Abstract.Temperatures in the Arctic regions are rising, thawing permafrost and exposing previously stable soil organic carbon (OC) to decomposition. This can result in northern latitude soils, which have accumulated large amounts of OC potentially shifting from atmospheric C sinks to C sources with positive feedback on climate warming. In this paper, we estimate the annual net C gas balance (NCB) of the subarctic mire Stordalen, based on automatic chamber measurements of CO 2 and total hydrocarbon (THC; CH 4 and NMVOCs) exchange. We studied the dominant vegetation communities with different moisture and permafrost characteristics; a dry Palsa underlain by permafrost, an intermediate thaw site with Sphagnum spp. and a wet site with Eriophorum spp. where the soil thaws completely. Whole year accumulated fluxes of CO 2 were estimated to 29.7, −35.3 and −34.9 gC m −2 respectively for the Palsa, Sphagnum and Eriophorum sites (positive flux indicates an addition of C to the atmospheric pool). The corresponding annual THC emissions were 0.5, 6.2 and 31.8 gC m −2 for the same sites. Therefore, the NCB for each of the sites was 30.2, −29.1 and −3.1 gC m −2 respectively for the Palsa, Sphagnum and Eriophorum site. On average, the whole mire was a CO 2 sink of 2.6 gC m −2 and a THC source of 6.4 gC m −2 over a year. Consequently, the mire was a net source of C to the atmosphere by 3.9 gC m −2 (based on area weighted estimates for each of the three plant communities). Early and late snow season efflux of CO 2 and THC emphasize the importance of winter measurements for complete annual C budgets. Decadal vegetation changes at Stordalen indicate that both the productivity and the THC emissions increased between 1970 and 2000. Considering the GWP 100 of CH 4 , the Correspondence to: K. Bäckstrand (kristina.backstrand@gmail.com) net radiative forcing on climate increased 21% over the same time. In conclusion, reduced C compounds in these environments have high importance for both the annual C balance and climate.

Abstract. The amount of nitrous oxide (N20) continues to increase in the atmosphere. Agricultural use of nitrogen fertilizers in the tropics is thought to be an important source of atmospheric N20. High frequency, highly precise measurements of the N20 flux were made with an automated system deployed in N fertilized and unfertilized agricultural plots of papaya and corn in Costa Rica for an entire corn crop growth to harvest cycle. N20 fluxes were as high as 64 ng N-N20 cm -2 h -• from fertilized versus 12 ng N-N20 cm -2 h '• from unfertilized corn and 28 ng N-N20 cm -2 h -• from fertilized versus 4.6 ng N-N20 cm -2 h '• from unfertilized papaya. Fertilized corn released more N20 than fertilized papaya over the 125 days of the crop cycle, 1.83 kg N ha -• versus 1.37 kg N ha -•. This represents a loss as N2 ¸ of 1.1 and 0.9% of the total N applied as ammonium nitrate to the corn and papaya, respectively. As has often been observed, N20 fluxes were highly variable. The fastest rates of emission were associated with fertilization and high soil moisture. A diurnal cycle in the fluxes was not evident probably due to the minimal day/night temperature fluctuations. Each chamber was measured between 509 and 523 times over the course of the experiment. This allows us to evaluate the effect on constructed mean fluxes of lowered sampling frequencies. Sampling each collar about once a day throughout the crop cycle (25% of the data set) could result in a calculated mean flux from any individual chamber that can vary by as much as 20% even though the calculated mean would probably be within 10% of the mean of the complete data set. The uncertainty increases very rapidly at lower sampling frequencies. For example, if only 10% of the data set were used which would be the equivalent of sampling every other day, a very high sampling frequency in terms of manual measurements, the calculated mean flux could vary by as much as 40% or more at any given site.

Abstract. This study compares the CH 4

Abstract. Conversion of humid tropical forest to agriculture significantly alters trace gas emissions from soils. We report nitrous oxide (N20), nitric oxide (NO), and methane (CH4) fluxes from secondary forest soils prior to and during deforestation, and throughout the first agricultural cropping. Annual average nitrogen oxide emissions from forest soils were 1.5 ng N cm '2 h -1 for N20 and 0.9 ng N cm '2 h -1 for NO. Forest clearing increased the level of extractable nitrate in soils and average nitrogen oxides fluxes (2.7 ng N cm -2 h -I for N20, and 8.1 ng N cm -2 h -I for NO). Immediately after biomass burning, short-term peaks of N20 and NO (123 ng N cm -2 h -I for N20 , and 41 ng N cm -2 h -1 for NO) were superimposed on generally increased fluxes. Peak emissions declined within 3 days after burning. Postbum fluxes stayed higher than measured on adjacent forest sites for 3-4 months (averages for postbum fluxes were 17.5 ng N cm -2 h -1 for N20 , and 19.2 ng N cm -2 h -1 for NO). Increased N20 and NO emissions after clearing and until cropping were

Abstract. This study compares the CH4 fluxes from two arctic wetland sites of different annual temperatures during 2004 to 2006. The PEATLAND-VU model was used to simulate the emissions. The CH4 module of PEATLAND-VU is based on the Walter-Heimann model. The first site is located in northeast Siberia, Indigirka lowlands, Kytalyk reserve (70° N, 147° E) in a continuous permafrost region with mean annual temperatures of –14.3°C. The other site is Stordalen mire in the eastern part of Lake Torneträsk (68° N, 19° E), ten kilometres east of Abisko, northern Sweden. It is located in a discontinuous permafrost region. Stordalen has a sub arctic climate with a mean annual temperature of –0.7°C. Model input consisted of observed temperature, precipitation and snow cover data. In all cases, modelled CH4 emissions show a direct correlation between variations in water table and soil temperature variations. The differences in CH4 emissions between the two sites are caused by different climate, hydrology, soil physical properties, vegetation type and NPP. For Kytalyk the simulated CH4 fluxes show similar trends during the growing season, having average values for 2004 to 2006 between 1.29–2.09 mg CH4 m−2 h−1. At Stordalen the simulated fluxes show a slightly lower average value for the same years (3.52 mg CH4 m−2 h−1) than the observed 4.7 mg CH4 m−2 h−1. The effect of the longer growing season at Stordalen is simulated correctly. Our study shows that modelling of arctic CH4 fluxes is improved by adding a relatively simple hydrological model that simulates the water table position from generic weather data. We conclude that CH4 fluxes at these sites are less sensitive to temperature variation than to water table variations. Furthermore, parameter uncertainty at site level in wetland CH4 process models is an important factor in large scale modelling of CH4 fluxes.