Annual balances and extended seasonal modelling of carbon fluxes from a temperate fen cropped to festulolium and tall fescue under two‐cut and three‐cut harvesting regimes

This study reports the annual carbon balance of a drained riparian fen under two‐cut or three‐cut managements of festulolium and tall fescue. CO₂ fluxes measured with closed chambers were partitioned into gross primary production (GPP) and ecosystem respiration (ER) for modelling according to environmental factors (light and temperature) and canopy reflectance (ratio vegetation index, RVI). Methodological assessments were made of (i) GPP models with or without temperature functions (Ft) to adjust GPP constraints imposed by low temperature (<10 °C) and (ii) ER models with RVI or GPP parameters as biomass proxies. The sensitivity of the models was also tested on partial datasets including only alternate measurement campaigns and on datasets only from the crop growing period. Use of Ft in GPP models effectively corrected GPP overestimation in cold periods, and this approach was used throughout. Annual fluxes obtained with ER models including RVI or GPP parameters were similar, and also annual GPP and ER fluxes obtained with full and partial datasets were similar. Annual CO₂ fluxes and biomass yield were not significantly different in the crop/management combinations although the individual collars (n = 12) showed some variations in GPP (?1818 to ?2409 g CO₂‐C m^?2), ER (1071 to 1738 g CO₂‐C m^?2), net ecosystem exchange (NEE, ?669 to ?949 g CO₂‐C m^?2) and biomass yield (556 to 1044 g CO₂‐C m^?2). Net ecosystem carbon balance (NECB), as the sum of NEE and biomass carbon export, was only slightly negative to positive in all crop/management combinations. NECBs, interpreted as emission factors, tended to favour the least biomass producing systems as the best management options in relation to climate saving carbon balances. Yet, considering the down‐stream advantages of biomass for fossil fuel replacement, yield‐scaled carbon fluxes are suggested to be given additional considerations for comparison of management options in terms of atmospheric impact.     

Variability in exchange of CO2 across 12 northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO₂) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1–7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO₂ (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO₂. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R_eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R_eco. After this correction neither R_eco (P= 0.162) nor GPP (P= 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R_eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R_eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R_eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R_eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.     

Intensified inundation shifts a freshwater wetland from a CO2 sink to a source

Climate change has altered global precipitation patterns and has led to greater variation in hydrological conditions. Wetlands are important globally for their soil carbon storage. Given that wetland carbon processes are primarily driven by hydrology, a comprehensive understanding of the effect of inundation is needed. In this study, we evaluated the effect of water level (WL) and inundation duration (ID) on carbon dioxide (CO₂) fluxes by analysing a 10‐year (2008–2017) eddy covariance dataset from a seasonally inundated freshwater marl prairie in the Everglades National Park. Both gross primary production (GPP) and ecosystem respiration (ER) rates showed declines under inundation. While GPP rates decreased almost linearly as WL and ID increased, ER rates were less responsive to WL increase beyond 30 cm and extended inundation periods. The unequal responses between GPP and ER caused a weaker net ecosystem CO₂ sink strength as inundation intensity increased. Eventually, the ecosystem tended to become a net CO₂ source on a daily basis when either WL exceeded 46 cm or inundation lasted longer than 7 months. Particularly, with an extended period of high‐WLs in 2016 (i.e., WL remained >40 cm for >9 months), the ecosystem became a CO₂ source, as opposed to being a sink or neutral for CO₂ in other years. Furthermore, the extreme inundation in 2016 was followed by a 4‐month postinundation period with lower net ecosystem CO₂ uptake compared to other years. Given that inundation plays a key role in controlling ecosystem CO₂ balance, we suggest that a future with more intensive inundation caused by climate change or water management activities can weaken the CO₂ sink strength of the Everglades freshwater marl prairies and similar wetlands globally, creating a positive feedback to climate change.     

Warming impacts on boreal fen CO2 exchange under wet and dry conditions

Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub‐arctic sedge fen carbon dioxide (CO₂) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO₂ exchange is more strongly affected by drying than warming. Experimental water level draw‐down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO₂ exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO₂ uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO₂ exchange.     

