Net Ecosystem Carbon Exchange Research Articles

Precipitation Pattern Determines the Inter-annual Variation of Herbaceous Layer and Carbon Fluxes in a Phreatophyte-Dominated Desert Ecosystem

Arid and semi-arid ecosystems dominated by shrubby species are an important component in the global carbon cycle but are largely under-represented in studies of the effect of climate change on carbon flux. This study synthesizes data from long-term eddy covariance measurements and experiments to assess how changes in ecosystem composition, driven by precipitation patterns, affect inter-annual variability of carbon flux and their components in a halophyte desert community dominated by deep-rooted shrubs (phreatophytes, which depend on groundwater as their primary water source). Our results demonstrated that the carbon balance of this community responded strongly to precipitation variations. Both pre-growing season precipitation and growing season precipitation frequency significantly affected inter-annual variations in ecosystem carbon flux. Heavy pre-growing season precipitation (November–April, mostly as snow) increased annual net ecosystem carbon exchange, by facilitating the growth and carbon assimilation of shallow-rooted annual plants, which used spring and summer precipitation to increase community productivity. Sufficient pre-growing season precipitation led to more germination and growth of shallow-rooted annual plants. When followed by high-frequency growing season precipitation, community productivity of this desert ecosystem was lifted to the level of grassland or forest ecosystems. The long-term observations and experimental results confirmed that precipitation patterns and the herbaceous component were dominant drivers of the carbon dynamics in this phreatophyte-dominated desert ecosystem. This study illustrates the importance of inter-annual variations in climate and ecosystem composition for the carbon flux in arid and semi-arid ecosystems. It also highlights the important effect of changing frequency and seasonal pattern of precipitation on the regional and global carbon cycle in the coming decades.

Resilience to seasonal heat wave episodes in a Mediterranean pine forest.

Short-term, intense heat waves (hamsins) are common in the eastern Mediterranean region and provide an opportunity to study the resilience of forests to such events that are predicted to increase in frequency and intensity. The response of a 50-yr-old Aleppo pine (Pinus halepensis) forest to hamsin events lasting 1-7d was studied using 10yr of eddy covariance and sap flow measurements. The highest frequency of heat waves was c. four per month, coinciding with the peak productivity period (March-April). During these events, net ecosystem carbon exchange (NEE) and canopy conductance (gc ) decreased by c. 60%, but evapotranspiration (ET) showed little change. Fast recovery was also observed with fluxes reaching pre-stress values within a day following the event. NEE and gc showed a strong response to vapor pressure deficit that weakened as soil moisture decreased, while sap flow was primarily responding to changes in soil moisture. On an annual scale, heat waves reduced NEE and gross primary productivity by c. 15% and 4%, respectively. Forest resilience to short-term extreme events such as heat waves is probably a key to its survival and must be accounted for to better predict the increasing impact on productivity and survival of such events in future climates.

Carbon exchange over four growing seasons for a subarctic sedge fen in northern Manitoba, Canada

Net ecosystem exchange of carbon was measured using eddy covariance for four growing seasons at a subarctic hummocky fen in northern Manitoba, Canada. Over a 115 day measurement period each year, cumulative net ecosystem exchange of carbon ranged from a gain of 49 g C m−2 to a loss of 16 g C m−2 with a mean loss of 6 g C m−2 from the fen, with an uncertainty of about ±34 g C m−2. Ecosystem respiration decreased with higher water tables (r2 = 0.3), especially in one summer when flooding occurred to 0.12 m above the peat surface. Additional methane emissions previously documented for the site of 4–5.7 g C m−2 year−1 added to the carbon loss. Carbon loss was measured from this same fen in the 1990s and it is likely that the carbon gain (peat accumulation) during past centuries has not continued in recent decades. Scaling to annual greenhouse gas emissions as a 100 year global warming potential showed that this fen is currently a source of 192–490 g CO2-equivalents m−2 year−1 based on both carbon dioxide and methane flux measurements, indicating that peat is decomposing.

