中国东部亚热带森林土壤呼吸的时空格局

土壤呼吸是陆地碳循环中仅次于全球总初级生产力的第二大碳通量途径, 揭示土壤呼吸的时空格局对整个陆地碳循环具有重要意义。该文在中国东部亚热带季风气候区, 按纬度梯度由南向北选取深圳梧桐山、杨东山十二度水保护区、宁波天童山3个区域作为研究对象, 于2009年8月至2010年10月测定了不同季节各个区域内代表性植被类型的土壤呼吸速率及地下5 cm处土壤温度, 旨在初步了解中国东部亚热带森林地区土壤呼吸的时空格局及其影响因素。结果显示: 3个区域的土壤呼吸速率均存在显著的季节变化, 其变幅为2.64-6.24 μmol CO₂·m ^-2·s^-1, 总体趋势和地下5 cm处土壤温度的季节变化一致, 均为夏季最高冬季最低; 土壤温度的变化可以解释不同样地土壤呼吸季节变化的58.3%-90.2%; 各样地全年的Q₁₀值从1.56到3.27; 通过离样地最近的气象站点的日平均气温与试验样地地下5 cm处土壤温度之间的线性正相关关系推算出日土壤温度的变化, 利用土壤呼吸速率和地下5 cm处土壤温度之间的指数关系, 估算出各样地全年的土壤CO₂通量为1 077-2 058 g C·m^-2·a^-1, 在全球所有生态系统类型中处于较高水平。     

Soil respiration in Pinus tabulaeformis forest during dormant period at Huoditang forest zone in the Qinling Mountains, China

Soil CO₂ efflux in forest ecosystems during dormant season is one of the key components of the forest ecosystem carbon balance. Little work has been done to quantify soil CO₂ efflux in most forests in China in special time because of difficulty in taking measurements. Soil respiration in a natural secondary Pinus tabulaeformis forest at Huoditang in the Qinling Mountains was measured from October to December in 2006 by means of open-path dynamic chamber technique. Relationships of soil respiration rate (R_s) with mean soil temperature (MST) and mean volumetric soil moisture content (MVSC) in different depths (0-5 cm and 5-10 cm) were examined in the current study. We found that (1) At the same observation site (upper-part, middle-part or under-part), there were tremendous temporal and spatial variations in R_s with variation coefficients of 48.38%, 82.51% and 81.88% in October, November and December, respectively; (2) There was a significant exponent relationship between diurnal mean soil respiration rate (F_c) and diurnal mean soil temperature (DMST) when DMST > 8.5°C for both soil depths (0-5 cm and 5-10 cm) examined. The temperature sensitivity of soil respiration, known as the Q₁₀ value, was 1.297 and 1.323 in soil depths of 0-5 cm and 5-10 cm, respectively; (3) Relationship between R_s and MVSC was complex in soil depths of 0-5 cm and 5-10 cm; (4) Soil CO₂ efflux from October to December in 2006 in the experimental area was (977.37 ± 88.43) to (997.19 ± 80.73) gCm⁻² (p = 0.005).     

秦岭火地塘林区油松(Pinus tabulaeformis)林休眠期的土壤呼吸

林木休眠期林地土壤CO2释放是森林生态系统碳平衡关键组成部分之一.由于绝大多数森林生态系统林木休眠期土壤CO2释放过程测定困难,国内有关林木休眠期CO2释放,量化方面的研究开展较少.采用动态开路气室法对秦岭火地塘林区天然次生油松(Pinus tabulaeformis)林土壤呼吸的日变化进行了测定,分析了土壤呼吸速率(mgCO2m-2h-1)与土壤温度和体积含水率的关系,基于土壤日均呼吸速率和土壤日均温度指数方程与观测季的总天数,估算了林木休眠期林地土壤CO2释放量.结果表明:(1)研究区林地土壤呼吸速率存在较大的时、空变异.不同观测部位土壤呼吸速率的峰值出现时间各异,呼吸作用较弱的时段也不一致.同一观测部位不同观测月中,土壤日均呼吸速率变异系数分别为48.38%,82.51%和81.88%;(2)当土温＞8.5 ℃时,0～5 cm和5～10 cm土层,土壤日均温与土壤日均呼吸速率间存在极显著(p＜0.001)的指数关系,Q10分别为1.297和1.323;(3)0～5 cm和5～10 cm土层,土壤体积含水率与土壤呼吸速率间关系复杂;(4) 林木休眠期研究区林地土壤CO2释放量变化于(977.37±88.43)～(997.19±80.73) gCm-2(p=0.005)间.     

