Average Net Ecosystem CO2 Exchange Research Articles

Satellite-based measurements of temporal and spatial variations in C fluxes of irrigated and rainfed cotton grown in India

The small number of carbon dioxide (CO2) observation networks and the prohibitively high equipment cost restrict the estimation of net ecosystem CO2 exchange (NEE). Satellite-based remote sensing techniques have made it possible to obtain NEE and component carbon (C) fluxes. One-third of the world's cotton area is in India, but the information on NEE is limited. We used the Level 4 Carbon (L4_C) product from the Soil Moisture Active Passive (SMAP) mission to estimate C fluxes based on satellite-derived soil moisture, weather, and vegetation data. For our study (2018-19 to 2020-21), we chose two ecosystems (rainfed central India vs. irrigated northern India). Seasonal variations were observed in C fluxes. Gross primary productivity was the highest during the boll formation phase. This phase was the strongest sink and coincided with the highest CO2 uptake, followed by the flowering and square formation phases. The cotton crop was a C source during the initial vegetative phase and after the boll opening. Overall, the cotton crop was a sink for atmospheric CO2 with an average NEE value of −189.6 g C m−2 under irrigated and −245.6 g C m−2 in rainfed cotton. Higher ecosystem respiration in irrigated cotton resulted in lower C sink strength than rainfed cotton. Our studies indicate that the SMAP L4_C product model estimates can be used to obtain information on C fluxes in real-world situations. Moreover, such satellite-based remote sensing techniques will enable large-scale environmental monitoring with different cropping systems and support policymaking.

Cultivated Grassland Types Differently Affected Carbon Flux Downstream of the Yellow River

Cultivated grasslands are an important part of grassland ecosystems and have been proven to be major carbon sinks, then playing an important role in the global carbon balance. The effect of cultivated grassland type (Medicago sativa, Triticum aestivum, Secale cereale, and Vicia villosa grasslands) on carbon flux (including net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP)) downstream of the Yellow River was studied via the static chamber technique and a portable photosynthetic system. Bare land was used as a control. The results showed that the four cultivated grassland types were mainly carbon sinks, and bare land was a carbon source. The cultivated grassland types significantly affected carbon flux. The average NEE and GEP of the grassland types were in the following order from high to low: Medicago sativa, Secale cereale, Triticum aestivum, and Vicia villosa grassland. Stepwise regression analysis showed that among all measured environmental factors, soil pH, soil bulk density (BD), soil organic carbon (SOC), and soil microbial carbon (MBC) were the main factors affecting CO2 flux. The combined influence of soil BD, SOC, and pH accounted for 77.6% of the variations in NEE, while soil BD, SOC, and MBC collectively explained 79.8% of changes in ER and 72.9% of the changes in GEP. This finding indicates that Medicago sativa grassland is a cultivated grassland with a high carbon sink level. The changes in carbon flux were dominated by the effects of soil physicochemical properties.

Extreme temperature events reduced carbon uptake of a boreal forest ecosystem in Northeast China: Evidence from an 11-year eddy covariance observation.

Boreal forests, the second continental biome on Earth, are known for their massive carbon storage capacity and important role in the global carbon cycle. Comprehending the temporal dynamics and controlling factors of net ecosystem CO2 exchange (NEE) is critical for predicting how the carbon exchange in boreal forests will change in response to climate change. Therefore, based on long-term eddy covariance observations from 2008 to 2018, we evaluated the diurnal, seasonal, and interannual variations in the boreal forest ecosystem NEE in Northeast China and explored its environmental regulation. It was found that the boreal forest was a minor CO2 sink with an annual average NEE of -64.01 (± 24.23) g CO2 m-2 yr-1. The diurnal variation in the NEE of boreal forest during the growing season was considerably larger than that during the non-growing season, and carbon uptake peaked between 8:30 and 9:30 in the morning. The seasonal variation in NEE demonstrated a "U" shaped curve, and the carbon uptake peaked in July. On a half-hourly scale, photosynthetically active radiation and vapor pressure deficit had larger impacts on daytime NEE during the growing season. However, temperature had major control on NEE during the growing season at night and during the non-growing season. On a daily scale, temperature was the dominant factor controlling seasonal variation in NEE. Occurrence of extreme temperature days, especially extreme temperature events, would reduce boreal forest carbon uptake; interannual variation in NEE was substantially associated with the maximum CO2 uptake rate during the growing season. This study deepens our understanding of environmental controls on NEE at multiple timescales and provides a data basis for evaluating the global carbon budget.

