Soil Surface CO2 Efflux Research Articles

Differential responses of soil CO2 dynamics along soil depth to rainfall patterns in the Chinese Loess Plateau

Soil surface carbon dioxide (CO2) efflux not only originates from topsoils, but also significantly involves contributions from deeper soil layers. Soil surface CO2 efflux significantly fluctuated with rainfall patterns in arid and semiarid regions. However, how soil CO2 dynamics respond at different soil depths to varying rainfall patterns remains largely unclear. To address this gap, we continuously monitored soil CO2 concentrations, temperature, and moisture content at 10 cm, 50 cm, and 100 cm depths in situ under cropland and orchards located in the semiarid Loess Plateau over a full year. Rainfall events were meticulously recorded, categorizing them into light (<10 mm), moderate (10 mm–40 mm), and heavy (>40 mm) to discern their impact on soil CO2 dynamics. Specifically, soil CO2 flux was not affected during light rainfall. Moderate and heavy rainfall decreased soil CO2 flux at 0–10 cm by an average of 70% and 83%, respectively. This decrease was associated with reduced gas diffusivity across rainfall patterns. For instance, heavy rainfall reduced gas diffusivity by an average of 83% and 53% at 10 cm and 50 cm soil depths, respectively. Furthermore, soil CO2 concentrations slightly dropped as soil temperature decreased at 10 cm depth during light rainfall. Soil CO2 concentrations at 10 cm and 50 cm depths initially decreased by up to 15% and subsequently increasing by up to 52% during moderate and heavy rainfall. This response was likely influenced by temperature reductions and subsequent rises in moisture content, with a hysteretic response of soil CO2 concentrations to temperature. The rapid increase in soil CO2 concentrations was mainly due to a substantial decrease in gas diffusivity. Notably, heavy rainfall induced a delayed increase in soil moisture content at 50 cm depth and a significant decrease in CO2 concentration by 16% at 100 cm depth. A substantial decrease in soil CO2 concentrations in deep soil layers was primarily related to decreased soil temperature. Additionally, the observed soil CO2 dynamics were partly attributed to biotic factors (microbial biomass carbon and root density) mainly on cropland, but mainly abiotic factors (soil organic carbon and bulk density) under orchards. Overall, these results suggest that reduced gas diffusivity triggered by increased soil moisture content in topsoils and weakened biological processes caused by decreased soil temperature in deep soils typically drive the differential responses of soil CO2 dynamics to rainfall patterns.

Effects of plastic mulching on soil CO2 efflux in a cotton field in northwestern China

In Northwestern China, more and more traditional cultivation system (TC) with no mulching and flood irrigation have been replaced by modern cultivation technology (MC) combining plastic film mulching with drip irrigation. Does plastic film mulching increase or reduce soil CO2 emission in arid areas? In order to study the effects of plastic mulching on soil CO2 efflux, a field study was conducted to compare soil CO2 concentration, soil CO2 efflux, soil temperature and moisture between the TC treatment and the MC treatment during a cotton growing season in Northwestern China. The seasonal patterns of soil profile temperature and soil moisture in the TC treatment were similar to that in the MC treatment. The mean value of soil profile temperature in the MC treatment was higher than that in the TC treatment. Except for soil moisture at 15 cm depth, the mean value of soil moisture at 5 cm and 10 cm depths in the MC treatment was higher than that in the TC treatment. The variation patterns of soil CO2 concentration and soil CO2 efflux in MC treatment were different to that in the TC treatment. Although the peak of soil CO2 concentration in the TC treatment was earlier than that in the MC treatment, the duration of soil CO2 concentration with high values in TC treatment was shorter than that in the MC treatment. Based on the model of Fick’s first diffusion law, soil surface CO2 efflux in the MC and TC treatments were determined. The surface CO2 efflux in the TC treatment calculated by Fick’s first diffusion law model was in good agreement with the value measured by chamber method. The seasonal curve of soil surface CO2 efflux in the MC treatment indicate the similar pattern with that in the TC treatment, and the rate of CO2 efflux was lower in the MC system. In the MC treatment, the seasonal variation of soil surface efflux was explained more by soil moisture than by soil temperature. However, in the TC treatment, the seasonal variation of soil surface efflux was explained more by soil temperature than by soil moisture. Over the completely experimental period, accumulated rates of soil CO2 efflux were 361 g C m−2 and 474 g C m−2 for the MC and TC system, respectively. We concluded that converting agricultural practices from traditional cultivation to the plastic mulching cultivation could reduce soil CO2 efflux by approximately 110 g C m−2 year−1 in agricultural land in arid areas of Northwestern China.