神农架海拔梯度上4种典型森林的土壤呼吸组分及其对温度的敏感性

量化森林土壤呼吸及其组分对温度的响应对准确评估未来气候变化背景下陆地生态系统的碳平衡极其重要。该文通过对神农架海拔梯度上常绿阔叶林、常绿落叶阔叶混交林、落叶阔叶林以及亚高山针叶林4种典型森林土壤呼吸的研究发现: 4种森林类型的年平均土壤呼吸速率和年平均异养呼吸速率分别为1.63、1.79、1.74、1.35 μmol CO₂·m^-2·s^-1和1.13、1.12、1.12、0.80 μmol CO₂·m^-2·s^-1。该地区的土壤呼吸及其组分呈现出明显的季节动态, 夏季最高, 冬季最低。4种森林类型中, 阔叶林的土壤呼吸显著高于针叶林, 但阔叶林之间的土壤呼吸差异不显著。土壤温度是影响土壤呼吸及其组分的主要因素, 二者呈显著的指数关系; 土壤含水量与土壤呼吸之间没有显著的相关关系。4种典型森林土壤呼吸的Q₁₀值分别为2.38、2.68、2.99和4.24, 随海拔的升高土壤呼吸对温度的敏感性增强, Q₁₀值随海拔的升高而增加。     

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）。     

Mechanisms of carbon and nutrient release and retention in beech forest gaps

Fluxes of CO₂ and N₂O were measured along a microclimatic gradient stretching from the centre of a gap into a mature beech stand using an automated chamber method. Simultaneously the regulating factors like soil water tensions, soil temperatures, nitrate concentrations were measured along the gradient. The daily mean values of the fluxes of CO₂ and N₂O were divided into classes of temperature and furthermore subdivided into classes of soil water tension to assess the significance of each regulating factor.Soil respiration at the centre of the gap was 40% lower compared to the rooted mature stand. The difference was explained by root respiration. At both sites soil respiration was primarily controlled by the soil temperature with an average Q₁₀ value of 2.3 over the different classes of temperature and soil water tension. Soil water tension reduced the soil respiration by up to 20% only by soil water tension above 400–600 hPa at the mature stand. The formation of N₂O was reduced when the soil temperature was below 10°C or the soil water tension exceeded 200 hPa. Therefore the N₂O emission was 6 times higher at the unrooted centre of the gap due to the high moisture content in the growing season. Higher nitrate concentration doubled the N₂O emission at the unrooted edge of the canopy and resulted in losses of 6.4 kg N ha^-1 within six months. Above 10°C and below 200 hPa the N₂O emission depended strongly upon the temperature with varying Q₁₀ values over the different classes of temperature and soil water tension. High Q₁₀ values up to 14.4 have been calculated below 14°C and were explained by several processes with synergetic effects.     

Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest

Variation in soil temperature can account for most of the seasonal and diel variation in soil CO₂ efflux, but the temperature effect is not always consistent, and other factors such as soil water content are known to influence soil respiration. The objectives of this research were to study the spatial and temporal variation in soil respiration in a temperate forested landscape and to evaluate temperature and soil water functions as predictors of soil respiration. Soil CO₂ fluxes were measured with chambers throughout an annual cycle in six study areas at the Harvard Forest in central Massachusetts that include soil drainage classes from well drained to very poorly drained. The mean annual estimate of soil CO₂ efflux was 7.2 Mg ha^–1, but ranged from 5.3 in the swamp site to 8.5 in a well-drained site, indicating that landscape heterogeneity is related to soil drainage class. An exponential function relating CO₂ fluxes to soil temperature accounted for 80% of the seasonal variation in fluxes across all sites (Q₁₀ = 3.9), but the Q₁₀ ranged from 3.4 to 5.6 for the individual study sites. A significant drought in 1995 caused rapid declines in soil respiration rates in August and September in five of the six sites (a swamp site was the exception). This decline in CO₂ fluxes correlated exponentially with decreasing soil matric potential, indicating a mechanistic effect of drought stress. At moderate to high water contents, however, soil water content was negatively correlated with soil temperature, which precluded distinguishing between the effects of these two confounded factors on CO₂ flux. Occurrence of high Q₁₀ values and variation in Q₁₀ values among sites may be related to: (i) confounding effects of high soil water content; (ii) seasonal and diel patterns in root respiration and turnover of fine roots that are linked to above ground phenology and metabolism; and (iii) variation in the depth where CO₂ is produced. The Q₁₀ function can yield reasonably good predictions of annual fluxes of CO₂, but it is a simplification that masks responses of root and microbial processes to variation in temperature and water content throughout the soil.     

