Quantifying soil respiration in response to short-term tillage practices: a case study in southern Turkey

Abstract

The study aimed at quantifying the rates of soil CO₂ efflux under the influence of common tillage systems of moldboard plow (PT), chisel plow (CT), rotary tiller (RT), heavy disc harrow (DT), and no-tillage (NT) for 46 days in October and November in a field left fallow after wheat harvest located in southern Turkey. The NT and DT plots produced the lowest soil CO₂ effluxes of 0.3 and 0.7 g m^?2 h^?1, respectively, relative to the other plots (P < 0.001). Following the highest rainfall amount of 87 mm on the tenth day after the tillage, soil CO₂ efflux rates of all the plots peaked on the 12th day, with less influence on soil CO₂ efflux in the NT plot than in the conventional tillage plots. Soil evaporation in NT (64 mmol m^?2 s^?1) was significantly lower than in the PT (85 mmol m^?2 s^?1) and RT (89 mmol m^?2 s^?1) tillage treatments (P < 0.01). The best multiple-regression model selected explained 46% of variation in soil respiration rates as a function of the tillage treatments, soil temperature, and soil evaporation (P < 0.001). The tillage systems of RT, PT, and CT led, on average, to 0.23, 0.22, and 0.18 g m^?2 h^?1 more soil CO₂ efflux than the baseline of NT, respectively (P≤0.001).     

设施菜田土壤呼吸速率日变化特征分析

研究设施菜田土壤呼吸速率日变化特征对于了解CO₂排放对环境和作物生长的影响十分重要。本研究采用CO2红外分析仪 动态箱法在2009年秋冬季和2010年冬春季监测了不同有机肥和氮肥处理下设施菜田土壤呼吸速率的日变化特征。结果表明: 施用有机肥和秸秆明显提高设施菜田土壤呼吸速率, 尤其是在高氮投入下, 鸡粪和小麦秸秆混施土壤呼吸速率明显高于其他处理; 不同季节各处理土壤呼吸速率的日变化特征基本一致, 土壤呼吸速率的最大值出现在14:00-17:00; 随着温度升高, 土壤呼吸速率逐渐增加, 但是过高的温度和CO₂浓度均会抑制土壤呼吸速率; 上午8:00-11:00测定的土壤呼吸速率值与土壤呼吸速率日平均值基本一致, 可采用上午8:00-11:00土壤呼吸速率的观测值评估设施菜田CO₂的排放量; 施肥、温度和温室内近地面CO₂浓度是影响不同季节土壤呼吸速率日变化的主要因素, 合理调控对于实现设施蔬菜的可持续发展具有重要意义。     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

退化草地暗沃寒冻雏形土CO2释放的日变化和季节动态

采用CI-301PS红外CO     

Calibrating soil respiration measures with a dynamic flux apparatus using artificial soil media of varying porosity

Measurement of soil respiration to quantify ecosystem carbon cycling requires absolute, not relative, estimates of soil CO₂ efflux. We describe a novel, automated efflux apparatus that can be used to test the accuracy of chamber‐based soil respiration measurements by generating known CO₂ fluxes. Artificial soil is supported above an air‐filled footspace wherein the CO₂ concentration is manipulated by mass flow controllers. The footspace is not pressurized so that the diffusion gradient between it and the air at the soil surface drives CO₂ efflux. Chamber designs or measurement techniques can be affected by soil air volume, hence properties of the soil medium are critical. We characterized and utilized three artificial soils with diffusion coefficients ranging from 2.7 × 10^?7 to 11.9 × 10^?7 m² s^?1 and porosities of 0.26 to 0.46. Soil CO₂ efflux rates were measured using a commercial dynamic closed‐chamber system (Li‐Cor 6400 photosynthesis system equipped with a 6400‐09 soil CO₂ flux chamber). On the least porous soil, small underestimates (< 5%) of CO₂ effluxes were observed, which increased as soil diffusivity and soil porosity increased, leading to underestimates as high as 25%. Differential measurement bias across media types illustrates the need for testing systems on several types of soil media.     

