Soil CO2 efflux and its spatial variation in a Florida slash pine plantation

The efflux of CO2 from the soil surface can vary markedly in magnitude both in time and space and its correct determination is crucial in many ecological studies. In this paper, we report results of field measurements, using an open-top dynamic chamber, of soil CO2 efflux in a mature Florida slash pine (Pinus elliottii Engelm. var.elliottii) plantation. The daily average efflux was 0.217 mg CO2 m-2s-1 in the autumn and 0.087 mg CO2 m-2s-1 in the winter. Soil temperature, which accounts for most of the temporal variability in CO2 efflux, is by far the most influential factor controlling soil respiration rate and its temporal variation. The CO2 efflux in the slash pine plantation is highly spatially variable and effluxes from the soil under palmetto is significantly higher than that from the open floor. The CO2 efflux generally increases with increase in soil fine root biomass, litter and humus amount on the forest floor but is inversely related to the amount of organic matter in the mineral soil. The spatial variation in CO2 efflux can be well characterised by a simple multiple regression model incorporating live and dead biomass and soil total porosity as predictor variables. Understorey plants, mostly Serenoa repens, are an important component of the C cycle and the major contributor to the spatial heterogeneity of soil CO2 efflux. The influence of understorey plants on soil respiration is probably via two approaches: increasing litterfall and root metabolism, both consequently stimulating microbial activity in the mineral soil.     

Soil CO2 efflux measurements in a mixed forest: impact of chamber disturbances,spatial variability and seasonal evolution

A closed‐dynamic‐chamber system (CDCS) was used to measure the spatial and seasonal variability of the soil CO₂ efflux (F_s) in beech and in Douglas fir patches of the Vielsalm forest (Belgium). First the difference between natural and measured soil CO₂ efflux induced by the presence of the CDCS was studied. The impact on the measurements of the pressure difference between the outside (natural condition) and the inside of the chamber was found to be small (0.4%). The influence of wind disturbance in the closed chamber was tested by comparison with an open‐chamber system characterized by a different wind distribution. A very good correlation between the two systems was found (r² = 0.99) but the open system yielded slightly lower fluxes than the closed one (slope = 0.88 ± 0.05). A measurement procedure has been developed to minimize the effect of the other sources of perturbation. The spatial and seasonal evolution of the soil CO₂ efflux was obtained by performing regular measurements on 29 spots in the beech patch over a period of 12 months and on 24 spots in the Douglas fir patch over 8 months. For each spot, the experimental relationship between Fs and soil temperature was compared with the fitted line for an Arrhenius relationship with a soil temperature‐dependent activation energy. Soil temperature explains 73% of the seasonal variation for all the data. The spatial average of the soil CO₂ efflux at 10 °C (Fs₁₀) in the beech patch is 2.57 ± 0.41 μmol m^?2 s^?1, approximately twice the average in the Douglas fir patch recorded at 1.42 ± 0.22 μmol m^?2 s^?1. The litter fall analysis seems to indicate that soil organic matter quality and quantity may be one the reasons for this difference. Finally the annual soil CO₂ efflux was calculated for the beech and Douglas fir patches (870 ± 140 and 438 ± 68 gC m^?2 y^?1, respectively). The beech value would represent 92 ± 15% of the annual ecosystem respiration estimated from the eddy covariance measurements.     

Seasonal soil‐surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

Elevated atmospheric CO₂ concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO₂ concentration were higher than those at ambient CO₂ concentration, and whether this was related to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol^?1) CO₂ concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m^?2 s^?1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO₂ concentrations at four depths. For the second year of growth, the annual carbon flux from the plots was 1671 g y^?1 and 1895 g y^?1 at ambient and elevated CO₂ concentrations, respectively, although this was not a significant difference. Higher f at elevated CO₂ concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.     