内蒙古农牧交错区不同土地利用方式下土壤呼吸速率及其温度敏感性变化

2009年8-10月, 采用动态气室法观测了内蒙古农牧交错区多伦县农田、弃耕和围封3种土地利用方式下, 土壤呼吸速率从6:00到18:00的变化规律, 分析了不同深度的土壤温度与土壤含水量对土壤呼吸速率的控制作用。结果表明, 空间尺度上, 不同土地利用方式的土壤呼吸速率由高到低依次为: 农田>弃耕>围封; 时间尺度上, 土壤呼吸速率在6:00-18:00的变化趋势为单峰曲线, 在12:00-15:00达到峰值, 随后降低, 在18:00基本恢复到6:00左右的呼吸水平, 同时, 土壤呼吸速率在9、10月显著降低。利用Van’t Hoff指数模型研究不同深度土壤温度对土壤呼吸速率的影响发现, 10-15 cm深度的土壤温度对土壤呼吸速率的影响最为显著, 其中, 土壤呼吸温度敏感性由高到低分别为: 农田>围封>弃耕。相反, 由于8-10月土壤含水量变化较小, 故土壤含水量与土壤呼吸速率间的相关性不显著, 土壤含水量不能解释该时段土壤呼吸速率的变化。     

Determinants of growing season soil CO2flux in a Minnesota grassland

Soil CO₂ flux was measured across 947 plots at 7 experimentalgrassland sites at the Cedar Creek Natural History Area in order to determinethe relationships between soil CO₂ flux and environmental factors,living plant biomass, and soil C and N. Soil CO₂ flux increased asthe day progressed, and was positively related to aboveground biomass,belowground biomass, and soil % C. However, most of the variation in soilCO₂ flux explained by a multiple regression model(r ² = 0.55) was attributed to the different experimental sites (61%).Soil CO₂ flux increased with increasing aboveground plant biomass(explaining 16% of the model variation),belowground plant biomass (12%), and soil C and C:N ratio(6%). The length of time between aboveground biomass in aplot was clipped and soil CO₂ flux variedamong plots. Soil CO₂ flux declined with increased timesince clipping, supporting the idea that recently fixedcarbon is a significant component of soil CO₂ flux.Soil CO₂ flux did not follow standard Q₁₀relationships. Over a 20 °C temperature range,soil CO₂ flux tended to be lower in warmer plots.More work is necessary to understand what factors explainthe large differences that were seen among experimentalsites in soil CO₂ flux that could not be explainedby biomass or soil properties.     

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。     

Experimental forest soil warming: response of autotrophic and heterotrophic soil respiration to a short-term 10°C temperature rise

We warmed the top soil of a mature coniferous forest stand by means of heating cables on control and trenched plots within 24 h by 10°C at 1 cm soil depth (9°C at 5 cm depth) and measured the effect on the autotrophic (R_A) and heterotrophic (R_H) component of total soil CO₂ efflux (R_S). The short time frame of warming enabled us to exclude confounding fluctuations in soil moisture and carbon (C) flow from the canopy. The results of the field study were backed up by a lab soil incubation experiment. During the first 12 h of warming, R_A strongly responded to soil warming; The Q ₁₀ values were 5.61 and 6.29 for 1 and 5 cm soil depth temperature. The Q ₁₀ values for R_A were almost twice as high as the Q ₁₀ values of R_H (3.04 and 3.53). Q ₁₀ values above 5 are above reasonable plant physiological values for root respiration. We see interactions of roots, mycorrhizae and heterotrophic microbes, combined with fast substrate supply to the rhizosphere as an explanation for the high short-term temperature response of R_A. When calculated over the whole duration (24 h) of the field soil-warming experiment, temperature sensitivities of R_A and R_H were similar (no significant difference at P < 0.05); Q ₁₀ values were 3.16 and 3.96 for R_A and 2.94 and 3.35 for R_H calculated with soil temperatures at 1 and 5 cm soil depth, respectively. Laboratory incubation showed that different soil moisture contents of trenched and control plots affected rates of R_H, but did not affect the temperature sensitivity of R_H. We conclude that a single parameter is sufficient to describe the temperature sensitivity of R_S in soil C models which operate on larger temporal and spatial scales. The strong short-term response of R_A may be of relevance in soils suspected to experience increasingly strong diurnal temperature variations.     