Soil CO2 efflux vs. soil respiration: Implications for flux models

In the long term, all CO₂ produced in the soil must be emitted by the surface and soil CO₂ efflux (FCO₂) must correspond to soil respiration (R_soil). In the short term, however, the efflux can deviate from the instantaneous soil respiration, if the amount of CO₂ stored in the soil pore-space (SCO₂) is changing. We measured FCO₂ continuously for one year using an automated chamber system. Simultaneously, vertical soil profiles of CO₂ concentration, moisture, and temperature were measured in order to assess the changes in the amount of CO₂ stored in the soil. R_soil was calculated as the sum of the rate of change of the CO₂ storage over time and FCO₂. The experiment was split into a warm and a cold season. The dependency of soil respiration and soil efflux on soil temperature and on soil moisture was analyzed separately. Only the moisture-driven model of the warm season was significantly different for FCO₂ and R_soil. At our site, a moisture-driven soil-respiration model derived from CO₂ efflux data would underestimate the importance of soil moisture. This effect can be attributed to a temporary storage of CO₂ in the soil pore-space after rainfalls where up to 40% of the respired CO₂ were stored.     

Hierarchical Bayesian models for soil CO2 flux using soil texture: a case study in central Hokkaido,Japan

Hierarchical Bayesian (HB) methods are useful tools for modeling multifaceted, nonlinear phenomena such as those encountered in ecology, and have been increasingly applied in environmental sciences, e.g., to estimate soil gas flux from different soil textures or sites. We have developed a model of soil carbon dioxide (CO₂) flux based on soil temperature (T, 5 cm depth) and water-filled pore space (WFPS, 5 cm depth) using HB theory. The HB model was calibrated using a dataset of CO₂ flux measured from bare soils belonging to four texture classes in 14 upland field sites in a watershed in central Hokkaido, Japan, in the nonsnow-cover season from 2003 to 2011. The numerical software HYDRUS-1D was used to simulate daily WFPS, and the estimated values were significantly correlated with the measured WFPS (R² = 0.68, P < 0.001). Compared to a nonhierarchical Bayesian model (Bayesian pooled model), the CO₂ predictions with the HB model more accurately represented texture-specific observations. The simulation–observation fit of the CO₂ flux model was R² = 0.64 (P < 0.001). More than 90% of the observed daily data were within the 95% confidence interval. The HB model exhibited high uncertainty for high CO₂ flux values. The HB model calibration revealed differing sensitivity of CO₂ flux to T and WFPS in different soil texture classes. CO₂ flux increased with an increase in T, and it increased to a lesser degree with a finer texture, possibly because the clay and silt facilitated soil aggregation, thus reducing temperature fluctuations. WFPS values between 0.48 and 0.64 resulted in optimal conditions for CO₂ flux. The minimum WFPS value increased with an increase in clay content (P < 0.05). Although only a small number of soil types were studied in only one season in this study, the HB model may provide a method for predicting how the effects of soil temperature and moisture on CO₂ flux change with texture, and soil texture could be regarded as an upscaling factor in future research on regional extrapolation.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C (F₁₀) of 2.6 μmol m⁻² s⁻¹ and the Q₁₀ of 3.7. No evidence of displacement of CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

Differences in soil CO2 flux between different land use types in mid-subtropical China

