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1.
Many studies on global climate have forecast major changes in the amounts and spatial patterns of precipitation that may significantly affect temperate grasslands in arid and semi-arid regions. As a part of ChinaFLUX, eddy covariance flux measurements were made at a semi-arid Leymus chinensis steppe in Inner Mongolia, China during 2003–2004 to quantify the response of carbon exchange to environmental changes. Results showed that gross ecosystem production ( F GEP) and ecosystem respiration ( R eco) of the steppe were significantly depressed by water stress due to lack of precipitation during the growing season. Temperature was the dominant factor affecting F GEP and R eco in 2003, whereas soil moisture imposed a significant influence on both R eco and F GEP in 2004. Under wet conditions, R eco showed an exponentially increasing trend with temperature ( Q 10 = 2.0), but an apparent reduction in the value of R eco and its temperature sensitivity were observed during the periods of water stress ( Q 10=1.6). Both heat and water stress can cause decrease in F GEP. The seasonality of ecosystem carbon exchange was strongly correlated with the variation of precipitation. With less precipitation in 2003, the steppe sequestrated carbon in June and July, and went into a senescence in early August due to water stress. As compared to 2003, the severe drought during the spring of 2004 delayed the growth of the steppe until late June, and the steppe became a CO 2 sink from early July until mid-September, with ample precipitation in August. The semi-arid steppe released a total of 9.7 g C·m ?2 from May 16 to the end of September 2003, whereas the net carbon budget during the same period in 2004 was close to zero. Long-term measurements over various grasslands are needed to quantify carbon balance in temperate grasslands. 相似文献
2.
Long-term measurement of carbon metabolism of old-growth forests is critical to predict their behaviors and to reduce the uncertainties of carbon accounting under changing climate. Eddy covariance technology was applied to investigate the long-term carbon exchange over a 200 year-old Chinese broad-leaved Korean pine mixed forest in the Changbai Mountains (128°28′E and 42°24′N, Jilin Province, P. R. China) since August 2002. On the data obtained with open-path eddy covariance system and CO2 profile measurement system from Jan. 2003 to Dec. 2004, this paper reports (i) annual and seasonal variation of F
NEE, F
GPP and R
E; (ii) regulation of environmental factors on phase and amplitude of ecosystem CO2 uptake and release Corrections due to storage and friction velocity were applied to the eddy carbon flux. LAI and soil temperature determined the seasonal and annual dynamics of FGPP and RE separately. VPD and air temperature regulated ecosystem photosynthesis at finer scales in growing seasons. Water condition at the root zone exerted a significant influence on ecosystem maintenance carbon metabolism of this forest in winter. The forest was a net sink of atmospheric CO2 and sequestered −449 g C·m−2 during the study period; −278 and −171 gC·m−2 for 2003 and 2004 respectively. F
GPP and F
RE over 2003 and 2004 were −1332, −1294 g C·m−2. and 1054, 1124 g C·m−2 respectively. This study shows that old-growth forest can be a strong net carbon sink of atmospheric CO2. There was significant seasonal and annual variation in carbon metabolism. In winter, there was weak photosynthesis while the ecosystem emitted CO2. Carbon exchanges were active in spring and fall but contributed little to carbon sequestration on an annual scale. The summer is the most significant season as far as ecosystem carbon balance is concerned. The 90 days of summer contributed 66.9, 68.9% of F
GPP, and 60.4, 62.1% of R
E of the entire year. 相似文献
3.
Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO2 flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO2 fluxes with a depression of CO2 flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO2 were −7.4 and 5.4 g·m−2·d−1 respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO2 were −12.8 and 5.8 g·m−2·d−1 and occurred both in August. The grassland fixed 294.66 and 467.46 g CO2·m−2 in 2003 and 2004, and released 333.14 and 437.17 g CO2·m−2 in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation (PAR) are two important factors of controlling CO2 flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO2 flux is dependent on PAR. CO2 flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m−2·s−1. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature. 相似文献
4.
