Greenhouse gas flux in a temperate grassland as affected by landform and disturbance

Much of the remaining native rangelands in Canada are topographically complex. The flux of greenhouse gases (GHGs) in rangelands of hummocky terrain has not been adequately studied, leaving a gap in the national GHG sources and sinks budget. The objectives of this study were to determine the effects of topography and mowing on carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) flux and to correlate these fluxes to abiotic and biotic factors. GHG flux was studied on six landform elements, including north-facing concave and, south-facing concave and convex, upland and depressions, in the Northern Mixedgrass Prairie of Canada over 2 years with mowing being imposed in early spring. GHG fluxes showed strong temporal variations, ranging from 3.0 to 40.4 kg CO₂–C ha^?1 d^?1, 0.1 to 2.6 g N₂O–N ha^?1 d^?1, and ?0.25 to ?0.01 g CH₄–C ha^?1 d^?1. GHG fluxes responded to changes in soil water and soil temperature across the landscape. The largest production of CO₂ was recorded in depression mainly due to its more favourable soil water conditions. Mowing enhanced CO₂ flux more than CH₄ and N₂O fluxes. Dominant plant species varied among the six landform elements, but using plant community type as the direct indicator for GHG emissions in grasslands may not always be reliable when precipitation is low. The net emissions of GHGs from Saskatchewan rangeland was relatively low, but the potential to increase emissions through changes in land management could be high. Our results suggest that in the Mixedgrass Prairie, best management practices for maintaining grassland health such as moderate grazing may also reduce GHG emissions.     

On the challenges of modeling the net radiative forcing of wetlands: reconsidering Mitsch et al. 2013

Wetlands play a role in regulating global climate by removing carbon dioxide (CO₂) from the atmosphere and sequestering it as soil carbon, and by emitting methane (CH₄) and nitrous oxide (N₂O) to the atmosphere. In a recent article in this journal (Mitsch et al. Landscape Ecol 28:583–597, 2013), CO₂ sequestration and CH₄ emissions were modeled for several freshwater wetlands that vary in vegetation type, climate, and hydrology. The authors of that study made significant errors that caused them to underestimate the importance of wetland CH₄ emissions on climate dynamics. Here, I reanalyze the Mitsch et al. dataset and show that all of their wetlands had an initial warming effect but eventually caused negative net radiative forcing within ~60–14,000 years, depending on the ratio of CO₂ sequestration to CH₄ emissions. The addition of a N₂O component to the model suggested that typical wetland N₂O emission rates would contribute only a minor burden to wetland radiative forcing, although specific application of this three-gas model is limited by the paucity of sites where CO₂ sequestration, CH₄ emission, and N₂O exchange rates have all been measured. Across the landscape, many natural wetlands may already cause negative net radiative forcing when integrated over their lifetime. However, caution should be applied when using carbon sequestration as a rationale for designing wetland construction and restoration projects since freshwater wetlands may have a net positive (warming) effect on climate for decades to centuries or longer.     

A strain of Bacillus subtilis stimulates sunflower growth (Helianthus annuus L.) temporarily

Preliminary studies showed that a Bacillus subtilis strain stimulates plant growth. We investigated how inoculating seeds of a sunflower cultivar (Helianthus annuus L.) with this strain stimulated plant growth, soil properties and emissions of greenhouse gasses, i.e. carbon dioxide (CO₂) and nitrous oxide (N₂O), when cultivated in a greenhouse. Unfertilized sunflowers or fertilized with urea served as controls. After one month, root length and fresh and dry root weight of the sunflower was significantly higher in the bacteria amended plant than in the urea and unfertilized plants. However, at harvest, no positive effect was observed. The number of seeds per plant and seed weight was not significantly different between the treatments, but total plant N was significantly higher in urea-amended plants than in unfertilized plants. The CO₂ production rate was not affected by treatment, but the N₂O emission rate was significantly higher in soil amended with urea plus bacteria soil compared to the unfertilized treatments. It was found that the B. subtilis strain used in this study had a positive, but only temporarily effect on growth of the sunflower cultivar used.     