Soil CO2 efflux and net ecosystem exchange following biomass harvesting: Impacts of harvest intensity, residue retention and vegetation control

Biomass harvesting removes more woody material than would be taken with conventional forest harvesting. Harvesting residues, left on site are an important substrate for micro-organisms that maintain nutrient cycles essential for future forest productivity by mineralizing organic matter, and releasing carbon dioxide (CO2) as a respiratory bi-product. We assessed the impact of biomass removal intensity (stem-only [SO], full-tree biomass [FT], full-tree biomass plus stumping [FT+S], full-tree biomass plus stumps and forest floor removed [FT+B]), and herbicide application on soil respiration and net ecosystem exchange of carbon (C) in a harvested 40-yr-old jack pine stand. Soil respiration (surface CO2 efflux) normalized to 15°C (R15) was lower in biomass harvest treatments than in the uncut stand and a mature 80-yr-old fire-origin natural stand. Among harvest treatments, R15 was positively related to the amount of C retained, with the general pattern of FT+B<FT+S<FT≈SO. Differences in R15 among treatments were primarily related to residue and soil organic matter quantity and quality (i.e., presence of mineral soil and forest floor polysaccharide). Herbicide application further reduced R15 by diminishing root respiration, although herbicide treatments in the SO, FT and FT+S resulted in greater net CO2 fluxes to the atmosphere in August because herbaceous photosynthesis was greatly reduced. We suggest that criteria for determining site-specific biomass retention should take into account the amount and type of residue required to maintain microbial soil respiration driving nutrient cycling.

Heat waves reduce ecosystem carbon sink strength in a Eurasian meadow steppe

BackgroundAs a consequence of global change, intensity and frequency of extreme events such as heat waves (HW) have been increasing worldwide. MethodsBy using a combination of continuous 60-year meteorological and 6-year tower-based carbon dioxide (CO2) flux measurements, we constructed a clear picture of a HWs effect on the dynamics of carbon, water, and vegetation on the Eurasian Songnen meadow steppe. ResultsThe number of HWs in the Songnen meadow steppe began increasing since the 1980s and the rate of occurrence has advanced since the 2010s to higher than ever before. HWs can reduce the grassland carbon flux, while net ecosystem carbon exchange (NEE) will regularly fluctuate for 4–5 days during the HW before decreasing. However, ecosystem respiration (Re) and gross ecosystem production (GEP) decline from the beginning of the HW until the end, where Re and GEP will decrease 30% and 50%, respectively. When HWs last five days, water-use efficiency (WUE) will decrease by 26%, soil water content (SWC) by 30% and soil water potential (SWP) will increase by 38%. In addition, the soil temperature will still remain high after the HW although the air temperature will recover to its previous state. ConclusionsHWs, as an extreme weather event, have increased during the last two decades in the Songnen meadow steppe. HWs will reduce the carbon flux of the steppe and will cause a sustained impact. Drought may be the main reason why HWs decrease carbon flux. At the later stages of or after a HW, the ecosystem usually lacks water and the soil becomes so hot and dry that it prevents roots from absorbing enough water to maintain their metabolism. This is the main reason why this grassland carbon exchange decreases during and after HWs.

Nonlinear responses of ecosystem carbon fluxes and water‐use efficiency to nitrogen addition in Inner Mongolia grassland

Summary Nitrogen (N) deposition is a continuous process and likely to affect ecosystem carbon (C) and water fluxes in a nonlinear way. However, experimental evidence is still lacking because most previous studies on these impacts usually used two discrete levels of N treatment. By a 12‐year, 6‐level N addition experiment in Inner Mongolia grassland, we found that the responses of C fluxes, including net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), all exhibited nonlinear patterns with increasing N addition rate while that of evapotranspiration did not significantly change. As a result, the response of ecosystem water‐use efficiency (EWUE) followed a similar pattern with NEE. These N‐induced changes in C fluxes were greatly affected by the distribution of precipitation among different stages of growing seasons and mainly driven by the alterations in biomass production rather than soil temperature and soil moisture. This study highlights the importance of the nonlinearity of N addition impacts on ecosystem C fluxes, which should be incorporated into the global‐C‐cycling models for better predicting future C balance. Our results also have implications for the use of fertilization in restoring the degraded grasslands given that N addition can promote biomass production and ecosystem C uptake without additional water evapotranspiration but also change ecosystem composition.