Soil carbon dioxide effluxes from riparian areas of two hydrogeomorphic settings in the Ozark National Forest, USA

Riparian buffers serve as conduits through which carbon (C) cycles from terrestrial landscapes to aquatic systems and, ultimately, to oceanic or atmospheric sinks. Riparian areas have also been identified as zones of concentrated biogeochemical activity that often produce greater amounts of greenhouse gases than neighboring upland terrestrial and adjacent aquatic systems. Little work has explored how hydrologic classification of a stream may influence the magnitude of riparian soil processes. A three-way factorial field study examined soil carbon dioxide (CO2) effluxes across flow regime (Groundwater and Runoff streams), season (autumn, winter, spring, and summer), and distance from stream edge (0, 10, and 20 m). Soil CO2 efflux differed between flow regimes across seasons (P = 0.01), where Runoff sites had greater spring soil surface CO2 efflux (i.e., soil respiration mean ± 1 SE = 2.98 ± 0.29 μmol CO2 m−2 s−1) than Groundwater sites (1.40 ± 0.30 μmol CO2 m−2 s−1). Soil CO2 effluxes from Runoff sites were related to soil temperature (rs value range = 0.58 to 0.82, all P < 0.0001) and soil moisture (rs value range = −0.35 to 0.27, P = 0.02 to 0.03). However, effluxes were unrelated to soil moisture at Groundwater sites. Results of this study suggest that consideration of the riparian zone and demonstrated differences in flow regime are needed to understand forested stream C budgets. Anthropogenic landscape alteration and climate change can modify the natural flow regime which, in turn, may have consequences for aquatic C dynamics in streams draining altered riparian zones, particularly in forested settings.

Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest.

Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil-surface CO2 efflux ("soil respiration"; FS ) in nine plots organized across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q10 ) of above- and belowground C fluxes; and (c) quantify stand-level C allocation responses to MAT and AR. Averaged across all years, FS , litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF and FS , but increased litterfall, with patterns representing interacting responses to declining light. The Q10 of FS , litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above- and belowground C fluxes to rising MAT (Q10 of FS , litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

The influence of stocking levels, clone, fertilization, and weed control on surface CO2 efflux in a mid-rotation Pinus radiata D. Don plantation in Canterbury, New Zealand

Silvicultural practices applied in managed forest plantations may help counteract the effects of climate change by influencing soil surface CO2 efflux (Fs). Understanding the effects of silvicultural practices on Fs will provide unbiased estimates of carbon fluxes and allow better silvicultural decisions for carbon sequestration. Therefore, we assessed how Fs differed seasonally across silvicultural practices (i.e., stocking levels, clone, fertilization and weed control treatments) and evaluated the effects of soil temperature (Ts) and soil volumetric water content (θv) on Fs across these practices for a mid-rotation (14 year-old) Pinus radiata plantation in the Canterbury region of New Zealand. There were significant differences in Fs (p < 0.05) over the four seasons, three levels of stocking, and five clones. The effects of fertilization and weed control applied 12 years previously on Fs were insignificant. Annual estimate of Fs (mean ± 1 standard deviation) from the study site was 22.7 ± 7.1 t ha−1 a−1 in the form of CO2 (6.2 ± 2.1 t ha−1 a−1 in the form of C). Fs values were consistently higher in plots with 1250 stems ha−1 compared to 2500 stems ha−1, which may be related to a strong soil resource limitation because of the close spacing in the latter plantation. Significant differences in Fs across clones suggest that variations in carbon partitioning might explain their growth performance. Silvicultural treatments influenced Fs response to soil temperature (p < 0.05), resulting in models explaining 28–49% of the total variance in Fs. These findings provide insights into how silvicultural management decisions may impact Fs in mid-rotation radiata pine plantations, contributing towards developing more precise and unbiased plantation carbon budgets.

