INFLUENCE OF FIRE ON SOIL RESPIRATION RATES IN FOREST STANDS ADJACENT TO MADRID. SPAIN Carla Uribe 1. Rosa Inclán 1. Dolores M. Sánchez 1. Ramón Morante 1. Ana María Fernández 1. Ángeles Clavero 1. Ana Cardeña 1. Daniel De la Torre 1 and Helga Van Miegroet 2 1 Department of environment. CIEMAT. MADRID. SPAIN. 2 Department of Wildland Resources. Utah State University. LOGAN. USA. INTRODUCTION Mediterranean forests are vulnerable to numerous threats including wildfires due to a combination of climatic factors and increased urbanization. In the last decades the number of wildfires has doubled around Madrid. Increased temperatures and summer drought lead to increased risk of forest fires (IPCC 2007). This may have important consequences for the C dynamics and the C balance in these ecosystems. Soil respiration represents ~67-76% of total ecosystem respiration and changes in this flux can have a major impact on the global C cycle. In this study we characterize the effects of fire on soil respiration and physic-chemical and biological soil properties in three ecosystem type characteristic of the Central System in Spain: Quercus ilex subsp. Ballota. Quercus pyrenaica Willd. and Pinus sylvestris L. In each forest type we established two study plots one burned and one in the unburned control. Near each plot a pit was dug to determine the soil characteristics. In addition we took soil samples (0-10cm) to characterize carbon content and the microbial community structure using the PLFA technique (Phospholipids Fatty Acids). In 2008 the following monthly measurements were made: Soil respiration (12 random locations): closed dynamic system LI-6400 (LI-COR INC. Lincoln. NE. USA); no measurements after rain to avoid inhibition of CO 2 diffusion due to saturation of soil pore space or CO 2 pulse with rehydration of the soil (Rey et al. 2002). Temperature: thermocouple (Omega Engineering). Soil moisture: TDR (Time Domain Reflectometry ; System TRIMEG IMKO GmbH). Litterfall: collected in 4 boxes per plot (0.29 m 2 each). Differences were analyzed using Statistic 6.0 software (StatSoft. Inc.. Tulsa USA). RESULTS STUDY OBJECTIVES EXPERIMENTAL METHODS LOCATION CONCLUSIONS Quercus ilex Quercus pyrenaica Willd Pinus Sylvestris L. Figure 1. Seasonal variation of the soil respiration by the three species of trees: CO 2 flows are generally greater in unburned than in burned treatments. Also most burned treatments have higher temperatures (ºC) and lower moisture (vol%) than the unburned. Figure 2. Differences in soil carbon pools are small and species/location dependent. Table1. Soil characteristics under the tree species of study. SpeciesTreatmentGram + Bacteria(%)Gram – Bacteria(%)Actinomycetes(%) Fungi(%) Quercus ilex Unburned Burned Quercus pyrenaica Unburned Burned Pinus sylvestris Unburned Burned Table 2. The only change in the microbial community structure in the burned site is a decline in Gram – bacteria and an increase in Gram + bacteria. DATE OF FIRE:2004 DATE OF FIRE: 2002 DATE OF FIRE:2001 Fire affects soil temperature, moisture and soil respiration. The most significant effect (decline in respiration) is observed in the most recent fire (Quercus ilex in 2004). In the other two locations (Quercus pyrenaica and Pinus sylvestris) the effects are more heterogeneous, possibly to longer time since fire. The lower soil respiration in the burned treatment of Quercus ilex coincides with a lower soil carbon content and lower C/N. Future directions: To investigate the interaction of all parameters involved in carbon dynamics and better interpret the complex effects of fires on forest soils and on the overall carbon balance of these ecosystems. Quercus ilex Soil classification (FAO1998): UMBRISOL HÁPLICO Depth (cm)C/NpHSilt and Clay% (Fine fraction) UnburnedBurnedUnburnedBurnedUnburnedBurnedUnburnedBurned Quercus pyrenaica Soil classification (FAO1998): CAMBISOL ÉUTRICO Depth (cm)C/NpHSilt and Clay% (Fine fraction) UnburnedBurnedUnburnedBurnedUnburnedBurnedUnburnedBurned Pinus sylvestris Soil classification (FAO1998): UMBRISOL HÁPLICO Depth (cm)C/NpHSilt and Clay% (Fine fraction) UnburnedBurnedUnburnedBurnedUnburnedBurnedUnburnedBurned