Response of soil surface CO2 flux in a boreal forest to ecosystem warming

Soil surface carbon dioxide (CO₂) flux (R_S) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m × 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species and Epilobium angustifolium was the dominant understory. Soil temperature was maintained (～5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3‐m‐diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ～5 °C above control ambient air temperature allowing for the testing of soil‐only warming and soil+air warming. Soil surface CO₂ flux (R_S) was positively correlated (P < 0.0001) to soil temperature at 10 cm depth. Soil surface CO₂ flux (R_S) was 24% greater in the soil‐only warming than the control in 2004, but was only 11% greater in 2005, while R_S in the soil+air warming treatments was 31% less than the control in 2004 and 23% less in 2005. Live fine root mass (< 2 mm diameter) was less in the heated than control treatments in 2004 and statistically less (P < 0.01) in 2005. Similar root mass between the two heated treatments suggests that different heating methods (soil‐only vs. soil+air warming) can affect the rate of decomposition.     

Vertical partitioning of CO2 production within a temperate forest soil

The major driving factors of soil CO₂ production – substrate supply, temperature, and water content – vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO₂ efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO₂ production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO₂ production by genetic soil horizon vary seasonally and interannually. We measured surface CO₂ efflux and vertical soil profiles of CO₂ concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO₂ flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40–48% of the total annual soil CO₂ efflux. Although the temperature sensitivity of CO₂ production varied across soil horizons, the partitioning of CO₂ production by horizon did not improve the overall prediction of surface CO₂ effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO₂ production only in the O horizon. Large interannual variations in estimates of O horizon CO₂ production indicate that this layer could be an important transient interannual source or sink of ecosystem C.     

Seasonal patterns of soil CO2 efflux and soil air CO2 concentration in a Scots pine forest: comparison of two chamber techniques

A non‐vented non‐steady state flow‐through chamber and a non‐vented non‐steady state non‐flow‐through chamber technique were used to measure CO₂ efflux of a young Scots pine forest on a fertile till soil in southern Finland. Soil temperature, soil moisture and soil CO₂ concentration were measured concurrently with CO₂ efflux for two and a half successive years. The CO₂ efflux showed a seasonal pattern, effluxes ranging from low 0.0–0.1 g CO₂ m ^{? 2} h ^{? 1} in winter to peak values of 2.3 g CO₂ m ^{? 2} h ^{? 1} occurring in late June and in July. The daily average effluxes in July measured by flow through chambers were 1.23 and 0.98 g CO₂ m ^{? 2} h ^{? 1} in 1998 and 1999, respectively. The annual accumulated CO₂ efflux was 3117 and 3326 g CO₂ m ^{? 2} in 1998 and 1999, respectively. The spatial variation in CO₂ efflux was high (CV 0.18–0.45) and increased with increasing efflux. Soil air CO₂ concentration showed similar seasonal pattern the peak concentrations occurring in July–August. The CO₂ concentrations ranged from 580 to 780 µ mol mol ^{? 1} in the humus layer to 13 620–14 470 µ mol mol ^{? 1} in the C‐horizon. In winter the soil air CO₂ concentrations were lower, especially in deeper soil layers. Drought decreased CO₂ efflux and soil air CO₂ concentration. The in situ comparison on forest soil between the chamber methods showed the non‐flow‐through chamber to give ～～50% lower efflux values than that of the flow‐through chamber. When calibrated against known CO₂ efflux ranging from 0.4 to 0.8 g CO₂ m ^{? 2} h ^{? 1} generated with a diffusion box method developed by Widén and Lindroth [Acta Universitatis Agriculturae Suecia Silvestria, 2001], the flow‐through chamber gave equal effluxes at the lower end of the calibration range, but overestimated high effluxes by 20%. Non‐flow‐through chamber underestimated the CO₂ efflux by 30%.     

Experimental warming and burn severity alter soil CO2 flux and soil functional groups in a recently burned boreal forest

Global warming is projected to be greatest in northern regions, where forest fires are also increasing in frequency. Thus, interactions between fire and temperature on soil respiration at high latitudes should be considered in determining feedbacks to climate. We tested the hypothesis that experimental warming will augment soil CO₂ flux in a recently burned boreal forest by promoting microbial and root growth, but that this increase will be less apparent in more severely burned areas. We used open‐top chambers to raise temperatures 0.4–0.9°C across two levels of burn severity in a fire scar in Alaskan black spruce forest. After 3 consecutive years of warming, soil respiration was measured through a portable gas exchange system. Abundance of active microbes was determined by using Biolog EcoPlates^? for bacteria and ergosterol analysis for fungi. Elevated temperatures increased soil CO₂ flux by 20% and reduced root biomass, but had no effect on bacterial or fungal abundance or soil organic matter (SOM) content. Soil respiration, fungal abundance, SOM, and root biomass decreased with increasing burn severity. There were no significant interactions between temperature and burn severity with respect to any measurement. Higher soil respiration rates in the warmed plots may be because of higher metabolic activity of microbes or roots. All together, we found that postfire soils are a greater source of CO₂ to the atmosphere under elevated temperatures even in severely burned areas, suggesting that global warming may produce a positive feedback to atmospheric CO₂, even in young boreal ecosystems.     