Importance of CO2 production in subsoil layers of drained tropical peatland under mature oil palm plantation

Expansion of oil palm (Elaeis guineensis Jacq.) plantations in tropical peatlands is always followed by a lowering groundwater level, which exposes peat to oxidic condition and releases large amounts of carbon dioxide (CO2) to atmosphere. This study is concerned with decomposition of organic matter in the soil profile of tropical peatland under oil palm plantation. For 2 years, CO2 fluxes were measured from the soil surface (CO2-efflux) and from the surface where peat was removed to the depths of 10, 30 and 50 cm (CO2 efflux potential/CO2-EP). CO2-EP is a potential estimation of organic matter decomposition from subsoil. Groundwater level, soil moisture and soil temperature were measured as soil environmental factors. The annual CO2 efflux was 139 mg m–2 h–1 while CO2-EP increased with depth which were 1032, 1209, 2304 mg m–2 h–1 from 10, 30, 50 cm depths, respectively. CO2 efflux did not significantly correlated with soil environmental factors. CO2-EP had a significant negative correlation with groundwater level and soil moisture at depths of 30 cm and 50 cm. There are significant correlations among CO2 released from 0, 10, 30, and 50 cm depths. These results indicated that the formation of CO2 in the deeper peat contributes greatly to the emission of CO2, and that keeping the groundwater level at a high level will greatly contribute to the suppression of CO2 emissions from peatland. The subsoil of cultivated peatland is considered to be vulnerable, and its exploitation should be avoided.

Effects of nitrogen fertilizer, soil temperature and moisture on the soil-surface CO2 efflux and production in an oasis cotton field in arid northwestern China

In several studies of agricultural ecosystems, researchers have focused on the soil-surface carbon dioxide (CO2) effluxes, but the nature of CO2 production in the soil profile and its influencing factors remain unclear. In this study, the soil-surface CO2 effluxes in an oasis cotton field were measured using the chamber method, and the CO2 concentrations were used to estimate the CO2 production in different layers of the soil profile using the gradient method. The soil CO2 concentrations increased with increasing soil depth, whereas CO2 production decreased with increasing soil depth. Both soil-surface CO2 effluxes and CO2 production in the 0–40cm layers exponentially increased with increasing temperature. Irrigation temporarily reduced the soil-surface CO2 effluxes by 19–63% through inhibiting CO2 production in the 10–40cm layer but did not affect the CO2 production in the 0–10cm layer. CO2 production mainly occurred in the 0–10cm layer, and this cumulative production accounted for 63–67% of the total production throughout the soil profile (0–40cm). The application of nitrogen (N) fertilizer enhanced the rate of CO2 production in the 0–20cm layer by increasing the root biomass and soil mineral N content. A positive correlation was detected between the soil-surface CO2 efflux and soil NO3− content in 2015, but no significant correlations were found between the soil-surface CO2 efflux and soil NH4+ contents in any treatment. A higher soil-surface CO2 efflux was observed under high soil temperature and a certain soil moisture range (0.21–0.23cm3cm−3). An analysis of the soil profile revealed higher CO2 production rates detected in the 0–10cm layer under high soil temperature and moisture conditions, but higher rates were observed under high soil temperature and low soil moisture conditions in the 10–20cm layer. Therefore, our results suggest that the effects of fertilization, soil temperature and moisture on CO2 production vary depending on the soil depth. These findings might improve our understanding of the mechanisms underlying soil respiration in soil profiles.