Impacts of seasonal grazing on net ecosystem carbon exchange in alpine meadow on the Tibetan Plateau

Understanding the effect of grazing season on net ecosystem CO2 exchange (NEE) and its components is crucial to predict the feedback of grazing management to climate change. We estimated NEE, gross primary productivity (GPP) and ecosystem respiration (Re) under different seasonal grazing practices (i.e. no-grazing (NG), warm season grazing (WG) and cold season grazing (CG)) by sheep during the growing seasons from 2008 to 2012 on the Tibetan Plateau. Our results show that the impacts of seasonal grazing on daily GPP, Re and NEE in the alpine meadow ecosystem varied with sampling date and year. Compared with NG and CG, WG significantly reduced average seasonal NEE by 22.7 %, because grazing impact was exacerbated by drought in July in 2010. Soil temperature only explained 19–31 % of the variation in daily GPP, Re and NEE for all grazing treatments. The interannual variabilities of GPP, Re and NEE were mainly determined by root biomass and/or average soil temperature during the growing season. Our results suggest that although WG may decrease sequestration of CO2 under continuous drought conditions after grazing, it would have little impact on CO2 sequestration during the growing season under conditions of future warming with greater rainfall in alpine meadows on the Tibetan Plateau.

Effects of grazing exclusion on CO2 fluxes in a steppe grassland on the Loess Plateau (China)

Abstract Understanding how grazing exclusion affects carbon exchange by the plant and soil features is essential for clarifying the contribution of grassland management to the carbon budget. This paper studied effects of grazing exclusion on soil respiration, ecosystem respiration, net ecosystem carbon exchange, plant and soil characterizes for two growing-seasons (2012 and 2013) in a steppe grassland on the Loess Plateau, China. Grazing exclusion markedly increased soil respiration, ecosystem respiration and net ecosystem exchange about 0.36, 0.65 and −0.10 g C m−2 d−1, respectively. Grazing exclusion enhanced aboveground biomass, belowground biomass and cover, and which were positive correlated with CO2 flux. Soil water content, rather than soil temperature, was the major environmental factor controlling soil respiration and ecosystem respiration by partial least-squares regression. The ratios of soil respiration, ecosystem respiration and net ecosystem exchange to atmospheric CO2 assimilated by photosynthesis at fenced grassland were similarly with them at grazed grassland. Results suggested that grazing exclusion was a positive management practice to help maintain or increase carbon stocks in a steppe grassland on the Loess Plateau.

Effects of continuous drought stress on soil respiration in a tropical rainforest in southwest China

Drought is predicted to have a profound impact on soil respiration. This study aimed to assess the effects of long-term precipitation decrease on soil respiration in a tropical rainforest. A precipitation reduction experiment was conducted in a tropical forest in southwest China at the beginning of 2011. Soil respiration and environmental parameters were measured monthly for three years. The continuous precipitation reduction treatment did not affect the seasonal patterns of soil respiration, but it significantly increased soil respiration in the study plot during the rainy season, and the relationship between soil respiration and soil moisture differed in the control and reduction treatment in the rainy season. Compared with the net ecosystem exchange of carbon in this system, the increment of annual soil carbon emissions in the reduction treatment was considerable and should not be ignored. Our results indicate that the responses of soil respiration to precipitation decrease may vary seasonally and the variation of volumetric water content in different seasons may be an important factor leading to the seasonal variation. The variation of soil moisture among different ecosystems as well as in different seasons should be taken into consideration when predicting the future response of soil respiration to drought globally.