Long-term measurement of carbon metabolism of old-growth forests is critical to predict their behaviors and to reduce the uncertainties of carbon accounting under changing climate. Eddy covariance technology was applied to investigate the long-term carbon exchange over a 200 year-old Chinese broad-leaved Korean pine mixed forest in the Changbai Mountains (128°28′E and 42°24′N, Jilin Province, P. R. China) since August 2002. On the data obtained with open-path eddy covariance system and CO 2 profile measurement system from Jan. 2003 to Dec. 2004, this paper reports (i) annual and seasonal variation of F NEE, F GPP and R E; (ii) regulation of environmental factors on phase and amplitude of ecosystem CO 2 uptake and release Corrections due to storage and friction velocity were applied to the eddy carbon flux. LAI and soil temperature determined the seasonal and annual dynamics of FGPP and RE separately. VPD and air temperature regulated ecosystem photosynthesis at finer scales in growing seasons. Water condition at the root zone exerted a significant influence on ecosystem maintenance carbon metabolism of this forest in winter.The forest was a net sink of atmospheric CO 2 and sequestered ?449 g C·m ?2 during the study period; ?278 and ?171 gC·m ?2 for 2003 and 2004 respectively. F GPP and F RE over 2003 and 2004 were ?1332, ?1294 g C·m ?2. and 1054, 1124 g C·m ?2 respectively. This study shows that old-growth forest can be a strong net carbon sink of atmospheric CO 2.There was significant seasonal and annual variation in carbon metabolism. In winter, there was weak photosynthesis while the ecosystem emitted CO 2. Carbon exchanges were active in spring and fall but contributed little to carbon sequestration on an annual scale. The summer is the most significant season as far as ecosystem carbon balance is concerned. The 90 days of summer contributed 66.9, 68.9% of F GPP, and 60.4, 62.1% of R E of the entire year. 相似文献
5.
Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO 2 flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO 2 fluxes with a depression of CO 2 flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO 2 were ?7.4 and 5.4 g·m ?2·d ?1 respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO 2 were ?12.8 and 5.8 g·m ?2·d ?1 and occurred both in August. The grassland fixed 294.66 and 467.46 g CO 2·m ?2 in 2003 and 2004, and released 333.14 and 437.17 g CO 2·m ?2 in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation ( PAR) are two important factors of controlling CO 2 flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO 2 flux is dependent on PAR. CO 2 flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m ?2·s ?1. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature. 相似文献
6.
As one component of ChinaFLUX, the measurement of CO 2 flux using eddy covariance over subtropical planted coniferous ecosystem in Qianyanzhou was conducted for a long term. This paper discusses the seasonal dynamics of net ecosystem exchange (NEE), ecosystem respiration (RE) and gross ecosystem exchange (GEE) between the coniferous ecosystem and atmosphere along 2003 and 2004. The variations of NEE, RE and GEE show obvious seasonal variabilities and correlate to each other, i.e. lower in winter and drought season, but higher in summer; light, temperature and soil water content are the main factors determining NEE; air temperature and water vapor pressure deficit (VPD) influence NEE with stronger influence from VPD. Under the proper light condition, drought stress could decrease the temperature range for carbon capture in planted coniferous, air temperature and precipitation controlled RE; The NEE, RE, and GEE for planted coniferous in Qianyanzhou are ?387.2 g C·m ?2 a ?1, 1223.3 g C·m ?2 a ?1, ?1610.4 g C·m ?2 a ?1 in 2003 and ?423.8 g C·m ?2 a ?1, 1442.0 g C·m ?2 a ?1, ?1865.8 g C·m ?2 a ?1 in 2004, respectively, which suggest the intensive ability of plantation coniferous forest on carbon absorbing in Qianyanzhou. 相似文献
7.
