Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO₂) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO₂ uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BM_ag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.     

Effects of Water and Nitrogen Addition on Ecosystem Carbon Exchange in a Meadow Steppe

A changing precipitation regime and increasing nitrogen deposition are likely to have profound impacts on arid and semiarid ecosystem C cycling, which is often constrained by the timing and availability of water and nitrogen. However, little is known about the effects of altered precipitation and nitrogen addition on grassland ecosystem C exchange. We conducted a 3-year field experiment to assess the responses of vegetation composition, ecosystem productivity, and ecosystem C exchange to manipulative water and nitrogen addition in a meadow steppe. Nitrogen addition significantly stimulated aboveground biomass and net ecosystem CO₂ exchange (NEE), which suggests that nitrogen availability is a primary limiting factor for ecosystem C cycling in the meadow steppe. Water addition had no significant impacts on either ecosystem C exchange or plant biomass, but ecosystem C fluxes showed a strong correlation with early growing season precipitation, rather than whole growing season precipitation, across the 3 experimental years. After we incorporated water addition into the calculation of precipitation regimes, we found that monthly average ecosystem C fluxes correlated more strongly with precipitation frequency than with precipitation amount. These results highlight the importance of precipitation distribution in regulating ecosystem C cycling. Overall, ecosystem C fluxes in the studied ecosystem are highly sensitive to nitrogen deposition, but less sensitive to increased precipitation.     

Non-Additive Effects of Water and Nitrogen Addition on Ecosystem Carbon Exchange in a Temperate Steppe

Changes in precipitation and nitrogen (N) deposition can influence ecosystem carbon (C) cycling and budget in terrestrial biomes, with consequent feedbacks to climate change. However, little is known about the main and interactive effects of water and N additions on net ecosystem C exchange (NEE). In a temperate steppe of northern China, a field-manipulated experiment was conducted to evaluate the responses of NEE and its components to improve N and water availability from 2005 to 2008. The results showed that both water and N additions stimulated gross ecosystem productivity (GEP), ecosystem respiration (ER), and NEE. Water addition increased GEP by 17%, ER by 24%, and NEE by 11% during the experimental period, whereas N addition increased GEP by 17%, ER by 16%, and NEE by 19%. The main effects of both water and N additions changed with time, with the strongest water stimulation in the dry year and a diminishing N stimulation over time. When water and N were added in combination, there were non-additive effects of water and N on ecosystem C fluxes, which could be explained by the changes in species composition and the shifts of limiting resources from belowground (water or N) to aboveground (light). The positive water and N additions effects indicate that increasing precipitation and N deposition in the future will favor C sequestration in the temperate steppe. The non-additive effects of water and N on ecosystem C fluxes suggest that multifactor experiments are better able to capture complex interactive processes, thus improving model simulations and projections.     

Spatial and Temporal Variability in Growing-Season Net Ecosystem Carbon Dioxide Exchange at a Large Peatland in Ontario,Canada

We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas--poor fen and beaver pond margin--had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities.     

1981—2000年中国陆地生态系统碳通量的年际变化

应用一个生物地球化学模型(CEVSA)估算了中国陆地净初级生产力(NPP)、土壤异养呼吸(HR)和净生态系统生产力(NEP)在1981—1998年期间对气候和大气CO2浓度变化的动态响应。结果显示,全国NPP总量波动于2．89—3．37Gt／a之间,平均值为3．09Gt C／a,年平均增长趋势约为0．32％。HR总量变化范围为2．89—3．21Gt C／a,平均值为3．02Gt C／a,年均增长0．40％。NEP总量变动于-0．32和0．25Gt C／a之间,在统计上没有明显的年际变化趋势。在研究时段内,年平均NEP约为0．07Gt C／a,表明中国陆地生态系统在气候与大气CO2浓度变化的条件下吸收了碳,为碳汇,总的吸收量为1．22Gt C,约占全球碳吸收总量的10％,与同期内美国由大气CO2和气候变化所产生的碳吸收量大致相当。尽管由于较高的年际变率,NEP在统计上没有明显的变化趋势,但NPP的增长率低于HR的增长率,说明在研究时段内,中国陆地生态系统的吸碳能力由于气候变化降低了。全国大多数地区年平均NEP接近零,明显的NEP正值区(即碳汇)出现在东北平原、西藏东南部和黄淮平原等地区,而大小兴安岭、黄土高原和云贵高原等地区NEP为负值(即碳源)。研究认为,1981～1998年期间中国气候温暖、干旱,因此估算的NEP可能低于其他时段。如果气候进入一个比较湿润的时期,碳吸收量可显著增加,但若当前干旱和暖化趋势以此为继,中国的NEP可能会变成一个负值。     