Ecosystem carbon exchange under different land use on the Qinghai-Tibetan plateau

There is a great uncertainty about the effect of land use change on grassland ecosystem in the Tibetan Plateau. Net ecosystem carbon exchange (NEE) was measured for native alpine meadow with winter grazing (NAM), abandoned cropland/pasture (APL), perennial Elymus nutans (PEN), and annual oat pasture (AO) on the Tibetan plateau, during the growing seasons in 2009 and 2010 using a transparent chamber technique (Licor-6400). AO significantly decreased annual average NEE by 21.6, 23.7, and 15.7% compared to PEN, NAM, and APL during the growing season in 2010. Compared to PEN, NAM, and APL, AO significantly decreased average ecosystem respiration (Re) by 21.1, 52.3, and 39.3%, respectively, during the growing season in 2009. Soil moisture and total aboveground and belowground biomass together explained 39.6% of NEE variation and 71% in gross primary productivity variation. Soil moisture and belowground biomass explained about 83.1% of the Re variation. Our results indicated that it is possible to convert APL to PEN in the region because it could result in a higher NEE together with higher forage production compared to AO.

Carbon dioxide exchange in a tropical wet grassland

Wet grasslands in the Brazilian Cerrado occupy around 1 % of the Cerrado extension, acting as ecotones connecting well-drained savanna vegetation and gallery forest. Wet grasslands help maintain biodiversity and water balance, and are an important component of the carbon (C) balance, storing an average of 241 MgC h−1 in the top 60 cm soils. However, in addition to increased climactic variability, Brazilian wet grasslands are under pressure from domestic water use and agricultural irrigation. Here, we present 2 years of carbon balance data from a Brazilian wet grassland, which was exposed to below average seasonal and annual precipitation during the first year and excess precipitation during the second year. We used the eddy covariance approach to measure net ecosystem carbon exchange (NEE) and ecosystem respiration (R e). Results indicated that the wet grassland was a carbon sink during both years, taking up similar amounts each year (−86.5 gC m−2 year−1 in 2005 and −81.3 gC m−2 year−1 in 2006). However, seasonal NEE was variable and driven by fluctuations in the water table depth (P < 0.05), shifting the balance from a carbon sink to a carbon source from 1 month to the next by as much as 50 gC m−2 when water table dropped below 20 cm depth. The gross ecosystem carbon uptake (GEE) decreased with water table depth, while Re increased due to the exposure of old organic carbon to microbial decomposition. Results indicated that a prolonged drought, which is probable under future projected climatic variability, could lead the ecosystem to be a carbon source and highlight the importance of management strategy in preserving soil water in these ecosystems.

Modeling net ecosystem carbon exchange of alpine grasslands with a satellite-driven model.

Estimate of net ecosystem carbon exchange (NEE) between the atmosphere and terrestrial ecosystems, the balance of gross primary productivity (GPP) and ecosystem respiration (Reco) has significant importance for studying the regional and global carbon cycles. Using models driven by satellite data and climatic data is a promising approach to estimate NEE at regional scales. For this purpose, we proposed a semi-empirical model to estimate NEE in this study. In our model, the component GPP was estimated with a light response curve of a rectangular hyperbola. The component Reco was estimated with an exponential function of soil temperature. To test the feasibility of applying our model at regional scales, the temporal variations in the model parameters derived from NEE observations in an alpine grassland ecosystem on Tibetan Plateau were investigated. The results indicated that all the inverted parameters exhibit apparent seasonality, which is in accordance with air temperature and canopy phenology. In addition, all the parameters have significant correlations with the remote sensed vegetation indexes or environment temperature. With parameters estimated with these correlations, the model illustrated fair accuracy both in the validation years and at another alpine grassland ecosystem on Tibetan Plateau. Our results also indicated that the model prediction was less accurate in drought years, implying that soil moisture is an important factor affecting the model performance. Incorporating soil water content into the model would be a critical step for the improvement of the model.