It is crucial to advance the understanding of the soil carbon dioxide (CO₂) flux and environmental factors for a better comprehension of carbon dynamics in subtropical ecosystems. Red soil, one of the typical agricultural soils in subtropical China, plays important roles in the global carbon budget due to their large potential to sequester C and replenish atmospheric C through soil CO₂ flux. We examined the relationship between soil CO₂ flux and environmental determinants in four different land use types of subtropical red soil-paddy (P), orchard (O), woodland (W) and upland (U) using static closed chamber method. Objectives were to evaluate the relationship of soil temperature, water-filled pore space (WFPS), and dissolved organic carbon (DOC) with the soil CO₂ flux. Soil CO₂ fluxes were measured on each site about every 14 days between 09:00 and 11:00 a.m. during 14-July 2004 to 25-April 2007 at the experimental station of Heshengqiao at Xianning, Hubei, China. Soil CO₂ fluxes revealed seasonal fluctuations, with the tendency that maximum values occurred in summer, minimum in winter and intermediate values in spring and autumn except for paddy soil when it was submerged. Further, significant differences in soil CO₂ fluxes were observed among the four soils, following the order of P > O > U  W. Average soil CO₂ fluxes were estimated as 901 ± 114, 727 ± 55, 554 ± 22 and 533 ± 27 (±S.D.) g CO₂ m⁻² year⁻¹ in paddy, orchard, upland and woodland soils, respectively. Variations in soil CO₂ flux were related to soil temperature, WFPS, and dissolved organic carbon with a combined R² of 0.49–0.75. Soil temperature was an important variable controlling 26–59% of soil CO₂ flux variability. The interaction of soil temperature and WFPS could explain 31–60% of soil CO₂ flux variations for all the land use types. We conclude that soil CO₂ flux from red soil is under environmental controls, soil temperature being the main variable, which interact with WFPS and DOC to control the supply of readily mineralizable substrates.     

Explaining temporal variation in soil CO2 efflux in a mature spruce forest in Southern Germany

An open dynamic chamber system was used to measure the soil CO₂ efflux intensively and continuously throughout a growing season in a mature spruce forest (Picea abies) in Southern Germany. The resulting data set contained a large amount of temporally highly resolved information on the variation in soil CO₂ efflux together with environmental variables. Based on this background, the dependencies of the soil CO₂ efflux rate on the controlling environmental factors were analysed in-depth. Of the abiotic factors, soil temperature alone explained 72% of the variation in the efflux rate, and including soil water content (SWC) as an additional variable increased the explained variance to about 83%. Between April and December, average rates ranged from 0.43 to 5.15 μmol CO₂ m⁻² s⁻¹ (in November and July, respectively) with diurnal variations of up to 50% throughout the experiment. The variability in wind speed above the forest floor influenced the CO₂ efflux rates for measuring locations with a litter layer of relatively low bulk density (and hence relatively high proportions of pore spaces). For the temporal integration of flux rates for time scales of hours to days, however, wind velocities were of no effect, reflecting the fact that wind forcing acts on the transport, but not the production of CO₂ in the soil. The variation in both the magnitude of the basal respiration rate and the temperature sensitivity throughout the growing season was only moderate (coefficient of variation of 15 and 25%, respectively). Soil water limitation of the CO₂ production in the soil could be best explained by a reduction in the temperature-insensitive basal respiration rate, with no discernible effect on the temperature sensitivity. Using a soil CO₂ efflux model with soil temperature and SWC as driving variables, it was possible to calculate the annual soil CO₂ efflux for four consecutive years for which meteorological data were available. These simulations indicate an average efflux sum of 560 g C m⁻² yr⁻¹ (SE=22 g C m⁻² yr⁻¹). An alternative model derived from the same data but using temperature alone as a driver over-estimated the annual flux sum by about 7% and showed less inter-annual variability. Given a likely shift in precipitation patterns alongside temperature changes under projected global change scenarios, these results demonstrate the necessity to include soil moisture in models that calculate the evolution of CO₂ from temperate forest soils.     

Long‐term effects of vehicular passages on soil carbon sequestration and carbon dioxide emission in a no‐till corn‐soybean rotation on a Crosby silt loam in Central Ohio,USA

