Variation In Soil CO2 Efflux Research Articles

Temporal Variation of Soil CO2 Efflux on Sloping Pasture of Heihe River Basin and Effects of Temperature and Soil Moisture on it

Employing LiCor 6400 gas exchange analyzer and soil respiration chamber attachment (LiCor Inc., Lincoln, NE, USA), this paper continuously measured the soil surface CO2 effluxes on the sloping pasture of Heihe River basin from early April to late October 2010 to investigate the soil CO2 efflux rate and its feedback to the changes of climate and land use. The results showed that during the growing season, the diurnal variation of pasture soil respiration in the mountain watershed of the Heihe River valley was low at night, with lowest appears at 7:00, 6:30, 5:30, 5:00, 6:00 and 7:00 from May to October, and started to rise rapidly during 7:00~8:30, and then descend during 16:00~18:30. The maximum soil CO2 efflux appears at 15:00, 14:30, 14:30, 13:30, 14:00 and 15:00. The maximum of average soil CO2 efflux occurred in July and August, and the second was in May and September, and the third was in April and October. And it was basically consistent in April and October. The diurnal average of pasture soil CO2 efflux was between 0.31~6.98?mol m-2s-1, and the Q10 value is 2.16. Soil CO2 efflux had an exponential and Boltzmann correlation with temperature and soil moisture, respectively.

Latitudinal distribution of soil CO2 efflux and temperature along the Dalton Highway, Alaska

In this paper, we investigate spatial variations in soil CO2 efflux and carbon dynamics across five sites located between 65.5°N and 69.0°N in tundra and boreal forest biomes of Alaska. Growing and winter mean CO2 effluxes for the period 2006–2010 were 261 ± 124 (Coefficients of Variation: 48%) and 71 ± 42 (CV: 59%) gCO2/m2, respectively. This indicates that winter CO2 efflux contributed 24% of the annual CO2 efflux over the period of measurement. In tundra and boreal biomes, tussock is an important source of carbon efflux to the atmosphere, and contributes 3.4 times more than other vegetation types. To ensure that representativeness of soil CO2 efflux was determined, 36 sample points were used at each site during the growing season, so that the experimental mean fell within ±20% of the full sample mean at 80% and 90% confidence levels. We found that soil CO2 efflux was directly proportional to the seasonal mean soil temperature, but inversely proportional to the seasonal mean soil moisture level, rather than to the elevation-corrected July air temperature. This suggests that the seasonal mean soil temperature is the dominant control on the latitudinal distribution of soil CO2 efflux in the high-latitude ecosystems of Alaska.

Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO 2 efflux in two semiarid grasslands in southern Botswana

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO(2) efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO(2) efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO(2) efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO(2) efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO(2) efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.

Soil carbon balance in a tropical grassland: Estimation of soil respiration and its partitioning using a semi-empirical model

In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share of ecosystem carbon is located below ground due to high root:shoot ratios. Temporal variations in soil CO2 efflux (RS) were investigated in a grassland of coastal Congo over two years. The objectives were (1) to identify the main factors controlling seasonal variations in RS and (2) to develop a semi-empirical model describing RS and including a heterotrophic component (RH) and an autotrophic component (RA). Plant above-ground activity was found to exert strong control over soil respiration since 71% of seasonal RS variability was explained by the quantity of photosynthetically active radiation absorbed (APAR) by the grass canopy. We tested an additive model including a parameter enabling RS partitioning into RA and RH. Assumptions underlying this model were that RA mainly depended on the amount of photosynthates allocated below ground and that microbial and root activity was mostly controlled by soil temperature and soil moisture. The model provided a reasonably good prediction of seasonal variations in RS (R2=0.85) which varied between 5.4μmolm−2s−1 in the wet season and 0.9μmolm−2s−1 at the end of the dry season. The model was subsequently used to obtain annual estimates of RS, RA and RH. In accordance with results reported for other tropical grasslands, we estimated that RH accounted for 44% of RS, which represented a flux similar to the amount of carbon brought annually to the soil from below-ground litter production. Overall, this study opens up prospects for simulating the carbon budget of tropical grasslands on a large scale using remotely sensed data.