Gross primary production controls the subsequent winter CO2 exchange in a boreal peatland

In high‐latitude regions, carbon dioxide (CO₂) emissions during the winter represent an important component of the annual ecosystem carbon budget; however, the mechanisms that control the winter CO₂ emissions are currently not well understood. It has been suggested that substrate availability from soil labile carbon pools is a main driver of winter CO₂ emissions. In ecosystems that are dominated by annual herbaceous plants, much of the biomass produced during the summer is likely to contribute to the soil labile carbon pool through litter fall and root senescence in the autumn. Thus, the summer carbon uptake in the ecosystem may have a significant influence on the subsequent winter CO₂ emissions. To test this hypothesis, we conducted a plot‐scale shading experiment in a boreal peatland to reduce the gross primary production (GPP) during the growing season. At the growing season peak, vascular plant biomass in the shaded plots was half that in the control plots. During the subsequent winter, the mean CO₂ emission rates were 21% lower in the shaded plots than in the control plots. In addition, long‐term (2001–2012) eddy covariance data from the same site showed a strong correlation between the GPP (particularly the late summer and autumn GPP) and the subsequent winter net ecosystem CO₂ exchange (NEE). In contrast, abiotic factors during the winter could not explain the interannual variation in the cumulative winter NEE. Our study demonstrates the presence of a cross‐seasonal link between the growing season biotic processes and winter CO₂ emissions, which has important implications for predicting winter CO₂ emission dynamics in response to future climate change.     

Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO₂ concentrations (C_a) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO₂ between the atmosphere and the biosphere (NEE), under both ambient and elevated C_a. Here we report data on annual sums of CO₂ (NEE_net) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C₃ photosynthetic pathway)‐ and Spartina patens (C₄ photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C_a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C_a on seasonally integrated CO₂ assimilation (NEE_net = NEE_day + NEE_night, kg C m^?2 y^?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE_net averaged 1.9 kg m^?2 y^?1 in ambient C_a and 2.5 kg m^?2 y^?1 in elevated C_a, for the C₃‐dominated community. Similarly, elevated C_a significantly (P < 0.01) increased carbon uptake in the C₄‐dominated community, as NEE_net averaged 1.5 kg m^?2 y^?1 in ambient C_a and 1.7 kg m^?2 y^?1 in elevated C_a. This resulted in an average CO₂ stimulation of 32% and 13% of seasonally integrated NEE_net for the C₃‐ and C₄‐dominated communities, respectively. Increased NEE_day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C_a, while decreased NEE_night was associated with lower canopy nitrogen content. These results suggest that rising C_a may increase carbon assimilation in both C₃‐ and C₄‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.     

Characterization of CO2 flux in three Kobresia meadows differing in dominant species

Aims Kobresia meadows, the dominant species of which differ in different habitats, cover a large area of alpine grassland on the Qinghai-Tibetan Plateau and act as potential CO₂ sinks. Kobresia meadows with different dominant species may differ in carbon sink strength. We aimed to test the hypothesis and to clarify the differences in CO₂ sink strength among three major Kobresia meadows on the plateau and the mechanisms underlying these differences.Methods We measured the net ecosystem exchange flux (NEE), ecosystem respiration flux (ER), aboveground biomass (AGB) and environmental variables in three Kobresia meadows, dominated by K. pygmaea, K. humilis, or K. tibetica, respectively, in Haibei, Qinghai. NEE and ER were measured by a closed-chamber method. Environmental variables, including photosynthetic photon flux density (PPFD), air and soil temperature and air and soil moisture, were monitored during the above flux measurements.Important findings The measured peak AGB increased with soil water content and was 365, 402 and 434 g dry weight m^{-2<-sup> for K. pygmaea, K. humilis and K. tibetica meadow, respectively. From the maximum ecosystem photosynthetic rate in relation to PPFD measured during the growing season, we estimated gross ecosystem photosynthetic potential (GEP max) as 22.2, 29.9 and 37.8 μmol CO₂ m-2<-sup> s-1 for K. pygmaea, K. humilis and K. tibetica meadow, respectively. We estimated the respective gross primary production (GPP) values as 799, 1-063 and 1?158 g C m-2<-sup> year-1 and ER as 722, 914 and 1-011 g C m-2<-sup> year-1. Average net ecosystem production (NEP) was estimated to be 76.9, 149.4 and 147.6 g C m-2<-sup> year-1 in K. pygmaea, K. humilis and K. tibetica meadows, respectively. The results indicate that (i) the three meadows were CO₂ sinks during the study period and (ii) Kobresia meadows dominated by different species can differ considerably in carbon sink strength even under the same climatic conditions, which suggests the importance of characterizing spatial heterogeneity of carbon dynamics in the future.}     