秦岭火地塘林区油松林土壤呼吸时空变异

土壤呼吸是陆地生态系统碳循环的关键生态过程,土壤呼吸的时空变异及其影响因子已成为生态学研究的主要内容之一。采用红外线开路气室法和便携式微气象站,连续测定了秦岭火地塘林区天然次生油松林地不同部位土壤呼吸速率和不同土层深度土壤温度和土壤体积含水率,结果表明:(1)植物生长季,试验地上部与中部、中部与下部,土壤呼吸日均值间存在显著差异。植物休眠季,全坡面土壤呼吸日均值差异不显著。同一观测部位植物生长季与休眠季,土壤呼吸日均值差异显著。观测期内全样地土壤呼吸日均值为(38.64±6.43)gm-2d-1;(2)同一地形部位不同观测月中和不同地形部位同一观测时间,土壤呼吸月均值大多存在显著差异,植物生长季和休眠季,全样地土壤呼吸均值分别为(46.98±2.21)gm-2d-1和(35.94±1.01)gm-2d-1,全样地土壤呼吸月均值为(1.18±0.20)kgm-2月-1,休眠季土壤日均呼吸约为整个观测季的43.34%;(3)当土壤温度9.0℃时,土壤温度与土壤呼吸速率间均存在显著的指数关系。回归模型的决定系数均大于0.87,均方差根不超过0.21,模型有效性系数不小于0.85,残差系数的绝对值不超过0.007。(4)植物生长季0-5cm和5-10cm土层及植物休眠季0-5cm土层,土壤呼吸日累积值均值与相应土层深度土壤体积含水率均值间存在三次函数关系,回归模型的决定系数分别为0.456,0.513和0.143;植物休眠季5-10cm土层,土壤呼吸日累积值均值与土壤体积含水率均值间存在幂函数关系,回归模型的决定系数为0.650。     

Temperature sensitivity of biomass‐specific microbial exo‐enzyme activities and CO2 efflux is resistant to change across short‐ and long‐term timescales

Accurate representation of temperature sensitivity (Q₁₀) of soil microbial activity across time is critical for projecting soil CO₂ efflux. As microorganisms mediate soil carbon (C) loss via exo‐enzyme activity and respiration, we explore temperature sensitivities of microbial exo‐enzyme activity and respiratory CO₂ loss across time and assess mechanisms associated with these potential changes in microbial temperature responses. We collected soils along a latitudinal boreal forest transect with different temperature regimes (long‐term timescale) and exposed these soils to laboratory temperature manipulations at 5, 15, and 25°C for 84 days (short‐term timescale). We quantified temperature sensitivity of microbial activity per g soil and per g microbial biomass at days 9, 34, 55, and 84, and determined bacterial and fungal community structure before the incubation and at days 9 and 84. All biomass‐specific rates exhibited temperature sensitivities resistant to change across short‐ and long‐term timescales (mean Q₁₀ = 2.77 ± 0.25, 2.63 ± 0.26, 1.78 ± 0.26, 2.27 ± 0.25, 3.28 ± 0.44, 2.89 ± 0.55 for β‐glucosidase, N‐acetyl‐β‐d ‐glucosaminidase, leucine amino peptidase, acid phosphatase, cellobiohydrolase, and CO₂ efflux, respectively). In contrast, temperature sensitivity of soil mass‐specific rates exhibited either resilience (the Q₁₀ value changed and returned to the original value over time) or resistance to change. Regardless of the microbial flux responses, bacterial and fungal community structure was susceptible to change with temperature, significantly differing with short‐ and long‐term exposure to different temperature regimes. Our results highlight that temperature responses of microbial resource allocation to exo‐enzyme production and associated respiratory CO₂ loss per unit biomass can remain invariant across time, and thus, that vulnerability of soil organic C stocks to rising temperatures may persist in the long term. Furthermore, resistant temperature sensitivities of biomass‐specific rates in spite of different community structures imply decoupling of community constituents and the temperature responses of soil microbial activities.     