CO 2 flux was measured continuously in a wheat and maize rotation system of North China Plain using the eddy covariance technique to study the characteristic of CO 2 exchange and its response to key environmental factors. The results show that nighttime net ecosystem exchange (NEE) varied exponentially with soil temperature. The temperature sensitivities of the ecosystem ( Q 10) were 2.94 and 2.49 in years 2002–2003 and 2003–2004, respectively. The response of gross primary productivity (GPP) to photosynthetically active radiation (PAR) in the crop field can be ex-pressed by a rectangular hyperbolic function. Average A max and α for maize were more than those for wheat. The values of α increased positively with leaf area index (LAI) of wheat. Diurnal variations of NEE were significant from March to May and from July to September, but not remarkable in other months. NEE, GPP and ecosystem respiration ( R ec) showed significantly seasonal variations in the crop field. The highest mean daily CO 2 uptake rate was ?10.20 and ?12.50 gC·m ?2?d ?1 in 2003 and 2004, for the maize field, respectively, and ?8.19 and ?9.50 gC?m ?2·d ?1 in 2003 and 2004 for the wheat field, respectively. The maximal CO 2 uptake appeared in April or May for wheat and mid-August for maize. During the main growing seasons of winter wheat and summer maize, NEE was controlled by GPP which was chiefly influenced by PAR and LAI. R ec reached its annual maximum in July when R ec and GPP contributed to NEE equally. NEE was dominated by R ec in other months and temperature became a key factor controlling NEE. Total NEE for the wheat field was ?77.6 and ?152.2 gC·m ?2·a ?1 in years 2002–2003 and 2003–2004, respectively, and ?120.1 and ?165.6 gC·m ?2·a ?1 in 2003 and 2004 for the maize field, respectively. The cropland of North China Plain was a carbon sink, with annual ?197.6 and ?317.9 gC·m ?2·a ?1 in years 2002–2003 and 2003–2004, respectively. After considering the carbon in grains, the cropland became a carbon source, which was 340.5 and 107.5 gC·m ?2·a ?1 in years 2002–2003 and 2003–2004, respectively. Affected by climate and filed managements, inter-annual carbon exchange varied largely in the wheat and maize rotation system of North China Plain. 相似文献
8.
Soil CO 2 flux is strongly influenced by precipitation in many ecosystem types, yet knowledge of the effects of precipitation on soil CO 2 flux in semi‐arid desert ecosystems remains insufficient, particularly for sandy soils. To address this, we investigated the response of sandy soil CO 2 flux to rainfall pulses in a desert ecosystem in northern China during August–September 2011. Significant changes ( P < 0.05) were found in diel patterns of soil CO 2 flux induced by small (2.1 mm), moderate (12.4 mm) and large (19.7 mm) precipitation events. Further analysis indicated that rainfall pulses modified the response of soil CO 2 flux to soil temperature, including hysteresis between soil CO 2 flux and soil temperature, with Fs higher when Ts was increasing than when Ts was decreasing, and the linear relationship between them. Moreover, our results showed that rainfall could result in absorption of atmospheric CO 2 by soil, possibly owing to mass flow of CO 2 induced by a gradient of gas pressure between atmosphere and soil. After each precipitation event, soil CO 2 flux recovered exponentially to pre‐rainfall levels with time, with the recovery times exhibiting a positive correlation with precipitation amount. On the basis of the amounts of precipitation that occurred at our site during the measurement period (August–September), the accumulated rain‐induced carbon absorption evaluated for rainy days was 1.068 g C m ?2; this corresponds approximately to 0.5–2.1% of the net primary production of a typical desert ecosystem. Thus, our results suggest that rainfall pulses can strongly influence carbon fluxes in desert ecosystems. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
9.
The study by the eddy covariance technique in the alpine shrub meadow of the Qinghai-Tibet Plateau in 2003 and 2004 showed that the net ecosystem carbon dioxide exchange (NEE) exhibited noticeable diurnal and annual variations, with more distinct daily changes during the warmer seasons. The CO2 emission of the shrub ecosystem culminated in April and September while the CO2 absorption capacity reached a maximum in July and August. The absorbed carbon dioxide during the two consecutive years was 231.4 and 274.8 g CO2·m−2 respectively, yielding an average of 253.1 gCO2·m−2 per year: that accounts for a large proportion of absorbed CO2 in the region. Obviously, the diurnal carbon flux was negatively related to temperature, radiation and other atmospheric factors. Still, minute discrepancies in kurtosis and duration of carbon emission/absorption were detected between 2003 and 2004. It was found that the CO2 flux in the daytime was similarly affected by photosynthetic photon flux density in both years. Temperature appears to be the most important determinant of CO2 flux: specifically, the high temperature during the plant growing season inhibits the carbon absorption capacity. One potential explanation is that soil respiration is enhanced under such condition. Analysis of biomass revealed that the annual net carbon fixed capacity of aboveground and belowground biomass was 544.0 in 2003 and 559.4 g C·m−2 in 2004, which coincided with the NEE absorption capacity (63.1 g C·m−2 in 2003 and 74.9 g C·m−2 in 2004) in the corresponding plant growing season. 相似文献
10.