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

Modeling 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 (R_h). 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 (R_a) and R_h. 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 R_a. This study showed that a decrease in soil CO₂ efflux in response to a warm year was most likely dominated by a decrease in R_a instead of an increase in R_h.     

Responses of soil respiration to elevated CO2, air warming, and changing soil water availability in a model old-field grassland

Responses of soil respiration to atmospheric and climatic change will have profound impacts on ecosystem and global carbon (C) cycling in the future. This study was conducted to examine effects on soil respiration of the concurrent driving factors of elevated atmospheric CO₂ concentration, air warming, and changing precipitation in a constructed old‐field grassland in eastern Tennessee, USA. Model ecosystems of seven old‐field species were established in open‐top chambers and treated with factorial combinations of ambient or elevated (+300 ppm) CO₂ concentration, ambient or elevated (+3 °C) air temperature, and high or low soil moisture content. During the 19‐month experimental period from June 2003 to December 2004, higher CO₂ concentration and soil water availability significantly increased mean soil respiration by 35.8% and 15.7%, respectively. The effects of air warming on soil respiration varied seasonally from small reductions to significant increases to no response, and there was no significant main effect. In the wet side of elevated CO₂ chambers, air warming consistently caused increases in soil respiration, whereas in the other three combinations of CO₂ and water treatments, warming tended to decrease soil respiration over the growing season but increase it over the winter. There were no interactive effects on soil respiration among any two or three treatment factors irrespective of time period. Treatment‐induced changes in soil temperature and moisture together explained 49%, 44%, and 56% of the seasonal variations of soil respiration responses to elevated CO₂, air warming, and changing precipitation, respectively. Additional indirect effects of seasonal dynamics and responses of plant growth on C substrate supply were indicated. Given the importance of indirect effects of the forcing factors and plant community dynamics on soil temperature, moisture, and C substrate, soil respiration response to climatic warming should not be represented in models as a simple temperature response function, and a more mechanistic representation including vegetation dynamics and substrate supply is needed.     

The response of soil CO2 flux to changes in atmospheric CO2, nitrogen supply and plant diversity

We measured soil CO₂ flux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmospheric carbon dioxide (CO₂) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO₂ and N fertilization increased soil CO₂ flux by 0.57 µmol m^?2 s^?1 (13% increase) and 0.37 µmol m^?2 s^?1 (8% increase) above average control soil CO₂ flux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO₂ flux by 0.23, 0.41 and 1.09 µmol m^?2 s^?1 (5%, 8% and 21% decreases), respectively. There were no statistically significant pairwise interactions among the three treatments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO₂ increased soil CO₂ flux most when soil moisture was low and soils were warm. Effects on soil CO₂ flux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no significant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correlated over time; neither were correlated with the CO₂ effect. Models of soil CO₂ flux will need to incorporate ecosystem CO₂ and N availability, as well as ecosystem plant diversity, and incorporate different environmental factors when determining the magnitude of the CO₂, N and diversity effects on soil CO₂ flux.     

干旱对土壤剖面不同深度土壤CO2通量的影响

由于全球气候变化,预计未来我国亚热带地区干旱频率和持续时间将会增加。森林土壤CO₂的释放是陆地生态系统碳循环的重要组成部分,然而,有关不同深度土壤CO₂通量对干旱响应的理解仍相当有限。选择武夷山针叶林（Coniferous Forest,CF）和常绿阔叶林（Evergreen Broadleaved Forest,EBF）为研究对象,于2014年6月至2015年12月,采用梯度法计算10、30 cm和50 cm深度各层土壤CO₂通量,探讨模拟干旱对其影响。结果表明：CF和EBF样地土壤CO₂浓度均随土壤深度的增加而升高。CF和EBF样地对照（CK）处理10 cm深度土壤CO₂生产量分别占总CO₂生产量的53.5%和55.7%,表明土壤CO₂生产量主要来源于浅层土壤,这可能与浅层土壤有高的有机碳含量及细根生物量主要分布区有关。干旱处理使CF和EBF样地不同深度土壤CO₂通量均显著减少。在两个样地土壤CO₂通量的温度敏感性（Q₁₀）值均随着土壤深度的增加而减少。干旱处理显著减少了CF样地浅层土壤的Q₁₀值（P=0.02）,对深层土壤影响不显著（30 cm：P=0.30;50 cm：P=0.23）;而在EBF样地干旱处理显著减少了深层土壤的Q₁₀值（30 cm：P=0.02;50 cm：P=0.01）,对浅层土壤影响不显著（P=0.32）。     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