Carbon fluxes in coniferous and deciduous forest soils

Our aims were to identify responsible factors for the site-to-site variability in soil CO2 efflux and to assess the sources of soil CO2 of different forest types on a regional scale. Soil CO2 effluxes were measured over 1–4 years in four coniferous and three deciduous forests of Bavaria, Germany, and related to climate, soil properties and forest productivity. Total belowground carbon allocation (TBCA) was assessed using soil CO2 effluxes and aboveground litterfall. Additionally, CO2 production of organic layers was examined over 10 months under constant conditions in an incubation experiment. Annual soil CO2 effluxes were not different among the forest sites, but predicted effluxes at a given temperature of 10°C revealed some significant differences and correlated with the phosphorous stock of the organic layers. The incubation study indicated 50% faster decomposition of organic layers from deciduous than from coniferous forests. TBCA related to soil CO2 efflux was smaller in the deciduous than in the coniferous forests. The ratio of TBCA to soil CO2 efflux was positively correlated with the C stock of organic layers. Our results suggest that marked differences in site characteristics have little impact on soil CO2 effluxes at the regional scale, but the contribution of soil CO2 sources varies among the forest types.

Seasonality in a boreal forest ecosystem affects the use of soil temperature and moisture as predictors of soil CO<sub>2</sub> efflux

Abstract. Our objectives were to identify factors related to temporal variation of soil CO2 efflux in a boreal pine forest and to evaluate simple predictive models of temporal variation of soil CO2 efflux. Soil CO2 efflux was measured with a portable chamber in a Finnish Scots pine forest for three years, with a fourth year for model evaluation. Plot averages for soil CO2 efflux ranged from 0.04 to 0.90 g CO2 m−2 h−1 during the snow-free period, i.e. May–October, and from 0.04 to 0.13 g CO2 m−2 h−1 in winter. Soil temperature was a good predictor of soil CO2 efflux. A quadratic model of ln-transformed efflux explained 76–82 % of the variation over the snow-free period. The results revealed an effect of season: at a given temperature of the organic layer, soil CO2 efflux was higher later in the snow-free period (in August and September) than in spring and early summer (in May and June). Regression coefficients for temperature (approximations of a Q10 value) of month-specific models decreased with increasing average soil temperatures. Efflux in July, the month of peak photosynthesis, showed no clear response to temperature or moisture. Inclusion of a seasonality index, degree days, improved the accuracy of temperature response models to predict efflux for the fourth year of measurements, which was not used in building of regression models. During peak efflux from mid-July to late-August, efflux was underestimated with the models that included degree days as well as with the models that did not. The strong influence of the flux of photosynthates belowground and the importance of root respiration could explain the relative temperature insensitivity observed in July and together with seasonality of growth of root and root-associated mycorrhizal fungi could explain partial failure of models to predict magnitude of efflux in the peak season from mid-July to August. The effect of moisture early in the season was confounded by simultaneous advancement of the growing season and increase in temperature. In a dry year, however, the effect of drought was evident as soil CO2 efflux was some 30 % smaller in September than in the previous wet year. Soil temperature was a good overall predictor of soil CO2 efflux, possibly partly because its apparent effect was strengthened by many environmental factors and ecosystem processes that varied in concert with its variation. However, the consistent underestimation by the predictive models for the peak season corroborates recent findings concerning the importance of seasonal changes in carbon inputs to processes producing CO2 in soil.

Annual variation in soil respiration and its component parts in two structurally contrasting woody savannas in Central Brazil

Background and aims: Due to the high spatial and temporal variation in soil CO₂ efflux, terrestrial carbon budgets rely on a detailed understanding of the drivers of soil respiration from a diverse range of ecosystems and climate zones. In this study we aim to evaluate the independent influence of vegetation structure and climate on soil CO₂ efflux within cerrado ecosystems. Methods: We examine the seasonal and diel variation of soil CO₂ efflux, including its autotrophic and heterotrophic components, within two adjacent and structurally contrasting woody savannas in central Brazil. Principle results: We found no significant difference in the annual soil CO₂ efflux between the two stands (p=0.53) despite a clear disparity in both LAI (p<0.01) and leaf litterfall (p<0.01). The mean annual loss of carbon from the soil was 17.32(± 1.48) Mg Cha⁻¹ of which approximately 63% was accounted for by autotrophic respiration. The relative contribution of autotrophic respiration varied seasonally between 55% in the wet season to 79% of the total soil CO₂ efflux in the dry season. Furthermore, seasonal fluctuations of all the soil respiration components were strongly correlated with soil moisture (R²=0.79-0.90, p<0.01). Conclusions: Across these two structurally distinct cerrado stands, seasonal variations in rainfall, was the main driver of soil CO₂ efflux and its components. Consequently, soil respiration within these ecosystems is likely to be highly sensitive to any changes in seasonal precipitation patterns.