Research information from a systematic planned study on the effects of vehicular passages and axle load on soil carbon dioxide (CO₂) fluxes and soil carbon (C) sequestration under long‐term NT farming is scanty. Therefore, the present study was conducted on an on‐going 20‐year experiment to assess the impacts of variable vehicular passages of a low axle load on soil CO₂ emission and soil C sequestration from a no‐till (NT) managed corn (Zea mays L.)–soybean (Glycine max Linneo) rotation in comparison with that a soil under woodlots (soils under natural wooded plantation). The experimental treatment consisted of an empty wagon [0 Mg load for compaction (C‐0; control)] compared with 2 (C‐2) and 4 (C‐4) passages of 2.5 Mg water wagon axle load, applied to the entire plot every year during April/May for 20 consecutive years. Soil samples were obtained in November 2016 to determine the effects of various vehicular passages on C and nitrogen (N) contents and CO₂ emissions. Soil CO₂ fluxes were measured from November 16, 2016, to May 30, 2017, on the bi‐weekly (November to December and April to May) and monthly (January to March) basis by using high‐density polyvinyl chloride static gas chambers. The soil CO₂ fluxes ranged from –1.05 to 9.03 g CO₂ m^?2 d^?1. The lowest soil CO₂ fluxes were observed in December coinciding with the minimum soil temperature. In general, daily soil CO₂ fluxes were higher under C‐0 than those under other treatments. Vehicular traffic and axle load reduced the cumulative emission of CO₂ by 22.6 and 29.8% under C‐2 and C‐4, respectively, compared with that under C‐0 (6.09 Mg ha^?1). Soil and air temperatures had a significant positive correlation with the diurnal fluxes of soil CO₂ in all the treatments except that under C‐4. Electrical conductivity, soil C and N contents and pools did not differ significantly among the treatments. Further, 2 to 4 passages of vehicles with 2.5 Mg of axle load decreased the soil CO₂ emission on Crosby silt loam under NT as compared to that under the control. Therefore, continuous cultivation of row crops with moderate trafficking under NT and residue retention is recommended, and it also reduces the potential of soil CO₂ emission while improving the soil organic C pools of well‐drained soils of Central Ohio.     

Soil CO2 emission and its relationship to soil properties under different tillage systems

There is a lack of understanding as to which soil property is the most important at regulating the temporal variability of soil CO₂ emissions on China’s Loess Plateau. The objective of this study was to evaluate the CO₂ emissions and their relationships to certain soil properties in a winter wheat (Triticum aestivum L.) field subject to no-till (NT) and conventional tillage (CT) practices. The CO₂ emissions were signi?cantly higher in the CT (257.6 mg CO₂ m^?2 h^?1), compared with the NT (143.8 mg CO₂ m^?2 h^?1), treatment. Soil organic matter content and carbon stock were 8% and 14% higher, respectively, in the NT, compared with the CT, treatment. Regression analyses between the CO₂ emissions and soil properties, including soil temperature and carbon stock, explained up to 88% and 60% of the temporal variability in CO₂ emissions in the NT and CT treatments, respectively. Linear correlations between the soil temperature and CO₂ emissions were recorded in both the NT and CT treatments. Soil temperature was the most important factor in terms of understanding the temporal variability in CO₂ emissions in wheat fields of the study area.     

不同管理措施对滨海盐渍农田土壤CO2排放及碳平衡的影响

为探讨不同管理措施对滨海盐渍农田碳平衡的影响,本文通过玉米–小麦轮作试验,研究农田土壤的CO_2释放规律,及其农田碳收支状况。试验设计6个处理:1常规对照(CK);2有机肥常量(OF);3氮肥增施(NF);4秸秆还田(S);5有机肥加秸秆(OF+S);6免耕(NT)。研究表明,秸秆还田和有机肥的施用增加了土壤呼吸的强度,而免耕处理的CO_2平均释放量最低,不同处理下土壤呼吸总体表现为OF+SSOMNFCKNT。各处理土壤有机碳含量随着作物的收获逐渐升高,其中OF与NT增加最多,而增施氮肥处理并没有显著提高土壤的有机碳水平。各处理间的有机碳含量没有显著性差异。在两季作物种植结束后,各处理的碳输入均高于碳输出,均为碳净输入,表现出较强的碳汇特征。秸秆还田和单施有机肥的碳净输入均显著高于对照,可有效减缓因农田土壤CO_2排放而造成的全球气候变化问题。     

Factors controlling soil CO2 effluxes and the effects of rewetting on effluxes in adjacent deciduous, coniferous, and mixed forests in Korea