Weather effect on thermal and energy performance of an extensive tropical green roof

This study investigated the weather effect on thermal performance of a retrofitted extensive green roof on a railway station in humid-subtropical Hong Kong. Absolute and relative (reduction magnitude) ambient and surface temperatures recorded for two years were compared amongst antecedent bare roof, green roof, and control bare roof. The impacts of solar radiation, relative humidity, soil moisture and wind speed were explored. The holistic green-roof effect reduced daily maximum tile surface temperature by 5.2 °C and air temperature at 10 cm height by 0.7 °C, with no significant effect at 160 cm. Green-roof passive cooling was enhanced by high solar radiation and low relative humidity typical of sunny summer days. High soil moisture supplemented by irrigation lowered air and vegetation surface temperature, and dampened diurnal temperature fluctuations. High wind speed increased evapotranspiration cooling of green roof, but concurrently cooled bare roof. Heat flux through green roof was also weather-dependent, with less heat gain and more heat loss on sunny days, but notable decline in both attributes on cloudy days. On rainy days, green roof assumed the energy conservation role with slight increase instead of reduction in cooling load. Daily cooling load was 0.9 kWh m^?2 and 0.57 kWh m^?2, respectively for sunny and cloudy summer days, with negligible effect on rainy days. The 484 m² green roof brought potential air-conditioning energy saving of 2.80 × 10⁴ kWh each summer, equivalent to electricity tariff saving of HK$2.56 × 10⁴ and upstream avoidance of CO₂ emission of 27.02 t at the power plant. The long-term environmental and energy benefits could justify the cost of green roof installation on public buildings.     

Growth responses of some greenhouse plants to environment. I. Experimental techniques

A system for continuous measurement of net photosynthesis of small stands of greenhouse plants in an almost air-tight cabinet is described. The accuracy of control of the CO₂ concentration in the cabinet is within ± 1% of the desired value. The control of air temperature is within ± 0.2°C and the relative humidity within ± 2% from the adjusted value. The amount of CO₂ used in photosynthesis is measured by the number of injections of controlled volumes of CO₂ in a specific time interval.A system for soil temperature regulation is described. In the range from 5 to 35°C, the accuracy of control of the soil temperature is usually within ± 0.5°C from the desired value. At high solar irradiance, however, the temperature may rise about 2°C at the lower, and somewhat less at the higher, soil temperatures.Results from preliminary experiments with roses and chrysanthemums are presented.     

Effects of air temperature,soil temperature and soil moisture on growth and development of tomato itself and grafted on its own and egg-plant rootstock

The problem was studied whether tomatoes, grown in a hot and arid climate, benefit from grafting on egg-plant, which is highly efficient in water uptake. Growth and development of tomato (T), tomato grafted on its own rootstock ($\text{T}\text{T}$) and tomato grafted on egg-plant rootstock ($\text{T}\text{E}$) were compared at air temperatures of 28°C during the day and 18°C during the night ($\text{28}\text{18}$) and at 28°C constantly ($\text{28}\text{28}$), at soil temperatures of 14, 21 and 28°C with the following soil moisture regimes: wet (W₁), medium (W₂) and dry (W₃).At $\text{28}\text{18}$ and $\text{28}\text{28}$ water consumption was about equal, but the transpiration ratio at $\text{28}\text{28}$ was twice as high as that at $\text{28}\text{18}$. The latter conditions gave a much stronger plant with more fruits. At a soil temperature of 14°C water use was strongly reduced. The transpiration ratio increased with the soil temperature. Differences in plant type were small. At the highest soil temperature of 28°C fruit growth was strongly reduced. At lower soil moisture levels less water was used and the transpiration was lower. Plant type was correlated herewith.Vegetative growth of $\text{T}\text{T}$ was weaker than of T, but generative growth was stimulated. The strong E rootstock stimulated vegetative growth at high air and soil temperature, but fruit growth was very poor under these conditions; at a low soil temperature of 14°C vegetative growth was also reduced.The hope that the E rootstock would be beneficial for fruit growth at high temperatures was not fulfilled.An additional experiment in a growth-room at 23°C showed that under conditions of moisture stress there was no difference in water potential between leaves of $\text{T}\text{T}$ and $\text{T}\text{E}$.     