The study by the eddy covariance technique in the alpine shrub meadow of the Qinghai-Tibet Plateau in 2003 and 2004 showed that the net ecosystem carbon dioxide exchange (NEE) exhibited noticeable diurnal and annual variations, with more distinct daily changes during the warmer seasons. The CO 2 emission of the shrub ecosystem culminated in April and September while the CO 2 absorption capacity reached a maximum in July and August. The absorbed carbon dioxide during the two consecutive years was 231.4 and 274.8 g CO 2·m ?2 respectively, yielding an average of 253.1 gCO 2·m ?2 per year: that accounts for a large proportion of absorbed CO 2 in the region. Obviously, the diurnal carbon flux was negatively related to temperature, radiation and other atmospheric factors. Still, minute discrepancies in kurtosis and duration of carbon emission/absorption were detected between 2003 and 2004. It was found that the CO 2 flux in the daytime was similarly affected by photosynthetic photon flux density in both years. Temperature appears to be the most important determinant of CO 2 flux: specifically, the high temperature during the plant growing season inhibits the carbon absorption capacity. One potential explanation is that soil respiration is enhanced under such condition. Analysis of biomass revealed that the annual net carbon fixed capacity of aboveground and belowground biomass was 544.0 in 2003 and 559.4 g C·m ?2 in 2004, which coincided with the NEE absorption capacity (63.1 g C·m ?2 in 2003 and 74.9 g C·m ?2 in 2004) in the corresponding plant growing season. 相似文献
11.
As one component of ChinaFLUX, the measurement of CO2 flux using eddy covariance over subtropical planted coniferous ecosystem in Qianyanzhou was conducted for a long term. This paper discusses the seasonal dynamics of net ecosystem exchange (NEE), ecosystem respiration (RE) and gross ecosystem exchange (GEE) between the coniferous ecosystem and atmosphere along 2003 and 2004. The variations of NEE, RE and GEE show obvious seasonal variabilities and correlate to each other, i.e. lower in winter and drought season, but higher in summer; light, temperature and soil water content are the main factors determining NEE; air temperature and water vapor pressure deficit (VPD) influence NEE with stronger influence from VPD. Under the proper light condition, drought stress could decrease the temperature range for carbon capture in planted coniferous, air temperature and precipitation controlled RE; The NEE, RE, and GEE for planted coniferous in Qianyanzhou are −387.2 g C·m−2 a−1, 1223.3 g C·m−2 a−1, −1610.4 g C·m−2 a−1 in 2003 and −423.8 g C·m−2 a−1, 1442.0 g C·m−2 a−1, −1865.8 g C·m−2 a−1 in 2004, respectively, which suggest the intensive ability of plantation coniferous forest on carbon absorbing in Qianyanzhou. 相似文献
12.
The long-term and continuous carbon fluxes of Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical evergreen coniferous forest (QYZ), Dinghushan subtropical evergreen mixed forest (DHS) and Xishuangbana tropical rainforest (XSBN) have been measured with eddy covariance techniques. In 2003, different responses of carbon exchange to the environment appeared across the four ecosystems. At CBS, the carbon exchange was mainly determined by radiation and temperature. 0°C and 10°C were two important temperature thresholds; the former determined the length of the growing season and the latter affected the magnitude of carbon exchange. The maximum net ecosystem exchange ( N EE) of CBS occurred in early summer because maximum ecosystem photosynthesis ( G PP) occurred earlier than maximum ecosystem respiration ( R e). During summer, QYZ experienced severe drought and N EE decreased significantly mainly as a result of the depression of G PP. At DHS and XSBN, N EE was higher in the drought season than the wet season, especially the conversion between carbon sink and source occurring during the transition season at XSBN. During the wet season, increased fog and humid weather resulted from the plentiful rainfall, the ecosystem G PP was dispressed. The Q 10 and annual respiration of XSBN were the highest among the four ecosystems, while the average daily respiration of CBS during the growing season was the highest. Annual N EE of CBS, QYZ, DHS and XSBN were 181.5, 360.9, 536.2 and ?320.0 g·C·m ?2·a ?1, respectively. From CBS to DHS, the temperature and precipitation increased with the decrease in latitude. The ratio of N EE/ R e increased with latitude, while R e/ G PP, ecosystem light use efficiency ( L UE), precipitation use efficiency and average daily G PP decreased gradually. However, XSBN usually escaped such latitude trend probably because of the influence of the south-west monsoon climate which does not affect the other ecosystems. Long-term measurement and more research were necessary to understand the adaptation of forest ecosystems to climate change and to evaluate the ecosystem carbon balance due to the complexity of structure and function of forest ecosystems. 相似文献
13.