Separating component parts of soil respiration under Robinia pseudoacacia plantation in the Taihang Mountains, China

Partitioning the respiratory components of soil surface CO2 efflux is important in understanding carbon turnover and in identifying the soil carbon sink/source function in response to land-use change. The sensitivities of soil respiration components on changing climate patterns are currently not fully understood. We used trench and isotopic methods to separate total soil respiration into autotrophic (RA) and heterotrophic components (RH). This study was undertaken on a Robinia pseudoacacia L. plantation in the southern Taihang Mountains, China. The fractionation of soil 13CO2 was analyzed by comparing the δ13C of soil CO2 extracted from buried steel tubes with results from Gas Vapor Probe Kits at a depth of 50 cm at the preliminary test (2.03‰). The results showed that the contribution of autotrophic respiration (fRA) increased with increasing soil depth. The contribution of heterotrophic respiration (fRH) declined with increasing soil depth. The contribution of autotrophic respiration was similar whether estimated by the trench method (fRA, 23.50%) or by the isotopic method in which a difference in value of 13C between soil and plant prevailed in the natural state (RC, 21.03%). The experimental error produced by the trench method was insignificant as compared with that produced by the isotopic method, providing a technical basis for further investigations.

CO2 emission response to different water conditions under simulated karst environment

Habitat degradation has been proven to result associated with drought in karst region in south China. However, how this drought condition relates to CO2 efflux is not clear. In this study, we designed a simulated epikarst water–rock (limestone)–soil–plant columns, under varying water levels (treatment), and monitored CO2 concentration and efflux in soil in different seasons during 2011. The results showed that increased soil water greatly enhanced CO2 concentrations. With which treatment with epikarst water (WEW) had higher CO2 concentration than without epikarst water (WOEW). This was particularly high in low soil water treatment and during high temperature in the summer season. Under 30–40 % relative soil water content (RSWC), CO2 concentration in WEW treatment was 1.44 times of WOEW; however, under 90–100 % RSWC, this value was smaller. Comparatively, soil surface CO2 efflux (soil respiration) was 1.29–1.94 μmol m−2 s−1 in WEW and 1.35–2.04 μmol m−2 s−1 in WOEW treatment, respectively. CO2 efflux increased with increasing RSWC, but it was not as sensitive to epikarst water supply as CO2 concentration. WEW tended to weakly influence CO2 efflux under very dry or very wet soil condition and under low temperature. High CO2 efflux in WEW occurred under 50–80 % RSWC during summer. Both CO2 concentrations and CO2 efflux were very sensitive to temperature increase. As a result, at degraded karst environment, increased temperature may enhance CO2 concentration and CO2 emission; meanwhile, the loss of epikarst and soil water deficiency may decrease soil CO2 concentration and CO2 emission, which in turn may decrease karst corrosion.

Minimising methodological biases to improve the accuracy of partitioning soil respiration using natural abundance 13C

Microbial degradation of soil organic matter (heterotrophic respiration) is a key determinant of net ecosystem exchange of carbon, but it is difficult to measure because the CO2 efflux from the soil surface is derived not only from heterotrophic respiration, but also from plant root and rhizosphere respiration (autotrophic). Partitioning total CO2 efflux can be achieved using the different natural abundance stable isotope ratios (δ(13)C) of root and soil CO2. Successful partitioning requires very accurate measurements of total soil efflux δ(13)CO2 and the δ(13)CO2 of the autotrophic and heterotrophic sources, which typically differ by just 2-8‰. In Scottish moorland and grass mesocosm studies we systematically tested some of the most commonly used techniques in order to identify and minimise methodological errors. Typical partitioning methods are to sample the total soil-surface CO2 efflux using a chamber, then to sample CO2 from incubated soil-free roots and root-free soil. We investigated the effect of collar depth on chamber measurements of surface efflux δ(13)CO2 and the effect of incubation time on estimates of end-member δ(13)CO2. (1) a 5 cm increase in collar depth affects the measurement of surface efflux δ(13)CO2 by -1.5‰ and there are fundamental inconsistencies between modelled and measured biases; (2) the heterotrophic δ(13)CO2 changes by up to -4‰ within minutes of sampling; we recommend using regression to estimate the in situ δ(13)CO2 values; (3) autotrophic δ(13)CO2 measurements are reliable if root CO2 is sampled within an hour of excavation; (4) correction factors should be used to account for instrument drift of up to 3‰ and concentration-dependent non-linearity of CRDS (cavity ringdown spectroscopy) analysis. Methodological biases can lead to large inaccuracies in partitioning estimates. The utility of stable isotope partitioning of soil CO2 efflux will be enhanced by consensus on the optimum measurement protocols and by minimising disturbance, particularly during chamber measurements.