氮素添加对内蒙古草甸草原生态系统CO2交换的影响

氮沉降增加将影响草原生态系统固碳, 但如何影响草原生态系统CO₂交换目前为止还没有定论。同时, 不同类型和剂量氮素对生态系统CO₂交换影响的差异也不明确。选取内蒙古额尔古纳草甸草原, 开展了不同类型氮肥和不同剂量氮素添加条件下生态系统CO₂交换的野外测定。实验设置尿素和缓释尿素2种类型氮肥各5个剂量水平(0、5.0、10.0、20.0和50.0 g N·m^-2·a^-1)。结果显示, 生长季初期及中期降雨量低时, 氮素添加抑制生态系统CO₂交换; 而生长季末期降雨量较高时促进生态系统CO₂交换。随着氮素添加水平的提高, NEE和GEP均显著增加, 当氮素添加量达到10 g N·m^-2·a^-1时, NEE和GEP的响应趋于饱和。2种氮肥(尿素和缓释尿素)仅在施氮量为5 g N·m^-2·a^-1时, 缓释尿素对生态系统CO₂交换的促进作用显著大于尿素, 在其它添加剂量时差异不显著。研究结果表明: 氮素是该草甸草原生态系统的重要限制因子, 但氮沉降增加对生态系统CO₂交换的影响强烈地受降雨量与降雨季节分配的限制, 不同氮肥(尿素和缓释尿素)对生态系统CO₂交换作用存在差异。     

CO2储存通量估算方法对森林生态系统碳收支估测的影响

准确测定森林生态系统中CO₂储存通量(F_s)对于以涡动协方差(EC)法估算生态系统碳收支具有重要意义,而F_s不同算法引起的森林碳收支估测误差还未被全面评估。本研究利用2018年帽儿山落叶阔叶林的开路EC系统和8层CO₂/H₂O廓线系统(AP100, Campbell Scientific Inc., USA)数据,比较了2-min平均廓线(P_{2 min})、30-min平均廓线(P_{30 min})和30-min平均EC单点法(P_s)3种不同方法估算的F_s对净生态系统交换(NEE)、生态系统呼吸(R_e)和总初级生产力(GPP)估算结果的影响。结果表明: F_s估算方法对森林碳通量的影响总体上随时间尺度增大而不断增大,表明通量数据插补和拆分会进一步放大F_s估算方法的影响。在年尺度上,P_{2 min}法和P_s法的NEE分别比P_{30 min}法的低36.3%和29.4%;P_{2 min}法的R_e比P_{30 min}法和P_s法高8.7%;而P_{2 min}法的GPP比P_{30 min}法的高5.4%,P_s法则比P_{30 min}法的低2.1%。传统的P_{30 min}法忽略了CO₂浓度的瞬时变化,P_s法缺少林冠层内部CO₂浓度变化,因此两者低估了真实R_e。近似瞬时廓线的方法(2-min平均)具有更高的时间与空间分辨率,能够更加准确地估算非平坦地形和复杂冠层结构的森林碳收支,这对解决EC法在复杂条件下森林R_e和GPP低估、净碳汇高估具有重要启示。     

Ecosystem CO2 exchange and plant biomass in the littoral zone of a boreal eutrophic lake