Rooftop temperature reduction from unirrigated modular green roofs in south-central Texas

Modular green roofs were investigated to better understand surface and membrane level temperature expectations of unirrigated green roofs during hot summer conditions in south-central Texas. We used three succulent monocultures, Sedum kamtschaticum, Delosperma cooperi, Talinum calycinum syn. Phemeranthus calycinus and one unplanted control module, each replicated 3 times. Media surface and below media temperatures were monitored, as well as soil water content and general weather conditions (RH, air temperature). Temperatures at the surface and below the media surface were compared with temperatures of a standard roof surface. We found that diurnal surface temperature reductions were very stable throughout the summer. Much larger temperature reductions were achieved below the modules than at the soil surface. Temperature reductions at the soil surface were predominantly driven by soil volumetric water content (VWC) and, to a lesser degree, air temperature while species and percent cover had small modifying effects through interactions with VWC and air temperature. Temperature reductions below the modules were driven by surface soil temperature, while increasing VWC led to a small decrease in temperature reductions at the membrane level. Mean daily temperature reductions achieved were 18.0 °C at the soil surface and 27.5 °C below the module, thus demonstrating that unirrigated, succulent-based green roofs can provide significant rooftop temperature reductions during hot, dry summer conditions.     

不同生育期黄瓜土壤热通量特征分析

以“博耐-13号”黄瓜为试材,运用土壤热通量板及常规气象数据观测,对不同生育期黄瓜土壤热通量进行了研究.结果表明:在黄瓜生长初期,土壤相对含水量50％～60％(SW1)和土壤相对含水量75％～90％(SW2)的土壤热通量变化是同升同降的同向单峰曲线变化,变化较一致,其土壤热通量变化均为放热-吸热-放热的过程;在黄瓜生长中期和末期,SW1和SW2的土壤热通量变化是相反,均呈异向近似单峰变化.SW1的土壤热通量变化为吸热-放热-吸热的过程,而SW2为放热-吸热-放热的过程.SW1土壤相对含水量50％～60％的黄瓜土壤热通量与X1二氧化碳浓度呈正相关,与X2光合有效辐射、X6空气相对湿度、X7饱和水气压差呈负相关;SW2土壤相对含水量75％～90％的黄瓜土壤热通量与X2光合有效辐射、X8水气压呈现正相关;与X1二氧化碳浓度、X13地下25 cm处土壤温度呈负相关.在黄瓜生长发育时期,二氧化碳浓度、光合有效辐射这2个气象因子的大小均直接影响黄瓜植株土壤热通量.     

Effects of water deficit on olive trees cv. Chemlali under field conditions in arid region in Tunisia

Water scarcity in the Mediterranean basin in addition to the extension of irrigated lands is one of the main factors limiting agricultural development. The need for supplementary irrigation of the Chemlali olive cultivar (Olea europaea L.) during summer and autumn periods was investigated. Leaf water content, gas exchange parameters, fruit development and yield in rain-fed and in irrigated plants have been monitored in 12-year-old olive trees grown under environmental conditions in semi arid regions characterized by high temperatures and high light intensity. Trees were subjected to three irrigation treatments, T0, T1 and T2 corresponding respectively to 0, 33 and 66% of crop evapotranspiration (ET_c) by a drip irrigation system. The water deficit during the summer (from June to August) led to the decrease of soil moisture, leaf water content and gas exchange parameters. Irrigated trees showed the same slow activity in the three summer months as the rain-fed trees. For all treatments, net CO₂ assimilation, stomatal conductance and transpiration rates were markedly decreased by environmental conditions (high air temperature and high light intensity) during the summer period. At the partial active growth phase of the Chemlali olive cultivar (September–November), a significant re-increase in all parameters was observed. However, net photosynthesis and stomatal conductance of control plants (T0) were, respectively, 57 and 40% lower than those of plants conducted under milder water contribution (T1). The decrease of physiological activity in irrigated plants during hot and dry (summer time) and cold (winter) seasons was a clear evidence that water supply during such periods will be without a great benefit for photosynthetic activity, and thus growth, if applied under critical conditions inducing the rest phase of the plant. The non-statistically significant slight differences as well in photosynthetic performances activities (Pn, Gs and E rates), as in olive production between the two irrigated treatments will not cover the expenses of water loss when applying irrigation at 66% of ET_c especially in arid region characterized by scant and irregular rainfall. On the light of these results, we can conclude that the irrigation of this species during the vegetative growth phase (in spring and autumn), and stopping it during the olive rest phase (in summer and winter) could be recommended at least under the experimental conditions of this study; and that the contribution of 600 mm of water per year (33% of ET_c) can respond to the needs of the Chemlali olive cultivar in a semi arid region without impairing photosynthetic activity and olive production.     