鼎湖山不同演替阶段森林土壤水分时空变异研究

土壤水分作为森林生态系统水分蓄库的主体,森林土壤水分储量及其时空动态与变异对揭示区域植被恢复与气候变化背景下的森林生态系统水文过程响应与服务功能变化机制具有重要意义。本研究以南亚热带地区典型森林植被演替序列马尾松人工林（Pinus massoniana coniferous forest,PF）-马尾松针阔叶混交林（mixed Pinus massoniana/broad-leaved forest,MF）-季风常绿阔叶林（monsoon evergreen broad-leaved forest,MEBF）为研究对象,依托中国生态系统研究网络森林样地建设与监测统一规范对鼎湖山森林生态系统定位站站区内分布的上述森林类型土壤水分的长期定位观测（2005-2015年）,通过分析各演替阶段森林土壤不同土层（0-15、15-30、30-45、45-60、60-75和75-90 cm）土壤体积含水量观测数据,探究该区域森林植被恢复过程中的土壤水分变化及其时空变异。结果表明：在雨热同期且干湿季明显的南亚热带地区,鼎湖山森林土壤储水量及其时间动态受降雨量的影响显著,森林土壤层对降雨具有强烈的调蓄和稳定作用,伴随PF→MF→MEBF自然演替进程,调蓄水分能力逐步增强。林型间,由初期阶段PF到顶级群落MEBF,森林土壤水分储量逐渐提高,且演替后期林型相对于早期林型,土壤储水量均呈现为较小的年际与年内变幅。干、湿季而言,干季时林型间的土壤储水量差异大于湿季,干季时MEBF和MF土壤含水量分别是PF的1.33倍和1.11倍。从土壤含水量的干、湿季期间变异来看,不同林型各土层土壤含水量的变异系数大小均表现为干季大于湿季;垂直剖面方向上,突出表现为无论干湿季MEBF各层土壤含水量变异均比其他两种林型较为缓和,充分体现了MEBF优越的土壤水分时空调配能力。整体上,伴随PF→MF→MEBF自然演替进程,土壤水分储量及其稳定性逐步提升。     

Temporal and spatial variations of stem CO2 efflux of three species in subtropical China

Aims Although stem CO₂ efflux is critical to ecosystem carbon and energy balance and its feedback to future climate change, little information is available on stem CO₂ efflux and its responses to temperature, especially in subtropical China. This study aims to (i) evaluate the temporal and spatial variations of stem CO₂ efflux of three species, including oak (Quercus acutissima Carr.), masson pine (Pinus massoniana Lamb.) and loblolly pine (Pinus taeda Linn.) in subtropical China and (ii) analyze the temperature sensitivity of stem CO₂ efflux in the three species based on 2-year field measurements.Methods We measured stem CO₂ efflux and stem temperature (at 3 cm depth) of the three species using the horizontally oriented soil chamber technique from September 2008 to August 2010. We also conducted a 24-h measurement to examine the diurnal variation of stem CO₂ efflux in three consecutive days in April 2009.Important findings The temporal dynamics of stem CO₂ efflux followed the change of the stem temperature in a 3-cm depth with a bell-shaped curve in the three species. Stem temperature explained 77–85% of the seasonal variations of stem CO₂ efflux over the entire study period in the three species. The temperature sensitivity (Q 10) of stem CO₂ efflux was obviously different among the three species with higher Q 10 value found in oak (2.24) and lower values in the coniferous species (1.76 and 1.63). Our results also showed that the Q 10 values of stem CO₂ efflux in all the three species were lower in the growing season than that in the non-growing season, indicating that the growth and maintenance respiration had different temperature responses. Moreover, we found that the temperature-normalized stem CO₂ efflux (R 10) changed greatly between the growing and non-growing seasons in oak and masson pine, but not in loblolly pine. Additionally, we also found that in the non-growing season, the principal factor responsible for the spatial variation of stem CO₂ efflux among the 15 sampling trees was sapwood volume, whereas in the growing season, stem CO₂ efflux was closely related to annual dry-matter production in the three subtropical species.     

Soil CO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> efflux and fungal and bacterial biomass in a plantation and a secondary forest in wet tropics in Puerto Rico

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO₂ efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO₂ efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO₂ efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO₂ efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO₂ efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO₂ efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO₂ efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO₂ efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R² > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests.     