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake.

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub-dominated and evergreen/graminoid-dominated community-level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant-community-specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra-specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10-day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.

Effects of an extremely dry winter on net ecosystem carbon exchange and tree phenology at a cork oak woodland

In seasonally dry climates, such as the Mediterranean, lack of rainfall in the usually wet winter may originate severe droughts which are a main cause of inter-annual variation in carbon sequestration. Leaf phenology variability may alter the seasonal pattern of photosynthetic uptake, which in turn is determined by leaf gas exchange limitations. The current study is based on the monitoring of an extremely dry winter in an evergreen cork oak woodland under the Mediterranean climate of central Portugal. Results are focused on net ecosystem CO2 exchange (NEE), phenology and tree growth measurements during two contrasting years: 2011, a wet year with a typical summer drought pattern and 2012, with an extremely unusual dry winter (only 10mm of total rainfall) that exacerbated the following summer drought effects. Main aims of this study were to assess the effects of an extreme dry winter in (1) annual and seasonal net ecosystem CO2 exchange, and in (2) cork oak phenology. The dry year 2012 was marked by a 45% lower carbon sequestration (−214 vs. −388gCm−2year−1) and a 63% lower annual tree diameter growth but only a 9% lower leaf area index compared to the wet year 2011. A significant reduction of 15% in yearly carbon sequestration was associated with leaf phenological events of canopy renewal in the early spring. In contrast to male flower production, fruit setting was severely depressed by water stress with a 54% decrease during the dry year. Our results suggest that leaf growth and leaf area maintenance are resilient ecophysiological processes under winter drought and are a priority carbon sink for photoassimilates in contrast to tree diameter growth. Thus, carbon sequestration reductions under low water availabilities in cork oak woodland should be ascribed to stomatal regulation or photosynthetic limitations and to a lesser extent to leaf area reductions.