Evapotranspiration and carbon exchange of the main agroecosystems and their responses to agricultural land use change in North China Plain

The North China Plain (NCP) plays an important role in food production while there is serious groundwater overexploitation in this area. The quantified relationship between the variation in evapotranspiration (ET) and net ecosystem CO2 exchange (NEE) and agricultural land use change in NCP remains unclear. In this study, based on the eddy covariance method, water balance method, and remote sensing data, ET and NEE variations were quantified and the impacts of agricultural land use change from 2002 to 2012 on regional ET and NEE variation were analyzed. The average ET and NEE of the winter wheat – summer maize cropland ecosystem were 714.2 mm yr-1 and − 534.3 gC m-2 yr-1, respectively. Correspondingly, the above values were 786.2 mm yr-1 and 667.5 gC m-2 yr-1 for the pear orchard ecosystem, and 778.6 mm yr-1 and 814.8 gC m-2 yr-1 for the vegetable field ecosystem, which had relatively high ET and ecosystem productivity. The average annual ET and NEE of the cotton field ecosystem were only 592.0 mm and − 351.3 gC m-2, respectively. Agricultural land area increased by 331.2 × 103 ha over 2002–2012, which was caused by the increasing land area of spring maize, forest/fruit trees, and vegetables. As a result, the ET and NEE of the agroecosystems in NCP increased by 25.6 × 108 m3 and − 257.9 × 104 tonC from 2002 to 2012, which are beneficial for carbon sequestration. However, the increase in carbon sink was at the cost of greater groundwater overexploitation due to increased regional water consumption. The planting of crops with high water consumption should be reduced as the surpluses of these agricultural products compared to the food demand in this region. These results are meaningful for reasonable planting structure adjustment of water-adaptive sustainable agricultural development in the NCP.

Seasonal not annual precipitation drives 8-year variability of interannual net CO2 exchange in a salt marsh

Salt marshes are significant contributors to global “blue carbon” resources, and these habitats are sensitive to precipitation events due to periodically dry-wet alternation induced by tides. However, whether annual marsh-atmospheric CO2 flux responds more to annual or seasonal precipitation remains unclear. Here, eight years (2012-2019) of eddy covariance data were evaluated to determine typical CO2 budgets, and we assessed the effect of annual and seasonal precipitation on interannual net ecosystem CO2 exchange (NEE) in a salt marsh of the Yellow River Delta, China. The salt marsh was a sink for atmospheric CO2 in each of the eight study years, with an 8-year average NEE of -51.7 ± 9.7 g C m−2 a−1 varying from -8 to -85 g C m−2 a−1. Annual NEE was mainly regulated by precipitation levels during the early growth stage of plants, which modulated maximal plant biomass accumulation via water-salt transport. Besides, a spring precipitation distribution experiment showed that wetter early growth stage of plants enhanced carbon assimilation capacity in the salt marsh by decreasing soil salinity and promoting plant biomass accumulation. These findings suggest that soil water-salt conditions induced by precipitation during the onset of the growing season are crucial for interannual variations of NEE in salt marshes.

Divergence in ecosystem carbon fluxes and soil nitrogen characteristics across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone

Alpine ecosystem carbon cycling is sensitive to climate change, particularly in the transition zones between biomes. Soil nitrogen conditions, including the ammonium to nitrate (NH4+/NO3−) ratio, regulate ecosystem carbon uptake by coupling carbon‑nitrogen cycle. The largest alpine pasture on Earth is distributed on the Tibetan Plateau, where alpine biome transition zones are also widely distributed. However, it is largely unknown how the soil NH4+/NO3− ratio and net ecosystem CO2 exchange vary among vegetation types in the alpine biome transition zones due to a lack of in situ field observations. Here, we investigated soil NH4+/NO3− ratio and ecosystem carbon fluxes across alpine steppe, alpine meadow and alpine swamp ecosystems in a biome transition zone on the central Tibetan Plateau. The results showed that soil NH4+/NO3− ratio was lowest in the alpine steppe (driest environment), which had the highest soil pH, and highest in the alpine swamp (wettest environment), which had the lowest soil pH. We proposed a theoretical framework describing how soil moisture regulates soil NH4+/NO3− ratio by altering both the denitrification process and soil pH. We further found that the growing season average net ecosystem CO2 exchange for the alpine steppe, alpine meadow and alpine swamp was −1.46, −1.90 and −5.43 μmol m−2 s−1, respectively. This divergence in net ecosystem CO2 exchange across the three grasslands is primarily explained by divergence in gross ecosystem photosynthesis, rather than ecosystem respiration. The air temperature sensitivity of ecosystem respiration (Q10) for the alpine steppe, alpine meadow and alpine swamp was 1.73 ± 0.05, 1.44 ± 0.03 and 2.43 ± 0.45, respectively. Our study highlights large differences in both soil nutrient and ecosystem carbon uptake across different vegetation types in an alpine biome transition zone. More in situ investigations in various biome transition zones are urgently needed to quantitatively understand the spatial pattern of alpine ecosystem carbon‑nitrogen cycling processes.

Repackaging precipitation into fewer, larger storms reduces ecosystem exchanges of CO2 and H2O in a semiarid steppe

General circulation models predict that precipitation will become more extreme, i.e. rainfall events of larger size but reduced frequency. Studies in North American grasslands have shown that such repackaging of precipitation into fewer, larger events enhanced above ground net primary productivity (ANPP), likely due to deeper soil moisture infiltration favoring plant water use over evaporation. However, ANPP responses in other regions remain poorly understood, and responses of carbon and water exchanges with the atmosphere remain unknown. Here we manipulated rainfall in a steppe ecosystem of northern China over 4 years to investigate how temporal packaging of precipitation impacts ANPP, evapotranspiration (ET), net ecosystem CO2 exchange (NEE) and the component fluxes gross primary productivity (GPP) and ecosystem respiration (RE). Experimental plots received precipitation equivalent to the 60-year growing-season average of 240 mm, variously packaged into 6, 10, 16, or 24 events representing extreme (P6) to historical average (P24) rainfall frequency. Extraordinarily extreme frequency (6 large events) reduced NEE, GPP, RE, ET and water use efficiency (WUE = |NEE|/ET). The average NEE, GPP and RE declined 35%, 45% and 48% respectively in the P6 treatment as compared to P16, which showed maximum ET and CO2 exchange. After peaking in the 16-event treatment, GPP and WUE in P24 were not distinguishable from P6. These peaks suggest that P16 was optimal for photosynthesis, with sufficiently frequent rain to maintain unregulated plants and adequately deep soil moisture infiltration to favour transpiration, with associated carbon uptake, over evaporation. Path analysis indicated the lower CO2 fluxes were influenced by reduced soil water content and leaf area index and higher soil temperature, with ET regulating the effects of these microclimatic drivers. ANPP showed a monotonic but non-significant decline with decreasing precipitation frequency, consistent with reduced CO2 fluxes. We found an increase in ANPP of xerophyte plants partially compensated for the ANPP decline in the dominant eurytopic xerophyte plants. Our results suggest that extreme temporal repackaging of precipitation into few events with correspondingly long dry intervals may reduce the capacity of steppe ecosystems to assimilate atmospheric CO2, although community diversity may moderate impacts.

Growing season variability in carbon dioxide exchange of irrigated and rainfed soybean in the southern United States

Measurement of carbon dynamics of soybean (Glycine max L.) ecosystems outside Corn Belt of the United States (U.S.) is lacking. This study examines the seasonal variability of net ecosystem CO2 exchange (NEE) and its components (gross primary production, GPP and ecosystem respiration, ER), and relevant controlling environmental factors between rainfed (El Reno, Oklahoma) and irrigated (Stoneville, Mississippi) soybean fields in the southern U.S. during the 2016 growing season. Grain yield was about 1.6tha−1 for rainfed soybean and 4.9tha−1 for irrigated soybean. The magnitudes of diurnal NEE (~2-weeks average) reached seasonal peak values of −23.18 and −34.78μmolm−2s−1 in rainfed and irrigated soybean, respectively, approximately two months after planting (i.e., during peak growth). Similar thresholds of air temperature (Ta, slightly over 30°C) and vapor pressure deficit (VPD, ~2.5kPa) for NEE were observed at both sites. Daily (7-day average) NEE, GPP, and ER reached seasonal peak values of −4.55, 13.54, and 9.95gCm−2d−1 in rainfed soybean and −7.48, 18.13, and 14.93gCm−2d−1 in irrigated soybean, respectively. The growing season (DOY 132–243) NEE, GPP, and ER totals were −54, 783, and 729gCm−2, respectively, in rainfed soybean. Similarly, cumulative NEE, GPP, and ER totals for DOY 163–256 (flux measurement was initiated on DOY 163, missing first 45days after planting) were −291, 1239, and 948gCm−2, respectively, in irrigated soybean. Rainfed soybean was a net carbon sink for only two months, while irrigated soybean appeared to be a net carbon sink for about three months. However, grain yield and the magnitudes and seasonal sums of CO2 fluxes for irrigated soybean in this study were comparable to those for soybean in the U.S. Corn Belt, but they were lower for rainfed soybean.