喀斯特地区土壤表层CO2释放通量的影响因素Ⅰ:规律

测定了贵州喀斯特地区土壤表层CO2释放通量,同时还测定了土壤微生物生物量碳以及土壤可溶性有机质含量和土壤湿度。研究表明,贵州喀斯特地区全年土壤表层CO2释放通量与温度变化呈正相关关系,与土壤微生物生物量碳呈负相关关系;当温度＞20℃时,土壤表层CO2释放通量与土壤湿度呈正相关,与土壤可溶性有机碳含量呈负相关。     

Soil CO2 effluxes,temporal and spatial variations,and root respiration in shrub willow biomass crops fields along a 19‐year chronosequence as affected by regrowth and removal treatments

In shrub willow biomass crop (SWBC) production systems, the soil CO₂ efflux (F_c) component in the carbon cycle remains poorly understood. This study assesses (i) differences of F_c rates among the 5‐, 12‐, 14‐, and 19‐year‐old SWBCs with two treatments: continuous production (regrowth) willow fields that were harvested and allowed to regrow, and willow fields that were harvested, killed, and then stools and roots were ground into the soil (removal); (ii) temporal and spatial variations of F_c rates; (iii) root respiration contributions to total F_c; and (iv) climatic variables affecting F_c. During the growing season (May to September), F_c rates showed no statistically significant differences across different ages (P = 0.664), and between treatments (P = 0.351); however, there was an interaction between age and treatment (P = 0.001). Similarly, during the dormant season (October to April), F_c rates revealed no statistically significant differences across different ages (P = 0.305) and treatment interaction with age (P = 0.097). F_c rates differed significantly (P < 0.001) among different times of the day and times of the year. F_c rates, between 00 and 1059 h, between 1100 and 1659 h, and between 1700 and 2400 h displayed consistency from May to November; however, F_c rates in these three time intervals showed significant differences (P < 0.0001). In December, F_c rates remained constant over 24 h. F_c rates demonstrated higher temporal and spatial variations among willow age classes than between regrowth and removal treatments. Temporal and spatial variations of F_c were higher during the dormant season than during the growing season. The proportion of root respiration to total F_c ranged from 18 to 33% across age classes. F_c rates showed strong association with soil and air temperatures, and relative humidity.     

Differential controls by climate and substrate over the heterotrophic and rhizospheric components of soil respiration

The partitioning of soil respiration rates into the component processes of rhizospheric respiration (because of live roots and those microorganisms that subsist on root exudations) and heterotrophic respiration (because of decomposer microorganisms that subsist on the oxidation of soil organic matter) is difficult to accomplish through experimental observation. In order to minimize disturbance to the soil and maximize preservation of the natural relationships among roots, rhizospheric microorganisms, and decomposers, we conducted a girdling experiment in a subalpine forest dominated by lodgepole pine trees. In two separate years, we girdled trees in small forest plots (5–7 m in diameter) and trenched around the plots to sever invading roots in order to experimentally stop the transport of photosynthate from needles to roots, and eliminate rhizospheric respiration. Soil respiration rates in plots with trees girdled over 1 year prior to measurement were higher than those in plots with trees girdled 2–3 months prior to measurement. These results suggest that any stimulation of respiration because of the experimental artifact of fine root death and addition of labile carbon to the pool of decomposer substrates is slow, and occurs beyond the first growing season after girdling. Compared with control plots with nongirdled trees, soil respiration rates in plots with girdled trees were reduced by 31–44% at the mid‐summer respiratory maximum. An extreme drought during one of the 2 years used for observations caused greater reductions in the heterotrophic component of soil respiration compared with the rhizospheric component. In control plots, we observed a pulse in K₂SO₄‐extractable carbon during the spring snowmelt period, which was absent in plots with girdled trees. In control plots, soil microbial biomass increased from spring to summer, coincident with a seasonal increase in the rhizospheric component of soil respiration. In plots with girdled trees, the seasonal increase in microbial biomass was lower than in control plots. These results suggest that the observed seasonal increase in rhizospheric respiration rate in control plots was because of an increase in rhizospheric microbial biomass following ‘soil priming’ by a spring‐time pulse in dissolved organic carbon. Winter‐time, beneath‐snow microbial biomass was relatively high in control plots. Soil sucrose concentrations were approximately eight times higher during winter than during spring or summer, possibly being derived from the mechanical damage of shallow roots that use sucrose as protection against low‐temperature extremes. The winter‐time sucrose pulse was not observed in plots with girdled trees. The results of this study demonstrate that (1) the rhizospheric component of soil respiration rate at this site is significant in magnitude, (2) the heterotrophic component of soil respiration rate is more susceptible to seasonal drought than the rhizospheric component, and (3) the trees in this ecosystem exert a major control over soil carbon dynamics by ‘priming’ the soil with sugar exudates during the late‐spring snowmelt period and releasing high concentrations of sucrose to the soil during winter.     