Evaluation of methods for estimating soil carbon dioxide efflux across a gradient of forest disturbance

AbstractBetter understanding of variation in soil carbon dioxide (CO2) efflux caused by measurement techniques is needed, especially over gradients of site disturbance, to accurately estimate the global carbon cycle. We present soil CO2 efflux data from a gradient of disturbance to ponderosa pine (Pinus ponderosa C. Lawson var. scopulorum Engelm.) forests in northern Arizona, USA that were obtained using four different techniques: vented static chambers, a Licor 6400‐09, and soil CO2 diffusion profiles using two different models (Moldrup, Millington–Quirk) to estimate soil gas diffusivity. We also compared soil CO2 efflux measured by the Moldrup and Millington–Quirk diffusion profile methods to nighttime total ecosystem respiration (TER) data from an eddy covariance tower. We addressed four questions: (1) Does the use of a given method to measure soil CO2 efflux bias results across a disturbance gradient? (2) Does the magnitude of difference between observed and modeled estimates of soil CO2 differ between methods and across sites? (3) What is the spatial variability of each method at each site? (4) Which method is closest to the estimate of TER measured by the eddy covariance tower? Although soil CO2 efflux varied significantly among methods the differences were consistent among sites. Measured and modeled total growing season fluxes were generally higher for the Licor 6400‐09 and Millington–Quirk diffusion gradient methods compared with static chamber and the Moldrup diffusion gradient methods. A power analysis showed that the larger static chamber was the most efficient method at sampling spatial variation in soil CO2 efflux. Nighttime measurements of soil CO2 efflux from the Moldrup diffusion gradient method were most strongly related to nighttime TER assessed with eddy covariance. The use of a single, well‐implemented method to measure soil CO2 efflux is unlikely to create bias in comparisons across a gradient of forest disturbance.

Landscape structure control on soil CO2 efflux variability in complex terrain: Scaling from point observations to watershed scale fluxes

We investigated the spatial and temporal variability of soil CO2 efflux across 62 sites of a 393‐ha complex watershed of the northern Rocky Mountains. Growing season (83 day) cumulative soil CO2 efflux varied from ∼300 to ∼2000 g CO2 m−2, depending upon landscape position, with a median of 879.8 g CO2 m−2. Our findings revealed that highest soil CO2 efflux rates were observed in areas with persistently high soil moisture (riparian meadows), whereas lower soil CO2 efflux rates were observed on forested uplands (98% of watershed area). Furthermore, upslope accumulated area (UAA), a surrogate measure of the lateral redistribution of soil water, was positively correlated with seasonal soil CO2 efflux at all upland sites, increasing in explanatory power when sites were separated by the major aspects of the watershed (SE/NW). We used the UAA–soil CO2 efflux relationship to upscale measured CO2 efflux to the entire watershed and found watershed‐scale soil CO2 efflux of 799.45 ± 151.1 g CO2 m−2 over 83 days. These estimates compared well with independent eddy covariance estimates of nighttime ecosystem respiration measured over the forest. We applied this empirical model to three synthetic watersheds with progressively reduced complexity and found that seasonal estimates of soil CO2 efflux increased by 50, 58, and 98%, demonstrating the importance of landscape structure in controlling CO2 efflux magnitude. Our study represents an empirical quantification of seasonal watershed‐scale soil CO2 efflux and demonstrates that UAA (i.e., landscape position) and drainage patterns are important controls on the spatial organization of large‐scale (∼km2) soil CO2 efflux, particularly in semiarid, subalpine ecosystems.

Temperature controls temporal variation in soil CO2 efflux in a secondary beech forest in Appi Highlands, Japan

Forest soil is a huge reserve of carbon in the biosphere. Therefore to understand the carbon cycle in forest ecosystems, it is important to determine the dynamics of soil CO2 efflux. This study was conducted to describe temporal variations in soil CO2 efflux and identify the environmental factors that affect it. We measured soil CO2 efflux continuously in a beech secondary forest in the Appi Highlands in Iwate Prefecture for two years (except when there was snow cover) using four dynamic closed chambers that automatically open after taking measurements. Temporal changes in soil temperature and volumetric soil water content were also measured at a depth of 5 cm. The soil CO2 efflux ranged from 14 mg CO2 m−2 h−1 to 2,329 mg CO2 m−2 h−1, the peak occurring at the beginning of August. The relationship between soil temperature and soil CO2 efflux was well represented by an exponential function. Most of temporal variation in soil CO2 efflux was explained by soil temperature rather than volumetric soil water content. The Q10 values were 3.7 ± 0.8 and estimated annual carbon emissions were 837 ± 210 g C m−2 year−1. These results provide a foundation for further development of models for prediction of soil CO2 efflux driven by environmental factors.

