Cumulative Net Ecosystem CO2 Exchange Research Articles

Partitioning evapotranspiration and carbon flux in ungrazed and grazed tallgrass prairie

The effect of cattle grazing on water and carbon cycling in grasslands is not fully understood. The goal of this study was to evaluate how grazing affects carbon and water cycles in a tallgrass prairie. Eddy covariance measurements were carried out from 2003 to 2005 growing seasons (from May to September) in Manhattan, Kansas, U.S., in grazed (GR) and ungrazed (UG) tallgrass prairie paddocks. Net ecosystem CO2 exchange (NEE) was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco) using the relationship between the air temperature and nighttime NEE data. Evapotranspiration (ET) was partitioned into transpiration (T) and evaporation (E) using an approach based on the concept of underlying water use efficiency. Our results revealed that grazing reduced maximum aboveground biomass (AGB) and green leaf area index (GLAI) by 22.5% and 24.3%, respectively. Grazing also reduced GPP, NEE and T in the middle of the growing seasons but enhanced these flux components at the end of the growing seasons compared to the UG paddock. Grazing reduced the growing season cumulative NEE by 11.9% and enhanced ET by 3.3% compared to the UG paddock. The reduction in NEE at UG was associated with an increase in Reco (9.4%). In turn, changes in ET in response to grazing were driven more by the increase in E (26.6%) than by the decrease in T (5.9%). Despite the reduction in AGB and GLAI caused by grazing, similar cumulative GPP values at the canopy scale at both paddocks suggest that there are compensatory mechanisms that maintain similar CO2 assimilation at UG and GR. Our findings suggest that the grazing regime adopted in this tallgrass prairie impacts the carbon cycle more strongly than the water cycle, given the higher reduction in NEE compared to the slight increase in ET on GR compared to UG. These results can be useful to outline strategies to improve management practices aimed at improving the efficiency of rangelands.

Responses of grassland ecosystem carbon fluxes to precipitation and their environmental factors in the Badain Jaran Desert.

Studying the effects of precipitation on carbon exchange in grassland ecosystems is critical for revealing the mechanisms of the carbon cycle. In this study, the eddy covariance (EC) technique was used to monitor the carbon fluxes in a grassland ecosystem in the Badain Jaran Desert (BJD) during the growing season from 2018 to 2020. The responses of net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), and gross primary productivity (GPP) to precipitation were analysed, as well as the effects of environmental factors on carbon fluxes at half-hour and daily scales. The results showed that (1) during the growing seasons in 2019 and 2020, the grassland ecosystem in a lake basin in the BJD was a net CO2 sink, and the cumulative NEE was - 91.9 and - 79.2g C m-2, respectively. The greater the total precipitation in the growing season, the stronger the carbon sequestration capacity of a grassland ecosystem. (2) The precipitation intensity, frequency, and timing significantly affected the carbon fluxes in the ecosystem. Isolated minor precipitation events did not trigger obvious NEE, GPP, and Reco pulses. However, large precipitation events or continuous minor precipitation events over several days caused delayed high assimilation; in addition, the greater the precipitation intensity, the greater the carbon flux pulse and carbon assimilation. The timing and frequency of precipitation events had more important effects on carbon exchange than total precipitation. Droughts create a shift in grasslands, causing them to move from being a carbon sink to a carbon source. (3) Correlation analysis showed that NEE was significantly negatively correlated with photosynthetically active radiation (PAR). On the half-hour scale, Reco and GPP were significantly positively correlated with soil temperature at 5 cm deep (Ts5) and PAR, respectively. However, they were strongly correlated with air temperature (Ta), soil surface temperature (Ts) and (Ts5)on the daily scale. The correlations between daily NEE, Reco, GPP, and precipitation varied across years and seasons. Due to warming and humidification in northwest China, precipitation events will have a greater impact on the carbon sequestration capacity of the BJD. The results are vital for predicting the possible effects of climate change on the carbon cycle.