• 1 In order to study the dynamics of primary production and decomposition in the lake littoral, an interface zone between the pelagial, the catchment and the atmosphere, we measured ecosystem/atmosphere carbon dioxide (CO₂) exchange in the littoral zone of an eutrophic boreal lake in Finland during two open water periods (1998–1999). We reconstructed the seasonal net CO₂ exchange and identified the key factors controlling CO₂ dynamics. The seasonal net ecosystem exchange (NEE) was related to the amount of carbon accumulated in plant biomass.
• 2 In the continuously inundated zones, spatial and temporal variation in the density of aerial shoots controlled CO₂ fluxes, but seasonal net exchange was in most cases close to zero. The lower flooded zone had a net CO₂ uptake of 1.8–6.2 mol m^?2 per open water period, but the upper flooded zone with the highest photosynthetic capacity and above‐ground plant biomass, had a net CO₂ loss of 1.1–7.1 mol m^?2 per open water period as a result of the high respiration rate. The excess of respiration can be explained by decomposition of organic matter produced on site in previous years or leached from the catchment.
• 3 Our results from the two study years suggest that changes in phenology and water level were the prime cause of the large interannual difference in NEE in the littoral zone. Thus, the littoral is a dynamic buffer and source for the load of allochthonous and autochthonous carbon to small lakes.

     

Atmospheric impact of bioenergy based on perennial crop (reed canary grass,Phalaris arundinaceae,L.) cultivation on a drained boreal organic soil

Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long‐term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land‐use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO₂‐equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon‐negative. An LCA sensitivity analysis revealed that net ecosystem CO₂ exchange and crop yield are the major LCA components, while non‐CO₂ GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO₂ uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long‐term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies.     

Carbon balance of rewetted and drained peat soils used for biomass production: a mesocosm study

Rewetting of drained peatlands has been recommended to reduce CO₂ emissions and to restore the carbon sink function of peatlands. Recently, the combination of rewetting and biomass production (paludiculture) has gained interest as a possible land use option in peatlands for obtaining such benefits of lower CO₂ emissions without losing agricultural land. This study quantified the carbon balance (CO₂, CH₄ and harvested biomass C) of rewetted and drained peat soils under intensively managed reed canary grass (RCG) cultivation. Mesocosms were maintained at five different groundwater levels (GWLs), that is 0, 10, 20 cm below the soil surface, representing rewetted peat soils, and 30 and 40 cm below the soil surface, representing drained peat soils. Net ecosystem exchange (NEE) of CO₂ and CH₄ emissions was measured during the growing period of RCG (May to September) using transparent and opaque closed chamber methods. The average dry biomass yield was significantly lower from rewetted peat soils (12 Mg ha^?1) than drained peat soils (15 Mg ha^?1). Also, CO₂ fluxes of gross primary production (GPP) and ecosystem respiration (ER) from rewetted peat soils were significantly lower than from drained peat soils, but net uptake of CO₂ was higher from rewetted peat soils. Cumulative CH₄ emissions were negligible (0.01 g CH₄ m^?2) from drained peat soils but were significantly higher (4.9 g CH₄ m^?2) from rewetted peat soils during measurement period (01 May–15 September 2013). The extrapolated annual C balance was 0.03 and 0.68 kg C m^?2 from rewetted and drained peat soils, respectively, indicating that rewetting and paludiculture can reduce the loss of carbon from peatlands.     

Nonlinear CO2 flux response to 7 years of experimentally induced permafrost thaw

Rapid Arctic warming is expected to increase global greenhouse gas concentrations as permafrost thaw exposes immense stores of frozen carbon (C) to microbial decomposition. Permafrost thaw also stimulates plant growth, which could offset C loss. Using data from 7 years of experimental Air and Soil warming in moist acidic tundra, we show that Soil warming had a much stronger effect on CO₂ flux than Air warming. Soil warming caused rapid permafrost thaw and increased ecosystem respiration (R_eco), gross primary productivity (GPP), and net summer CO₂ storage (NEE). Over 7 years R_eco, GPP, and NEE also increased in Control (i.e., ambient plots), but this change could be explained by slow thaw in Control areas. In the initial stages of thaw, R_eco, GPP, and NEE increased linearly with thaw across all treatments, despite different rates of thaw. As thaw in Soil warming continued to increase linearly, ground surface subsidence created saturated microsites and suppressed R_eco, GPP, and NEE. However R_eco and GPP remained high in areas with large Eriophorum vaginatum biomass. In general NEE increased with thaw, but was more strongly correlated with plant biomass than thaw, indicating that higher R_eco in deeply thawed areas during summer months was balanced by GPP. Summer CO₂ flux across treatments fit a single quadratic relationship that captured the functional response of CO₂ flux to thaw, water table depth, and plant biomass. These results demonstrate the importance of indirect thaw effects on CO₂ flux: plant growth and water table dynamics. Nonsummer R_eco models estimated that the area was an annual CO₂ source during all years of observation. Nonsummer CO₂ loss in warmer, more deeply thawed soils exceeded the increases in summer GPP, and thawed tundra was a net annual CO₂ source.     

