Daytime Net Ecosystem CO2 Exchange Research Articles

Ecosystem CO2 Exchange and Its Environmental Regulation of a Restored Wetland in the Liaohe River Estuary

Coastal wetlands are important carbon sinks， and they contribute to reducing the effects of global warming. This study used the eddy covariance method to detect the CO2 flux in the restoration wetland of the Liaohe River estuary in 2021 and investigate the characteristics of ecosystem CO2 exchange and its environmental control factors. The aim was to assess the carbon source/sink capacity of salt marshes in the restored area and to provide data support and theoretical basis for evaluating the effectiveness of ecological restoration projects. The study revealed "U" curves in spring and autumn， "V" curves in summer， and horizontal lines in winter for the average daily variation curve of net ecosystem CO2 exchange （NEE） in the restored area. Its carbon sink efficiencies were -40.06， -63.62， 2.33， and 34.43 g·m-2 in the spring， summer， autumn， and winter， respectively. In the restored area， the daily cumulative variation in NEE was "V" shaped， and the monthly cumulative changes in NEE， ecosystem respiration （Reco）， and gross primary productivity （GPP） were obviously different. Photosynthetically active radiation （PAR） was an important regulation factor of daytime NEE in the restored area in 2021， and they displayed a rectangular hyperbolic relationship. PAR could explain 53% of the variation in the daytime NEE. Air temperature （Ta） was the main control factor of Reco，night， and there was an exponential relationship between them. When Ta < 5.5 ℃， the temperature sensitivity of ecosystem respiration （Q10） was 2.19， and Ta could explain 42% of the variation in the Reco，night； when Ta ≥ 5.5 ℃， the Q10 was 1.81， and Ta could explain 51% of the variation in the Reco，night. Additionally， there were significant linear negative correlations between NEE and both soil water content （SWC） and vapor pressure deficit （VPD）， whereas NEE was not significantly correlated with soil temperature （Ts） or relative humidity （RH）. In 2021， the restored wetland in the Liaohe River estuary acted as a CO2 sink， and the total net carbon sequestration was -66.89 g·m-2. The restored salt plays a role as an important carbon sink and has long-term carbon sequestration potential.

Hydrometeorological controls on net carbon dioxide exchange over a temperate desert steppe in Inner Mongolia, China

Understanding the effect of environmental factors on the net ecosystem CO2 exchange (NEE) and the response of NEE to rainfall events is of great significance for an accurate understanding of the carbon cycle for desert steppe ecosystems. Based on the long-term (2011–2018) eddy covariance flux data of a temperate desert steppe in Inner Mongolia, China, this study used path analysis to analyze the combined impact of the environmental factors on NEE. The results showed that during the growing season, vapor pressure deficit (VPD) and soil water content (SWC) was the most prominent environmental factor for the daytime NEE and nighttime NEE, respectively. NEE responds differently to individual environmental factors among multi-year climatic conditions. The size of rainfall event has significant impacts on NEE, it can effectively promote the CO2 uptake of the desert steppe ecosystem when rainfall event size is greater than 5 mm, and the NEE response increased with the rainfall event size. Moreover, NEE peaked approximately 1–3 days after a 5–10 mm rainfall event, while the rainfall event size >10 mm, it would take 3–5 days for NEE to reach a peak value; and yet, small rainfall events (< 5 mm) slightly increased CO2 emissions. During the growing season, carbon uptake increased with monthly rainfall, except in May. Our results are important for understanding the carbon cycle and its control mechanisms in the temperate desert steppe of Inner Mongolia.

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.

Effect of No-Tillage Management on Soil Organic Matter and Net Greenhouse Gas Fluxes in a Rice-Oilseed Rape Cropping System