Alpine wetland ecosystem carbon sink and its controls at the Qinghai Lake

Wetland ecosystems play an important role in regulating the Earth’s climate. This paper presents the results of a study on the alpine wetland ecosystem CO2 fluxes at the Qinghai Lake at different vegetative development stages and CO2 controlling factors measured by eddy covariance system. The results showed that the alpine wetland ecosystem at the Qinghai Lake area was a carbon sink on the whole year basis. The average net ecosystem CO2 exchange (NEE) amount was −904.42 g CO2/m2; the average amount of the ecosystem reparation (Re) was 1450 g CO2/m2; the average gross primary productivity (GPP) was 2354.42 g CO2/m2 from July 2011 to June 2013 year. The NEE, Re and GPP amounts were the maximum during the growing stage of the whole year. There were significant negative linear correlations between monthly NEE amount and monthly average total solar radiation (SR) (R 2 = 0.27, p < 0.05), incident photosynthetically active radiation (PAR) (R 2 = 0.28, p < 0.05), ambient vapor pressure deficit (VPD) (R 2 = 0.38, p < 0.01), monthly total precipitation (P) (R 2 = 0.65, p < 0.0001), monthly mean leaf area index (LAI) (R 2 = 0.85, p < 0.0001) and enhanced vegetation index (EVI) (R 2 = 0.84, p < 0.0001). Extreme significant polynomial nonlinear negative correlations were found between NEE amounts and monthly average air temperature (T a ) (R 2 = 0.84, p < 0.0001), soil temperature at 0–5 cm depth (T s ) (R 2 = 0.90, p < 0.0001) and air relative humidity (RH) (R 2 = 0.76, p < 0.0001). The monthly Re amount of alpine wetland ecosystem at the Qinghai Lake at different stages showed significant nonlinear positive correlations with all the identified affecting factors except for monthly mean EVI, which showed a linear relationship (R 2 = 0.72, p < 0.0001). The monthly GPP amount at different vegetative development stages showed a significant exponential correlation with monthly average T a (R 2 = 0.92, p < 0.0001), a power function with VPD (R 2 = 0.24, p < 0.05), quadratic polynomial ones with monthly average T s (R 2 = 0.93, p < 0.0001), RH (R 2 = 0.74, p < 0.0001) and monthly P (R 2 = 0.55, p < 0.05). Extremely significant linear correlations of the monthly GPP amount at different developing stages with monthly average LAI (R 2 = 0.85, p < 0.0001) and EVI (R 2 = 0.92, p < 0.0001) were also found.

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.