Net CO2 storage in mediterranean olive and peach orchards

Agricultural practices can play an important role in atmospheric CO₂ emission and fixation. In this study, we present results on carbon fluxes in the biomass of two typical Mediterranean orchards indicating that proper canopy management coupled with other agricultural techniques could increase the absorption of atmospheric CO₂ and its storage. We also discuss the potential environmental contribution of the orchards to enhancement of both soil and air quality. Trials were carried out in southern Italy on olive (Olea europaea L.) and peach orchards (Prunus persica L.) at different age and plant densities. At the end of each vegetative season, values of fixed atmospheric CO₂ were calculated by measuring dry matter accumulation and partitioning in the different plant organs. In the early years, sequestered CO₂ was primarily distributed in the permanent structures and in the root system while in mature orchards the fixed CO₂ was distributed in leaves, pruning materials and fruit. Significant differences in amounts of fixed CO₂ were observed in peach orchards cultivated using different planting and training strategies. The results underline the importance of training system, plant density and cultivation techniques in the absorption of atmospheric CO₂ and its storage as organic matter in the soil.     

Measuring the effect of plant-community composition on carbon fixation on green roofs

Green roofs provide many ecosystem services such as regulation of building temperatures, reducing urban heat-island effects and draining rainwater. In addition, they are expected to reduce the high levels of CO₂ concentrations in big cities. Previous CO₂ fixation studies on green roofs were done by taking long-time-period samples using expensive equipment and with limited replication. To plan green roofs for optimal CO₂ reduction, a simple method to quantify CO₂ fixation rate in relation to plant species-composition is required. The method we tested is direct measurement of CO₂ concentrations with a portable air-quality meter, which allows a large number of samples. Here we focus on differences in the photosynthetic effect between plots containing the local Mediterranean succulent, Sedum sediforme and plots containing various annuals. In a factorial design (presence or absence of Sedum crossed with presence or absence of annuals), we tested the effect of sedum and annual treatments on CO₂ concentrations. To compare our results with a commonly used method, and to evaluate the role of the different species, we examined the photosynthetic activity at the single plant level under these treatments by using a portable gas-exchange measuring system. We found that our method can detect the effect of different green roof plots and can be used as a simple and reliable tool for green-roof planers. We found that annuals reduced CO₂ concentrations, but only in the absence of Sedum. Sedum alone had no effect on CO₂ concentrations. This emphasizes the importance of integrating plots with annual plants in Sedum-based green roofs.     

Growth responses of some greenhouse plants to environment. III. Design and function of a growth chamber prototype

Growth chambers for the study of effects of temperature, air humidity and CO₂-concentration on plant growth, with or without supplementary artificial light, are described. Each chamber has a volume of 1080 1. The mean airflow at plant level is 0.22 m s^?1. The temperature is controlled within ± 0.5°C in the range from 10°C lower to 20°C higher than the ambient temperature at low solar radiation. In direct sunshine, the temperature may rise 1°C at floor level and the gradient from the floor to the upper part of the chamber may be about 2°C.The relative humidity is generally controlled within ± 4%, in the range from 50 to 95%.The CO₂-concentration is controlled within ± 5% of the desired value. The number of air changes in the chambers may be controlled from 0 to 20 h^?1.Tests revealed no significant difference between the chambers with respect to fresh weight production of lettuce, rose or chrysanthemum. There was, however, a significant effect from the position within the chambers.     

Detecting different levels of drought stress in apple trees (Malus domestica Borkh.) with selected biochemical and physiological parameters

Rational irrigation scheduling based on sensing drought stress directly in plants is becoming more important due to increasing worldwide scarcity of fresh water supplies. In order to evaluate a set of potential biochemical and physiological stress indicators and select the best drought stress markers in apple trees, two experiments with potted trees and an experiment with intensive orchard grown apple trees ‘Elstar’ and ‘Jonagold Wilmuta’ were conducted in early summer in tree following years. Biochemical parameters: ascorbic acid, glutathione, tocopherols, chlorophylls, carotenoids, free amino acids, soluble carbohydrates, and physiological parameters already known as stress indicators in apple trees: predawn and midday leaf water potential, net photosynthesis (Pn), stomatal conductance (g_s), transpiration (Tr) and intercellular CO₂ concentration (Ci) were measured in leaves of apple trees subjected to different intensities of slowly progressing drought or no drought. Our study pointed out zeaxanthin and glutathione as the best drought stress markers in apple trees. Ascorbate and sorbitol appeared to be reliable indicators of moderate drought only. Responses of other tested biochemical parameters were not consistent enough to prove their role as drought stress markers in apple trees. Relative air humidity should be taken in consideration when physiological parameters g_s, Pn, Tr and Ci are used as drought stress markers in apple trees. Our study revealed that in situations where low relative air humidity affects g_s and with g_s connected physiological parameters, biochemical markers may be better tool for determination of drought stress intensities in apple trees.     