No-tillage (NT) management is considered a leading approach for sustaining crop production and improving soil and environmental quality. Based on a long-term no-tillage experiment in a rice–oilseed rape cropping system, we examined differences in soil organic matter (SOM), soil microbial carbon (C) and nitrogen (N) content, and methane (CH4) and carbon dioxide (CO2) fluxes between NT and conventional tillage (CT) management. SOM under NT was 21.0 g kg–1, and a significant difference was detected between 2004 and 2016. SOM increased under NT and CT by averages of 0.60 and 0.32 g kg–1 year–1, respectively. Soil microbial C and N content were higher under CT than under NT. However, soil C:N ratios under NT were 17.4 and 9.7% higher than the CT, respectively, whereas soil microbial C:N ratios under NT were on average 9.47 and 9.70% higher. In addition, about 70% of CO2 net uptake and over 99% of net CH4 emissions occurred during the rice season in May–September in the rice–oilseed rape cropping system. Annual cumulative CH4 and daytime net ecosystem CO2 exchange (NEE) under NT was 1813.9 g CO2 equiv. m–2, 10.8% higher than that under CT. Our results suggest that a higher soil microbial C:N ratio and NEE (CH4 and daytime CO2) could contribute to increasing SOM/C in the surface soil under NT management.

Tidal effects on ecosystem CO2 exchange at multiple timescales in a salt marsh in the Yellow River Delta

The tide plays a crucial role in maintaining the carbon sink strength in salt marsh ecosystems. Furthermore, the effects of tides on ecosystem carbon exchange could vary with different timescales. Using the eddy covariance technique combined with wavelet analysis, we analyzed the time-frequency characteristics of net ecosystem CO2 exchange (NEE) to address the tidal effects on NEE at multiple timescales. The wavelet analysis showed that NEE displayed a tidal-driven pattern with distinct characteristics at the multiday scale (i.e., 8–16 days) and the seasonal scale (i.e., 64–128 days). Moreover, a more significant controlling effect of light rather than air temperature on NEE was found at the diel scale. Tides also affected the variation of the diurnal pattern of NEE. Tidal flooding inhibited nighttime CO2 emissions (NEEnighttime) as well as decreased temperature sensitivity (Q10) of NEEnighttime from 1.37 to 1.16. In contrast, the response of daytime NEE (NEEdaytime) to tidal activities was more complicated, as the NEEdaytime reacted differently with different months during the growing season. Overall, our findings can contribute to a better understanding of tidal effects on ecosystem carbon exchange in salt marshes.

CO2 exchange under different vegetation covers in a coastal wetland of Jiaozhou Bay, China

This study applied the static chamber method to evaluate the CO2 fluxes from December 2013 to October 2014 in the coastal wetland of Jiaozhou Bay, China. The study area was divided into Spartina alterniflora flat, Phragmites australis flat, Suaeda salsa flat and mud flat. Results showed that net ecosystem CO2 exchange (NEE) presented obviously temporal and spatial variations during growing season, and slight fluctuation in non-growing season. Vegetated flats (S. alterniflora flat, P. australis flat and S. salsa flat) acted as CO2 sinks during growing season and CO2 sources during non-growing season; while mud flat acted as a CO2 sink through the entire period. Over the growing season, the estimated total NEE were −1202.83 g CO2 m−2 in S. alterniflora flat, −1129.16 g CO2 m−2 in P. australis flat and −237.43 g CO2 m−2 in S. salsa flat, respectively. Daytime NEE were significantly exponential related to light intensity in S. alterniflora flat and P. australis flat; whereas no overt exponential relationship was found between these two factors in S. salsa flat. In P. australis flat, ecosystem respiration (Reco) and gross primary production (GPP) in growing season were estimated to be 2874.05 and 1744.89 g CO2 m−2, respectively; Reco was significantly exponential related to air temperature and soil temperature at different depths, among which soil temperature at the depth of 5 cm was the most remarkable; temperature sensitivity (Q10) of Reco ranged from 2.72 to 3.67. Aboveground biomass of P. australis over the entire growing season showed non-linear relationships with NEE, GPP, and Reco; while it showed significant linear relationships with these three factors during early and peak growing season. As for the mud flat acting as a CO2 sink, it is supposed to be caused by inorganic process, however the mechanism is not definite. Further research on the long-term and complete observations of CO2 exchange at the mud flat-atmosphere interface is necessary.

Effects of drying and wetting cycles induced by tides on net ecosystem exchange of CO2 over a salt marsh in the Yellow River Delta, China.