黄河小浪底人工混交林冠层CO2储存通量变化特征

PDF HTML阅读 XML下载 导出引用 引用提醒 黄河小浪底人工混交林冠层CO2储存通量变化特征 DOI: 10.5846/stxb201404260833 作者: 作者单位: 北京林业大学林学院,中国林业科学研究院林业研究所 国家林业局林木培育重点实验室,中国林业科学研究院林业研究所 国家林业局林木培育重点实验室,中国科学院地理科学与资源研究所陆地水循环及地表过程重点实验室 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金 (31100322); 国家林业局公益性行业项目(GYHY20110400904) Variation characteristics of carbon storage flux over a mixed plantation of the Xiaolangdi area Author: Affiliation: College of Forestry,Beijing Forestry University,Research Institute of Forestry,CAF,Research Institute of Forestry,CAF,Key Laboratory of Water Cycle and Related Land Surface Processes,Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:基于黄河小浪底人工混交林2008年的CO2浓度和碳通量数据,分析了不同天气条件下CO2浓度在时间和空间上的变化特征,对比了CO2浓度廓线法和涡度相关法估算的CO2储存通量,研究了CO2储存通量的日、季变化特征。结果表明:人工混交林冠层上方月平均CO2浓度具有明显的季节变化规律。月平均CO2浓度最大值出现在3月(370 μmol/mol),最低值出现在8月(347 μmol/mol)。涡度相关法估算的CO2储存通量比廓线法所得结果偏低9%。生长季,冠层CO2储存通量和净生态系统碳交换量(NEE)日平均值分别为-0.0004和-0.091 mg CO2 m-2 s-1,冠层CO2储存通量在NEE中仅占0.4%。2008年CO2储存通量和NEE分别为-46.1、-1133 g CO2 m-2 a-1。在年尺度上,CO2储存通量占NEE的4.1%。因此,在日和年尺度上计算黄河小浪底人工混交林NEE时,CO2储存通量可以忽略。 Abstract:Forest plays an important role in carbon cycle and global climate change. To estimate accurately net ecosystem carbon exchange (NEE) between forest ecosystem and the atmosphere and understand the ecophysical factors influencing NEE, it is necessary to study the variation characteristics of CO2 storage flux. In this study, the eddy covariance method and the profile method were used to measure CO2 flux and CO2 concentration over a warm-temperate mixed plantation in the Xiaolangdi area in 2008, respectively. The temporal and spatial variations of CO2 concentration under sunny and cloudy sky conditions above the forest canopy were analyzed. CO2 storage flux obtained by the eddy covariance was compared with those derived from the profile method and the diurnal and seasonal variations of CO2 storage flux were investigated. The results showed that the diurnal and seasonal variations in CO2 concentration were obvious. In the sunny days of the growing season, CO2 concentrations above the plantation canopy decreased continuously and the minimal CO2 concentration appeared at 12:00. After 12:30, CO2 concentration increased slowly. However, in the cloudy days, the maximal and minimal CO2 concentrations occurred in the early morning and at about 14:30, respectively. At the annual scale, CO2 concentration peaked in March, with a value of 370 μmol/mol. The minimal CO2 concentration occurred in August, with a value of 347 μmol/mol. CO2 storage flux estimated by the eddy covariance method was 9% lower than that obtained by the profile method. During the growing season of 2008, monthly mean CO2 storage flux was -0.0004 mg CO2 m-2 s-1 and NEE was -0.091 mg CO2 m-2 s-1. At the daily scale, the proportion of CO2 storage flux to NEE was only 0.4%. In 2008, CO2 storage flux and NEE were -46.1 and -1133 g CO2 m-2 a-1, respectively. On an annual scale, CO2 storage flux only accounted for 4.1% of NEE. Therefore, at the longer time (daily or annual) scale, CO2 storage flux could be neglected when NEE is estimated in the mixed plantation of the Xiaolangdi area. 参考文献 相似文献 引证文献

Typhoons exert significant but differential impacts on net ecosystem carbon exchange of subtropical mangrove forests in China

Abstract. Typhoons are very unpredictable natural disturbances to subtropical mangrove forests in Asian countries, but little information is available on how these disturbances affect ecosystem level carbon dioxide (CO2) exchange of mangrove wetlands. In this study, we examined short-term effect of frequent strong typhoons on defoliation and net ecosystem CO2 exchange (NEE) of subtropical mangroves, and also synthesized 19 typhoons during a 4-year period between 2009 and 2012 to further investigate the regulation mechanisms of typhoons on ecosystem carbon and water fluxes following typhoon disturbances. Strong wind and intensive rainfall caused defoliation and local cooling effect during the typhoon season. Daily total NEE values decreased by 26–50% following some typhoons (e.g., W28-Nockten, W35-Molave and W35-Lio-Fan), but significantly increased (43–131%) following typhoon W23-Babj and W38-Megi. The magnitudes and trends of daily NEE responses were highly variable following different typhoons, which were determined by the balance between the variances of gross ecosystem production (GEP) and ecosystem respiration (RE). Furthermore, results from our synthesis indicated that the landfall time of typhoon, wind speed and rainfall were the most important factors controlling the CO2 fluxes following typhoon events. These findings indicate that different types of typhoon disturbances can exert very different effects on CO2 fluxes of mangrove ecosystems and that typhoon will likely have larger impacts on carbon cycle processes in subtropical mangrove ecosystems as the intensity and frequency of typhoons are predicted to increase under future global climate change scenarios.