Identifying Differences in Carbon Exchange among Arctic Ecosystem Types

Our objective was to determine how varied is the response of C cycling to temperature and irradiance in tundra vegetation. We used a large chamber to measure C exchange at 23 locations within a small arctic catchment in Alaska during summer 2003 and 2004. At each location, we determined light response curves of C exchange using shade cloths, twice during a growing season. We used data to fit a simple photosynthesis-irradiance, respiration-temperature model, with four parameters. We used a maximum likelihood technique to determine the acceptable parameter space for each light curve, given measurement uncertainty. We then explored which sites and time periods had parameter sets in common—an indication of functional similarity. We found that seven distinct parameter sets were required to explain observed C flux responses to temperature and light variation at all sites and time periods. The variation in estimated maximum photosynthetic rate (P_max) was strongly correlated with measurements of site leaf area index (LAI). The behavior of tussock tundra sites, the dominant vegetation of arctic tundra, could largely be described with a single parameter set, with a P_max of 9.7 μmol m⁻² s⁻¹. Tussock tundra sites had, correspondingly, similar LAI (mean = 0.66). Non-tussock sites (for example, sedge and shrub tundras) had larger spatial and temporal variations in both C dynamic parameters (P_max varying from 9.7–25.7 μmol m⁻² s⁻¹) and LAI (0.6–2.0). There were no clear relationships between dominant non-tussock vegetation types and a particular parameter set. Our results suggest that C dynamics of the acidic tussock tundra slopes and hilltops in northern Alaska are relatively simply described during the peak growing season. However, the foot-slopes and water tracks have more variable patterns of LAI and C exchange, not simply related to the dominant vegetation type.     

1981～2000年中国陆地生态系统碳通量的年际变化

应用一个生物地球化学模型(CEVSA)估算了中国陆地净初级生产力 (NPP)、土壤异养呼吸(HR)和净生态系统生产力 (NEP) 在1981～1998年期间对气候和大气CO2浓度变化的动态响应.结果显示,全国NPP总量波动于2.89～3.37 Gt C/a之间,平均值为3.09 Gt C/a,年平均增长趋势约为0.32%.HR总量变化范围为2.89～3.21 Gt C/a,平均值为3.02 Gt C/a, 年均增长0.40%.NEP总量变动于 -0.32和0.25 Gt C/a之间,在统计上没有明显的年际变化趋势.在研究时段内,年平均NEP约为0.07 Gt C/a,表明中国陆地生态系统在气候与大气CO2浓度变化的条件下吸收了碳,为碳汇,总的吸收量为1.22 Gt C,约占全球碳吸收总量的10%,与同期内美国由大气CO2和气候变化所产生的碳吸收量大致相当.尽管由于较高的年际变率,NEP在统计上没有明显的变化趋势,但NPP的增长率低于HR的增长率,说明在研究时段内,中国陆地生态系统的吸碳能力由于气候变化降低了.全国大多数地区年平均NEP接近零,明显的NEP正值区(即碳汇)出现在东北平原、西藏东南部和黄淮平原等地区,而大小兴安岭、黄土高原和云贵高原等地区NEP为负值(即碳源).研究认为,1981～1998年期间中国气候温暖、干旱,因此估算的NEP可能低于其他时段.如果气候进入一个比较湿润的时期,碳吸收量可显著增加,但若当前干旱和暖化趋势以此为继,中国的NEP可能会变成一个负值.     