青藏高原高寒草甸土壤CO2排放对模拟氮沉降的早期响应

研究大气氮沉降输入对青藏高原高寒草甸土壤-大气界面CO₂交换通量的影响,对于准确评价全球变化背景下区域碳平衡至关重要。通过构建多形态、低剂量的增氮控制试验,利用静态箱-气相色谱法测定土壤CO₂排放通量,同时测定相关土壤变量和地上生物量,分析高寒草甸土壤CO₂排放特征及其主要驱动因子。研究结果表明:低、高剂量氮输入倾向于消耗土壤水分,而中剂量氮输入有利于土壤水分的保持;施氮初期总体上增加了土壤无机氮含量,铵态氮累积效应更为显著;施氮显著增加地上生物量和土壤CO₂排放通量,铵态氮的促进效应显著高于硝态氮。另外,土壤CO₂排放通量主要受土壤温度驱动,其次为地上生物量和铵态氮储量。上述结果反映了氮沉降输入短期内可能刺激了植物生长和土壤微生物活性,加剧了土壤-大气界面CO₂排放。     

Estimation of autotrophic and heterotrophic components of soil respiration by trenching is sensitive to corrections for root decomposition and changes in soil water content

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO₂ efflux partitioning. Soil CO₂ efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO₂ efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO₂ efflux in control plots was 528 g C m⁻² year⁻¹ at H1 and 527 g C m⁻² year⁻¹ at H2. The annual soil CO₂ efflux in trenched plots was 353 g C m⁻² year⁻¹ at H1 and 425 g C m⁻² year⁻¹ at H2. By taking into account annual CO₂ efflux from decaying trenched roots, the autotrophic contribution to total soil CO₂ efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO₂ despite a limited effect on partitioning.     

Inversely Estimating the Vertical Profile of the Soil CO2 Production Rate in a Deciduous Broadleaf Forest Using a Particle Filtering Method

Carbon dioxide (CO₂) efflux from the soil surface, which is a major source of CO₂ from terrestrial ecosystems, represents the total CO₂ production at all soil depths. Although many studies have estimated the vertical profile of the CO₂ production rate, one of the difficulties in estimating the vertical profile is measuring diffusion coefficients of CO₂ at all soil depths in a nondestructive manner. In this study, we estimated the temporal variation in the vertical profile of the CO₂ production rate using a data assimilation method, the particle filtering method, in which the diffusion coefficients of CO₂ were simultaneously estimated. The CO₂ concentrations at several soil depths and CO₂ efflux from the soil surface (only during the snow-free period) were measured at two points in a broadleaf forest in Japan, and the data were assimilated into a simple model including a diffusion equation. We found that there were large variations in the pattern of the vertical profile of the CO₂ production rate between experiment sites: the peak CO₂ production rate was at soil depths around 10 cm during the snow-free period at one site, but the peak was at the soil surface at the other site. Using this method to estimate the CO₂ production rate during snow-cover periods allowed us to estimate CO₂ efflux during that period as well. We estimated that the CO₂ efflux during the snow-cover period (about half the year) accounted for around 13% of the annual CO₂ efflux at this site. Although the method proposed in this study does not ensure the validity of the estimated diffusion coefficients and CO₂ production rates, the method enables us to more closely approach the “actual” values by decreasing the variance of the posterior distribution of the values.     

Variability in soil respiration across riparian-hillslope transitions

The spatial and temporal controls on soil CO₂ production and surface CO₂ efflux have been identified as outstanding gaps in our understanding of carbon cycling. We investigated both across two riparian-hillslope transitions in a subalpine catchment, northern Rocky Mountains, Montana. Riparian-hillslope transitions provide ideal locations for investigating the controls on soil CO₂ dynamics due to strong, natural gradients in the factors driving respiration, including soil water content (SWC) and soil temperature. We measured soil air CO₂ concentrations (20 and 50 cm), surface CO₂ efflux, soil temperature, and SWC at eight locations. We investigated (1) how soil CO₂ concentrations differed within and between landscape positions; (2) how the timing of peak soil CO₂ concentrations varied across riparian and hillslope zones; and (3) whether higher soil CO₂ concentrations necessarily resulted in higher efflux (i.e. did surface CO₂ efflux follow patterns of subsurface CO₂)? Soil CO₂ concentrations were significantly higher in the riparian zones, likely due to higher SWC. The timing of peak soil CO₂ concentrations also differed between riparian and hillslope zones, with highest hillslope concentrations near peak snowmelt and highest riparian concentrations during the late summer and early fall. Surface CO₂ efflux was relatively homogeneous at monthly timescales as a result of different combinations of soil CO₂ production and transport, which led to equifinality in efflux across the transects. However, efflux was 57% higher in the riparian zones when integrated to cumulative growing season efflux, and suggests higher riparian soil CO₂ production.     