凋落物去除和添加处理对典型草原生态系统碳通量的影响

为揭示凋落物去除和添加处理对草原生态系统碳通量的影响, 2013和2014年连续两年在成熟群落围封样地进行凋落物去除实验、在退化群落放牧样地进行凋落物添加实验, 并运用静态箱法探讨碳通量变化规律并分析其主要影响因子。结果表明: 两种群落的净生态系统CO₂交换(NEE)有明显的季节性变化。对成熟群落而言, 去除50%凋落物显著增加了NEE, 去除100%凋落物显著降低了NEE, 而对生态系统总初级生产力(GEP)和生态系统呼吸(ER)均无显著影响; 对退化群落而言, 凋落物添加显著增加了GEP和NEE, 而对ER无显著影响。两种群落的GEP与10 cm土壤温度显著正相关, 但NEE和GEP的变化规律与土壤温度相反, 与10 cm土壤湿度相同。由此可见, 凋落物去除和添加处理对生态系统碳通量的影响主要是改变土壤湿度和地上生物量,而不是改变土壤温度。该研究为合理利用凋落物改善草地生态系统管理和促进草地恢复提供了理论依据。     

Comparative assessment of ecosystem C exchange in Miscanthus and reed canary grass during early establishment

Land‐use change to bioenergy crop production can contribute towards addressing the dual challenges of greenhouse gas mitigation and energy security. Realisation of the mitigation potential of bioenergy crops is, however, dependent on suitable crop selection and full assessment of the carbon (C) emissions associated with land conversion. Using eddy covariance‐based estimates, ecosystem C exchange was studied during the early‐establishment phase of two perennial crops, C₃ reed canary grass (RCG) and C₄ Miscanthus, planted on former grassland in Ireland. Crop development was the main determinant of net carbon exchange in the Miscanthus crop, restricting significant net C uptake during the first 2 years of establishment. The Miscanthus ecosystem switched from being a net C source in the conversion year to a strong net C sink (?411 ± 63 g C m^?2) in the third year, driven by significant above‐ground growth and leaf expansion. For RCG, early establishment and rapid canopy development facilitated a net C sink in the first 2 years of growth (?319 ± 57 (post‐planting) and ?397 ± 114 g C m^?2, respectively). Peak seasonal C uptake occurred three months earlier in RCG (May) than Miscanthus (August), however Miscanthus sustained net C uptake longer into the autumn and was close to C‐neutral in winter. Leaf longevity is therefore a key advantage of C₄ Miscanthus in temperate climates. Further increases in productivity are projected as Miscanthus reaches maturity and are likely to further enhance the C sink potential of Miscanthus relative to RCG.     

Seasonal and interannual variation in carbon dioxide exchange and carbon balance in a northern temperate grassland