Increased rainfall variability and reduced rainfall amount decreases soil CO2 flux in a grassland ecosystem

Predicted climate changes in the US Central Plains include altered precipitation regimes with increased occurrence of growing season droughts and higher frequencies of extreme rainfall events. Changes in the amounts and timing of rainfall events will likely affect ecosystem processes, including those that control C cycling and storage. Soil carbon dioxide (CO₂) flux is an important component of C cycling in terrestrial ecosystems, and is strongly influenced by climate. While many studies have assessed the influence of soil water content on soil CO₂ flux, few have included experimental manipulation of rainfall amounts in intact ecosystems, and we know of no studies that have explicitly addressed the influence of the timing of rainfall events. In order to determine the responses of soil CO₂ flux to altered rainfall timing and amounts, we manipulated rainfall inputs to plots of native tallgrass prairie (Konza Prairie, Kansas, USA) over four growing seasons (1998–2001). Specifically, we altered the amounts and/or timing of growing season rainfall in a factorial combination that included two levels of rainfall amount (100% or 70% of naturally occurring rainfall quantity) and two temporal patterns of rain events (ambient timing or a 50% increase in length of dry intervals between events). The size of individual rain events in the altered timing treatment was adjusted so that the quantity of total growing season rainfall in the ambient and altered timing treatments was the same (i.e. fewer, but larger rainfall events characterized the altered timing treatment). Seasonal mean soil CO₂ flux decreased by 8% under reduced rainfall amounts, by 13% under altered rainfall timing, and by 20% when both were combined (P<0.01). These changes in soil CO₂ flux were consistent with observed changes in plant productivity, which was also reduced by both reduced rainfall quantity and altered rainfall timing. Soil CO₂ flux was related to both soil temperature and soil water content in regression analyses; together they explained as much as 64% of the variability in CO₂ flux across dates under ambient rainfall timing, but only 38–48% of the variability under altered rainfall timing, suggesting that other factors (e.g. substrate availability, plant or microbial stress) may limit CO₂ flux under a climate regime that includes fewer, larger rainfall events. An analysis of the temperature sensitivity of soil CO₂ flux indicated that temperature had a reduced effect (lower correlation and lower Q₁₀ values) under the reduced quantity and altered timing treatments. Recognition that changes in the timing of rainfall events may be as, or more, important than changes in rainfall amount in affecting soil CO₂ flux and other components of the carbon cycle highlights the complex nature of ecosystem responses to climate change in North American grasslands.     

Initial cultivation of a temperate-region soil immediately accelerates aggregate turnover and CO2 and N2O fluxes

The immediate effects of tillage on protected soil C and N pools and on trace gas emissions from soils at precultivation levels of native C remain largely unknown. We measured the response to cultivation of CO₂ and N₂O emissions and associated environmental factors in a previously uncultivated U.S. Midwest Alfisol with C concentrations that were indistinguishable from those in adjacent late successional forests on the same soil type (3.2%). Within 2 days of initial cultivation in 2002, tillage significantly (P=0.001, n=4) increased CO₂ fluxes from 91 to 196 mg CO₂‐C m^?2 h^?1 and within the first 30 days higher fluxes because of cultivation were responsible for losses of 85 g CO₂‐C m^?2. Additional daily C losses were sustained during a second and third year of cultivation of the same plots at rates of 1.9 and 1.0 g C m^?2 day^?1, respectively. Associated with the CO₂ responses were increased soil temperature, substantially reduced soil aggregate size (mean weight diameter decreased 35% within 60 days), and a reduction in the proportion of intraaggregate, physically protected light fraction organic matter. Nitrous oxide fluxes in cultivated plots increased 7.7‐fold in 2002, 3.1‐fold in 2003, and 6.7‐fold in 2004 and were associated with increased soil NO₃^? concentrations, which approached 15 μg N g^?1. Decreased plant N uptake immediately after tillage, plus increased mineralization rates and fivefold greater nitrifier enzyme activity, likely contributed to increased NO₃^? concentrations. Our results demonstrate that initial cultivation of a soil at precultivation levels of native soil C immediately destabilizes physical and microbial processes related to C and N retention in soils and accelerates trace gas fluxes. Policies designed to promote long‐term C sequestration may thus need to protect soils from even occasional cultivation in order to preserve sequestered C.     