To better understand the factors that control forest soil CO₂ efflux and the effects of rewetting on efflux, we measured soil CO₂ efflux in adjacent deciduous, coniferous, and mixed forests in the central part of the Korean Peninsula over the course of one year. We also conducted laboratory rewetting experiments with soil collected from the three sites using three different incubation temperatures (4 °C, 10 °C, and 20 °C). Soil moisture (SM), soil organic matter (SOM), and total root mass values of the three sites were significantly different from one another; however, soil temperature (ST), observed soil CO₂ efflux and sensitivity of soil CO₂ efflux to ST (i.e., Q₁₀ = 3.7 ± 0.1) were not significantly different among the three sites. Soil temperature was a dominant control factor regulating soil CO₂ efflux during most of the year. We infer that soil CO₂ efflux was not significantly different among the sites due to similar ST and Q₁₀. Though a significant increase in soil CO₂ efflux following rewetting of dry soil was observed both in the field observations (60-170%) and laboratory incubation experiments (100-1000%), both the increased rates of soil CO₂ efflux and the magnitude of change in SM were not significantly different among the sites. The increased rates of soil CO₂ efflux following rewetting depended on the initial SM before rewetting. During drying phase after rewetting, a significant correlation between SM and soil CO₂ efflux was found, but the effect of ST on increased soil CO₂ efflux was not clear. Cumulative peak soil CO₂ efflux (11.3 ± 0.7 g CO₂ m⁻²) following rewetting in the field was not significantly different among the sites. Those evidences indicate that the observed similar rewetting effects on soil CO₂ efflux can be explained by the similar magnitude of change in SM after rewetting at the sites. We conclude that regardless of vegetation type, soil CO₂ efflux and the effect of rewetting on soil CO₂ efflux do not differ among the sites, and ST is a primary control factor for soil CO₂ efflux while SM modulates the effect of rewetting on soil CO₂ efflux. Further studies are needed to quantify and incorporate relationship of initial dryness of the soil and the frequency of the dry-wet cycle on soil CO₂ efflux into models describing carbon (C) processes in forested ecosystems.     

Field measurement of carbon dioxide evolution from soil by a flow-through chamber method using a portable photosynthesis meter

Extract

Since a rise in atmospheric carbon dioxide (CO₂) concentration is expected to lead to global warming, it is important to quantify the global carbon circulation. The CO₂ evolution rate from soil has usually been measured by one of three methods: 1) CO₂ absorption (Anderson 1982), where the evolved CO₂ is absorbed in an alkali solution and the content subsequently determined, 2) closed chamber (Rolston 1986) in which the CO₂ evolution rate is calculated from the increase of the CO₂ concentration in a closed chamber covering the soil surface, and 3) flow-through chamber (Rolston 1986) in which a fixed rate of ambient air is pumped through an open chamber and the difference in the. CO₂ concentration between the inlet and the outlet is measured. Although the CO₂ absorption method is very simple in terms of apparatus and procedure, the determined CO₂ evolution rate tends to be underestimated in cases where the evolved CO₂ is not fully absorbed in the alkali solution (Ewel et al. 1987; Sakamoto and Yoshida 1988), or overestimated in cases where the CO₂ concentration in the chamber is too low to stimulate microbial activity (Koizumi et al. 1991; Nakadai et al. 1993), In the closed chamber method, when the gas concentration in the chamber is higher than that of the ambient air, gas diffusion from the soil to the atmosphere is restricted (Denmead 1978). At this point, the flow-through chamber method seems to be most suitable for measuring the CO₂ evolution rate, because the rate is determined under nearly natural conditions. However, this method has a disadvantage in that the apparatus is composed of an infra-red CO₂ analyzer, air pumps, mass flow meters, a recorder, and other items, which are too large, heavy, and complex to use in the field (Freijer and Bouten 1991). Hence, in spite of the above limitations, most of the studies on CO₂ evolution in situ have been carried out using the CO₂ absorption method (Kowalenko et al. 1978; Seto et al. 1978a, b; Ewel et al 1981, 1987; Gupta and Singh 1981; Reinke et al. 1981; Edwards and Ros-Todd 1983; Grahammer et al. 1991) or the closed chamber method (Naganawa et al. 1989; Mariko et al. 1994). The flow-through chamber method has been used only at sites where electric power supply and other types of equipment were available (Mathes and Schriefer 1985; Ewel et al. 1987; Nakadai et al. 1993). In the present report a flow-through chamber method using a portable CO₂ analyzer system was examined, for the determination of CO₂ evolution from soil without an electric power supply or other special equipment.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

A new technique to measure soil CO2 efflux at constant CO2 concentration

A new principle for measuring soil CO₂ efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO₂ within the system to achieve a constant CO₂ concentration inside the soil chamber at ambient level, thus balancing the amount of CO₂ entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO₂ efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO₂ within the soil, while allowing for continuous capture of the CO₂ released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO₂ efflux and its δ¹³C, since both variables show sensitivity to a distortion of the soil CO₂ profile commonly found in conventional chamber techniques.     