Net ecosystem carbon dioxide (CO₂) exchange (NEE) was measured in a northern temperate grassland near Lethbridge, Alberta, Canada for three growing seasons using the eddy covariance technique. The study objectives were to document how NEE and its major component processes—gross photosynthesis (GPP) and total ecosystem respiration (TER)—vary seasonally and interannually, and to examine how environmental and physiological factors influence the annual C budget. The greatest difference among the three study years was the amount of precipitation received. The annual precipitation for 1998 (481.7 mm) was significantly above the 1971–2000 mean (± SD, 377.9 ± 97.0 mm) for Lethbridge, whereas 1999 (341.3 mm) was close to average, and 2000 (275.5 mm) was significantly below average. The high precipitation and soil moisture in 1998 allowed a much higher GPP and an extended period of net carbon gain relative to 1999 and 2000. In 1998, the peak NEE was a gain of 5 g C m^?2 d^?1 (day 173). Peak NEE was lower and also occurred earlier in the year on days 161 (3.2 g C m^?2 d^?1) and 141 (2.4 g C m^?2 d^?1) in 1999 and 2000, respectively. Change in soil moisture was the most important ecological factor controlling C gain in this grassland ecosystem. Soil moisture content was positively correlated with leaf area index (LAI). Gross photosynthesis was strongly correlated with changes in both LAI and canopy nitrogen (N) content. Maximum GPP (A_max: value calculated from a rectangular hyperbola fitted to the relationship between GPP and incident photosynthetic photon flux density (PPFD)) was 27.5, 12.9 and 8.6 µmol m^?2 s^?1 during 1998, 1999 and 2000, respectively. The apparent quantum yield also differed among years at the time of peak photosynthetic activity, with calculated values of 0.0254, 0.018 and 0.018 during 1998, 1999 and 2000, respectively. The ecosystem accumulated a total of 111.9 g C m^?2 from the time the eddy covariance measurements were initiated in June 1998 until the end of December 2000, with most of that C gained during 1998. There was a net uptake of almost 21 g C m^?2 in 1999, whereas a net loss of 18 g C m^?2 was observed in 2000. The net uptake of C during 1999 was the combined result of slightly higher GPP (287.2 vs. 272.3 g C m^?2 year^?1) and lower TER (266.6 vs. 290.4 g C m^?2 year^?1) than occurred in 2000.     

Pan-Arctic modelling of net ecosystem exchange of CO2

Net ecosystem exchange (NEE) of C varies greatly among Arctic ecosystems. Here, we show that approximately 75 per cent of this variation can be accounted for in a single regression model that predicts NEE as a function of leaf area index (LAI), air temperature and photosynthetically active radiation (PAR). The model was developed in concert with a survey of the light response of NEE in Arctic and subarctic tundras in Alaska, Greenland, Svalbard and Sweden. Model parametrizations based on data collected in one part of the Arctic can be used to predict NEE in other parts of the Arctic with accuracy similar to that of predictions based on data collected in the same site where NEE is predicted. The principal requirement for the dataset is that it should contain a sufficiently wide range of measurements of NEE at both high and low values of LAI, air temperature and PAR, to properly constrain the estimates of model parameters. Canopy N content can also be substituted for leaf area in predicting NEE, with equal or greater accuracy, but substitution of soil temperature for air temperature does not improve predictions. Overall, the results suggest a remarkable convergence in regulation of NEE in diverse ecosystem types throughout the Arctic.     

Ecosystem-scale flux of CO2 from a restored vacuum harvested peatland

At the ecosystem scale, the water and gasexchange processes are strongly coupled.Drainage and removal of a peatland'ssurface vegetation cover for peatharvesting alters its hydrology, and as adirect consequence the carbon budget.Previous studies have measuredpeatland-atmosphere carbon exchange usingthe chamber methodology. These studies haveindicated that the spatial and temporalvariability is large, suggesting the needfor continuous ecosystem-scalemeasurements. This paper presents ecosystemscale measurements of the atmosphericexchange of water and carbon dioxide(CO₂) from a restored vacuum-harvestedpeatland in eastern Québec, Canada,using the eddy correlation measurementapproach.Results indicate that the adoptedrestoration practices reduce the loss ofwater from the peat. Evapotranspirationfrom the restored site was 20 and 25% lessthan that from an adjacent abandonedcomparison site in 2000 and 2001respectively. However, CO₂ emissionsremain large during non-snow periods (478and 468 g C m^-2 in 2000 and 2001,respectively). The blockage of drainageditches and the existence of a mulch coverat the site keep the moisture and thermalconditions more or less constant.Consequently, the CO₂ flux, which ispredominantly soil respiration, is stronglycontrolled by peat temperaturefluctuations.     