Natural variations in snow cover do not affect the annual soil CO2 efflux from a mid‐elevation temperate forest

Climate change might alter annual snowfall patterns and modify the duration and magnitude of snow cover in temperate regions with resultant impacts on soil microclimate and soil CO₂ efflux (F_soil). We used a 5‐year time series of F_soil measurements from a mid‐elevation forest to assess the effects of naturally changing snow cover. Snow cover varied considerably in duration (105–154 days) and depth (mean snow depth 19–59 cm). Periodically shallow snow cover (<10 cm) caused soil freezing or increased variation in soil temperature. This was mostly not reflected in F_soil which tended to decrease gradually throughout winter. Progressively decreasing C substrate availability (identified by substrate induced respiration) likely over‐rid the effects of slowly changing soil temperatures and determined the overall course of F_soil. Cumulative CO₂ efflux from beneath snow cover varied between 0.46 and 0.95 t C ha^?1 yr^?1 and amounted to between 6 and 12% of the annual efflux. When compared over a fixed interval (the longest period of snow cover during the 5 years), the cumulative CO₂ efflux ranged between 0.77 and 1.18 t C ha^?1 or between 11 and 15% of the annual soil CO₂ efflux. The relative contribution (15%) was highest during the year with the shortest winter. Variations in snow cover were not reflected in the annual CO₂ efflux (7.44–8.41 t C ha^?1) which did not differ significantly between years and did not correlate with any snow parameter. Regional climate at our site was characterized by relatively high amounts of precipitation. Therefore, snow did not play a role in terms of water supply during the warm season and primarily affected cold season processes. The role of changing snow cover therefore seems rather marginal when compared to potential climate change effects on F_soil during the warm season.     

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

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

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO₂ efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO₃^? deposition (3 g N m^?2 yr^?1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO₃^?‐amended plots. In all subsequent measurement years, soil respiration rates from NO₃^?‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO₂ efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO₃^? additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO₂ efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO₃^?‐amended plots are consistent with this explanation.     

杉木人工林去除根系土壤呼吸的季节变化及影响因子

2007年1月至2008年12月,在长沙天际岭国家森林公园内,采用挖壕法研究杉木人工林去除根系后土壤呼吸速率季节动态及其与5 cm土壤温、湿度的相关关系。结果表明:去除根系与对照5 cm土壤温度的差异性不显著(P=0.987),5 cm土壤湿度差异显著(P=0.035)。杉木林去除根系处理后土壤呼吸速率明显降低,2007至2008两年实验期间去除根系与对照处理变化范围分别为0.19-2.01μmol.m-2s-1和0.26-2.61μmo.lm-2s-1,年均土壤呼吸速率分别为0.90μmo.lm-2s-1和1.30μmol.m-2s-1。去除根系土壤呼吸速率降低幅度为9.4%-59.7%,平均降低了30.4%。去除根系和对照的土壤呼吸速率与5 cm土壤温度之间均呈显著指数相关,模拟方程分别为:y=0.120e0.094t(R2=0.882,P=0.000),y=0.291e0.069t(R2=0.858,P=0.000)。Q10值分别为2.56和2.01。     