As a unique hydrological characteristic, the tidal action can strongly affect carbon balance in a salt marsh despite their short duration. Using the eddy covariance technique, we measured the net ecosystem CO2 exchange (NEE) and its environmental factors and tidal change over a salt marsh in the Yellow River Delta. It aimed to investigate the effect of tidal process and drying and wetting cycles induced by tides on NEE. The results showed that the tidal process promoted the daytime CO2 uptake, but it didn't clearly affect the nighttime CO2 release. Tidal inundation was a major factor influencing daytime NEE. The diurnal change of NEE showed a distinct U-shaped curve on both drought and wet stages, but not with substantial variation in its amplitude during the drought stage. The drying and wetting cycles enhanced the absorption of daytime CO2. Under drought stage, the mean of the maximum photosynthetic rate (Amax), apparent quantum yield (α) and ecosystem respiration (Reco) were higher than those in wet stage. In addition, the drying and wetting cycles suppressed the nighttime CO2 release from the salt marsh but increased its temperature sensitivity.

Diurnal and Seasonal Variations in the Net Ecosystem CO2 Exchange of a Pasture in the Three-River Source Region of the Qinghai-Tibetan Plateau.

Carbon dioxide (CO2) exchange between the atmosphere and grassland ecosystems is very important for the global carbon balance. To assess the CO2 flux and its relationship to environmental factors, the eddy covariance method was used to evaluate the diurnal cycle and seasonal pattern of the net ecosystem CO2 exchange (NEE) of a cultivated pasture in the Three-River Source Region (TRSR) on the Qinghai−Tibetan Plateau from January 1 to December 31, 2008. The diurnal variations in the NEE and ecosystem respiration (Re) during the growing season exhibited single-peak patterns, the maximum and minimum CO2 uptake observed during the noon hours and night; and the maximum and minimum Re took place in the afternoon and early morning, respectively. The minimum hourly NEE rate and the maximum hourly Re rate were −7.89 and 5.03 μmol CO2 m−2 s−1, respectively. The NEE and Re showed clear seasonal variations, with lower values in winter and higher values in the peak growth period. The highest daily values for C uptake and Re were observed on August 12 (−2.91 g C m−2 d−1) and July 28 (5.04 g C m−2 day−1), respectively. The annual total NEE and Re were −140.01 and 403.57 g C m−2 year−1, respectively. The apparent quantum yield (α) was −0.0275 μmol μmol−1 for the entire growing period, and the α values for the pasture’s light response curve varied with the leaf area index (LAI), air temperature (Ta), soil water content (SWC) and vapor pressure deficit (VPD). Piecewise regression results indicated that the optimum Ta and VPD for the daytime NEE were 14.1°C and 0.65 kPa, respectively. The daytime NEE decreased with increasing SWC, and the temperature sensitivity of respiration (Q10) was 3.0 during the growing season, which was controlled by the SWC conditions. Path analysis suggested that the soil temperature at a depth of 5 cm (Tsoil) was the most important environmental factor affecting daily variations in NEE during the growing season, and the photosynthetic photon flux density (PPFD) was the major limiting factor for this cultivated pasture.

Effect of environmental and biotic factors on net ecosystem CO2 exchange over a coastal wetland in the Yellow River Delta.

Using the eddy covariance technique, we measured the net ecosystem CO2 exchange (NEE) and its environmental and biotic factors over a coastal wetland in the Yellow River Delta to investigate the diurnal and seasonal variation in NEE and quantify the effect of environmental and biotic factors on NEE. The results showed that the diurnal change of NEE showed a distinct U-shaped curve during the growing season, but not with substantial variation in its amplitude during the non-growing season. During the growing season, the wetland acted as a significant net sink for CO2, while it became carbon source during the non-growing season. On the scale of a whole year, the wetland functioned as a strong carbon sink of -247 g C·m-2. Daytime NEE was mainly dominated by photosynthetically active radiation (PAR). Apparent quantum yield (α) and daytime respiration of ecosystem (Reco,d) reached maximum in August, while maximum photosynthesis rate (Amax) reached its maximum in July. Nighttime NEE had an exponential relationship with air temperature (Ta). The mean value of temperature sensibility coefficient (Q10) was 2.5, and it was positively related to soil water content (SWC). During the non-growing season, NEE was negatively correlated with net radiation (Rn), but not with other environmental factors significantly. However, during the growing season NEE was significantly correlated with Rn, Ta, soil temperature at 10 cm depth (Ts 10) and leaf area index (LAI), but not with aboveground biomass (AGB). Stepwise multiple regression analysis indicated that Rn and LAI explained 52% of the variation in NEE during the growing season.