Fruit quality and respiration of ‘McIntosh’ apples in response to ethylene,very low oxygen and carbon dioxide storage atmospheres

Storage of ‘McIntosh’ Apples (Malus domestica Borkh.) in a controlled atmosphere (CA) with very low O₂ (1.5% CO₂ + 1.0% O₂, 2.8°C) retained greater fruit firmness and titratable acids during storage and during subsequent air storage than apples stored in conventional CA (5.0% CO₂ + 3.0% O₂, 2.8°C). The rate of firmness loss during subsequent 0°C air storage decreased with length of storage in CA. Storage of apples in very low O₂ for 40 or 80 days decreased the rate of firmness loss in subsequent 0°C air storage as compared to the rate of firmness loss in conventional CA fruit, but the rate of firmness loss in 0°C air storage subsequent to 160 or 320 days of conventional CA was significantly less than the loss in similar fruit stored in very low O₂ atmospheres.A modified atmosphere with 1.0% O₂ decreased the rate of C₂H₄ accumulation in storage, and fruit production of both C₂H₄ and CO₂ after storage opening in comparison with similar fruit in conventional CA. The accumulation of C₂H₄ in storage chambers was increased with increasing O₂ levels, but the rate of increase depended upon the CO₂ level. C₂H₄ storage accumulation was stimulated by the presence of CO₂ at 0.5% O₂, but was suppressed by CO₂ when 3.0% O₂ was maintained.Retention of fruit firmness and titratable acids in apples stored in 1.5% CO₂ + 1.0% O₂ were insensitive to very low (0.231 ml l^?1) or very high (2440 ml l^?1) C₂H₄ levels in storage. Scrubbing C₂H₄ (0.304 ml l^?1) from chambers held at 5.0% CO₂ + 3.0% O₂ resulted in significantly firmer fruit after storage, but this effect was not significant after shelf life of 7 days at 20°C.     

Photosynthesis of field-grown Arracacha (Arracacia xanthorriza Bancroft) cultivars in relation to root-yield

There has been limited research on measuring potential differences in leaf gas exchange of Arracacha (Peruvian parsnip, Arracacia xanthorriza Bancroft) cultivars, as affected by different environments, as well as its relation to storage root-yield. The present paper reports field measurements of leaf CO₂ assimilation rates (A) for five contrasting cultivars grown at two different high-altitude locations. Using a design of plots chosen at random with three repetitions, commercial root production was determined in the two locations at different altitude (1580 and 1930 m). Daily leaf gas exchange was repeatedly monitored with a portable open-mode infrared gas analyzer at different times in both locations during the growth cycle. Root-yield, leaf area and dry weight were measured. Significant differences in leaf photosynthetic rate and in specific leaf area (SLA) were observed among cultivars. Cultivars with high SLA, had high CO₂ assimilation. Mean (A_n) and total (A_tot) of CO₂ assimilation and SLA were significantly correlated with storage root-yield across cultivars and locations. The three cultivars with the greatest commercial root production also had the highest maximum values for A and the highest specific leaf area, indicating that these two parameters can be used to select for highly productive cultivars of A. xanthorriza.     

Modelling the effect of air vapour pressure deficit on leaf photosynthesis of greenhouse tomatoes: The importance of leaf conductance to CO2

Summary

The response of photosynthesis of leaves of greenhouse tomato plants to leaf position and vapour pressure deficit (VPD) was studied by modelling the effect of these on leaf conductance to CO₂. The study was carried out in Avignon (southern France) on well-irrigated plants during spring and summer seasons, with VPD ranging from 0.7 to 3.4 kPa at midday, while the 24 h mean ranged from 0.6 to 1.8 kPa. Net photosynthesis (Pn) was measured on single leaves placed at three levels defined by the leaf position, and under different CO₂ concentrations in the range of 200 to 1100 µmol mol^–1. A model for leaf photosynthesis was used to evaluate the leaf conductance to CO₂ transfer. The leaf conductance to CO₂ transfer was maximum for top level leaves, and decreased with leaf depth in the canopy. Leaf conductance at the upper level was reduced when air VPD exceeded a threshold value of 1 kPa.     