Integration of CO2 flux and remotely-sensed data for primary production and ecosystem respiration analyses in the Northern Great Plains: potential for quantitative spatial extrapolation

Aim Extrapolation of tower CO₂ fluxes will be greatly facilitated if robust relationships between flux components and remotely sensed factors are established. Long‐term measurements at five Northern Great Plains locations were used to obtain relationships between CO₂ fluxes and photosynthetically active radiation (Q), other on‐site factors, and Normalized Difference Vegetation Index (NDVI) from the SPOT VEGETATION data set. Location CO₂ flux data from the following stations and years were analysed: Lethbridge, Alberta 1998–2001; Fort Peck, MT 2000, 2002; Miles City, MT 2000–01; Mandan, ND 1999–2001; and Cheyenne, WY 1997–98. Results Analyses based on light‐response functions allowed partitioning net CO₂ flux (F) into gross primary productivity (P_g) and ecosystem respiration (R_e). Weekly averages of daytime respiration, γ_day, estimated from light responses were closely correlated with weekly averages of measured night‐time respiration, γ_night (R² 0.64 to 0.95). Daytime respiration tended to be higher than night‐time respiration, and regressions of γ_day on γ_night for all sites were different from 1 : 1 relationships. Over 13 site‐years, gross primary production varied from 459 to 2491 g CO₂ m^?2 year^?1, ecosystem respiration from 996 to 1881 g CO₂ m^?2 year^?1, and net ecosystem exchange from ?537 (source) to +610 g CO₂ m^?2 year^?1 (sink). Maximum daily ecological light‐use efficiencies, ?_d,max = P_g/Q, were in the range 0.014 to 0.032 mol CO₂ (mol incident quanta)^?1. Main conclusions Ten‐day average P_g was significantly more highly correlated with NDVI than 10‐day average daytime flux, P_d (R² = 0.46 to 0.77 for P_g‐NDVI and 0.05 to 0.58 for P_d‐NDVI relationships). Ten‐day average R_e was also positively correlated with NDVI, with R² values from 0.57 to 0.77. Patterns of the relationships of P_g and R_e with NDVI and other factors indicate possibilities for establishing multivariate functions allowing scaling‐up local fluxes to larger areas using GIS data, temporal NDVI, and other factors.     

Carbon dioxide exchange dynamics over a mature switchgrass stand

Switchgrass (Panicum virgatum L.) has gained importance as feedstock for bioenergy over the last decades due to its high productivity for up to 20 years, low input requirements, and potential for carbon sequestration. However, data on the dynamics of CO₂ exchange of mature switchgrass stands (>5 years) are limited. The objective of this study was to determine net ecosystem exchange (NEE), ecosystem respiration (Re), and gross primary production (GPP) for a commercially managed switchgrass field in its sixth (2012) and seventh (2013) year in southern Ontario, Canada, using the eddy covariance method. Average NEE flux over two growing seasons (emergence to harvest) was ?10.4 μmol m^?2 s^?1 and reached a maximum uptake of ?42.4 μmol m^?2 s^?1. Total annual NEE was ?380 ± 25 and ?430 ± 30 g C m^?2 in 2012 and 2013, respectively. GPP reached ?1354 ± 23 g C m^?2 in 2012 and ?1430 ± 50g C m^?2 in 2013. Annual Re in 2012 was 974 ± 20 g C m^?2 and 1000 ± 35 g C m^?2 in 2013. GPP during the dry year of 2012 was significantly lower than that during the normal year of 2013, but yield was significantly higher in 2012 with 1090 g  m^?2, compared to 790 g m^?2 in 2013. If considering the carbon removed at harvest, the net ecosystem carbon balance came to 106 ± 45 g C  m^?2 in 2012, indicating a source of carbon, and to ?59 ± 45 g C m^?2 in 2013, indicating a sink of carbon. Our results confirm that switchgrass can switch between being a sink and a source of carbon on an annual basis. More studies are needed which investigate this interannual variability of the carbon budget of mature switchgrass stands.