Diurnal and seasonal variations of CO2 fluxes and their climate controlling factors for a subtropical forest in Ningxiang

In this study, the diurnal and seasonal variations of CO2 fluxes in a subtropical mixed evergreen forest in Ningxiang of Hunan Province, part of the East Asian monsoon region, were quantified for the first time. The fluxes were based on eddy covariance measurements from a newly initiated flux tower. The relationship between the CO2 fluxes and climate factors was also analyzed. The results showed that the target ecosystem appeared to be a clear carbon sink in 2013, with integrated net ecosystem CO2 exchange (NEE), ecosystem respiration (RE), and gross ecosystem productivity (GEP) of −428.8, 1534.8 and 1963.6 g C m−2yr−1, respectively. The net carbon uptake (i.e. the -NEE), RE and GEP showed obvious seasonal variability, and were lower in winter and under drought conditions and higher in the growing season. The minimum NEE occurred on 12 June (−7.4 g C m−2 d−1), due mainly to strong radiation, adequate moisture, and moderate temperature; while a very low net CO2 uptake occurred in August (9 g C m−2 month−1), attributable to extreme summer drought. In addition, the NEE and GEP showed obvious diurnal variability that changed with the seasons. In winter, solar radiation and temperature were the main controlling factors for GEP, while the soil water content and vapor pressure deficit were the controlling factors in summer. Furthermore, the daytime NEE was mainly limited by the water-stress effect under dry and warm atmospheric conditions, rather than by the direct temperature-stress effect.

High vapor pressure deficit constrains GPP and the light response of NEE at a Southern Hemisphere bog

We used eddy covariance measurements of net ecosystem CO2 exchange (NEE) in an undisturbed New Zealand bog to determine the major controls on gross primary production (GPP). In particular, we were interested in the relative importance of air temperature, saturation vapor pressure deficit (VPD), and diffuse fraction of incoming radiation (fdiff) in driving increases and restrictions to ecosystem photosynthesis. We found that NEE light response parameters for this peatland were remarkably similar year-round. We used four different light response functions to explore the influence of driving meteorological variables on daytime CO2 uptake. This revealed that including VPD as a model input reduced the bias associated with estimates of NEE relative to the standard rectangular hyperbola. An apparent temperature restriction of light-saturated photosynthetic capacity was relieved when high VPD data were excluded from light response curve fits. We thus, utilized the model that incorporated VPD to fill gaps in daytime NEE data and to estimate GPP. Increases in mid-day GPP were associated with increasing total photosynthetic radiation and air temperature, especially during winter and spring/autumn seasons. However, in summer, mid-day GPP was capped by high VPD, likely due to a stomatal response triggered to reduce transpiration water losses. Although GPP did not directly respond to fdiff, both temperature and VPD were coupled to sky conditions. Furthermore, the highest mid-day mean GPP for high light levels always occurred during the periods of highest fdiff. These results have important implications for the future C sink strength of New Zealand peatlands given a trend toward drier summers with clearer skies.

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

Biophysical controls on net ecosystem CO<sub>2</sub> exchange over a semiarid shrubland in northwest China

Abstract. The carbon (C) cycling in semiarid and arid areas remains largely unexplored, despite the wide distribution of drylands globally. Rehabilitation practices have been carried out in many desertified areas, but information on the C sequestration capacity of recovering vegetation is still largely lacking. Using the eddy-covariance technique, we measured the net ecosystem CO2 exchange (NEE) over a recovering shrub ecosystem in northwest China throughout 2012 in order to (1) quantify NEE and its components and to (2) examine the dependence of C fluxes on biophysical factors at multiple timescales. The annual budget showed a gross ecosystem productivity (GEP) of 456 g C m−2 yr−1 (with a 90% prediction interval of 449–463 g C m−2 yr−1) and an ecosystem respiration (Re) of 379 g C m−2 yr−1 (with a 90% prediction interval of 370–389 g C m−2 yr−1), resulting in a net C sink of 77 g C m−2 yr−1 (with a 90% prediction interval of 68–87 g C m−2 yr−1). The maximum daily NEE, GEP and Re were −4.7, 6.8 and 3.3 g C m−2 day−1, respectively. Both the maximum C assimilation rate (i.e., at the optimum light intensity) and the quantum yield varied over the growing season, being higher in summer and lower in spring and autumn. At the half-hourly scale, water deficit exerted a major control over daytime NEE, and interacted with other stresses (e.g., heat and photoinhibition) in constraining C fixation by the vegetation. Low soil moisture also reduced the temperature sensitivity of Re (Q10). At the synoptic scale, rain events triggered immediate pulses of C release from the ecosystem, followed by peaks of CO2 uptake 1–2 days later. Over the entire growing season, leaf area index accounted for 45 and 65% of the seasonal variation in NEE and GEP, respectively. There was a linear dependence of daily Re on GEP, with a slope of 0.34. These results highlight the role of abiotic stresses and their alleviation in regulating C cycling in the face of an increasing frequency and intensity of extreme climatic events.