Measurement system of whole-canopy carbon dioxide exchange rates in grafted cucumber transplants in which scions were exposed to different water regimes using a semi-open multi-chamber

A continuous CO₂ measurement system was developed to monitor the CO₂ exchange rate of the whole canopy of grafted transplants using semi-open multiple chambers. Air heating or cooling and humidification inside a healing box were under control, if needed. To test the system, the gas exchange rate of the cucumber (Cucumis sativus L.) transplants grafted onto pumpkin (Cucurbita maxima cv. ‘New-Shintozwa’) was analysed. During the healing and acclimatisation of the grafted cucumber plants, the air temperature in the box remained constant at night but ranged above 1 °C of a set value under high humidity in daytime. The relative humidity was kept within the set point during the daytime, and it nearly reached 100% at night when not controlled. The cucumber seedlings were exposed to different water stresses before grafting, and the water potentials of each treatment were −0.579 (non-stressed), −0.814 (mildly water-stressed), and −0.870 MPa (strongly water-stressed) on grafting. At the water-stressed scions, leaf expansion was inhibited by 30.9–53.3% compared with the non-stressed scions. Therefore, the gas exchange rates of the strongly water-stressed scions based on the leaf area were decreased to 72.7% compared with the non-stressed scions. After grafting, the apparent photosynthesis of the transplants of all treatments was negative, with higher respiration in the strongly water-stressed scions during the initial period of healing. However, they turned to positive values and exceeded those of the non-stressed scions from three days after grafting. This result provides critical information that the water column is physiologically connected between the stock and scion within two days after grafting. As a result of water stress, the leaf area and dry weight of the transplants in the strongly water-stressed scions were inhibited by 67.5% and 83% compared with the non-stressed scions at the end of acclimatisation. In contrast, the relative growth rate and graft-take of the strongly water-stressed transplants were slightly increased, which suggests that the water stress prior to grafting alleviated the water demand of the scion. This system may provide useful information for diagnosis at the early stage by monitoring the whole canopy's photosynthesis over a long term.     

Integration of soil solarization and chemical sterilization with beneficial microorganisms for the control of white root rot and growth of nursery apple

White root rot (Dematophora necatrix (Mart.)) is a serious disease of apple (Malus domestica Borkh) in nurseries and orchards in India. In 2002 and 2003, field experiments were conducted to integrate soil solarization with native isolates of Azotobacter chrococcum and vesicular–arbuscular mycorrhizal fungi and observe its effect on the incidence of white root rot and growth of the saplings. Apple seeds coated with two native isolates of A. chrococcum (AZUHF₁ and AZUHF₂) were sown in plots inoculated with 4 native isolates of va-mycorrhiza, i.e. AMUHF₁ (Glomus fesiculatum), AMUHF₂ (Glomus macrocarpum), AMUHF₃ (Glomus mosseae) and AMUHF₄ (Gigaspora sp.) in 14 different combinations and these plots were solarized with transparent polyethylene mulch (25 μm thick) for 40 days in summer months. Soil solarization resulted around 9 °C higher temperature with average maximum temperature of 38–39 °C. Inoculation of saplings with AMUHF₁ isolate of va-mycorrhiza and AZUHF₁ isolate of A. chrococcum and then their planting in solarized soil was found most effective with no incidence of white root rot in comparison to 33.6–35.4% in control accompanied with 78–113% increase in shoot length and 81.6–84.3% increase in root length. Shoot and root length of the saplings was 9.6–10.6 and 9.2–16.0% higher, respectively, in solarized plots in comparison to sterilized plots.     