The use of soil respiration as an ecological indicator in arid ecosystems of the SE of Spain: Spatial variability and controlling factors

In the context of an ongoing monitoring study of the Cabo de Gata-Níjar Natural Park (SE of Spain), we explored the use of soil respiration as an indicator of ecosystem functioning reflecting changes in ecological processes in semiarid environments. With this purpose, we measured soil CO₂ efflux in six different and representative ecosystems of the Natural Park, with different land uses (forest and agricultural sites) and under different soil covers (under plant and bare soil) in two distinctive periods of the year: summer (dry period) and spring (growing season). We also measured the main soil properties and environmental variables. Soil CO₂ efflux ranged from 0.40 μmol m⁻² s⁻¹ in the dry period to 1.93 μmol m⁻² s⁻¹ in the growing season. Soil CO₂ efflux showed a large spatial variability, with different behaviour between the measured periods. Whereas in the dry period differences among ecosystems were larger (CVs 75-80%) than within them (CVs 40-55%), in the growing season the CVs were smaller (40-50%) and no differences were observed between or within ecosystem. The factors controlling soil CO₂ efflux also differed in the two measurement occasions. Whereas in the dry period soil CO₂ efflux was mainly the result of transport processes in the soil and therefore related to local factors (OC content, CN ratio, clay, rock outcrop, etc.) assigned to ecosystem conditions, in the growing season soil CO₂ efflux was dominated by soil CO₂ production and thus related only to organic carbon content and plant cover. In the growing season environmental variables explained ca. 10% of the variation in soil CO₂ efflux. In order to capture these different processes in different times of the year, i.e., diffusion versus production processes we calculated a new index, normalised seasonal difference in soil respiration (SDSR), which is proposed as a good indicator of the state and functioning of the ecosystem.     

Base Cation Cycling in a Pristine Watershed of the Canadian Boreal Forest

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO₂ efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO₂ efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO₂ effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (R _soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO₂ efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO₂ efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO₂ efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R _soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization.     

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

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

Temperature Sensitivity and Basal Rate of Soil Respiration and Their Determinants in Temperate Forests of North China

The basal respiration rate at 10°C (R₁₀) and the temperature sensitivity of soil respiration (Q₁₀) are two premier parameters in predicting the instantaneous rate of soil respiration at a given temperature. However, the mechanisms underlying the spatial variations in R₁₀ and Q₁₀ are not quite clear. R₁₀ and Q₁₀ were calculated using an exponential function with measured soil respiration and soil temperature for 11 mixed conifer-broadleaved forest stands and nine broadleaved forest stands at a catchment scale. The mean values of R₁₀ were 1.83 µmol CO₂ m⁻² s⁻¹ and 2.01 µmol CO₂ m⁻² s⁻¹, the mean values of Q₁₀ were 3.40 and 3.79, respectively, for mixed and broadleaved forest types. Forest type did not influence the two model parameters, but determinants of R₁₀ and Q₁₀ varied between the two forest types. In mixed forest stands, R₁₀ decreased greatly with the ratio of coniferous to broadleaved tree species; whereas it sharply increased with the soil temperature range and the variations in soil organic carbon (SOC), and soil total nitrogen (TN). Q₁₀ was positively correlated with the spatial variances of herb-layer carbon stock and soil bulk density, and negatively with soil C/N ratio. In broadleaved forest stands, R₁₀ was markedly affected by basal area and the variations in shrub carbon stock and soil phosphorus (P) content; the value of Q₁₀ largely depended on soil pH and the variations of SOC and TN. 51% of variations in both R₁₀ and Q₁₀ can be accounted for jointly by five biophysical variables, of which the variation in soil bulk density played an overwhelming role in determining the amplitude of variations in soil basal respiration rates in temperate forests. Overall, it was concluded that soil respiration of temperate forests was largely dependent on soil physical properties when temperature kept quite low.     

Temporal variation in CO<Subscript>2</Subscript> efflux from soil and snow surfaces in a Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) plantation,central Japan

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