Environmental control of daytime net ecosystem exchange of carbon dioxide in switchgrass

Net ecosystem CO2 exchange (NEE) over a young switchgrass (Panicum virgatum) stand was measured with the eddy covariance technique across two growing seasons in the southern Great Plains of the United States at Chickasha, OK. The objectives of the study were to characterize the effects of environmental factors on daytime NEE and to explore the underlying mechanisms. Photosynthetic photon flux density (PPFD) was the most significant driver of NEE and explained over 90% of the NEE variation during optimum environmental conditions. The light-response curve showed hysteresis as carbon uptake by the ecosystem decreased up to 62% (monthly average) from morning to afternoon at similar light levels because of the stomatal closure control of photosynthesis at high vapor pressure deficit (VPD). This resultant large hysteresis led to the failure of the rectangular hyperbolic light-response function in explaining the NEE–PPFD relationship. The NEE exhibited an optimum temperature range of 28–34°C and decreased markedly beyond 35°C. Our results demonstrated that warm temperature and high VPD altered the NEE–PPFD relationship and thereby affected the ecosystem light-response parameters (respiration, quantum yield, and light saturated photosynthetic capacity). Thus, it is essential to incorporate the effects of temperature and VPD on ecosystem light-response into both empirical and mechanistic models. This study also suggests including the VPD effect in the NEE flux partitioning technique can account for the systematic presence of NEE hysteresis during non-optimal environmental conditions. The results of this study are useful for the modeling community to develop, improve, and validate the models for global change studies, and for the eddy covariance community to develop more robust gap filling methods.

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.

Differences in plant cover and species composition of semiarid grassland communities of central Mexico and its effects on net ecosystem exchange

Abstract. Changes in land use across the semiarid grasslands of northern Mexico have driven a decline of plant cover and alteration of plant species composition. A number of different plant communities have resulted from these changes. Their implications, however, on the carbon (C) cycle and regional carbon balance are still poorly understood. Here, we examined the effects of plant cover loss and changes in species composition on net ecosystem CO2 exchange (NEE) and their biotic and abiotic controls. NEE was measured in five representative plant community types within a semiarid grassland by temporarily enclosing the entire aboveground ecosystem using a chamber method (i.e., geodesic dome). Sites included an oat crop (crop), a moderately grazed grassland (moderate grazing), a 28 yr-old grazing exclosure (exclosure), an overgrazed site with low perennial grass cover (overgrazed), and an overgrazed site presenting shrub encroachment (shrub encroachment). For natural vegetation, rates of standardized daytime NEE for sites with a high plant cover (exclosure and moderate grazing) were similar (P > 0.05) as compared to sites with low plant cover (overgrazed and shrub encroachment). However, yearly total nighttime NEE (carbon loss) was more than double (P < 0.05) for sites with high plant cover compared to sites with low cover, resulting to slight C sinks for the low plant cover sites, and neutral or sources for the high plant cover sites as accounted by daytime and nighttime NEE annual balance. Differences in plant cover and its associated biomass defined the sensitivity to environmental controls. Thus, daytime NEE in low plant cover sites reached light compensation points at lower photosynthetic photon flux density than those from high plant cover sites. Differences in species composition did not influence NEE rates even though there were transient or permanent changes in C3 vs. C4 functional groups. Our results allowed the detection of the large variability and contribution of different plant communities to regional C balance in patchy landscapes. Identification of the role of landscape patches in the regional C balance as either sinks or sources may provide tools allowing land use management strategies that could favor C uptake in patchy landscapes.