Carbon Dioxide Effects on Carbohydrate Status and Partitioning in Rice

The atmospheric carbon dioxide (CO₂) concentration has beenrising and is predicted to reach double the present concentrationsometime during the next century. The objective of this investigationwas to determine the long-term effects of different CO₂ concentrationson carbohydrate status and partitioning in rice (Oryza sativaL cv. IR-30). Rice plants were grown season-long in outdoor,naturally sunlit, environmentally controlled growth chamberswith CO₂ concentrations of 160, 250, 330, 500, 660, and 900µmolCO₂ mol¹ air. In leaf blades, the priority between the partitioningof carbon into storage carbohydrates or into export changedwith developmental stage and CO₂ concentration. During vegetativegrowth, leaf sucrose and starch concentrations increased withincreasing CO₂ concentration but tended to level off above 500µmolmol^–1 CO₂. Similarly, photosynthesis also increased withCO2 concentrations up to 500µmol mol^–1 and thenreached a plateau at higher concentrations. The ratio of starchto sucrose concentration was positively correlated with theCO₂ concentration. At maturity, increasing CO₂ concentrationresulted in an increase in total non-structural carbohydrate(TNC) concentration in leaf blades, leaf sheaths and culms.Carbohydrates that were stored in vegetative plant parts beforeheading made a smaller contribution to grain dry weight at CO₂concentrations below 330µmol mol^–1 than for treatmentsat concentrations above ambient Increasing CO₂ concentrationhad no effect on the carbohydrate concentration in the grainat maturity Key words: CO₂ enrichment, starch, sucrose     

Inorganic Nitrogen Losses from a Forested Ecosystem in Responseto Physical,Chemical, Biotic,and Climatic Perturbations

Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems. Two primary theories predicting long-term (decadal) changes in nitrate loss rates (N saturation theory and the nutrient retention hypothesis) both predict increasing dissolved inorganic nitrogen (DIN) losses for watershed 6 (W6), the biogeochemical reference watershed at the Hubbard Brook Experimental Forest (HBEF). Measured values, however, have declined substantially since measurements began in the mid-1960s. Are these theories wrong, or are there other important controls on DIN losses at the annual to decadal time scale that have obscured the tendency toward higher losses over time? We tested the individual and combined effects of several forms of disturbance on DIN loss rates from northern hardwood forests by comparing predictions from a relatively simple model of forest carbon, nitrogen, and water dynamics (PnET-CN) with the long-term record of annual DIN loss from W6 at HBEF. Perturbations tested include interannual climate variation, changes in atmospheric chemistry (CO₂, O₃, N deposition), and physical and biotic disturbances (two harvests, a hurricane salvage, and a defoliation event). No single disturbance caused changes in DIN losses to mimic measured values. Only when run with all of the disturbances combined did the model-predicted pattern of interannual change in DIN loss approach the measured record. Single-disturbance simulations allow an estimation of the role of each in the total pattern of DIN loss. We conclude that DIN losses from W6 were elevated in the 1960s by a combination of recovery from extreme drought and a significant defoliation event. N deposition alone, in the absence of other disturbances, would have increased DIN losses by 0.35 g N m^?2y^?1. These findings indicate that predictions of DIN losses must take into account the full spectrum of disturbance events and changes in environmental conditions impacting the systems examined.     

土地利用变化对草原生态系统土壤碳贮量的影响

本文概述了长期非持续性土地利用（草原开垦和过度放牧）对草原生态系统土壤碳贮量影响的有关研究成果和进展,指出了今后这方面研究的重点内容和对策。     

Effects of Carbon Dioxide Enrichment and Nitrogen Addition on Inorganic Carbon Leaching in Subtropical Model Forest Ecosystems

