Higher CO2 Efflux Research Articles

Subsurface CO2 Dynamics in Temperate Beech and Spruce Forest Stands

Rates of soil respiration (CO2 effluxes), subsurface pore gas CO2/O2 concentrations, soil temperature and soil water content were measured for 15 months in two temperate and contrasting Danish forest ecosystems: beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.). Soil CO2 effluxes showed a distinct seasonal trend in the range of 0.48–3.3 μmol CO2 m−2 s−1 for beech and 0.50–2.92 μmol CO2 m−2 s−1 for spruce and were well-correlated with near-surface soil temperatures. The soil organic C-stock (upper 1 m including the O-horizon) was higher in the spruce stand (184±23 Mg C ha−1) compared to the beech stand (93±19 Mg C ha−1) and resulted in a faster turnover time as calculated by mass/flux in soil beneath the beech stand (28 years) compared to spruce stand (60 years). Observed soil CO2 concentrations and effluxes were simulated using a Fickian diffusion-reaction model based on vertical CO2 production rates and soil diffusivity. Temporal trends were simulated on the basis of observed trends in the distribution of soil water, temperature, and live roots as well as temperature and water content sensitivity functions. These functions were established based on controlled laboratory incubation experiments. The model was successfully validated against observed soil CO2 effluxes and concentrations and revealed that temporal trends generally could be linked to variations in subsurface CO2 production rates and diffusion over time and with depths. However, periods with exceptionally high CO2 effluxes (> 20 μmol CO2 m−2 s−1) were noted in March 2000 in relation to drying after heavy rain and after the removal of snow from collars. Both cases were considered non-steady state and could not be simulated.

Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecol- ogy of agricultural soil. Carbon dioxide (CO2) and nitrous oxide (N2O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs ("organic" manage- ment) or one of low OM and inorganic fertilizer inputs ("conventional" management). In microcosms, where short- termmicrobialresponsestowettinganddryingwerestudied, the highest CO2 efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient avail- ability were elevated and a PLFA stress indicator was de- pressed,suggestingpeakmicrobialactivity.ThehighestN2O efflux in the organically managed soil (0.94 mg N2O-N m −2 h −1 ) occurred after manure and legume cover crop incorpo- ration, and in the conventionally managed soil (2.12 mg N2O-N m −2 h −1 ) after inorganic N fertilizer inputs. Elevated N2O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0-150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N2O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.

CO2 exchange in a temperate Japanese cypress forest compared with that in a cool‐temperate deciduous broad‐leaved forest

AbstractTo examine the characteristics of carbon exchange in coniferous forests, we analysed the seasonal and diurnal patterns of CO2 exchange, as measured using the eddy covariance method, in a Japanese cypress forest in the Kiryu Experimental Watershed (KEW) in central Japan. The net CO2 exchange data during periods of low‐friction velocity conditions and during periods of missing data were interpolated. The daily CO2 uptake was observed throughout the year, with maximum values occurring in early summer. Periods of low carbon uptake were seen in late summer owing to high respiratory CO2 efflux. The diurnal and seasonal patterns of daytime CO2 exchange at KEW were compared with those in a cool‐temperate deciduous forest of the Tomakomai Experimental Forest (TOEF) in Japan. The environmental differences between evergreen and deciduous forests affected the seasonal patterns of carbon uptake. Although there were great differences in the mean monthly air temperatures between the sites, the mean monthly daytime carbon uptake was almost equal at both sites during the peak growing period. The carbon‐uptake values at the same PAR level were greater before noon than after noon, especially at TOEF, suggesting the stomatal regulation of carbon uptake.

Carbon dioxide emissions from soils at Hakkoda, north Japan

A soil gas and CO2 efflux survey was carried out at Hakkoda, north Japan, to investigate the spatial distribution and origin of the gas which caused the accidental death of three Japanese soldiers on 12 July 1997. In the study area, no fumarolic and/or thermal activity is observed. Chemical composition of the gas from areas with high CO2 effluxes (&gt;5,000 g m−2 d−1) and high CO2 contents (&gt;15 vol %) indicate a strong deep sourced contribution for CO2, whereas air and/or air saturated water is suggested for the rest of the discharged gases, which are mainly released from a shallow aquifer. Isotopic analysis of soil gas and hot spring discharges suggests a combination of magmatic degassing and thermal decarbonation, with magmatic degassing being the main contributior to the high CO2 emission. Approximately 74 t d−1 of deep‐seated (magmatic plus thermal decarbonation origin) CO2 were estimated to contribute to the total output of CO2 from this area.