Accounting for Time-Variable Soil Porosity Improves the Accuracy of the Gradient Method for Estimating Soil Carbon Dioxide Production

Soil porosity is usually taken as a constant over time for a given field, although in reality it decreases with time after tillage. For the gradient method, estimating soil CO2 production with a fixed porosity may lead to large errors when soil porosity varies over time. In this study, we compared soil air-filled porosity, gas diffusivity, and CO2 production based on a temporally variable soil porosity (ϕV) with those based on a constant porosity, either initial porosity just after soil tillage (ϕi) or final porosity at harvest after a tilled soil has settled (ϕf). Soil porosity was measured seven times during a maize (Zea mays L.) growing season, and an exponential relationship of soil porosity with time was developed to describe ϕV for the 0- to 5-cm soil layer. Soil CO2 production was estimated from the gradient method and the mass conservation law. Soil-surface CO2 efflux was measured with a dynamic chamber throughout the growing season. The ϕi value was 0.49 m3 m−3 and the ϕf value was 0.43 m3 m−3. Compared with results obtained from ϕV, soil air-filled porosity, gas diffusivity, and CO2 production values obtained from ϕf were 6, 11, and 22% lower, whereas values obtained from ϕi were 17, 36, and 70% larger. The soil-surface CO2 effluxes estimated with ϕV better matched the chamber values than did the estimates with ϕi or ϕf. We conclude that use of variable soil porosity improves estimations of soil-surface CO2 effluxes and soil CO2 production with the gradient method.

Contrasting genotypes, soil amendments, and their interactive effects on short-term total soil CO2 efflux in a 3-year-old Pinus taeda L. plantation

Intensively managed pine forests in the southeastern United States are considered an important C sink and may play a critical role in offsetting increased global CO2 emissions. The combination of improved silvicultural methods and the use of superior genotypes are estimated to result in future volume gains of up to 60 percent. However to date, no work has looked at whether selection of elite genotypes could influence soil C dynamics, which could decrease the time necessary for the stand to function as a C sink. We evaluated the effects of contrasting loblolly pine genotypes on total soil surface CO2 efflux (FS) and heterotrophic respiration (RH) under two soil amendment treatments: 1.) fertilization and 2.) logging residue (LR) incorporation. We found an immediate and sustained difference in FS (p = 0.05) and RH (p < 0.01) among our two genotypes throughout the first two years of stand development. Our soil amendment treatments did not significantly change FS, but did influence RH. LR increased (p = 0.05) RH while N and P fertilization induced a slight (p = 0.06) decrease throughout the study. Our genotypes differed (p = 0.05) in their temperature response of FS, which resulted in an 11% difference in total cumulative C loss from the soil over the duration of the study. We hypothesize that observed treatment effects in FS and RH are largely due to differences in belowground C allocation among genotypes, which is supported by others that have looked at fine-root standing crop and turnover on these same genotypes. This work underscores the importance of accounting for differences among genotypes when developing stand-level C estimates.