Carbon dioxide and water vapor fluxes of multi-purpose winter wheat production systems in the U.S. Southern Great Plains

• We synthesized CO 2 fluxes and ET from differently managed eight winter wheat fields. • Wheat fields, including graze-out, were large sinks of CO 2 during growing seasons. • Wheat fields were from near CO 2 neutral to large sinks of CO 2 at annual scales. • In general, evapotranspiration (ET) was smaller under no-till than tilled fields. • CO 2 fluxes and ET were poorly correlated with biometric variables in grazed fields. Eddy fluxes collected during 2016 to 2019 from eight production-scale multi-purpose winter wheat fields (grain only, graze-grain, and graze-out), managed under conventional till (CT) and no-till (NT), were synthesized to determine seasonality, daily magnitudes, seasonal, and annual budgets of carbon dioxide (CO 2 ) fluxes and evapotranspiration (ET), and to investigate spatio-temporal variability of the fluxes. Maximum daily net ecosystem CO 2 exchange (NEE), gross primary production (GPP), and ecosystem respiration (ER) approximated -11, 19, and 12 g C m −2 , respectively, and daily ET approximated 7 mm. Wheat fields, including graze-out, were large sinks of CO 2 (ranged from -149 ± 8 to -564 ± 9 g C m −2 ) during growing seasons (October–May). Wheat fields, left fallow during summer, were from near neutral to large sinks of CO 2 at annual (calendar year) scales. Cumulative annual NEE was up to -242 ± 12 g C m −2 in a NT and -183 ± 12 g C m −2 in a CT field, which had grain-only wheat in spring followed by graze-grain wheat in fall. Cumulative seasonal ET ranged from 260 mm to 521 mm, and maximum annual ET approximated 800 mm. In general, ET was smaller under NT than CT. Eddy fluxes showed stronger relationships with remotely-sensed enhanced vegetation index and in-situ biometric variables in grain-only fields than grazed fields. Across-site analysis for grain-only wheat showed biomass and leaf area index alone explained >80% of variations in NEE, GPP, and ET, and ∼70% of variations in ER. Similarly, Canopy coverage explained >80% of variations in NEE and GPP, and ∼60% of variations in ER and ET. Strong relationships of biometric observations with the fluxes demonstrated their potential to model and explain spatio-temporal variability of CO 2 fluxes and ET. Results also indicated huge implications of management practices on carbon and water budgets by altering vegetative properties.

Hydrometeorological sensitivities of net ecosystem carbon dioxide and methane exchange of an Amazonian palm swamp peatland

Tropical peatlands are a major, but understudied, biophysical feedback factor on the atmospheric greenhouse effect. The largest expanses of tropical peatlands are located in lowland areas of Southeast Asia and the Amazon basin. The Loreto Region of Amazonian Peru contains ~63,000 km2 of peatlands. However, little is known about the biogeochemistry of these peatlands, and in particular, the cycling of carbon dioxide (CO2) and methane (CH4), and their responses to hydrometeorological forcings. To address these knowledge gaps, we established an eddy covariance (EC) flux tower in a natural palm (Mauritia flexuosa L.f.) swamp peatland near Iquitos, Peru. Here, we report ecosystem-scale CO2 and CH4 flux observations for this Amazonian palm swamp peatland over a two-year period in relation to hydrometeorological forcings. Seasonal and short-term variations in hydrometeorological forcing had a strong effect on CO2 and CH4 fluxes. High air temperature and vapor pressure deficit (VPD) exerted an important limitation on photosynthesis during the dry season, while latent heat flux appeared to be insensitive to these climate drivers. Evidence from light-response analyses and flux partitioning support that photosynthetic activity was downregulated during dry conditions, while ecosystem respiration (RE) was either inhibited or enhanced depending on water table position. The cumulative net ecosystem CO2 exchange indicated that the peatland was a significant CO2 sink ranging from −465 (−279 to −651) g C m−2 y−1 in 2018 to −462 (−277 to −647) g C m−2 y−1 in 2019. The forest was a CH4 source of 22 (20 to 24) g C m−2 y−1, similar in magnitude to other tropical peatlands and larger than boreal and arctic peatlands. Thus, the annual carbon budget of this Amazonian palm swamp peatland appears to be a major carbon sink under current hydrometeorological conditions.