Site and temporal variation of soil respiration in European beech, Norway spruce, and Scots pine forests

AbstractGlobal warming and changes in rainfall amount and distribution may affect soil respiration as a major carbon flux between the biosphere and the atmosphere. The objectives of this study were to investigate the site to site and interannual variation in soil respiration of six temperate forest sites. Soil respiration was measured using closed chambers over 2 years under mature beech, spruce and pine stands at both Solling and Unterlüß, Germany, which have distinct climates and soils. Cumulative annual CO2 fluxes varied from 4.9 to 5.4 Mg C ha−1 yr−1 at Solling with silty soils and from 4.0 to 5.9 Mg C ha−1 yr−1 at Unterlüß with sandy soils. With one exception soil respiration rates were not significantly different among the six forest sites (site to site variation) and between the years within the same forest site (interannual variation). Only the respiration rate in the spruce stand at Unterlüß was significant lower than the beech stand at Unterlüß in both years. Soil respiration rates of the sandy sites at Unterlüß were limited by soil moisture during the rather dry and warm summer 1999 while soil respiration at the silty Solling site tended to increase. We found a threshold of −80 kPa at 10 cm depth below which soil respiration decreased with increasing drought. Subsequent wetting of sandy soils revealed high CO2 effluxes in the stands at Unterlüß. However, dry periods were infrequent, and our results suggest that temporal variation in soil moisture generally had little effect on annual soil respiration rates. Soil temperature at 5 cm and 10 cm depth explained 83% of the temporal variation in soil respiration using the Arrhenius function. The correlations were weaker using temperature at 0 cm (r2 = 0.63) and 2.5 cm depth (r2 = 0.81). Mean Q10 values for the range from 5 to 15 °C increased asymptotically with soil depth from 1.87 at 0 cm to 3.46 at 10 cm depth, indicating a large uncertainty in the prediction of the temperature dependency of soil respiration. Comparing the fitted Arrhenius curves for same tree species from Solling and Unterlüß revealed higher soil respiration rates for the stands at Solling than in the respective stands at Unterlüß at the same temperature. A significant positive correlation across all sites between predicted soil respiration rates at 10 °C and total phosphorus content and C‐to‐N ratio of the upper mineral soil indicate a possible effect of nutrients on soil respiration.

Short‐Term Dynamics of Nitrogen, Microbial Activity, and Phospholipid Fatty Acids after Tillage

Little is known about the short-term effects (hours to days) of tillage on labile pools of C and N, or microbial activity and community composition. We examined the effects of rototillage on microbial biomass C (MBC) and N (MBN), respiration (i.e., soil CO2 production in 1-h incubations), CO2 efflux from the soil surface, inorganic N, nitrification potential, denitrification rate, and phospholipid fatty acids (PLFA). A fallow silt loam soil was rototilled in the field and soil cores were immediately obtained from tilled and adjacent control soils. The soil cores were then incubated at constant temperature and sampled throughout a 2-wk period. Tilled soil had higher CO2 efflux than the control soil. This increase occurred immediately after tillage and lasted for 4 d. Respiration was similar in both soils until the fourth day after tillage, and then declined in the tilled soil. Tilled soil showed increases in MBN, nitrate, and denitrification rates, suggesting that tillage makes available previously protected organic N. The overall reduction in respiration together with the lack of response in MBC, however, suggests that tillage did not make available significant amounts of readily decomposable C. These combined C and N dynamics suggest that low C/N ratio compounds may have been mineralized following tillage. Denitrification rates increased in the tilled soil even though the bulk of the soil had reduced respiration and bulk density. Tillage caused temporary changes in PLFA profiles, suggesting changes in soil microbial community structure. Phospholipid fatty acid 18:1 ω7t, which marks the presence of eubacteria, decreased in the tilled soil. In contrast, 19:0 cy, a marker for anaerobic eubacteria, increased in the tilled soil. Our results show that tillage causes short-term changes in nutrient dynamics that may potentially result in N losses through denitrification and nitrate leaching, as well as C losses through degassing of dissolved CO2 These changes are accompanied by concomitant shifts in microbial community structure, suggesting a possible relationship between microbial composition and ecosystem function.

Physiological Basis for Genotypic Variation in Net Photosynthesis of Oat Leaves1

The net CO2 exchange rate of flag‐leaf blades of 20 oat (Avena spp.) genotypes was measured by infrared gas analysis in 1968 and 1969. Net photosynthetic rate and rate of CO2 efflux into CO2‐free air in light differed significantly among genotypes. Rankings were consistent across years. The CO2 compensation point was the least variable of the factors measured, and it was not related to net photosynthetic rate. A positive correlation between net photosynthesis and CO2 efflux was found; genotypes showing high CO2 efflux rates were concluded to have a lower resistance to CO2 diffusion between leaf respiratory sites and the external atmosphere. Differences in diffusion resistances, particularly the residual or mesophyll resistance, seemed to explain best the differences in net photosynthesis.Mean genotypic net photosynthetic rates were correlated with mean specific leaf dry weights (g dry weight dm‐2 leaf area). That mean residual resistance values were negatively correlated with mean specific leaf dry weight values indicates that internal leaf morphology or biochemical factors associated with morphological characteristics of the oat leaf may be associated with genotypic differences in net photosynthesis.