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.

珠三角城市绿地CO2通量的季节特征

Urban green space plays a significant role on carbon cycling in urban areas.Accurate estimation of CO2 fluxes above urban green space using the long-term field observation method provides critical information about climate change reactions and regional carbon sequestration evaluations.Continuous eddy-covariance measurements of CO2 fluxes were conducted to analyze the carbon exchange status in 2009 and 2010.The analyses included daily variations in the CO2 fluxes during different seasons,annual variations in light response parameters,including apparent light use efficiency(α) and maximum photosynthetic productivity(Pmax).Annual variations in the net ecosystem CO2 exchange(NEE) and the ecosystem respiration(Reco) were also studied. A tower located in the botanical garden of Dongguan with 4 levels and a total height of 20 m was used for the measurements.The CO2 flux at a height of 20 m was analyzed to study the CO2 exchange between an urban green space and the atmosphere interface.According to the eddy-covariance method and the detection limits of the instrument,abnormal data were removed to ensure accuracy.The daytime NEE and the photosynthetic active radiation(PAR) were used to formulate the Michaelis-Menten model as the gap-filling model during the daytime,while the night respiration(Reco) and soil temperature at 5 cm depth(Ts) were used to calculate the exponential equation as the gap-filling model at night. The major results were:(1) the NEE was-91.9 gC·m-2·a-1 in 2009 and-116.5 gC·m-2·a-1 in 2010,indicating that urban green space was a net carbon sink.The NEE varied seasonally with environmental factors.Urban green space was a carbon source from December-March and a carbon sink from April-November each year.(2) The daily total CO2 flux(Fc) was negative during the winter,but positive during the other seasons.CO2 storage(Fs) occupied a larger proportion during winter than during the other seasons.Daily CO2 concentrations(Cc) were highest before sunrise and lowest after sunset.Cc was highest in the winter and lowest in the summer.(3) The annual average α was(0.00134±0.00035) mgCO2·μmol-1Photons and Pmax was(1.006±0.283) mgCO2·m-2·s-1.(4) The annual Reco was 1370.4 gC·m-2·a-1 in 2009 and 1384.8 gC·m-2·a-1 in 2010.Reco varied with changes in the soil temperature during the year.The simulated Reco during the night was less than the daytime result produced by the Michaelis-Menten model at the same temperature.(5) Because the urban green space was a mixture of forest and grassland,the NEE of the urban green space was smaller than that of a forest ecosystem,but larger than a grassland ecosystem.Compared with other urban green spaces,the NEE in Dongguan was lower than northern urban forests due to the high Reco caused by high temperatures.(6) Correlation analyses between the NEE and meteorological factors like the PAR,air temperature(Ta) and saturation vapor pressure differential(VPD) were performed.The partial correlation coefficient of the PAR was larger than the others,indicating that the PAR was the most important factor to the NEE.The partial correlation coefficient of Ta was negative during the winter and positive during the summer.The partial correlation coefficient of VPD was positive year-round and was higher under high VPD conditions.

Hysteresis response of daytime net ecosystem exchange during drought

Abstract. Continuous measurements of net ecosystem CO2 exchange (NEE) using the eddy-covariance method were made over an agricultural ecosystem in the southeastern US. During optimum environmental conditions, photosynthetically active radiation (PAR) was the primary driver controlling daytime NEE, accounting for as much as 67 to 89% of the variation in NEE. However, soil water content became the dominant factor limiting the NEE-PAR response during the peak growth stage. NEE was significantly depressed when high PAR values coincided with very low soil water content. The presence of a counter-clockwise hysteresis of daytime NEE with PAR was observed during periods of water stress. This is a result of the stomatal closure control of photosynthesis at high vapor pressure deficit and enhanced respiration at high temperature. This result is significant since this hysteresis effect limits the range of applicability of the Michaelis-Menten equation and other related expressions in the determination of daytime NEE as a function of PAR. The systematic presence of hysteresis in the response of NEE to PAR suggests that the gap-filling technique based on a non-linear regression approach should take into account the presence of water-limited field conditions. Including this step is therefore likely to improve current evaluation of ecosystem response to increased precipitation variability arising from climatic changes.