Seasonal variation in net ecosystem CO2 exchange of a Brazilian seasonally dry tropical forest

Forest ecosystems sequester large amounts of atmospheric CO2, and the contribution from seasonally dry tropical forests is not negligible. Thus, the objective of this study was to quantify and evaluate the seasonal and annual patterns of CO2 exchanges in the Caatinga biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. In addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (GPP), ecosystem respiration (Reco) and net ecosystem CO2 exchange (NEE). Results showed that the dynamics of the components of the CO2 fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m−2) when compared to 2015 (−145.0 g C m−2) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the Caatinga acted as an atmospheric carbon sink during the years 2014 and 2015.

Dynamics of CO2 and H2O fluxes in Johnson grass in the U.S. Southern Great Plains

Johnson grass (Sorghum halepense (L.) Pers.) is rapidly spreading throughout the continental United States (U.S.). Thus, determining magnitudes and seasonal dynamics of carbon dioxide (CO2) and water vapor (H2O) fluxes in Johnson grass is crucial to understand regional changes in hydrology and carbon balance. Using eddy covariance (EC), CO2 and H2O fluxes were measured from June 2017 to October 2019 over a rainfed Johnson grass field in central Oklahoma. Hay was harvested from late May to early July each year, with biomass yield ~7.5 t ha−1. Weekly averaged daily integrated net ecosystem CO2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) reached −8.28 ± 0.76 g C m−2, 20.02 ± 1.62 g C m−2, and 5.42 ± 0.26 mm, respectively. Ecosystem water use efficiency (EWUE) and ecosystem light use efficiency (ELUE) ranged from 3.22 to 3.93 g C mm−1 ET and 0.34 to 0.41 g C mol−1 PAR (photosynthetically active radiation), respectively, during peak growths. Based on aggregated fluxes for each month over the three years (2017–2019), cumulative annual NEE was −434 ± 112 g C m−2, indicating a carbon gain by the Johnson grass field. Cumulative annual ET (858 ± 72 mm) was ~86% of the average annual rainfall (996 ± 100 mm). Results showed Johnson grass could be a carbon sink from May to September in the U.S. Southern Great Plains. Both NEE and ET did not decline up to air temperature (Ta) of ~33 °C and vapor pressure deficit (VPD) of ~2 kPa, suggesting optimum Ta of ≥33 °C and VPD of ≥2 kPa for the fluxes. Results indicated that Johnson grass might be well suited for dryland production in the region. Additionally, these findings provide initial baseline information on CO2 fluxes and ET for Johnson grass relative to other forage species in the region.

Carbon fluxes response of an artificial sand-binding vegetation system to rainfall variation during the growing season in the Tengger Desert

Revegetation is considered as an effective approach for desertification control, and artificial sand-binding vegetation exerts a significant contributor to carbon cycling in arid and semiarid regions; however, this is largely determined by the rainfall regime. We measured carbon fluxes (the net ecosystem CO2 exchange (NEE), the gross ecosystem productivity (GEP) and the ecosystem respiration (Reco)) during the growing season of 2014–2016 using the eddy covariance technique and explored the effects of rainfall variables (amount, timing distribution and pulse size) and environmental factors on carbon fluxes at different time scales. The system had NEE values of −117.5 and −98.9 g C m−2 during the growing seasons of 2015 (dry year) and 2016 (wet year), respectively. When the rainfall amount did not differ significantly between spring and autumn, the cumulative GEP was greater in spring than in autumn, whereas the cumulative Reco and NEE showed the opposite pattern. Small (<5 mm) rain events failed to trigger obvious GEP and NEE pulses, whereas ≥ 5 mm or a series of small rain events led to high assimilation but with hysteresis. The magnitude of Reco increased as the rain pulses increased. The Random Forest (RF) algorithm revealed that soil water contents had a great impact on carbon fluxes at different integration periods. A correlation analysis showed that the soil water contents were positively correlated with GEP and Reco and negatively correlated with NEE over different time scales in most cases. These findings suggest that artificial vegetation not only improves habitat restoration but is a significant carbon sink during both dry and wet growing season, which is likely to supplement our knowledge gap to accurately evaluate the current carbon budget in dry land.