Plant productivity and nitrogen gas fluxes in a tallgrass prairie landscape

We explored relationships between plant productivity and annual fluxes of nitrogen (N₂) and nitrous oxide (N₂O) in a tallgrass prairie landscape in central Kansas. Our objective was to develop predictive relationships between these variables that could be used in conjunction with remote sensing information on plant productivity to produce large-area estimates of N gas fluxes. Our hypothesis was that there are inherent relationships between plant productivity and N gas fluxes in tallgrass prairie because both are controlled by water and N availability. The research was carried out as part of a multi-investigator project, the First ISLSCP Field Experiment (FIFE, ISLSCP = International Satellite Land Surface Climatology Program), directed toward the use of remote sensing to characterize land-atmosphere interactions. Fluxes of N₂ (denitrification) and N₂O were measured using soil core techniques. Estimates of annual flux were produced by temporal extrapolation of measured rates. Annual aboveground net primary productivity (ANPP) was estimated from measurements of the maximum standing crop of plant biomass. There were strong relationships between ANPP and N gas fluxes, and between a satellite remote sensing-based index of plant productivity (normalized difference vegetation index, NDVI) and gas fluxes. We used these relationships to convert images of NDVI into images of N gas fluxes for one 83 ha watershed and for the entire 15 by 15 km FIFE site. These images were used to compute mean landscape gas fluxes (0.62 g N m^-2 y^-1 for N₂, 0.66 g N m^-2 y^-1 for N₂O) and total N gas production for the two areas. Our flux and production values are useful for comparison with values produced by simulation models and site-specific studies, and for assessing the significance of N gas production to ecosystem and landscape scale processes related to nutrient cycling, water quality and atmospheric chemistry.     

Seasonal changes in CO2 assimilation in leaves of five major Greek olive cultivars

Leaf CO₂ assimilation rate, stomatal conductance (g_s), internal CO₂ concentration (C_i), chlorophyll (a + b) content, specific leaf weight (SLW) and stomatal density were measured during the season, under field conditions, for five major Greek olive cultivars, ‘Koroneiki’, ‘Megaritiki’, ‘Konservolia’, ‘Lianolia Kerkiras’, and ‘Kalamon’, with different morphological and agronomic characteristics and diverse genetic background. Measurements were taken from current-season and 1-year-old leaves, and from fruiting and vegetative shoots, throughout the season, from March to November in years 2001 and 2002. CO₂ assimilation rates showed a substantial seasonal variation, similar in all cultivars, with higher values during spring and autumn and lower values during summer and late autumn. Stomatal conductance (g_s) followed similar trends to leaf CO₂ assimilation rates, increasing from March to July, following by a decrease during August and increasing again in autumn. ‘Koroneiki’ had the highest leaf CO₂ assimilation rate and g_s values (21 μmol m⁻² s⁻¹ and 0.45 mol m⁻² s⁻¹, respectively) while ‘Lianolia Kerkiras’ and ‘Kalamon’ showed the lowest leaf CO₂ assimilation rate and g_s values (13–14 μmol m⁻² s⁻¹ and 0.22 mol m⁻² s⁻¹, respectively). One-year-old leaves had significantly higher leaf CO₂ assimilation rate than current-season leaves from April to June, for all cultivars. From August and then, leaf CO₂ assimilation rate in current-season leaves was higher than in 1-year-old leaves. There were no significant differences in leaf CO₂ assimilation rate between fruiting and vegetative shoots. Total chlorophyll (a + b) content increased with leaf age in current-season leaves. In 1-year-old leaves chlorophyll content increased in spring, then started to decrease and increased slightly again late in the season. Chlorophyll content was higher in 1-year-old leaves than in current-season leaves throughout the season. Total specific leaf weight (SLW) increased throughout the season for all cultivars. Stomatal density in lower leaf surface was lowest for ‘Koroneiki’ (399 mm⁻²) and highest for ‘Megaritiki’ (550 mm⁻²). Our results showed differences in leaf CO₂ assimilation rate among the five different olive cultivars, with a diverse genetic background, ranging from 12 to 21 μmol m⁻² s⁻¹. From the five cultivars examined, ‘Koroneiki’, a drought resistant cultivar, performed better and was able to maintain higher leaf CO₂ assimilation rate, even under high air vapor pressure deficit. All cultivars had a pronounced seasonal variation in leaf CO₂ assimilation rate, affected by date of the year, depending on ambient conditions. The high temperatures and high air vapor pressure deficit occurring during summer caused a reduction in leaf CO₂ assimilation rate in all cultivars. Leaf CO₂ assimilation rate was also affected by leaf age for all cultivars, with old leaves having significantly higher leaf CO₂ assimilation rate than young leaves early in the season.