Understanding the spatial and temporal variation in soil respiration within small geographic areas is essential to accurately assess the carbon budget on a global scale. In this study, we investigated the factors controlling soil respiration in an altitudinal gradient in a southern Mediterranean mixed pine-oak forest ecosystem in the north face of the Sierra de Guadarrama in Spain. Soil respiration was measured in five Pinus sylvestris L. plots over a period of 1 year by means of a closed dynamic system (LI-COR 6400). Soil temperature and water content were measured at the same time as soil respiration. Other soil physico-chemical and microbiological properties were measured during the study. Measured soil respiration ranged from 6.8 to 1.4 lmol m -2 s -1 , showing the highest values at plots situated at higher elevation. Q 10 values ranged between 1.30 and 2.04, while R 10 values ranged between 2.0 and 3.6. The results indicate that the seasonal variation of soil respiration was mainly controlled by soil temperature and moisture. Among sites, soil carbon and nitrogen stocks regulate soil respiration in addition to soil temperature and moisture. Our results suggest that application of standard models to estimate soil respiration for small geographic areas may not be adequate unless other factors are considered in addition to soil temperature.

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We investigated N 2 0 and CH 4 fluxes from soils of Quercus ilex, Quercus pyrenaica and Pinus sylvestris stands located in the surrounding área of Madrid (Spain). The fluxes were measured for 18 months from both mature stands and post fire stands using the static chamber technique. Simultaneously with gas fluxes, soil temperature, soil water content, soil C and soil N were measured in the stands. Nitrous oxide fluxes ranged from -11.43 to 8.34 ug N 2 0-N m~2 h" 1 in Q.üex, -1.1 A to 13.52 ug N 2 0-N m -2 h _1 in Q. pyrenaica and -28.17 to 21.89 ug N 2 0-N m~2 h" 1 in P. sylvestris. Fluxes of CH 4 ranged from -8.12 to 4.11 ug CH 4 -C m~2 h" 1 in Q.üex, -7.74 to 3.0 ug CH 4 -C m 2 h l in Q. pyrenaica and -24.46 to 6.07 ug CHzL-Cm^h-1 in P. sylvestris. Seasonal differences were detected; N 2 0 fluxes being higher in wet months whereas N 2 0 fluxes declined in dry months. Net consumption of N 2 0 was related to low N availability, high soil C contents, high soil temperatures and low moisture content. Fire decreased N 2 0 fluxes in spring. N 2 0 emissions were closely correlated with previous day's rainfall and soil moisture. Our ecosystems generally were a sink for methane in the dry season and a source of CH 4 during wet months. The available water in the soil influenced the observed seasonal trend. The burned sites showed higher CH 4 oxidation rates in Q. ilex, and lower rates in P. sylvestris. Overall, the data suggest that fire alters both N 2 0 and CH 4 fluxes. However, the magnitude of such variation depends on the site, soil characteristics and seasonal climatic conditions.