Five‐Year Measurements of Net Ecosystem CO2 Exchange at a Fen in the Zoige Peatlands on the Qinghai‐Tibetan Plateau

AbstractLittle is known about the relationship between carbon sequestration and environment in some alpine peatland ecosystems, especially in remote areas such as the Qinghai‐Tibetan Plateau. In the present study, net ecosystem CO2 exchange (NEE) flux was measured from 2013 to 2017 at the Riganqiao fen in the Zoige peatlands on the eastern Qinghai‐Tibetan Plateau using the eddy covariance technique. In the growing season, diurnal CO2 uptake peaks occurred between 12:00 and 14:00, while emission peaks occurred between 21:00 and 22:00. Carbon uptake peak occurred mainly from June to July in the growing season, and the maximum daily net CO2 uptake of −8.4 g C m−2 d−1 was observed in mid‐June 2016. The Riganqiao peatland was a net CO2 sink, with uptake (g C m−2 yr−1) of average −171.4 ± 38.1 during the five years. Photosynthetically active radiation (PAR) and soil temperature (Ts) were significantly correlated with diurnal net CO2 uptake. In growing seasons, PAR was significantly correlated with daytime CO2 uptake, and Ts was a major determinant of ecosystem respiration during growing and nongrowing seasons, although the fitting functions differed between the two seasons. Rainfall influenced interannual variability of cumulative NEE and water table depth (WTD) and volumetric water content (VWC) were both significantly correlated with interannual NEE. Our results suggest that in the long term, peatland NEE is affected mainly by the water regime, which is influenced by soil temperature and water condition.

Annual dynamics of carbon dioxide fluxes over a rainfed alfalfa field in the U.S. Southern Great Plains

A thorough investigation of annual dynamics of carbon dioxide (CO2) fluxes with respect to major controlling factors and harvest management is lacking for rainfed alfalfa (Medicago sativa L.), a high quality perennial legume forage. To address this knowledge gap, this study reports two years (April 2016 - March 2018) of eddy covariance measurements of CO2 fluxes over a rainfed alfalfa field in central Oklahoma, USA. Alfalfa yields were strongly regulated by amount and timing of rainfall. As a result, cumulative dry forage yield was ∼7.5 t ha−1 (four harvests) in 2016 (dry year) and ∼10 t ha−1 (five harvests) in 2017 (wet year). An optimum air temperature (Ta) and vapor pressure deficit (VPD) for net ecosystem CO2 exchange (NEE) was approximately 25 °C and 2.2 kPa, respectively. The response of gross primary production (GPP) to photosynthetically active radiation (PAR) varied with growth stage of alfalfa and climatic conditions (i.e., dry or normal/wet periods). Daily (8-day averages) NEE and gross primary production (GPP) reached -8.17 and 16.69 g C m-2 d−1, respectively. Magnitudes of GPP (GPPMOD) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) were < 50% of tower-derived GPP, most likely due to a smaller value (0.15 g C mol−1 PAR) for light use efficiency in the GPPMOD algorithm. The observed 8-day composite ecosystem light use efficiency (ELUE) was up to 0.36 g C mol−1 PAR in this study. The rainfed alfalfa field with 4–5 hay harvests per year showed large carbon uptake potential (e.g., cumulative NEE of -454 g C m-2 in 2017) at an annual scale. The GPP and ELUE showed a strong correspondence with MODIS-derived vegetation indices, indicating the potential of applying satellite remote sensing to upscale site-level observations of CO2 fluxes for alfalfa to larger spatial scales.