太湖流域典型稻麦轮作农田生态系统碳交换及影响因素

PDF HTML阅读 XML下载 导出引用 引用提醒 太湖流域典型稻麦轮作农田生态系统碳交换及影响因素 DOI: 10.5846/stxb201404010614 作者: 作者单位: 中国科学院南京地理与湖泊研究所,中国科学院南京地理与湖泊研究所,中国科学院南京地理与湖泊研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金重点项目(41030745);国家自然科学基金(41101565,41371532);中国科学院南宁地理与湖泊研究所"135"学科前沿项目(NIGLAS2012135019);江苏省自然科学基金(BK2011882) Analysis of net ecosystem CO2 exchange (NEE) in the rice-wheat rotation agroecosystem of the Lake Taihu Basin, China Author: Affiliation: Nanjing Institute of Geography & Limnology, 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:利用涡度相关技术观测太湖流域典型稻麦轮作农田生态系统2a净生态系统碳交换(NEE)变化过程,分析其碳交换特征及影响机理,结果表明:太湖流域典型稻麦轮作农田年NEE为-749.49--785.38 gC m-2 a-1,考虑作物籽粒碳和秸秆还田后净吸收88.12 gC m-2 a-1,为弱碳汇;稻/麦季日均NEE和白天NEE季节变化直接受作物植被生长影响;麦季夜间NEE与10 cm土壤温度呈显著指数关系,2012/2013年温度敏感系数(Q10)分别为3.03和2.67;当土壤水分低于田间持水量时,麦季夜间NEE主要受土壤温度影响,反之,夜间NEE受土壤温度和水分双重影响;降水对麦季夜间NEE有短时的激发效应;稻季淹水对土壤呼吸产生较明显的阻滞效应,降低了夜间NEE对土壤温度的敏感性,2012和2013年分别为1.88和1.39,稻季淹水与烤田交替变化对土壤呼吸产生明显的抑制或激发的短时效应。 Abstract:The agroecosystem plays an important role in global and regional carbon balance due to its large area and high carbon sequestration potential. Due to limitations in observation techniques and the spatial heterogeneity of the environment, net ecosystem CO2 exchange (NEE) in the rice-wheat rotation system of the Lake Taihu Basin remains poorly understood. This paper aimed to investigate NEE in this agroecosystem based on observations of CO2 flux with the eddy covariance technique, from December 2011 to November 2013. Annual carbon balance was estimated from observed NEE and carbon contents of grain and straw. Half-hourly flux data were first corrected by removal of anomalous data points, coordinate rotation, frequency loss correction, and WPL correction. Two neural network models for daytime and nighttime NEE gap-filling were built. The results showed that annual daytime and nighttime NEE was -749.49 gC m-2 a-1 and -785.38 gC m-2 a-1, respectively. Accounting for grain removal and return of straw to the field, total net C absorption was 88.12 gC m-2 a-1, characterizing the agroecosystem as a weak carbon sink. This carbon sequestration capacity is greater than those of the Huaihe River Basin, subtropical region, and north China plain. Diurnal NEE exhibited a typical "W" bimodal seasonal pattern, and both seasonal NEE and average monthly diurnal NEE showed significant annual fluctuations. Cumulative diurnal NEE ranged from -12.88 gC m-2 d-1 to 5.94 gC m-2 d-1, with a mean of -2.10 gC m-2 d-1. Maximum cumulative diurnal NEE occurred in the wheat season of each year, with values of -12.88 gC m-2 d-1 on April 26, 2012 and -11.62 gC m-2 d-1 on April 11, 2013, respectively. Variation in diurnal NEE and daytime NEE in the rice-wheat season was significantly correlated with crop height, suggesting that diurnal NEE and daytime NEE were both influenced by crop growth. Nighttime NEE and soil temperature at 10 cm during the wheat season exhibit a significant exponential relationship, with a temperature sensitivity coefficient (Q10) of 3.03 and 2.67 in 2012-2013, respectively, larger than that of many soil respiration models (Q10 = 2.0). If soil moisture is lower than the field capacity, nighttime NEE is affected mainly by soil temperature; otherwise, nighttime NEE is dually controlled by soil temperature and moisture. There exists a short excitation effect to enhance nighttime NEE after heavy rainfall. Permanent flooding significantly reduces soil respiration, simultaneously decreasing the sensitivity of nighttime NEE to soil temperature. The temperature sensitivity coefficient (Q10) in the rice seasons of 2012-2013 were 1.88 and 1.39, respectively. Alternate change of water regime between the permanent flooding and soil drying in the rice season would generate significant short-term effects of inhibition or excitation on soil respiration, respectively. The effects of carbon flux observation techniques and data gap-filling methods on uncertainty are discussed. Finally, the impact of straw application on greenhouse gas emissions (CO2, CH4, N2O) from the agroecosystem is suggested as an avenue of further investigation. 参考文献 相似文献 引证文献

A downward CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux seems to have nowhere to go