Soil mineral weathering may serve as a sink for atmospheric carbon dioxide (CO₂). Increased weathering of soil minerals induced by elevated CO₂ concentration has been reported previously in temperate areas. However, this has not been well documented for the tropics and subtropics. We used model forest ecosystems in open-top chambers to study the effects of CO₂ enrichment alone and together with nitrogen (N) addition on inorganic carbon (C) losses in the leachates. Three years of exposure to an atmospheric CO₂ concentration of 700 ppm resulted in increased annual inorganic C export through leaching below the 70 cm soil profile. Compared to the control without any CO₂ and N treatments, net biocarbonate C (HCO₃ ⁻-C) loss increased by 42%, 74%, and 81% in the high CO₂ concentration treatment in 2006, 2007, and 2008, respectively. Increased inorganic C export following the exposure to the elevated CO₂ was related to both increased inorganic C concentrations in the leaching water and the greater amount of leaching water. Net annual inorganic C (HCO₃ ⁻-C and carbonate C: CO₃ ²⁻-C) loss via the leaching water in the high CO₂ concentration chambers reached 48.0, 49.5, and 114.0 kg ha⁻¹ y⁻¹ in 2006, 2007, and 2008, respectively, compared with 33.8, 28.4, and 62.8 kg ha⁻¹ y⁻¹ in the control chambers in the corresponding years. The N addition showed a negative effect on the mineral weathering. The decreased inorganic C concentration in the leaching water and the decreased leaching water amount induced by the high N treatment were the results of the adverse effect. Our results suggest that tropical forest soil systems may be able to compensate for a small part of the atmospheric CO₂ increase through the accelerated processing of CO₂ into HCO₃ ⁻-C during soil mineral weathering, which might be transported in part into ground water or oceans on geological timescales.     

Near Isometric Biomass Partitioning in Forest Ecosystems of China

Based on the isometric hypothesis, belowground plant biomass (M_B) should scale isometrically with aboveground biomass (M_A) and the scaling exponent should not vary with environmental factors. We tested this hypothesis using a large forest biomass database collected in China. Allometric scaling functions relating M_B and M_A were developed for the entire database and for different groups based on tree age, diameter at breast height, height, latitude, longitude or elevation. To investigate whether the scaling exponent is independent of these biotic and abiotic factors, we analyzed the relationship between the scaling exponent and these factors. Overall M_B was significantly related to M_A with a scaling exponent of 0.964. The scaling exponent of the allometric function did not vary with tree age, density, latitude, or longitude, but varied with diameter at breast height, height, and elevation. The mean of the scaling exponent over all groups was 0.986. Among 57 scaling relationships developed, 26 of the scaling exponents were not significantly different from 1. Our results generally support the isometric hypothesis. M_B scaled near isometrically with M_A and the scaling exponent did not vary with tree age, density, latitude, or longitude, but increased with tree size and elevation. While fitting a single allometric scaling relationship may be adequate, the estimation of M_B from M_A could be improved with size-specific scaling relationships.     

森林生态系统生物能流的瞬态分析

森林生态系统生物能流的瞬态分析欧阳兵（中国科学院大气物理所大气数值模拟国家重点实验室,北京１０００８０）Ｔｒａｎｓｉｅｎｔ－ＳｔａｔｅＡｎａｌｙｓｉｓｏｎＢｉｏｔｉｃＥｎｅｒｇｙＦｌｏｗｉｎＦｏｒｅｓｔＥｃｏｓｙｓｔｅｍ．￥ＯｕｙａｎｇＢｉｎｇ（ＬＡ...     

Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems

Dissolved organic matter (DOM) contains molecules that absorb light at various wavelengths. This chromophoric DOM (CDOM) influences the transmission of both visible and ultraviolet energy through water. The absorption of light by CDOM often causes structural changes that reduce its capacity to further absorb light, a process termed ‘photobleaching‘. A model was designed to assess photobleaching through the entire water column of lake ecosystems. The model uses lake morphometry and dissolved organic carbon (DOC) concentration in conjunction with a defined solar spectrum and experimentally measured photobleaching rates to compute the total water columm photobleaching. The model was initially applied to a theoretical ‘average‘ lake using solar spectra for both the north (N) and south (S) temperate western hemispheres and variable DOC from 0.3 to 30 mg L⁻¹. The consequences of varying waveband-specific photobleaching coefficients and lake morphometry were explored in a second set of simulations. Finally, the model was also applied to four temperate northern lakes for which we had prior measurements of CDOM photobleaching rates. The model demonstrates that all three wavebands of solar radiation (UVB, UVA, and PAR) contribute significantly to total water column photobleaching, with UVA being most important. The relative contributions of the three wavebands were invariant for DOC more than 3 mg L⁻¹. Total water column photobleaching at 440 nm was three to five times faster under the UV-enriched solar spectrum of the southern hemisphere. Increasing the lake’s mean depth (from 0.37 to 9.39 m) resulted in five- or 15-fold slower rates of total water column photobleaching for DOC concentrations of 1 or 10 mg L⁻¹, respectively. Varying the waveband-specific photobleaching coefficients by 10-fold resulted in a similar change in total water column photobleaching rates. Applying the model to four specific lakes revealed that photobleaching for the entire water column would reduce CDOM light absorption by 50% in 18–44 days under summer conditions. Received 17 November 1998; accepted 27 June 2000.     

Net Ecosystem Production and Carbon Greenhouse Gas Fluxes in Three Prairie Wetlands

In central North America, prairie wetlands provide many important ecosystem services including attenuating floods, improving water quality, and supporting biodiversity. However, over half of these wetlands have been drained for agriculture. Relatively little is known about the functioning of these ecosystems either in their natural state or restored after drainage. We characterized net ecosystem production and carbon greenhouse gas (GHG) fluxes (carbon dioxide [CO₂] and methane) in the open-water zones of three prairie wetlands over two ice-free seasons. These wetlands included a natural site and sites restored 3 and 14 years prior to study (hereafter “recently restored” and “older restored”). We also assessed how two techniques for estimating metabolic status, the diel oxygen method (used to measure NEP) and net CO₂ fluxes, compared. The diel oxygen method suggested that the restored wetlands were net heterotrophic across years, whereas the natural wetland was net heterotrophic in 1 year and net autotrophic in the other. Similar conclusions arose from quantifying net CO₂ fluxes, although this technique proved to be relatively insensitive for understanding metabolic status at a daily temporal scale owing to the influence of geochemical processes on CO₂ concentrations. GHG efflux was greatest from the recently restored wetland, followed by the older restored and natural wetlands. Overall, GHG flux rates were high and variable compared with other inland aquatic ecosystems. Although restoration may progressively return wetland functioning to near-natural states, our results highlight the necessity of preventing wetland drainage in the first place to preserve ecosystem functions and services.     

Modeling Net Ecosystem Carbon Exchange of Alpine Grasslands with a Satellite-Driven Model

Estimate of net ecosystem carbon exchange (NEE) between the atmosphere and terrestrial ecosystems, the balance of gross primary productivity (GPP) and ecosystem respiration (Reco) has significant importance for studying the regional and global carbon cycles. Using models driven by satellite data and climatic data is a promising approach to estimate NEE at regional scales. For this purpose, we proposed a semi-empirical model to estimate NEE in this study. In our model, the component GPP was estimated with a light response curve of a rectangular hyperbola. The component Reco was estimated with an exponential function of soil temperature. To test the feasibility of applying our model at regional scales, the temporal variations in the model parameters derived from NEE observations in an alpine grassland ecosystem on Tibetan Plateau were investigated. The results indicated that all the inverted parameters exhibit apparent seasonality, which is in accordance with air temperature and canopy phenology. In addition, all the parameters have significant correlations with the remote sensed vegetation indexes or environment temperature. With parameters estimated with these correlations, the model illustrated fair accuracy both in the validation years and at another alpine grassland ecosystem on Tibetan Plateau. Our results also indicated that the model prediction was less accurate in drought years, implying that soil moisture is an important factor affecting the model performance. Incorporating soil water content into the model would be a critical step for the improvement of the model.     

