 to 90% of total ecosystem CO emissions. Despite its importance, to date still lacking are reliable and robust databases of vadose zone CO contents that would improve knowledge of seasonal-annual aboveground-belowground CO balances. Here we study 2.5 years of vadose zone CO dynamics in a semiarid ecosystem. The experimental design includes an integrative approach to continuously measure CO in vertical and horizontal soil profiles, following gradients from surface to deep horizons and from areas of net biological CO production (under plants) to areas of lowest CO production (bare soil), as well as a bedrock borehole representing karst cavities and ecosystem-scale exchanges. We found that CO followed similar seasonal patterns for the different layers, with the maximum seasonal values of CO delayed with depth (deeper more delayed). However, the behavior of CO transport differed markedly among layers. Advective transport driven by wind induced CO emission both in surface soil and bedrock, but with negligible effect on subsurface soil, which appears to act as a buffer impeding rapid CO exchanges. Our study provides the first evidence of enrichment of CO under plant, hypothesizing that CO-rich air could come from root zone or by transport from deepest layers through cracks and fissures.These data were funded by the Andalusian regional government project GEOCARBO (P08-RNM-3721), including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects SOILPROF (CGL2011-15276-E), CARBORAD (CGL2011-27493), and GEISpain (CGL2014-52838-C2-1-R). This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme, DIESEL project (625988).Peer Reviewe