The key-role of topsoil moisture on CO₂ efflux from a Mediterranean Quercus ilex forest

Abstract

-CO 2 respiratory losses partly determine net carbon ecosystem exchanges. The main objective of this paper was to understand regulation imposed by soil water content and temperature on soil and ecosystem CO 2 efflux in a holm oak (Quercus ilex L.) Mediterranean forest. Soil CO 2 efflux was monitored monthly during 1999 and 2001. Moreover, experimental water treatments were conducted in 1999 over 9 small plots (0.3 m 2 ) during nine months. Results showed strong decreases of soil CO 2 efflux for a relative soil water content below 0.7. Ecosystem respiration measured by eddy covariance over a 4-year period showed strong sensitivity to soil water content and temperature. Severe limitations of soil and ecosystem efflux imposed by low values of soil water content occurred on about 90 days per year. The best adjustments of soil and ecosystem CO 2 efflux were obtained using regression models where the exponential effect of temperature is linearly related to soil water content (r 2 = 0.68 and 0.79 for soil and ecosystem respectively). Our results highlighted strong differences in respiration sensitivity to topsoil moisture between soil and ecosystem. When the relative water content (RWC) is low (0.4), an increase of 1°C provokes an increase of soil respiration of 5.7% and an increase of ecosystem respiration of 8.6%. For nonlimiting soil water conditions, at RWC = 1, the increases of respiration caused by a 1°C temperature increase are of 8.5% and 16.5% for soil and ecosystem respectively. These results emphasized the probable determinant influences of changes in soil water regime for respiratory fluxes and net carbon exchanges of Mediterranean forest ecosystems.CO 2 efflux / soil water content / soil temperature / ecosystem respiration / Mediterranean ecosystem / Quercus ilex Résumé -Le rôle-clé de l'humidité du sol superficiel sur les efflux de CO 2 d'une forêt méditerranéenne de chêne vert. Les pertes de CO 2 par respiration vont déterminer largement les échanges nets de carbone des écosystèmes. L'objectif principal de cet article est de comprendre les régulations imposées par la teneur en eau et la température du sol sur les efflux de CO 2 du sol et de l'écosystème dans une forêt méditerranéenne de chêne vert (Quercus ilex L.). La respiration du sol a été mesurée mensuellement en 1999 et 2001. Par ailleurs, une expérimentation, mise en place en 1999, comprenant trois régimes hydriques a été suivie pendant 9 mois sur 9 parcelles de 0.3 m 2 . Les résultats mettent en évidence la très forte limitation des efflux lorsque la teneur en eau du sol est inférieure à 70 % de sa capacité de rétention. La respiration de l'écosystème mesurée sur une période de 4 ans par la méthode des fluctuations turbulentes montre la même sensibilité aux deux facteurs. Les conditions de fortes limitations par une faible teneur en eau du sol affectent l'écosystème environ 90 jours par an. Les meilleurs ajustements pour la simulation des flux de CO 2 du sol et de l'écosystème sont obtenus pour un modèle dans lequel l'effet exponentiel d...

Biomass increment and carbon balance of ash (Fraxinus excelsior) trees in an experimental stand in northeastern France

“…However, respiration of trees may depend on other environmental factors such as soil water content that can determine partly soil CO 2 efflux [11,18] and then derived root respiration, but water content data were not available in our study. For beech [15], estimations of above-ground respiration at the stand level (325 g C m -2 year -1 ) were very close to scaledup values derived from actual respiration measurements made at the tree level in the same experiment [6,8].…”

Warm and Fertile Sub-Humid Conditions Enhance Litterfall to Sustain High Soil Respiration Fluxes in a Mediterranean Cork Oak Forest

“…Soil respiration (Rs) was modelled using equation (2) designed for water-limited Mediterranean ecosystems where the effect of temperature is expressed as a linear function of soil moisture. The model is described in Joffre et al [54].…”

“…In Mediterranean [54,80] and semi-arid [92] ecosystems, the temporal variation of soil respiration is controlled by both temperature and soil moisture, supporting our choice for the use of 'expo' model [54]. The temperature sensitivity of soil respiration in our study was more affected by the top soil water status than in a Mediterranean evergreen Quercus ilex forest (g(θ) = 5.7%, RWC = 0.4) [54], suggesting an eventual higher contribution of heterotrophic respiration in an ecosystem with high litterfall and a low root/shoot ratio.…”

“…The lowest simulated soil respiration values were observed during winter with 1.31 µmol CO2 m Finally, we tested for the temperature sensitivity of respiration to soil water status for both sites. The temperature sensitivity of respiration g(θ) (see Joffre et al [54] for details) was identified as a linear function of RWC (Figure 7). At low RWC (0.56), an increase of 1 °C induced an increase of soil respiration of 15.4% in the YS and 15.7% in the MS. For non-limiting soil water conditions, at RWC = 1, the increases of respiration caused by a 1 °C temperature increase are +16.3% and +17% for YS and MS, respectively.…”

Spatial Variations of Soil Respiration in Arid Ecosystems

“…Soil temperature and soil water content are commonly considered to be the key environmental factors that re-sponsible for variation in soil respiration [3] [36] [37]. Fluctuations of soil temperature and/or soil moisture can well explain the temporal variation of soil respiration, both diurnally and seasonally.…”