Carbon Partitioning in Mature Leaves of Pepper: Effects of Daylength

Grange, R. 1. 1985. Carbon partitioning in mature leaves ofpepper: effects of daylength.—J. exp. Bot. 36: 1749–1759. The partitioning of recently fixed carbon has been examinedin mature pepper leaves grown in 6, 10 or 14 h photoperiodsat different irradiances chosen to give similar radiation integralsand in a 6 h photoperiod at the lowest of these irradiances.The partitioning of carbon into export, starch, sugars and respirationwas followed over the photopenod and the subsequent night ina mature leaf. The maximum export rate during the day (approximately 18 µgC cm^–2 leaf h^–1) was not significantly differentamong the treatments. Net photosynthesis rate was directly relatedto irradiance; the proportion of net photosynthesis exportedduring the day was 33% in 6-h days and 57% in 14-h days. Leafstarch accumulation (as a proportion of net photosynthesis rate)increased slightly when plants were grown in 6-h days. The remobilization of starch and sugars at night allowed exportrates to remain similar over 24 h when plants were grown in10-h or 14-h photoperiods. Leaves grown in 6-h days showed nosignificant changes in export rate during the first few hoursof night but exhausted their starch reserves during the nightand export rates declined. Sucrose and hexose levels decreased at the onset of darkness,but did not fall below 40 µg cm^–2 in plants grownin 10-h or 14-h photoperiods; when this level was reached after3–4 h of darkness, starch breakdown began. In leaves grownin both 6-h treatments, sucrose levels fell below 40 µgcm^–2 when starch reserves were depleted during the nightand the export rate decreased concurrently. The results are discussed in relation to the control of exportand starch metabolism in the leaf. Key words: Pepper, partitioning, daylength     

Diurnal and Seasonal Variations in Carbon Dioxide Exchange in Ecosystems in the Zhangye Oasis Area,Northwest China

Quantifying carbon dioxide exchange and understanding the response of key environmental factors in various ecosystems are critical to understanding regional carbon budgets and ecosystem behaviors. For this study, CO₂ fluxes were measured in a variety of ecosystems with an eddy covariance observation matrix between June 2012 and September 2012 in the Zhangye oasis area of Northwest China. The results show distinct diurnal variations in the CO₂ fluxes in vegetable field, orchard, wetland, and maize cropland. Diurnal variations of CO₂ fluxes were not obvious, and their values approached zero in the sandy desert, desert steppe, and Gobi ecosystems. Additionally, daily variations in the Gross Primary Production (GPP), Ecosystem Respiration (R_eco) and Net Ecosystem Exchange (NEE) were not obvious in the sandy desert, desert steppe, and Gobi ecosystems. In contrast, the distributions of the GPP, R_eco, and NEE show significant daily variations, that are closely related to the development of vegetation in the maize, wetland, orchard, and vegetable field ecosystems. All of the ecosystems are characterized by their carbon absorption during the observation period. The ability to absorb CO₂ differed significantly among the tested ecosystems. We also used the Michaelis-Menten equation and exponential curve fitting methods to analyze the impact of Photosynthetically Active Radiation (PAR) on the daytime CO₂ flux and impact of air temperature on R_eco at night. The results show that PAR is the dominant factor in controlling photosynthesis with limited solar radiation, and daytime CO₂ assimilation increases rapidly with PAR. Additionally, the carbon assimilation rate was found to increase slowly with high solar radiation. The light response parameters changed with each growth stage for all of the vegetation types, and higher light response values were observed during months or stages when the plants grew quickly. Light saturation points are different for different species. Nighttime R_eco increases exponentially with air temperature. High Q₁₀ values were observed when the vegetation coverage was relatively low, and low Q₁₀ values occurred when the vegetables grew vigorously.     

Different Effects of Spring and Summer Droughts on Ecosystem Carbon and Water Exchanges in a Semiarid Shrubland Ecosystem in Northwest China

Ecosystems - Extensive revegetation and conservation programs have been initiated in semiarid desert areas in northern China for over 10 years. However, our knowledge on how drought...