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO2) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO2 efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO2 efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO2 efflux varied widely in space; the range of variation averaged 17.4 μmol m−2 s−1 in total. While most CO2 flux rates were under 10 μmol m−2 s−1, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO2 efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO2 efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO2 efflux, estimated from the relationship between CO2 efflux and SMC at 30 cm depth, was 165 mol m−2 year−1 (1,986 g C m−2 year−1). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO2 efflux, leading an annual variation in soil C balance.

Abiotic regulators of soil respiration in desert ecosystems

Soil temperature and soil moisture are the most important environmental factors controlling soil respiration in mesic ecosystems. However, soil respiration and associated abiotic regulators have been poorly studied in desert ecosystems. In this study, soil respiration was measured using an automated CO2 efflux system (LI-COR 8100), and the effects of soil temperature and moisture on the rate of soil respiration were examined in six desert sites [three communities—Haloxylon ammodendron, Halostachys caspica and Anabasis aphylla at high (B) and low (A) vegetation coverage respectively]. It was found that soil respiration was significantly and positively correlated with soil surface temperature. A multi-variable model of soil temperature and soil moisture could explain 61.9% of temporal variation in soil CO2 efflux at a larger scale. There were significantly negative correlations between soil respiration and soil moisture in Haloxylon ammodendron B and Halostachys caspica B sites, which represented the driest and wettest sites, respectively. The results also showed that soil respiration displayed obvious diurnal and seasonal patterns during the growing season. The Q10 values for Haloxylon ammodendron A and B, Halostachys caspica A and B, and Anabasis aphylla A and B sites were 1.3, 1.34, 1.58, 1.65, 1.31 and 1.17, respectively, with a cross-site average of 1.39. The results showed that soil respiration was not positively correlated with soil moisture unlike in most mesic ecosystems. However, soil respiration in desert ecosystems is less sensitive to temperature variation than most mesic ecosystems as indicated by the lower Q10 values possibly due to energy limitation.

Temporal variation in CO2 efflux from soil and snow surfaces in a Japanese cedar (Cryptomeria japonica) plantation, central Japan

AbstractCO2 efflux from soil and snow surfaces was measured continuously in a Japanese cedar (Cryptomeria japonica D. Don) forest in central Japan using an open dynamic chamber system. The chamber opens and closes automatically and records measurements based on an open‐flow dynamic method. Between May and December, mean soil CO2 efflux ranged from 1,529 mg CO2 m−2 h−1 in September to 255 mg CO2 m−2 h−1 in December. The seasonal change in CO2 efflux from the soil paralleled the seasonal pattern of soil temperature. No marked diurnal trends in soil CO2 efflux were observed on days without rainfall, whereas significant pulses in soil CO2 efflux were observed on days with rainfall. In this plantation, soil CO2 efflux frequently responded to rainfall. Measurements of changes from litter‐covered soil to snow‐covered surfaces revealed that CO2 efflux decreased from values of ca. 250 mg CO2 m−2 h−1 above soil to less than 33 mg CO2 m−2 h−1 above snow. Soil temperature alone explained 66% of the overall variation in soil CO2 efflux, but explained approximately 85% of the variation when data from two anomalous periods were excluded. Moreover, we found a significant correlation between soil CO2 efflux and soil moisture (which explained 44% of the overall variation) using a second‐order polynomial function. Our results suggest that the seasonality of CO2 efflux is affected not only by soil temperature and moisture, but also by drying and rewetting cycles and by litterfall pulses.

Source components and interannual variability of soil CO2 efflux under experimental warming and clipping in a grassland ecosystem