Impacts of nitrogen addition on the carbon balance in a temperate semiarid grassland ecosystem

Understanding carbon (C) cycling and sequestration in vegetation and soils, and their responses to nitrogen (N) deposition, is important for quantifying ecosystem responses to global climate change. Here, we describe a 2-year study of the C balance in a temperate grassland in northern China. We measured net ecosystem CO2 exchange (NEE), net ecosystem production (NEP), and C sequestration rates in treatments with N addition ranging from 0 to 25 g N m−2 year−1. High N addition significantly increased ecosystem C sequestration, whose rates ranged from 122.06 g C m−2 year−1 (control) to 259.67 g C m−2 year−1 (25 g N). Cumulative NEE during the growing season decreased significantly at high and medium N addition, with values ranging from −95.86 g C m−2 (25 g N) to 0.15 g C m−2 (5 g N). Only the highest N rate increased significantly cumulative soil microbial respiration compared with the control in the dry 2014 growing season. High N addition significantly increased net primary production (NPP) and NEP in both years, and NEP ranged from −5.83 to 128.32 g C m−2. The C input from litter decomposition was significant and must be quantified to accurately estimate NPP. Measuring C sequestration and NEP together may allow tracking of the effects of N addition on grassland C budgets. Overall, adding 25 or 10 g N m−2 year−1 improved the CO2 sink of the grassland ecosystem, and increased grassland C sequestration.

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.

Large net CO2 loss from a grass‐dominated tropical savanna in south‐central Brazil in response to seasonal and interannual drought

AbstractThe savanna vegetation of Brazil (Cerrado) accounts for 20–25% of the land cover of Brazil and is the second largest ecosystem following Amazonian forest; however, Cerrado mass and energy exchange is still highly uncertain. We used eddy covariance to measure the net ecosystem CO2 exchange (NEE) of grass‐dominated Cerrado (campo sujo) over 3 years. We hypothesized that soil water availability would be a key control over the seasonal and interannual variations in NEE. Multiple regression indicated that gross primary production (GPP) was positively correlated (Pearson's r = 0.69; p &lt; 0.001) with soil water content, radiation, and the Moderate Resolution Imaging Spectroradiometer (MODIS)‐derived enhanced vegetation index (EVI) but negatively correlated with the vapor pressure deficit (VPD), indicating that drier conditions increased water limitations on GPP. Similarly, ecosystem respiration (Reco) was positively correlated (Pearson's r = 0.78; p &lt; 0.001) with the EVI, radiation, soil water content, and temperature but slightly negatively correlated with rainfall and the VPD. While the NEE responded rapidly to temporal variations in soil water availability, the grass‐dominated Cerrado stand was a net source of CO2 to the atmosphere during the study period, which was drier compared to the long‐term average rainfall. Cumulative NEE was approximately 842 gC m−2, varying from 357 gC m−2 in 2011 to 242 gC m−2 in 2012. Our results indicate that grass‐dominated Cerrado may be an important regional CO2 source in response to the warming and drying that is expected to occur in the southern Amazon Basin under climate change.

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.

Carbon dioxide exchange of a perennial bioenergy crop cultivation on a mineral soil

Abstract. One of the strategies to reduce carbon dioxide (CO2) emissions from the energy sector is to increase the use of renewable energy sources such as bioenergy crops. Bioenergy is not necessarily carbon neutral because of greenhouse gas (GHG) emissions during biomass production, field management and transportation. The present study focuses on the cultivation of reed canary grass (RCG, Phalaris arundinacea L.), a perennial bioenergy crop, on a mineral soil. To quantify the CO2 exchange of this RCG cultivation system, and to understand the key factors controlling its CO2 exchange, the net ecosystem CO2 exchange (NEE) was measured from July 2009 until the end of 2011 using the eddy covariance (EC) method. The RCG cultivation thrived well producing yields of 6200 and 6700 kg DW ha−1 in 2010 and 2011, respectively. Gross photosynthesis (GPP) was controlled mainly by radiation from June to September. Vapour pressure deficit (VPD), air temperature or soil moisture did not limit photosynthesis during the growing season. Total ecosystem respiration (TER) increased with soil temperature, green area index and GPP. Annual NEE was −262 and −256 g C m−2 in 2010 and 2011, respectively. Throughout the study period from July 2009 until the end of 2011, cumulative NEE was −575 g C m−2. Carbon balance and its regulatory factors were compared to the published results of a comparison site on drained organic soil cultivated with RCG in the same climate. On this mineral soil site, the RCG had higher capacity to take up CO2 from the atmosphere than on the comparison site.