Measurement of soil CO2 efflux using soda lime absorption: both quantitative and reliable

Measurement of soil CO₂ efflux using a non-flow-through steady-state (NFT-SS) chamber with alkali absorption of CO₂ by soda lime was tested and compared with a flow-through non-steady-state (FT-NSS) IRGA method to assess suitability of using soda lime for field monitoring over large spatial scales and integrated over a day. Potential errors and artifacts associated with the soda lime chamber method were investigated and improvements made. The following issues relating to quantification and reliable measurement of soil CO₂ efflux were evaluated: (i) absorption capacity of the soda lime, (ii) additional and thus artifactual absorption of CO₂ by soda lime during the experimental procedure, (iii) variation in the CO₂ concentration inside the chamber headspace, and (iv) effects of chamber closure on soil CO₂ efflux. Soil CO₂ efflux, as measured using soda lime (with a range of quantities: 50, 100, and 200 g per 0.082 m² ground area enclosed in chamber), was compared with transient IRGA measurements as a reference method that is based on well-established physical principles, using several forms of spatial and temporal comparisons. Natural variation in efflux rates ranged from 2 to 5.5 g C m⁻² day⁻¹ between different chambers and over different days. A comparison of the IRGA-based assay with measurement based on soda lime yielded an overall correlation coefficient of 0.82. The slope of the regression line was not significantly different from the 1:1 line, and the intercept was not significantly different from the origin. This result indicated that measurement of CO₂ efflux by soda lime absorption was quantitatively similar and unbiased in relation to the reference method. The soda lime method can be a highly practical method for field measurements if implemented with due care (in terms of drying and weighing soda lime, and in minimizing leakages), and validated for specific field conditions. A detailed protocol is presented for use of the soda lime method for measurement of CO₂ efflux from field soils.     

Hot spots,hot moments,and spatio-temporal controls on soil CO2 efflux in a water-limited ecosystem

Soil CO₂ efflux is the primary source of CO₂ emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO₂ efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO₂ efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO₂ efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO₂ efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO₂ efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO₂ efflux, and during the wet season the emergence of a hot spot of soil CO₂ efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO₂ efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO₂ efflux in water-limited ecosystems.     

九龙江口红树林湿地的土壤呼吸变化研究

The diurnal and seasonal variations of soil respiration (SR) were studied at a subtropical mangrove wetland in the Jiulong River Estuary from May 2010 to April 2011.SR rates were measured continuously from 08:00 to 06:00 local time (24-h time system) on July8–9 and October 3–4,2010;and January 15–16 and April 11–12,2011.Similar patterns in the diurnal variation of SR were observed on October 2–3 and April 11–12,with the maximum values at 14:00 and the minimum at 00:00.However,the diurnal dynamics of SR on July 8–9,2010 and January 15–16,2011 showed diferent patterns,with the maximum values at 08:00–10:00 on above sampling dates and the minimum at 22:00 on July 8 and at 04:00 on January 16.The daily mean values of SR approximated to the values measured at 08:00.SR fluctuated with distinct seasonal patterns.The seasonal variation was characterized by a mono-peak pattern,with the highest rate (6.18μmol CO2m-2s-1) in July and the lowest rate (0.36μmol CO2m-2s-1) in December.The results showed that the variation of SR in mangrove wetland was mainly controlled by soil temperature,and there was no significant correlation between SR and soil water content.It also implied that the model of SR in mangrove wetland should not only consider the efect of soil temperature,but also incorporate other factors,such as water level,precipitation,microbial activity and photosynthesis,which also could affect SR.