AbstractPartitioning soil CO2 efflux into autotrophic (RA) and heterotrophic (RH) components is crucial for understanding their differential responses to climate change. We conducted a long‐term experiment (2000–2005) to investigate effects of warming 2°C and yearly clipping on soil CO2 efflux and its components (i.e. RA and RH) in a tallgrass prairie ecosystem. Interannual variability of these fluxes was also examined. Deep collars (70 cm) were inserted into soil to measure RH. RA was quantified as the difference between soil CO2 efflux and RH. Warming treatment significantly stimulated soil CO2 efflux and its components (i.e. RA and RH) in most years. In contrast, yearly clipping significantly reduced soil CO2 efflux only in the last 2 years, although it decreased RH in every year of the study. Temperature sensitivity (i.e. apparent Q10 values) of soil CO2 efflux was slightly lower under warming (P&gt;0.05) and reduced considerably by clipping (P&lt;0.05) compared with that in the control. On average over the 4 years, RH accounted for approximately 65% of soil CO2 efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of RH to soil CO2 efflux reached a maximum in winter (∼90%) and a minimum in summer (∼35%). Annual soil CO2 efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO2 efflux may be mainly caused by precipitation distribution and summer severe drought. Our results suggest that the effects of warming and yearly clipping on soil CO2 efflux and its components did not result in significant changes in RH or RA contribution, and rainfall timing may be more important in determining interannual variability of soil CO2 efflux than the amount of annual precipitation.

Impacts of an anomalously warm year on soil CO2 efflux in experimentally manipulated tallgrass prairie ecosystems

AbstractModeling analyses suggest that an increase in growth rate of atmospheric CO2 concentrations during an anomalously warm year may be caused by a decrease in net ecosystem production (NEP) in response to increased heterotrophic respiration (Rh). To test this hypothesis, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large dynamic flux chambers (Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs)). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures. During the second year, air temperature in two EcoCELLs was increased by 4°C throughout the year to simulate anomalously warm conditions. This paper reports on the effect of warming on soil CO2 efflux, representing the sum of autotrophic respiration (Ra) and Rh.During the pretreatment year, weekly average soil CO2 efflux was similar in all EcoCELLs. During the late spring, summer and early fall of the treatment year, however, soil CO2 efflux was significantly lower in the warmed EcoCELLs. In general, soil CO2 efflux was correlated with soil temperature and to a lesser extent with moisture. A combined temperature and moisture regression explained 64% of the observed variation in soil CO2 efflux. Soil CO2 efflux correlated well with a net primary production (NPP) weighted greenness index derived from digital photographs. Although separate relationships for control and warmed EcoCELLs showed better correlations, one single relationship explained close to 70% of the variation in soil CO2 efflux across treatments and years. A strong correlation between soil CO2 efflux and canopy development and the lack of initial response to warming indicate that soil CO2 efflux is dominated by Ra. This study showed that a decrease in soil CO2 efflux in response to a warm year was most likely dominated by a decrease in Ra instead of an increase in Rh.

Spatial and temporal variation in soil CO 2 efflux in an old-growth neotropical rain forest, LA Selva, Costa Rica

Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO2 release. We measured soil CO2 efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (‘old alluvium’) and the other three were developed on old lava flows (‘residual’). At the same time we measured soil CO2 concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between ‘old alluvium’ sites, the two-year average CO2 flux rates ranged from 117.3 to 128.9 mg C m−2 h−1. Significantly higher soil CO2 flux occurred on the ‘residual’ sites (141.1 to 184.2 mg C m−2 h−1). Spatial differences in CO2 efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO2 storage was high and the amount of CO2 stored in the upper and lower soil profile was different between ‘old alluvial’ and ‘residual’ sites. The major factor identified for explaining temporal variations in soil CO2 efflux was soil water content. During periods of high soil water content CO2 emission decreased, probably due to lower diffusion and CO2 production rates. During the 2-year study period inter-annual variation in soil CO2 efflux was not detected.

Soil‐surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California

AbstractSoil‐surface CO2 efflux and its spatial and temporal variations were examined in an 8‐y‐old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements of soil CO2 efflux, soil temperatures and moisture were conducted on two 20 × 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO2 efflux in the plantation was 4.43 µmol m−2 s−1 in the growing season and 3.12 µmol m−2 s−1 in the nongrowing season. These values are in the upper part of the range of published soil‐surface CO2 efflux data. The annual maximum and minimum CO2 efflux were 5.87 and 1.67 µmol m−2 s−1, respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation of CO2 efflux was relatively small (&lt; 20%) with the minimum appearing around 09.00 hours and the maximum around 14.00 hours. Using daytime measurements of soil CO2 efflux tends to overestimate the daily mean soil CO2 efflux by 4–6%. The measurements taken between 09.00 and 11.00 hours (local time) seem to better represent the daily mean with a reduced sampling error of 0.9–1.5%. The spatial variation of soil CO2 efflux among the 18 sampling points was high, with a coefficient of variation of approximately 30%. Most (84%) of the spatial variation was explained by fine root biomass, microbial biomass, and soil physical and chemical properties. Although soil temperature and moisture explained most of the temporal variations (76–95%) of soil CO2 efflux, the two variables together explained less than 34% of the spatial variation. Microbial biomass, fine root biomass, soil nitrogen content, organic matter content, and magnesium content were significantly and positively correlated with soil CO2 efflux, whereas bulk density and pH value were negatively correlated with CO2 efflux. The relationship between soil CO2 efflux and soil temperature was significantly controlled by soil moisture with a Q10 value of 1.4 when soil moisture was &lt;14% and 1.8 when soil moisture was &gt;14%. Understanding the spatial and temporal variations is essential to accurately assessment of carbon budget at whole ecosystem and landscape scales. Thus, this study bears important implications for the study of large‐scale ecosystem dynamics, particularly in response to climatic variations and management regimes.