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

Tropical low land rice ecosystem is a net carbon sink

A yearlong field study was undertaken to investigate the carbon dioxide (CO2) and methane (CH4) exchange in relation to ecosystem carbon (C) balance in tropical low land rice by employing open path eddy covariance (OPEC) system. The study was conducted on annual basis, included both wet and dry seasons crop. This is the first report from India on net ecosystem CH4 exchange (NEME) from low land flooded rice ecology through eddy covariance techniques. On annual basis the cumulative net ecosystem CO2 exchange (NEE) was −457gCm−2 and NEME was +262.62kgCH4ha−1, respectively. On the other hand, the annual gross primary production (GPP) was +1340gCm−2 and ecosystem respiration (RE) was +883gCm−2. On seasonal basis the rate of NEE was in the range of −3.29 and −2.74gCm−2 day−1, respectively and NEME was +2.80 and +4.34mgm−2h−1 in wet and dry cropping seasons, respectively. As a whole, the integrated NEE in dry and wet seasons was −355 and −341gCm−2, whereas, the integrated NEME in the two seasons was +84 and +129kgha−1, respectively. Considering the net ecosystem production (NEP or −NEE: +695gCm−2), rhizodeposition, algal biomass, root, stubble and compost added as ecosystem C input and NEME (+213kgha−1), C removal through harvest and dissolved organic C as ecosystem C output, the tropical rice acts as net C sink both seasonally and annually. Although CH4 is a source of C loss from low land flooded rice ecosystem, considering all the components of C balance on system basis, this ecology has a good potential to store considerable amount of C.

Agricultural reclamation effects on ecosystem CO2 exchange of a coastal wetland in the Yellow River Delta

Little is known about the impacts of agricultural exploitation of coastal wetlands on ecosystem CO2 exchange, although coastal wetlands have been widely reclaimed for agricultural use across the world. We measured net ecosystem CO2 exchange (NEE) and its major components, gross primary production (GPP) and ecosystem respiration (Reco) using an eddy covariance flux technique in a natural coastal wetland (reed) and an adjacent, newly reclaimed farmland (cotton) in the Yellow River Delta, China. The results showed that agricultural reclamation changed the ecosystem CO2 exchange of the coastal wetland at three distinct levels. Initially, the conversion from the wetland to farmland changed the light response parameters (α, Amax, and Reco, day) of NEE and temperature sensitivity (Q10) of Reco mainly by changing the dominant vegetation type. Over the growing season, NEE, Reco and GPP were significantly correlated with LAI at both sites and aboveground biomass at the farmland site. Next, the reclamation of wetland modified the diurnal and seasonal dynamics of ecosystem CO2 exchange. Significant differences in diurnal variations of NEE between the wetland and farmland sites were found during the growing season (with the exception of June and July). Seasonal means of daily GPP and Reco values at the wetland site were higher than those at the farmland. Ultimately, the agricultural reclamation altered the CO2 sequestration capacity of the coastal wetland. The cumulative NEE in the wetland (−237.4gCm−2) was higher than that in the farmland (−202.0gCm−2). When biomass removal was taken into account, the farmland was a strong source for CO2 of around 131.9gCm−2 during the growing season. Overall, land use changes by reclamation altered ecosystem CO2 exchange at several ecological scales by changing the dominant vegetation type and altering the ecosystem's natural development.