Knowledge of seasonal variation of net ecosystem CO2 exchange (NEE) and its biotic and abiotic controllers will further our understanding of carbon cycling process, mechanism and large-scale modelling. Eddy covariance technique was used to measure NEE, biotic and abiotic factors for nearly 3 years in the hinterland alpine steppe—Korbresia meadow grassland on the Tibetan Plateau, the present highest fluxnet station in the world. The main objectives are to investigate dynamics of NEE and its components and to determine the major controlling factors. Maximum carbon assimilation took place in August and maximum carbon loss occurred in November. In June, rainfall amount due to monsoon climate played a great role in grass greening and consequently influenced interannual variation of ecosystem carbon gain. From July through September, monthly NEE presented net carbon assimilation. In other months, ecosystem exhibited carbon loss. In growing season, daytime NEE was mainly controlled by photosynthetically active radiation (PAR). In addition, leaf area index (LAI) interacted with PAR and together modulated NEE rates. Ecosystem respiration was controlled mainly by soil temperature and simultaneously by soil moisture. Q
10 was negatively correlated with soil temperature but positively correlated with soil moisture. Large daily range of air temperature is not necessary to enhance carbon gain. Standard respiration rate at referenced 10°C (R
10) was positively correlated with soil moisture, soil temperature, LAI and aboveground biomass. Rainfall patterns in growing season markedly influenced soil moisture and therefore soil moisture controlled seasonal change of ecosystem respiration. Pulse rainfall in the beginning and at the end of growing season induced great ecosystem respiration and consequently a great amount of carbon was lost. Short growing season and relative low temperature restrained alpine grass vegetation development. The results suggested that LAI be usually in a low level and carbon uptake be relatively low. Rainfall patterns in the growing season and pulse rainfall in the beginning and at end of growing season control ecosystem respiration and consequently influence carbon balance of ecosystem. 相似文献
14.
We present an uncertainty analysis of ecological process parameters and CO 2 flux components ( R
eco, NEE and gross ecosystem exchange ( GEE)) derived from 3 years’ continuous eddy covariance measurements of CO 2 fluxes at subtropical evergreen coniferous plantation, Qianyanzhou of ChinaFlux. Daily-differencing approach was used to
analyze the random error of CO 2 fluxes measurements and bootstrapping method was used to quantify the uncertainties of three CO 2 flux components. In addition, we evaluated different models and optimization methods in influencing estimation of key parameters
and CO 2 flux components. The results show that: (1) Random flux error more closely follows a double-exponential (Laplace), rather
than a normal (Gaussian) distribution. (2) Different optimization methods result in different estimates of model parameters.
Uncertainties of parameters estimated by the maximum likelihood estimation (MLE) are lower than those derived from ordinary
least square method (OLS). (3) The differences between simulated R eco, NEE and GEE derived from MLE and those derived from OLS are 12.18% (176 g C·m −2·a −1), 34.33% (79 g C·m −2·a −1) and 5.4% (92 g C·m −2·a −1). However, for a given parameter optimization method, a temperature-dependent model (T_model) and the models derived from
a temperature and water-dependent model (TW_model) are 1.31% (17.8 g C·m −2·a −1), 2.1% (5.7 g C·m −2·a −1), and 0.26% (4.3 g C·m −2·a −1), respectively, which suggested that the optimization methods are more important than the ecological models in influencing
uncertainty in estimated carbon fluxes. (4) The relative uncertainty of CO 2 flux derived from OLS is higher than that from MLE, and the uncertainty is related to timescale, that is, the larger the
timescale, the smaller the uncertainty. The relative uncertainties of R eco, NEE and GEE are 4%−8%, 7%−22% and 2%−4% respectively at annual timescale.
Supported by the National Natural Science Foundation of China (Grant No. 30570347), Innovative Research International Partnership
Project of the Chinese Academy of Sciences (Grant No. CXTD-Z2005-1) and National Basic Research Program of China (Grant No.