Carbon cycling and sequestration in a Japanese red pine (Pinus densiflora) forest on lava flow of Mt. Fuji

AbstractBiometric‐based carbon flux measurements were conducted in a pine forest on lava flow of Mt. Fuji, Japan, in order to estimate carbon cycling and sequestration. The forest consists mainly of Japanese red pine (Pinus densiflora) in a canopy layer and Japanese holly (Ilex pedunculosa) in a subtree layer. The lava remains exposed on the ground surface, and the soil on the lava flow is still immature with no mineral soil layer. The results showed that the net primary production (NPP) of the forest was 7.3 ± 0.7 t C ha−1 year−1, of which 1.4 ± 0.4 t C ha−1 year−1 was partitioned to biomass increment, 3.2 ± 0.5 t C ha−1 year−1 to above‐ground fine litter production, 1.9 t C ha−1 year−1 to fine root production, and 0.8 ± 0.2 t C ha−1 year−1 to coarse woody debris. The total amount of annual soil surface CO2 efflux was estimated as 6.1 ± 2.9 t C ha−1 year−1, using a closed chamber method. The estimated decomposition rate of soil organic matter, which subtracted annual root respiration from soil respiration, was 4.2 ± 3.1 t C ha−1 year−1. Biometric‐based net ecosystem production (NEP) in the pine forest was estimated at 2.9 ± 3.2 t C ha−1 year−1, with high uncertainty due mainly to the model estimation error of annual soil respiration and root respiration. The sequestered carbon being allocated in roughly equal amounts to living biomass (1.4 t C ha−1 year−1) and the non‐living C pool (1.5 t C ha−1 year−1). Our estimate of biometric‐based NEP was 25 % lower than the eddy covariance‐based NEP in this pine forest, due partly to the underestimation of NPP and difficulty of estimation of soil and root respiration in the pine forest on lava flows that have large heterogeneity of soil depth. However, our results indicate that the mature pine forest acted as a significant carbon sink even when established on lava flow with low nutrient content in immature soils, and that sequestration strength, both in biomass and in soil organic matter, is large.

Responses of seasonal and diurnal soil CO2 effluxes to land-use change from paddy fields to Lei bamboo (Phyllostachys praecox) stands

Land-use change often markedly alters soil carbon (C) dynamics such as soil surface CO2 efflux. This study aims to test the hypotheses that converting paddy fields to bamboo stands would markedly reduce soil CO2 efflux and their temperature sensitivity (change of soil CO2 efflux rate by increasing 10 °C of temperature), and change the relationship between soil CO2 efflux and other environmental factors. A 12-month field study was conducted to measure the seasonal and diurnal soil CO2 effluxes in three adjacent paddy field-bamboo forest pairs with the automated soil CO2 flux system (LI-8100). Results showed that soil CO2 effluxes from both of the two land-uses had distinct seasonal patterns, and were reduced from 45.4 to 34.7 t CO2 ha−1 yr−1 in cumulative CO2 emissions when paddy fields were converted to bamboo stands. About 80% of the variation in soil respiration in the bamboo stands was explained by soil temperature; however, a positive relationship between soil CO2 efflux and soil temperature in the paddy field was observed only when the soil was not submerged under water, indicating that soil water saturation in the paddy fields altered the soil CO2 efflux–temperature relationship. A negative relationship (P < 0.01) between soil CO2 efflux and soil moisture was observed in the paddy fields, while no such relationship was observed in the bamboo stands. The apparent temperature sensitivity of soil respiration (Q10) was dependent on the depth of the soil temperature measurement and was increased by converting paddy fields to bamboo stands, rejecting the hypothesis. In Lei bamboo stands, the R2 for the soil respiration-temperature regression was higher using seasonal and diurnal CO2 efflux data together than using the seasonal data alone. We conclude that the conversion of paddy fields to Lei bamboo stands decreased the annual soil CO2 efflux but increased its temperature sensitivity, and altered the relationship between soil respiration and soil moisture. When calculating the Q10, the soil temperature measurement depth and data with diurnal timescale should be taken into account. If such land-use conversion effects are confirmed over the subtropical region in China in future research, this land-use conversion could increase C sequestration in the ecosystem and help mitigate climate change.