Abstract. Recent studies have suggested that deserts, which are a long-neglected region in global carbon budgeting, have strong downward CO2 fluxes and might be a significant carbon sink. This finding, however, has been strongly challenged because neither the reliability of the flux measurements nor the exact location of the fixed carbon has been determined. This paper shows, with a full chain of evidence, that there is indeed strong carbon flux into saline/alkaline land in arid regions. Based on continuous measurement of net ecosystem CO2 exchange (NEE) from 2002 to 2012 (except for 2003), the saline desert in western China was a carbon sink for 9 out of 10 years, and the average yearly NEE for the 10 years was −25.00 ± 12.70 g C m−2 year−1. Supporting evidence for the validity of these NEE estimates comes from the close agreement of NEE values obtained from the chamber and eddy-covariance methods. After ruling out the possibility of changes in C stored in plant biomass or soils, the C uptake was found to be leached downwards into the groundwater body in the process of groundwater fluctuation: rising groundwater absorbs soil dissolved inorganic carbon (DIC), and falling groundwater transports the DIC downward. Horizontal groundwater flow may send this DIC farther away and prevent it from being observed locally. This process has been called "passive leaching" of DIC, in comparison with the active DIC leaching that occurs during groundwater recharge. This passive leaching significantly expands the area where DIC leaching occurs and creates a literally "hidden" carbon sink process under the desert. This study tells us that when a downward CO2 flux is observed, but seems to have nowhere to go, it does not necessarily mean that the flux measurement is unreliable. By looking deeper and farther away, a place and a process may be found "hidden" underground.

Year-round growing conditions explains large CO2 sink strength in a New Zealand raised peat bog

We measured net ecosystem CO2 exchange (NEE) for two years in a remnant New Zealand raised peat bog using the eddy covariance technique and estimated the component CO2 fluxes gross primary production (GPP) and ecosystem respiration (ER). Vegetation was dominated by vascular plants belonging to the southern hemisphere family Restionaceae, comprising an overstory of the erect Sporadanthus ferrugineus and a dense understory of Empodisma robustum. Annual NEE (GPP, ER) totals for the two years were −218.2 (1124.2, 906.0)gCm−2yr−1 and −250.3 (1148.3, 897.8)gCm−2yr−1, indicating substantially larger CO2 sink strength than has been reported for peatland ecosystems in the northern hemisphere. The bog was a net sink for CO2 in nine months each year, with small differences in autumn and wintertime NEE accounting for most of the difference in annual NEE totals. Summer daytime average NEE were similar to published values for northern hemisphere peatlands, despite differences in vegetation type and a comparatively deep water table. Fitted ecosystem light response and respiration model parameters were also within the ranges of values reported for peatlands elsewhere. The major distinction compared to northern peatlands was the year-round productivity of the evergreen vegetation cover in New Zealand, which largely offset respiration losses during the short, mild winter season.

Comparison of net ecosystem CO2 exchange in cropland and grassland with an automated closed chamber system

Field measurements of net ecosystem CO2 exchange (NEE) with high temporal resolution are essential to construct a meaningful ecosystem C balance. The objectives of this study were to monitor NEE in high temporal resolution in cropland and grassland between middle August and middle November (2006) at Kleinhohenheim, Germany and to evaluate NEE in autumn. A fully automated temperature controlled closed chamber system with an infrared CO2 analyzer was used to measure NEE. The measured NEE varied between the two ecosystems depending on changes in above-ground vegetation and environmental factors. The diurnal NEE pattern of daytime CO2 uptake and night time CO2 release was evident in the grassland, but not in the cropland as the crops were harvested at the beginning of the measurement period. The grassland generally showed higher night time NEE, but lower daytime NEE than the cropland. Night time NEE showed exponential dependence on air and soil temperature, resulting in Q10 of 1.8 and 1.9 (for air temperature), 2.3 and 2.4 (for soil temperature) in the grassland and cropland, respectively. The average daily NEE was 2.77 and 1.86 g CO2-C m−2 day−1 in the cropland and grassland, respectively. Both ecosystems were sources of CO2, during 3 months in autumn, but the grassland emitted less CO2 by 87.9 g CO2-C m−2 than the cropland.