Separating the effects of moisture and temperature on soil CO2 efflux in a coniferous forest in the Sierra Nevada mountains

Separating the effects of soil temperature and moisture on soil CO2 efflux is critical to modeling and understanding the belowground carbon dynamics of forest ecosystems. We developed two analytical procedures to separate the effects of soil temperature and moisture, based on continuous measurements of the CO2 efflux, temperature and moisture of the soil at a ponderosa pine plantation in the Sierra Nevada Mountains in California, from May 1998 to August 1999. We found that the combined effects of temperature and moisture on the seasonal variation of soil CO2 efflux could be effectively separated and represented with the product of a temperature term and a moisture term. The relationship between soil CO2 efflux and temperature could be well described using a power function. This relationship was modified by soil moisture which affects only the coefficient, but not the exponent, of the power function. We also found that when soil moisture was held constant, the temperature effect explained 82% of the temporal variation in CO2 efflux of the soil. Similarly, when temperature was held constant, the moisture effects explained 84% of the variation. Temperature and moisture together explained 89% of the total temporal variations in soil CO2 efflux. A multiplicative formulation with power functions representing both temperature and moisture dependences was recommended for modeling soil CO2 efflux. This formulation can be used to model the seasonal trend of soil CO2 efflux of the forest based on temperature and moisture, two key variables influenced by climate change and management practices.

CO2 efflux from a Mediterranean semi-arid forest soil. I. Seasonality and effects of stoniness

We studied the seasonality of total soil CO2efflux and labeled C-CO2 released from 14Clabeled straw incubated in the H horizon of asemi-arid Mediterranean forest soil. Fieldmeasurements were carried out over 520 days in aseries of reconstructed soil profiles with and withouta gravel layer below the H horizon. We monitored soilclimate and related this to soil CO2 efflux.Seasonal variations in soil CO2 efflux in asemiarid Mediterranean forest were mainly related tochanges in soil temperature. In spite of drought, highrespiration rates were observed in mid summer. Highsoil CO2 efflux in hot and dry episodes wasattributed to increases in soil biological activity.The minimum soil CO2 efflux occurred in latesummer also under dry conditions, probably related toa decrease in soil biological activity in deephorizons. Biological activity in organic layers waslimited by water potential (Ψ) in summer and bytemperature in winter. Rewetting a dry soil resultedin large increases in soil CO2 efflux only at hightemperatures. These large increases represented asignificant contribution to the decomposition oforganic matter in the uppermost horizons. Soilbiological activity in the uppermost horizons was moresensitive to changes in soil Ψ and hence tosummer rainstorms than the bulk soil microbialactivity. The presence of a layer of gravel improvedboth moisture and temperature conditions for thedecomposition of organic matter. As a result, soilCO2 efflux increased in soils containing rockfragments. These effects were especially large for theorganic layers.

Spatial variability in forest soil CO2 efflux assessed with a calibrated soda lime technique

Accurate estimates of soil CO2 efflux are important in the current discussion on the carbon balance of forests, and can be used to validate models and remotely sensed data. Due to the typically large spatial variability, large sample numbers are required to estimate mean forest soil CO2 efflux with reasonable confidence intervals. Most infrared gas analysis (IRGA) systems are not well suited to simultaneously produce daily means and handle this spatial variability problem. The soda lime technique gives daily means and allows the required large sample numbers, but is less accurate than the IRGA systems. Using an elaborate cross‐calibration, we tried to combine the accuracy of an IRGA method with the spatial integration potential of the soda lime technique. This paper reports on the calibration technique used to improve the accuracy of the soda lime technique and confirms the spatial variability in soil CO2 efflux in a heterogeneous forest.