Effects of elevated atmospheric CO2 in agro-ecosystems on soil carbon storage

Increasing global atmospheric CO₂ concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (≈1.4 × 10¹⁵ g C y^–1) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects of CO₂ enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agro-ecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO₂ concentration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using δ¹³C were used to track the input of new C into the soil system. At the end of two years, shifts in δ¹³C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO₂ vs. ambient CO₂ (162 and 29 g m^–2, respectively), but with soybean the new C in soil organic matter was less for elevated CO₂ vs. ambient CO₂ (120 and 291 g m^–2, respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO₂ resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent.     

14C – a tool for separation of autotrophic and heterotrophic soil respiration

We assessed the potential of using ¹⁴C contents of soil respired CO₂ to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and ¹⁴C contents of soil respired CO₂ in girdled and control plots in the summers of 2001 or 2002. At all stands, CO₂ flux rates were slightly higher in the control plots, whereas the ¹⁴C contents of respired CO₂ tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO₂ cycles more slowly through the forest ecosystem than autotrophically respired CO₂. On the basis of these data, the contributions of hetero‐ and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or ¹⁴C). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the ¹⁴C contents of soil respired CO₂ in the girdled plots with the ¹⁴C contents of heterotrophically respired CO₂ calculated by three different ¹⁴C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the ¹⁴C content of CO₂ respired in the girdled plots to be lower than ‘true’ heterotrophically respired CO₂ in an undisturbed plot. Nevertheless, we argue that measurements and modeling of ¹⁴C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).     

Influence of atmospheric CO2 enrichment on methane consumption in a temperate forest soil

Rates of atmospheric CH₄ consumption of soils in temperate forest were compared in plots continuously enriched with CO₂ at 200 µL L^?1 above ambient and in control plots exposed to the ambient atmosphere of 360 µL CO₂ L^?1. The purpose was to determine if ecosystem atmospheric CO₂ enrichment would alter soil microbial CH₄ consumption at the forest floor and if the effect of CO₂ would change with time or with environmental conditions. Reduced CH₄ consumption was observed in CO₂‐enriched plots relative to control plots on 46 out of 48 sampling dates, such that CO₂‐enriched plots showed annual reductions in CH₄ consumption of 16% in 1998 and 30% in 1999. No significant differences were observed in soil moisture, temperature, pH, inorganic‐N or rates of N‐mineralization between CO₂‐enriched and control plots, indicating that differences in CH₄ consumption between treatments were likely the result of changes in the composition or size of the CH₄‐oxidizing microbial community. A repeated measures analysis of variance that included soil moisture, soil temperature (from 0 to 30 cm), and time as covariates indicated that the reduction of CH₄ consumption under elevated CO₂ was enhanced at higher soil temperatures. Additionally, the effect of elevated CO₂ on CH₄ consumption increased with time during the two‐year study. Overall, these data suggest that rising atmospheric CO₂ will reduce atmospheric CH₄ consumption in temperate forests and that the effect will be greater in warmer climates. A 30% reduction in atmospheric CH₄ consumption by temperate forest soils in response to rising atmospheric CO₂ will result in a 10% reduction in the sink strength of temperate forest soils in the atmospheric CH₄ budget and a positive feedback to the greenhouse effect.     

Elevated atmospheric CO2 effects on biomass production and soil carbon in conventional and conservation cropping systems

Increasing atmospheric CO₂ concentration has led to concerns about potential effects on production agriculture as well as agriculture's role in sequestering C. In the fall of 1997, a study was initiated to compare the response of two crop management systems (conventional and conservation) to elevated CO₂. The study used a split‐plot design replicated three times with two management systems as main plots and two CO₂ levels (ambient=375 μL L^?1 and elevated CO₂=683 μL L^?1) as split‐plots using open‐top chambers on a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudults). The conventional system was a grain sorghum (Sorghum bicolor (L.) Moench.) and soybean (Glycine max (L.) Merr.) rotation with winter fallow and spring tillage practices. In the conservation system, sorghum and soybean were rotated and three cover crops were used (crimson clover (Trifolium incarnatum L.), sunn hemp (Crotalaria juncea L.), and wheat (Triticum aestivum L.)) under no‐tillage practices. The effect of management on soil C and biomass responses over two cropping cycles (4 years) were evaluated. In the conservation system, cover crop residue (clover, sunn hemp, and wheat) was increased by elevated CO₂, but CO₂ effects on weed residue were variable in the conventional system. Elevated CO₂ had a greater effect on increasing soybean residue as compared with sorghum, and grain yield increases were greater for soybean followed by wheat and sorghum. Differences in sorghum and soybean residue production within the different management systems were small and variable. Cumulative residue inputs were increased by elevated CO₂ and conservation management. Greater inputs resulted in a substantial increase in soil C concentration at the 0–5 cm depth increment in the conservation system under CO₂‐enriched conditions. Smaller shifts in soil C were noted at greater depths (5–10 and 15–30 cm) because of management or CO₂ level. Results suggest that with conservation management in an elevated CO₂ environment, greater residue amounts could increase soil C storage as well as increase ground cover.     