Contrasting responses of growing season ecosystem CO2exchange to variation in temperature and water table depth in two peatlands in northern Alberta, Canada

[1] The large belowground carbon stocks in northern peatland ecosystems are potentially susceptible to release because of the expected differential responses of photosynthesis and respiration to climate change. This study compared net ecosystem CO2 exchange (NEE) measured using the eddy covariance technique at two peatland sites in northern Alberta, Canada, over three growing seasons (May–October). We observed distinct differences between the poor fen (Sphagnum moss dominated) and extreme-rich fen (Carex sedge dominated) sites for their responses of NEE to interannual variation in temperature and water table depth. The rates of growing season cumulative NEE at the poor fen were very similar among the three study years with an average (± standard deviation) of −110.1 ± 0.5 g C m−2 period−1. By contrast, the growing season cumulative NEE at the extreme-rich fen varied substantially among years (−34.5, −153.5, and −41.8 g C m−2 period−1 in 2004, 2005, and 2006, respectively), and net uptake of CO2 was lower (on average) than at the poor fen. Consistent with the eddy covariance measurements, analysis of 210Pb-dated peat cores also showed higher recent net rates of carbon accumulation in the poor fen than in the rich fen. Warm spring temperatures and sufficient water availability during the growing season resulted in the highest-magnitude ecosystem photosynthesis and NEE at the extreme-rich fen in 2005. Cool spring temperatures limited photosynthesis at the extreme-rich fen in 2004, while reduced water availability (lower water table) in 2006 constrained photosynthetic capacity relative to 2005, despite the warmer spring and summer temperatures in 2006. The combination of contrasting plant functional types and different peat water table features at our two study sites meant that the poor fen showed a reduced response of ecosystem CO2 exchange to environmental variation compared to the extreme-rich fen.

Scaling an Instantaneous Model of Tundra NEE to the Arctic Landscape

We scale a model of net ecosystem CO2 exchange (NEE) for tundra ecosystems and assess model performance using eddy covariance measurements at three tundra sites. The model, initially developed using instantaneous (seconds–minutes) chamber flux (~m2) observations, independently represents ecosystem respiration (ER) and gross primary production (GPP), and requires only temperature (T), photosynthetic photon flux density (I 0), and leaf area index (L) as inputs. We used a synthetic data set to parameterize the model so that available in situ observations could be used to assess the model. The model was then scaled temporally to daily resolution and spatially to about 1 km2 resolution, and predicted values of NEE, and associated input variables, were compared to observations obtained from eddy covariance measurements at three flux tower sites over several growing seasons. We compared observations to modeled NEE calculated using T and I 0 measured at the towers, and L derived from MODIS data. Cumulative NEE estimates were within 17 and 11% of instrumentation period and growing season observations, respectively. Predictions improved when one site-year experiencing anomalously dry conditions was excluded, indicating the potential importance of stomatal control on GPP and/or soil moisture on ER. Notable differences in model performance resulted from ER model formulations and differences in how L was estimated. Additional work is needed to gain better predictive ability in terms of ER and L. However, our results demonstrate the potential of this model to permit landscape scale estimates of NEE using relatively few and simple driving variables that are easily obtained via satellite remote sensing.

Interannual variability and timing of growing‐season CO2 exchange in a boreal forest

We studied the interannual variability of cumulative net ecosystem CO2 exchange (NEE) and its connection with cumulative or average climatic variables during five growing seasons. The analysis was based on a 5‐year‐long time series of CO2 flux measured from April 1996 to April 2001 in a Scots pine forest in southern Finland by the eddy covariance technique. The onset of the ecosystem growing season was best connected with air temperature, and the end of the growing season was best connected with day length. With these variables we were able to predict the timing and the length of each growing season within 0–3 days. The forest was a sink of carbon with little interannual variability: The uptake during the four full growing seasons varied by 80 g C m−2, ranging from 230 to 310 g C m−2. The estimated winter release each year varied between 60 and 90 g C m−2. The interannual variation in seasonal (spring, summer, autumn) carbon exchange ranged from 30 g C m−2 in autumn and spring to 80 g C m−2 in summer. The average climatic variables explained the variability of the seasonal or growing‐season cumulative NEE only partly. Both the daytime and the nighttime CO2 fluxes contributed markedly to the variability in carbon exchange, indicating that photosynthesis and respiration have an equally important influence on NEE.