Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20% reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10% v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130%, 26%, 58% and 220 %, respectively, as compared to the control. In contrast, the 20% reduction in throughfall (equivalent to 10% reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10% reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks.

The allocation of recently assimilated C to belowvs. aboveground plant components and the time this C remains inside the ecosystem are the key uncertainties in global terrestrial C models. Recent studies reported the time lag between the photosynthetic C uptake and its following respiration through the rooting systems in the magnitude from minutes to days. Studies in situ are still not numerous, covering few plant functional types and ecosystems. The methodology is not unique, destructive and non-destructive techniques with different shortcomings are involved and it is not clear if obtained results are comparable.

In regions characterized by arid seasons, such as the Mediterranean basin, soil moisture is a major driver of ecosystem CO 2 efflux during periods of drought stress. Here, a rain event can induce a disproportional respiratory pulse, releasing an amount of CO 2 to the atmosphere that may significantly contribute to the annual ecosystem carbon balance. The mechanisms behind this pulse are unclear, and it is still unknown whether it is due to the stimulation of autotrophic, heterotrophic and/or inorganic CO 2 fluxes. On the Mediterranean island of Pianosa, eddy flux measurements showed respiratory pulses after rain events following prolonged drought periods, which occurred in the summer of 2003 and 2006. To investigate the mechanisms of this observed enhanced respiration fluxes and partition of the soil CO 2 sources, two water manipulation experiments were performed. The first was designed to estimate the effect of soil rewetting on soil CO 2 efflux, in the different ecosystem types existing on the island (i.e. woodland, ex‐agricultural and Mediterranean shrubland). The second was a soil CO 2 partitioning experiment to investigate the relative contribution of inorganic and organic CO 2 sources to soil respiration, under dry and wet soil conditions. Our results suggest that the pulse in the CO 2 efflux is primarily due to the enhancement of heterotrophic respiration, likely caused by the degradation of easily decomposable substrates, accumulated in soils during the dry period. In fact, the vegetation at the site was senescent and did not play any significant role in CO 2 exchange, as suggested by the absence of diurnal CO 2 uptake in eddy covariance measurements. In addition, soil rewetting did not significantly enhance inorganic CO 2 efflux.

Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 degrees C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO2 carbon fluxes are not presently being adequately accounted for.

Will Mediterranean terrestrial ecosystems be affected by the expected changes in precipitation regimes? If so, by how much and in which direction? These questions are at the basis of the research performed in context of the EU MIND project, whose key objectives were: i) to investigate the potential effects of increasing drought on Mediterranean terrestrial ecosystems at the process, ecosystem and regional scales and ii) to assess ecosystem vulnerability to changes in rainfall patterns. A network of experimental study sites has been created in Portugal, Spain, France and Italy, where field manipulations alter the amount of water available to the ecosystem. The most up-to-date methods of ecophysiology, micrometeorology, soil ecology and remote sensing have been used to elucidate the mechanisms that regulate the response of vegetation and soil to changes in water availability. This information is providing the basis for the implementation and validation of simulation models capable of predicting the drought response of Mediterranean terrestrial ecosystems, and their vulnerability to future climate change, on a larger scale. The out-coming results are elucidating how water availability affects plant ecophysiological processes, the dynamics of soil carbon and the overall exchange of mass and energy between the land and the atmosphere. This paper focuses on some of the important, yet preliminary, results on C and energy fluxes that have been obtained at the large scale troughfall manipulation experiment (Tolfa, Italy), in a forest dominated by Arbutus unedo L.

Both biological responses and feedbacks to change are included in the journal, and may be considered at any level of organization from molecular to biome. Studies which integrate across levels of organization to provide a mechanistic understanding are also highlighted. Studies may be experimental, observational or theoretical, and may concern aquatic or terrestrial and managed or natural environments.

A new system for soil respiration measurement [P. Rochette, L.B. Flanagan, E.G. Gregorich. Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J., 63, 1207-1213 (1999).] was modified in order to collect soil-derived CO2 for stable isotope analysis. The aim of this study was to assess the suitability of this modified soil respiration system to determine the isotopic composition (delta13C) of soil CO2 efflux and to measure, at the same time, the soil CO2 efflux rate, with the further advantage of collecting only one air sample. A comparison between different methods of air collection from the soil was carried out in a laboratory experiment. Our system, as well as the other dynamic chamber approach tested, appeared to sample the soil CO2, which is enriched with respect to the soil CO2 efflux, probably because of a mass dependent fractionation during diffusion and because of the atmospheric contribution in the upper soil layer. On the contrary, the static accumulation of CO2 into the chamber headspace allows sampling of delta13C-CO2 of soil CO2 efflux.

A new principle for measuring soil CO2 efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO2 within the system to achieve a constant CO2 concentration inside the soil chamber at ambient level, thus balancing the amount of CO2 entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO2 efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO2 within the soil, while allowing for continuous capture of the CO2 released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO2 efflux and its δ13C, since both variables show sensitivity to a distortion of the soil CO2 profile commonly found in conventional chamber techniques.