黄河口潮间盐沼湿地生长季净生态系统CO2交换特征及其影响因素

Inter-tidal salt marsh plays a great important role in the global carbon cycle as a considerable potential capacity of the carbon sink. Compared to other types of wetlands,the inter-tidal salt marsh has a unique biogeochemical process under the combined action between freshwater and seawater. Therefore,there is considerable variability and uncertainty inits net ecosystem CO2exchange( NEE). However,few studies provide insights regarding the variability of NEE and its controlling factors in an inter-tidal salt marsh. Using the Eddy Covariance( EC) technique,we analyzed temporal variation in NEE and determined its control mechanisms coupled with meteorological and tidal inundation variables during the growing season( from April to October) of 2012 in an inter-tidal salt marsh in the Yellow River Estuary. The results showed that it was net CO2 absorption in the daytime and net CO2 release in the nighttime on a diurnal scale. The daily average NEE during the growing season was-0. 38 g CO2m-2d-1,with a maximum daily CO2 uptake rate of-3. 13 g CO2m-2d-1( June27) and a maximum release rate of 1. 47 g CO2m-2d-1( August 12). The monthly average NEE increased rapidly from May,and peaked in June,then decreasing gradually from July. The maximum monthly ecosystem respiration( Reco) was15. 16 g C /m2 in June when the hightest soil temperature was 27. 5 ℃. The monthly gross primary productivity( GPP)reached its peaking value( 25. 07 g C /m2) in July. During the growing season,NEE was mainly dominated by photosynthetic active radiation( PAR),soil temperature( Ts),soil water content( SWC) and tidal inundation. There was a rectangular hyperbolic relationship between the daytime net ecosystem CO2exchange( NEEdaytime) and PAR. The maximum ecosystem apparent quantum yield( α) and maximum photosynthesis rate( NEEsat) appeared in June(( 0.0086±0. 0019) μmol CO2μmol-1photons) and in May(( 4. 79±1. 52) μmol CO2m-2s-1),respectively. In addition,NEEdaytime also was positively correlated with Tsand SWC. During the growing season,NEEdaytimehad an exponential relationship with Ts. The mean value of Q10 was 1. 33,and it was positively related to SWC. During the typical sunny day of June 19 to June25,tidal inundation enhanced daytime net absorption of CO2 and nighttime CO2 release. As a result,tidal inundation increased the net ecosystem CO2 absorption by an average of 0. 76 g CO2m-2d-1. During the growing season,the inter-tidal salt marsh was an obvious CO2sink( 22. 28 g C /m2),with a cumulative emission of 96. 28 g C/m2 and a cumulative aborption of 118. 34 g C /m2.

Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

AbstractUnderstanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO2 exchange (NEE) and its components – gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD &gt; 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light–VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to −33.02 ± 1.96 μmol CO2 m−2 s−1 and the highest daily integrated NEE was −35.89 g CO2 m−2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO2 during 5–6 months and total seasonal uptakes were −1128 ± 130 and −1796 ± 217 g CO2 m−2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.

Environmental Controls on Net Ecosystem CO2 Exchange Over a Reed (Phragmites australis) Wetland in the Yellow River Delta, China

Using the Eddy Covariance (EC) technique, we analyzed temporal variation in net ecosystem CO2 exchange (NEE) and determined the effects of environmental factors on the balance between ecosystem photosynthesis and respiration in a reed (Phragmites australis) wetland in the Yellow River Delta, China. Our results indicated that diurnal and seasonal patterns of NEE and its components (ecosystem respiration (R (eco)), gross primary production (GPP)) varied markedly among months for the growing season (May to October). The cumulative CO2 emission was 1,657 g CO2 m(-2), while 2,612 g CO2 m(-2) was approximately accumulated as GPP, which resulted in the reed wetland being a net sink of 956 g CO2 m(-2). The ratio of R (eco) to GPP in reed wetland was 0.68, which was close to other temperate wetlands. Soil temperature and soil moisture exerted the primary controls on R (eco) during the growing season. Daytime NEE values during the growing season were strongly correlated with photosynthetically active radiation. Aboveground biomass showed significant linear relationships with 24-h average NEE, daytime GPP, and R (eco), respectively. Thus, we conclude that the coastal wetland acted as a carbon sink during the growing season despite the variations in environmental conditions, and long-term flux measurements over these ecosystems are undoubtedly necessary.