Vertical soil–air CO2 dynamics at the Takayama deciduous broadleaved forest AsiaFlux site

At the Takayama deciduous broadleaved forest Asiaflux site in Japan, the ecosystem carbon dynamics have been studied for more than two decades. In 2005, we installed non-dispersive infrared CO2 sensors in the soil below the site's flux tower to systematically study vertical soil–air CO2 dynamics and explain the behavior of soil surface CO2 efflux. Soil–air CO2 concentrations measured from June 2005 through May 2006 showed sinusoidal variation, with maxima in July and minima in winter, similar to the soil CO2 effluxes measured simultaneously using open-flow chambers. Soil–air CO2 concentrations increased with soil depth from 5 to 50 cm: from 2,000 to 8,000 ppm in the summer and from 2,000 to 3,000 ppm in the winter under snow. Summer soil–air CO2 concentrations were positively correlated with soil moisture on daily and weekly scales, indicating that the Oi, Oe, and A horizons, where decomposition is accelerated by high-moisture conditions, contributed substantially to CO2 emissions. This result is consistent with the short residence time (about 2 h) of CO2 in the soil and larger emissions in shallow soil layers based on our diffusion model. We revealed for the first time that soil–air CO2 concentrations in winter were correlated with both snow depth and wind speed. CO2 transfer through the snow was hundreds of times the gas diffusion rates in the soil. Our estimate of the CO2 efflux during the snow-cover season was larger than previous estimates at TKY, and confirmed the important contribution of the snow-cover season to the site's carbon dynamics.

Soil surface CO2 efflux measurements in Norway spruce forests: Comparison between four different sites across Europe — from boreal to alpine forest

Extensive measurements of soil surface (including vegetation cover) CO2 efflux were carried out on 80 positions at four different forest sites (Sweden, Germany, Czech Republic and Italy) using a closed dynamic chamber technique. The period of measurement was 4–5 consecutive days per site. Two approaches were used to analyze the measured data, the Q10 parameter and the Arrhenius relationship. Basic environmental factors such as soil temperature and moisture were measured. All the four investigated sites showed a positive dependence of the soil surface CO2 efflux on soil temperature. The four datasets generally resulted in good agreement (up to 93%) between the approaches and residual analysis showed no significant difference between approaches (less than 8%). The Q10 ranged between 2.0 and 2.3 among the sites. The highest spatial variation of the soil surface CO2 efflux at 10°C (expressed by the coefficient of variation CV) ranged from 30 to 65% between sites.

Effects of precipitation pulses on water and carbon dioxide fluxes in two semiarid ecosystems: measurement and modeling

Modeling of soil–water, –heat and –carbon (C) fluxes provides an important tool for predicting mass and energy transfers based on a hydraulic-, thermal- and C-mass balance approach. Model predictions were evaluated using measured data from two water-limited study sites, one pasture and one supporting an alfalfa crop, to indentify differences between these ecosystems. Soil water content, temperature, and evapotranspiration (ET) data were used to validate soil water dynamics components of a process-based numerical model. Soil surface CO2 efflux estimates (i.e., fluxes from soil respiration) were also made to estimate soil CO2 emissions. The results show that the Hydrus-1D numerical model can be parameterized to simulate the soil hydrodynamics and CO2 fluxes measured at both locations. Rainfall and irrigation events triggering increases in plant root and microbial respiration rates were simulated to recreate observed pulsed CO2 fluxes. There were distinct differences in ET and soil CO2 effluxes between the ecosystems and watering events significantly modified the fluxes. Differences in potential evapotranspiration and soil texture could help explain these discrepancies. The results demonstrate that numerical modeling can be a useful tool for estimating soil surface fluxes in calibrated ecosystems when micrometeorological methods may not be suitable.