2002CB412502) 相似文献
15.
The eddy covariance technique has emerged as an important tool to directly measure carbon dioxide, water vapor and heat fluxes between the terrestrial ecosystem and the atmosphere after a long history of fundamental research and technological developments. With the realization of regional networks of flux measurements in North American, European, Asia, Brazil, Australia and Africa, a global-scale network of micrometeorological flux measurement (FLUXNET) was established in 1998. FLUXNET has made great progresses in investigating the environmental mechanisms controlling carbon and water cycles, quantifying spatial-temporal patterns of carbon budget and seeking the “missing carbon sink” in global terrestrial ecosystems in the past ten years. The global-scale flux measurement also built a platform for international communication in the fields of resource, ecology and environment sciences. With the continuous development of flux research, FLUXNET will introduce and explore new techniques to extend the application fields of flux measurement and to answer questions in the fields of bio-geography, eco-hydrology, meteorology, climate change, remote sensing and modeling with eddy covariance flux data. As an important part of FLUXNET, ChinaFLUX has made significant progresses in the past three years on the methodology and technique of eddy covariance flux measurement, on the responses of CO 2 and H 2O exchange between the terrestrial ecosystem and the atmosphere to environmental change, and on flux modeling development. Results showed that the major forests on the North-South Transect of Eastern China (NSTEC) were all carbon sinks during 2003 to 2005, and the alpine meadows on the Tibet Plateau were also small carbon sinks. However, the reserved natural grassland, Leymus chinensis steppe in Inner Mongolia, was a carbon source. On a regional scale, temperature and precipitation are the primary climatic factors that determined the carbon balance in major terrestrial ecosystems in China. Finally, the current research emphasis and future directions of ChinaFLUX were presented. By combining flux network and terrestrial transect, ChinaFLUX will develop integrated research with multi-scale, multi-process, multi-subject observations, placing emphasis on the mechanism and coupling relationships between water, carbon and nitrogen cycles in terrestrial ecosystems. 相似文献
16.
Water use efficiency (WUE) links carbon and water exchanges between farmlands and the atmosphere. Understanding the variation and attribution of WUE is essential to reveal the physiological and ecological adaptation mechanisms of crops to the changing environment, and to better allocate, regulate and conserve water resources. However, few studies on the variation and attribution of WUE have been conducted in irrigated arid or semi-arid farmlands. Therefore, in this study, water and carbon fluxes were measured using eddy covariance systems in two farmlands (one sunflower field and one maize field) in a semi-arid irrigation district in China. It was found that the average WUE of sunflower during its full growth period was 1.72 g C kg −1 H 2O, much lower than that of maize (4.07 g C kg −1 H 2O). At each growth stage, the WUE of both crops were negatively correlated with vapour pressure deficit (VPD), net radiation ( Rn) and soil water content (SWC). The negative correlations could be attributed to the arid meteorological condition and the relatively abundant soil moisture due to irrigation and shallow groundwater levels. VPD was the main factor affecting WUE, followed by Rn and SWC. It was also found that the response of WUE to crop leaf area index (LAI) and to canopy conductance ( gc) depended on the VPD ranges: when VPD increased, the response of WUE to LAI and to gc decreased. Our findings could improve the understanding of the coupling effect of water and carbon fluxes over farmland ecosystems in arid and semi-arid irrigation areas and help improve agricultural production and save water resources in such areas. 相似文献
17.
Since 1986, with a sharp decrease in water dis-charges, the Yellow River has entered a period charac-terized by low discharges and seasonally occurring dry-ups[1,2]. Since 1999, more strict management of water diversion has been imposed, and therefore the dry-ups have been well under control. However, the lower reaches of the Yellow River is still predominated by low-discharges, and has become a man-induced shrinking river. In the past 40 years, significant effect of soil and water conservat… 相似文献
18.