皆伐火烧对亚热带森林不同深度土壤CO2通量的影响

评估不同深度土壤的CO_2通量是研究土壤碳动态的重要手段。目前有关皆伐火烧对森林土壤碳排放的影响研究仅局限于表层土壤,而对不同深度土壤碳排放影响鲜见报道。以米槠(Castanopsis carlesii)次生林(对照)及其皆伐火烧后林地为研究对象,利用非红外散射CO_2探头测定土壤CO_2浓度,并结合Fick第一扩散法则估算不同深度(0—80 cm)土壤CO_2通量。结果表明:(1)皆伐火烧改变土壤向大气排放的表观CO_2通量,在皆伐火烧后的2个月内土壤表观CO_2通量显著高于对照68%;2个月后,土壤表观CO_2通量低于对照37%。(2)皆伐火烧后,除10—20 cm的CO_2通量提高外,其余各深度(0—10、20—40、40—60 cm和60—80 cm)的CO_2通量均降低。同时,皆伐火烧改变不同土层对土壤呼吸的贡献率,降低0—10 cm土层的贡献率,提高10—20 cm土层的贡献率。(3)对照样地仅0—10 cm土壤CO_2通量与温度呈显著指数相关,10—40 cm深度CO_2通量则与土壤含水率呈显著线性相关。皆伐火烧后0—10 cm和10—20 cm处土壤的CO_2通量均与温度呈指数相关。说明皆伐火烧改变了不同深度土壤CO_2通量对于环境因子的响应。因此为准确评估和预测皆伐火烧对土壤与大气间碳交换的影响,应考虑皆伐火烧后不同时期土壤CO_2通量的变化,以及不同深度土壤CO_2通量对皆伐火烧的响应。     

Productivity and water use of wheat under free-air CO2 enrichment

A free-air CO₂ enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE apparatus maintained the CO₂ concentration, [CO₂], at 550 μmol mol^?1 across four replicate 25-m-diameter circular plots under natural conditions in an open field. Four matching Control plots at ambient [CO₂] (about 370 μmol mol^?1) were also installed in the field. In addition to the two levels of [CO₂], there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system in a strip, split-plot design. Measurements were made of net radiation, R_n; soil heat flux, G_o; soil temperature; foliage or surface temperature; air dry and wet bulb temperatures; and wind speed. Sensible heat flux, H, was calculated from the wind and temperature measurements. Latent heat flux, λET, and evapotranspiration, ET, were determined as the residual in the energy balance. The FACE treatment reduced daily total R_n by an average 4%. Daily FACE sensible heat flux, H, was higher in the FACE plots. Daily latent heat flux, λET, and evapotranspiration, ET, were consistently lower in the FACE plots than in the Control plots for most of the growing season, about 8% on the average. Net canopy photosynthesis was stimulated by an average 19 and 44% in the Wet and Dry plots, respectively, by elevated [CO₂] for most of the growing season. No significant acclimation or down regulation was observed. There was little above-ground growth response to elevated [CO₂] early in the season when temperatures were cool. Then, as temperatures warmed into spring, the FACE plants grew about 20% more than the Control plants at ambient [CO₂], as shown by above-ground biomass accumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 °C warmer than the Controls from February through April in the well-watered plots, and we speculate that this temperature rise contributed to the earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, there was a narrowing of the final biomass and yield differences. The 20% mid-season growth advantage of FACE shrunk to about an 8% yield advantage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.     

Effects of an induced drought on soil carbon dioxide (CO2) efflux and soil CO2 production in an Eastern Amazonian rainforest, Brazil

In the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO₂) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO₂ efflux and soil CO₂ concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO₂ efflux from 4.3±0.1 μmol CO₂ m⁻² s⁻¹ (control) to 3.2±0.1 μmol CO₂ m⁻² s⁻¹ (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO₂ production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO₂ efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO₂ production was dominated by a water stress‐induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.