Soil CO2 concentration and efflux from three forests in subtropical China

Measurements of soil CO2 efflux and soil CO2 concentration concurrently are important for understanding the mechanism and regulation of CO2 in the soil. We have analysed CO2 concentration in a soil profile and soil CO2 efflux in three typical forests in subtropical China: monsoon evergreen broad-leaved forest (BF, 400 years old), pine and broad-leaved mixed forest (MF, 80 years old), and pine forest (PF, 70 years old). A portable soil CO2 sampler of simple sample operation was designed and used. The seasonal patterns of soil surface CO2 efflux and soil CO2 concentration were observed, and were positively correlated with rainfall, soil temperature, and moisture. The mean values of soil CO2 concentrations at the 15, 30, 45, and 60 cm soil depth were higher in BF (3368–9243 μL L–1) than in MF (1495–7662 μL L–1) and PF (1566–5730 μL L–1), while the mean values of soil surface CO2 efflux (Rsurface) were 0.55 ± 0.11 g m–2 h–1 in BF, 0.52 ± 0.10 g m–2 h–1 in MF, and 0.45 ± 0.07 g m–2 h–1 in PF. Soil CO2 concentration and Rsurface increased gradually with the age of the forests, but the incremental increase in soil CO2 concentration will be greater than that of Rsurface in MF and PF compared with BF. The data suggested that, although older forests have more C, younger forests probably will sequester C as CO2 faster than older forests.

Effects of forest type, stand age, and altitude on soil respiration in subtropical forests of China

An automated chamber system was used to study how soil respiration (R S) differed with stand age (SA), altitude (AL), and forest type (FT), in subtropical China. From 2006 to 2008, measurements of soil-surface CO2 efflux and associated environmental factors were made in an evergreen broad-leaved forest (EB), and five aged Chinese fir [Cunninghamia lanceolata Hook.] plantations (CF), and three altitudinal Moso bamboo [Phyllostachys pubescens Mazel ex J. Houz.] plantations (MB), in Dagangshan mountain range, Jiangxi Province. Multivariate analysis of covariance with soil temperature (T) and soil water content (SWC) as the covariates showed that FT and AL as the independent factors had a significant effect on R S (P<0.01, P<0.05, respectively). The R S in the CF differed significantly from EB and MB (P<0.001), but not between EB and MB. The R S was positively correlated with T, and responded to changes in SWC at higher Ts. The correlation between R S and SWC was improved when the R S was normalized to 10°C. Annual mean R S differed significantly with SA and were ranked in the order (from high to low) of 21–25, 26–35, 11–20, >36, and 1–10 years. The conclusion of this study is that T, SWC, AL, FT, and SA all have significant effects on R s in subtropical China.

Rhizospheric and heterotrophic respiration of a warm-temperate oak chronosequence in China

Abstract Plot trenching and root decomposition experiments were conducted in a warm-temperate oak chronosequence (40-year-old, 48-year-old, 80-year-old, and 143-year-old) in China. We partitioned total soil surface CO2 efflux (RS) into heterotrophic (RH) and rhizospheric (RR) components across the growing season of 2009. We found that the temporal variation of RR and RH can be well explained by soil temperature (T5) at 5 cm depth using exponential equations for all forests. However, RR of 40-year-old and 48-year-old forests peaked in September, while their T5 peaks occurred in August. RR of 80-year-old and 143-year-old forests showed a similar pattern to T5. The contribution of RR to RS (RC) of 40-year-old and 48-year-old forests presented a second peak in September. Seasonal variation of RR may be accounted for by the different successional stages. Cumulative RH and RR during the growing season varied with forest age. The estimated RH values for 40-year-old, 48-year-old, 80-year-old and 143-year-old forests averaged 431.72, 452.02, 484.62 and 678.93 g C m−2, respectively, while the corresponding values of RR averaged 191.94, 206.51, 321.13 and 153.03 g C m−2. The estimated RC increased from 30.78% in the 40-year-old forest to 39.85% in the 80-year-old forest and then declined to 18.39% in the 143-year-old forest. We found soil organic carbon (SOC), especially the light fraction organic carbon (LFOC), stock at 0–10 cm soil depth correlated well with RH. There was no significant relationship between RR and fine root biomass regardless of stand age. Measured apparent temperature sensitivity (Q10) of RH (3.93 ± 0.27) was significantly higher than that of RR (2.78 ± 0.73). Capillary porosity decreased as stand age increased and it was negatively correlated to cumulative RS. Our results emphasize the importance of partitioning soil respiration in evaluating the stand age effect on soil respiration and its significance to future model construction.