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

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

太阳辐射对黄河小浪底人工混交林净生态系统碳交换的影响

太阳辐射对黄河小浪底人工混交林净生态系统碳交换的影响

Effects of different gap filling methods and land surface energy balance closure on annual net ecosystem exchange in a semiarid area of China

Based on eddy covariance measurements over two kinds of land surfaces (a degraded grassland and a maize cropland) in a semiarid area of China in 2005 and 2008, the effects of different gap filling methods, energy balance closure and friction velocity threshold (u*) on annual net ecosystem exchange (NEE) were analyzed. Six gap filling methods, including mean diurnal variation (MDV), marginal distribution sampling (MDS), and nonlinear regressions method, were investigated by generating secondary datasets with four different artificial gap lengths (ranging in length from single half-hours to 12 consecutive days). The MDS generally showed a good overall performance especially for long gaps, with an annual sum bias error less than 5 g C m−2 yr−1. There was a large positive annual sum bias error for nonlinear regressions, indicating an overestimate on net ecosystem respiration. The offset in the annual sum NEE for four nonlinear regressions was from 8.0 to 30.8 g C m−2 yr−1. As soil water content was a limiting factor in the semiarid area, the nonlinear regressions considering both soil temperature and soil water content as controlling variables had a better performance than others. The performance of MDV was better in daytime than in nighttime, with an annual sum bias error falling between −2.6 and −13.4 g C m−2 yr−1. Overall, the accuracy of the gap filling method was dependent on the type of the land surface, gap length, and the time of day when the data gap occurred. The energy balance ratio for the two ecosystems was nearly 80%. Turbulent intensity had a large impact on energy balance ratio. Low energy balance ratio was observed under low friction velocity during the night. When there was a large fetch distance in a wind direction, a low energy balance ratio was caused by mismatch of the footprints between the available energy and turbulent fluxes. The effect of energy balance correction on CO2 flux was evaluated by assuming the imbalance caused by the underestimation of sensible heat flux and latent heat flux. The results showed an average increase of 10 g C m−2 yr−1 for annual NEE in both ecosystems with an energy balance correction. On the other hand, the u* threshold also have a large impact on annual sum NEE. Net carbon emission increased 37.5 g C m−2 yr−1 as u* threshold increased from 0.1 to 0.2 m s−1, indicating a large impact of imposing u* threshold on net ecosystem carbon exchange.

Interannual changes in PAR and soil moisture during warm season may be more important than temperature fluctuations in directing annual carbon balance in Tundra

The results of 16-year instrumental monitoring (1996–2011) of the state of permafrost, soils, and vegetation influenced by environmental factors in the shrub tundra subzone of northeastern European Russia (Bol’shezemel’skaya Tundra) are described. The field measurement data for CO2 fluxes in 1996 and 1998–1999 have been used to construct regression equations and to simulate intraanual carbon fluxes with a step of 3 h. Over the observation period, the average annual and summer air temperatures have been considerably elevated relative to the local climate norms as of a climatically stable period of 1980–1996. According to our instrumental observations, the average winter and annual temperatures, total number of positive degree days, the input of photosynthetically active radiation (PAR), and average seasonal soil moisture increased during the next period (1996–2011). Correspondingly, the average annual depth of seasonal soil thawing, and seasonal maximum of vascular leaf phytomass stock also increased considerably. According to the model, the net ecosystem exchange of carbon (NEE) increased from an equilibrium state in 1996–2002 to a considerable elevation in the carbon stock in 1998–2011. Thus, the total stock over the 16-year period amounted to 109 g C m−2. An independent confirmation of the effect assumed by the model is an increase in the aboveground stocks of the live and dead organic matter in ecosystem. A statistical analysis of the model has demonstrated that, in addition to an increase in PAR and temperature, acting over the long-term period in the observed range in a mode of mutual compensation relative to NEE, the main reason underlying the observed increase in long-term carbon stock is the amount of vascular plant leaf stock, which has emerged as a major predictor for NEE and, correspondingly, acts as an independent parameter of the carbon balance. This increase is most likely determined by an increase in soil moisture.