The Dinghushan flux observation site, as one of the four forest sites of ChinaFLUX, aims to acquire long-term measurements of CO2 flux over a typical southern subtropical evergreen coniferous and broad-leaved mixed forest ecosystem using the open path eddy covariance method. Based on two years of data from 2003 to 2004, the characteristics of temporal variation in CO2 flux and its response to environmental factors in the forest ecosystem are analyzed. Provided two-dimensional coordinate rotation, WPL correction and quality control, poor energy-balance and underestimation of ecosystem respiration during nighttime implied that there could be a CO2 leak during the nighttime at the site. Using daytime (PAR > 1.0 μmol−1·m−2·s−1) flux data during windy conditions (u* > 0.2 m·s−1), monthly ecosystem respiration (Reco) was derived through the Michaelis-Menten equation modeling the relationship between net ecosystem C02 exchange (NEE) and photosynthetically active radiation (PAR). Exponential function was employed to describe the relationship between Reco and soil temperature at 5 cm depth (Ts05), then Reco of both daytime and nighttime was calculated respectively by the function. The major results are: (i) Derived from the Michaelis-Menten equation, the apparent quantum yield (α) was 0.0027±0.0011 mgCO2·μmol−1 photons, and the maximum photosynthetic assimilation rate (Amax) was 1.102±0.288 mgCO2·m−2·s−1. Indistinctive seasonal variation of α or Amax was consistent with weak seasonal dynamics of leaf area index (LAf) in such a lower subtropical evergreen mixed forest, (ii) Monthly accumulated Reco was estimated as 95.3±21.1 gC·m−2mon−1, accounting for about 68% of the gross primary product (GPP). Monthly accumulated WEE was estimated as −43.2±29.6 gC·m−2·mon−1. The forest ecosystem acted as carbon sink all year round without any seasonal carbon efflux period. Annual NEE of 2003 and 2004 was estimated as −563.0 and −441.2 gC·m−2·a−1 respectively, accounting for about 32% of GPP. 相似文献
19.
Previous “fraction of young water” ( Fyw) estimates based on relative annual isotopic amplitudes in precipitation ( Ap) and streamflow ( As) produced low Fyw values in mountain catchments, which is contrary to extensive research that reports rapid water transmission in mountains. This study investigated this discrepancy by testing the effect of snow accumulation on the model that underpins the Fyw method. A Monte-Carlo analysis of simulations for 20,000 randomly-generated catchment model configurations used 10 years of precipitation inputs for the Upper Elbow River catchment in the Rocky Mountains (Alberta, Canada) to model discharge with and without snowpack storage of winter precipitation. Neither direct nor modified precipitation input produced a 1:1 relationship between As/ Ap and Fyw, undermining the applicability of the original Fyw method in mountain watersheds with large seasonal snow accumulation. With snowpack-modified input a given As/ Ap ratio corresponds to a range of Fyw values, which can still provide semi-quantitative information. In the small (435 km 2) Elbow River catchment a Fyw range of 7–23% supports previous findings of rapid transmission in mountain catchments. Further analysis showed that the improved discharge prediction (Nash–Sutcliffe efficiency > 0.9) correlates with higher Fyw values and demonstrated that the interannual shifts in δ 18O can be used to estimate of new water (<1 year) fraction in winter streamflow, and the estimate of 20% for the Elbow River further supports rapid transmission in mountain catchments. 相似文献
20.
To estimate seasonal changes in recharge to the underlying sandy aquifer, the soil water dynamics of the unsaturated zone was monitored down to a depth of 20 m over a period of three years (1985 to 1987). The measurements were made by a neutron probe at eight locations beneath a native vegetation in a semiarid region, Western Australia, receiving precipitation of 775 mm yr ?1. A relatively simple method, based on the analyses of sequentially measured soil water profiles involving utilization of zero flux plane in the unsaturated zone, is presented and used to compute seasonal recharge rates. Drainage fluxes (recharge rates) below two specified depths were estimated. These were: R1 (water flux at a depth of 10 m, just below the maximum rooting depth) and R2 (water flux at a depth of 18 m, just above the water table). These two estimates were significantly different both on a seasonal and annual basis, but their cumulative values for the three year period were very similar. While the annual precipitation varied from 525 to 850 mm yr ?1, the corresponding spatially averaged R1 varied from 34 to 149 mm yr ?1, and R2 varied from 65 to 80 mm yr ?1. A significant difference in recharge between the upslope and downslope positions on a hillslope was ascribed to differences in vegetation density of the understorey and differences in hydraulic properties of subsoils. For the three year period, the average R1 and R2 were 13 per cent and 10 per cent of the precipitation respectively. These values compare favourably with a long-term estimate based on an environmental tracer technique. 相似文献
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