Temporal patterns of net CO2exchange for a tropical semideciduous forest of the southern Amazon Basin

[1] The carbon cycling of tropical ecosystems has received considerable attention over the last 1–2 decades; however, interactions between climate variation and tropical forest net ecosystem CO2 exchange (NEE) are still uncertain. To reduce this uncertainty, and assess the biophysical controls on NEE, we used the eddy covariance method over a 3 year period (2005–2008) to measure the CO2 flux and energy balance for a 25–28 m tall, mature tropical semideciduous forest located near Sinop Mato Grosso, Brazil. The study period encompassed warm-dry, cool-wet, and cool-dry climate conditions, and based on previous research, we hypothesized that the net CO2 accumulation of the semideciduous forest would be lower during periods of drought. Using time series of the enhanced vegetation index (EVI), a NEE-light-use model, and path analysis, we found that the estimated quantum yield (a′, μmol CO2μmol photons−1) was directly affected by temporal variations in the EVI, precipitation, and photosynthetically active radiation (PAR), while the optimal rate of gross primary production (FGPP,opt, μmol m−2 s−1) was directly affected by the EVI and PAR. However, indirect effects of precipitation on the a′ and FGPP,opt were stronger than direct effects because variations in precipitation also lead to variations in the EVI and the atmospheric vapor pressure deficit (VPD). Daytime ecosystem respiration (FRE,day, μmol m−2 s−1) was directly affected by temporal variations in temperature and VPD and indirect effects of other variables were of lesser importance. Net ecosystem CO2 uptake was often higher in the dry season than the wet season, not because of a dry season “green-up” but because rates of ecosystem respiration declined relatively more than rates of canopy photosynthesis. Over interannual timescales, average daily NEE increased over the 3 year study period and was highest in 2007–2008, which was also the driest year in terms of rainfall. However, 2007–2008 was also the coolest year during the 3 year study period, and the low temperature appeared to compensate for the low rainfall. Overall, our data suggest that the NEE of tropical semideciduous forests is sensitive to temporal variations in surface water availability but that indirect effects of other variables, such as temperature and VPD, are important in controlling CO2 gain and loss. Such interactions will be important for the future NEE under warmer and drier conditions that are anticipated with anthropogenic climate change.

Variations of net ecosystem CO2 exchange in a tidal inundated wetland: Coupling MODIS and tower‐based fluxes

Tidal activity is a major factor determining the distribution of plant species and ecosystem functions, including carbon fluxes. To explore the spatial variations of net ecosystem CO2 exchange (NEE) and related regulatory mechanisms along the tidal inundation gradient (i.e., middle or low tidal flat), an NEE estimation model using piecewise regression analysis was developed by coupling the Moderate Resolution Imaging Spectroradiometer (MODIS)‐ and tower‐based measurements. The results showed that our model achieved an adequate NEE estimation (slope = 0.70, R2 = 0.78). Then the model was applied to estimate NEE variation along a transect with a tidal inundation gradient. The average NEE was −1.75 g C m−2 d−1, varying from −2.02 g C m−2 d−1 to −1.42 g C m−2 d−1 from island‐ to oceanside. Generally, our empirical model captured the spatiotemporal patterns of NEE and the variation of the regulatory factors along the gradient. The sensitivity analysis of various regulatory variables showed that the variations of NEE near the islandside were primarily caused by seasonal shift and annual cycle of vegetation, whereas at the oceanside, NEE was more influenced by tidal activity with no clear phenological influence. In the middle area, NEE seemed to be subjected to both phenological changes of vegetation and tidal activity. In conclusion, this study illustrates that the estimates derived from MODIS‐ and tower‐based flux data are reliable for quantifying the spatiotemporal variations of NEE and reflecting the effect of tidal activity on NEE.

Year‐round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

SummaryAn Ameriflux site was established in mid 1996 to study the exchange of CO2 in a native tallgrass prairie of north‐central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid‐July–early August 1996), moisture‐stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid‐day net ecosystem CO2 exchange (NEE), under well‐watered conditions, reached a maximum magnitude of 1.4 mg CO2 m−2 s−1 (flux toward the surface is positive) during peak growth (mid‐July 1997), with green leaf area index of 2.8. In contrast, under moisture‐stress conditions in the same growth stage in 1996, mid‐day NEE was reduced to near‐zero. Average night NEE ranged from near‐zero, during winter dormancy, to − 0.50 mg CO2 m−2 s−1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y‐intercept, so obtained, can be potentially used as an estimate of night‐time CO2 emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO2 m−2 d−1 (8.4 g C m−2 d−1) in mid‐July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m−2 y−1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near‐zero in 1998.