Large‐scale variations in the vegetation growing season and annual cycle of atmospheric CO2 at high northern latitudes from 1950 to 2011

We combine satellite and ground observations during 1950–2011 to study the long‐term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO₂ concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days (P < 0.01, 1982–2011), which is unprecedented in the context of the past 60 years. The overall lengthening has been stronger and more significant in Eurasia (12.6 days, P < 0.01) than North America (6.2 days, P > 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over Eurasia rather than North America, is associated with a long‐term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO₂ annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO₂ uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO₂ to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.     

土壤温度和水分对油松林土壤呼吸的影响

用LI-COR 6400-09土壤呼吸测定系统,在太原天龙山自然保护区对油松林的土壤呼吸进行了4a测定.结果表明,土壤呼吸具有明显的季节变化特点,最大值出现在8月份,在6～10 μmol m~(-2) s~(-1) 之间,最小值出现在12月份和3月份,在0.5 μmol m~(-2) s~(-1)左右.2005、2006、2007和2008年土壤呼吸CO_2的年平均值分别为(4.71±3.74)、(3.08±2.91)、(2.96±2.58) μmol m~(-2) s~(-1)和(2.12±1.54) μmol m~(-2) s~(-1);4a的CO_2总平均值为(3.27±2.95) μmol m~(-2) s~(-1).4个测定年土壤呼吸的平均值总体差异显著.4个测定年土壤CO_2释放C量分别为1103.5、882.8、918.4 g m~(-2)和666.3 g m~(-2),总C平均值为892.8 g m~(-2),具有明显的年际差异.指数方程可以很好的表达土壤呼吸与10 cm深度土壤温度的关系,R~2值4a分别为0.39,0.60,0.68和0.71,Q_(10)值分别为3.10,4.41、4.05和5.18,用4a全部数据计算的Q_(10)值为4.31.土壤水分对土壤呼吸的作用较弱,R~2值4a分别仅为0.31、0.25、0.13和0.02,但是夏季土壤干旱对土壤呼吸的抑制作用非常明显,可使土壤呼吸下降50%以上.夏季土壤干旱是导致土壤呼吸年际变化的主要原因.4个包括土壤温度和水分的双变量模型均可以很好地模拟土壤呼吸的季节变化, 拟合方程的R~2值从0.58到0.79.     

Soil [N] modulates soil C cycling in CO2‐fumigated tree stands: a meta‐analysis

Under elevated atmospheric CO₂ concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO₂ effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO₂ stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO₂ induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO₂. Soil N concentration strongly interacted with CO₂ fumigation: the effect of elevated CO₂ on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO₂ are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.     

Plant respiration and photosynthesis in global‐scale models: incorporating acclimation to temperature and CO2

To realistically simulate climate feedbacks from the land surface to the atmosphere, models must replicate the responses of plants to environmental changes. Several processes, operating at various scales, cause the responses of photosynthesis and plant respiration to temperature and CO₂ to change over time of exposure to new or changing environmental conditions. Here, we review the latest empirical evidence that short‐term responses of plant carbon exchange rates to temperature and CO₂ are modified by plant photosynthetic and respiratory acclimation as well as biogeochemical feedbacks. We assess the frequency with which these responses have been incorporated into vegetation models, and highlight recently designed algorithms that can facilitate their incorporation. Few models currently include representations of the long‐term plant responses that have been recorded by empirical studies, likely because these responses are still poorly understood at scales relevant for models. Studies show that, at a regional scale, simulated carbon flux between the atmosphere and vegetation can dramatically differ between versions of models that do and do not include acclimation. However, the realism of these results is difficult to evaluate, as algorithm development is still in an early stage, and a limited number of data are available. We provide a series of recommendations that suggest how a combination of empirical and modeling studies can produce mechanistic algorithms that will realistically simulate longer term responses within global‐scale models.     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].