Variations In CO2 Concentration Research Articles

Ginkgo leaf cuticle chemistry across changing pCO2 regimes

Cuticles have been a key part of palaeobotanical research since the mid-19th Century. Recently, cuticular research has moved beyond morphological traits to incorporate the chemical signature of modern and fossil cuticles, with the aim of using this as a taxonomic and classification tool. For this approach to work, cuticle chemistry would have to maintain a strong taxonomic signal, with a limited input from the ambient environment in which the plant grew. Here, we use attenuated total reflectance Fourier Transform infrared (ATR-FTIR) spectroscopy to analyse leaf cuticles from Ginkgo biloba plants grown in experimentally enhanced CO2 conditions, to test for the impact of changing CO2 regimes on cuticle chemistry. We find limited evidence for an impact of CO2 on the chemical signature of Ginkgo cuticles, with more pronounced differences demonstrated between the abaxial (lower leaf surface) and adaxial (upper leaf surface) cuticles. These findings support the use of chemotaxonomy for plant cuticular remains across geological timescales, and the concomitant large-scale variations in CO2 concentrations.

Atmospheric Water Demand Dominates Daily Variations in Water Use Efficiency in Alpine Meadows, Northeastern Tibetan Plateau

AbstractEcosystem water use efficiency (WUE) acts as an integrated functional indicator for understanding land‐atmosphere interactions. The temporal patterns in the daily variations of WUE and their underlying drivers during different seasons in alpine meadow ecosystems, which are particularly vulnerable to changing climate, still remain poorly understood in spite of increasing efforts. In this study, we investigated the potential divergence in the response of WUE to climatic and biological drivers during different seasons at two alpine meadow ecosystems in the northeastern Tibetan Plateau using continuous eddy‐covariance measurements of carbon and water fluxes between 2013 and 2015. We found that variations in CO2 concentration exert significantly positive effects on variations in WUE in spring, but not in summer and autumn. Notably, vapor pressure deficit (VPD) overrode other factors playing a dominant role in regulating daily variations in WUE during all seasons in these alpine meadow ecosystems. Variations in VPD explained 29.5 to 52.3% of the variance in WUE between different seasons. We further showed that carbon gain and water loss processes responded divergently to different drivers; higher VPD significantly increased ecosystem evapotranspiration; whereas, variations in soil moisture and leaf area index significantly and positively affected gross primary productivity. Our findings highlighted the increasing importance of atmospheric drought in shaping land‐atmosphere interactions in alpine meadow ecosystems, particularly in a warming climate.

VCSEL-Based Atmospheric Trace Gas Sensor Using First Harmonic Detection

This paper presents a measurement system based on the first harmonic in tunable diode laser absorption spectroscopy using a vertical-cavity surface-emitting laser to measure the atmospheric CO2 and H2O concentrations. The developed system separates the residual amplitude modulation signal from the harmonics and then eliminates it. A digital signal processing is developed to autonomously infer the wavelength and light intensities of the laser. The gas concentrations are determined without extra calibration. The long-term measurements are taken to validate the precision and accuracy of the system. Based on the Allan variance analysis, the broad wavelength scanning enhances the measurement precision, and the first-harmonic detection can achieve about two times as high precision as the traditional second-harmonic detection. The field measurements implemented in early spring of 2018 in Munich were compared with the commercial nondispersive infrared (NDIR) sensor. The outcomes have revealed that our system has high accuracy for the gas concentration measurements and high consistency with the NDIR sensor. The diurnal variations of CO2 concentration have demonstrated that CO2 concentration in urban areas is affected by the biosphere and meteorological conditions and the daily anthropogenic activities. Furthermore, the air trajectory analysis based on the HYSPLIT model has found that the CO2 emission sources primarily come from the southeast of Munich. The developed system described in this paper has a great potential for in situ trace gas concentration measurements, the analysis on the polluted gas distributions, and the verification of the pollutant transport model in urban areas.

Sources and variability of CO2 in a prealpine stream gravel bar

AbstractGravel bars (GBs) contribute to carbon dioxide (CO2) emissions from stream corridors, with CO2 concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical conditions. We investigated CO2 concentrations and fluxes across a GB in a prealpine stream over three different discharge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant hydrological and biogeochemical processes contributing to CO2 dynamics. GB CO2 concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO2 evasion fluxes, measured CO2 concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO2 transport via streamwater downwelling contributed the largest fraction of the measured GB CO2 concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all discharge‐temperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO2 concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO2 inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO2 concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO2 concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO2 release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors.

A synergistic approach for the simultaneous decarbonisation of power and industry via bioenergy with carbon capture and storage (BECCS)

There is a need for a rapid and large scale decarbonisation to reduce CO2 emissions by 45% within 12 years. Thus, we propose a method that accelerates decarbonisation across multiple sectors via a synergistic approach with bioenergy with CCS (BECCS), which is able to remove 740 kgCO2 from air per MWh electricity generated. Industry is a hard-to-decarbonise sector which presents a unique set of challenges where, unlike the power sector, there are no obvious alternatives to CCS. One of these challenges is the significant variation of CO2 concentration, which directly influences CO2 capture costs, ranging from $10/tCO2 to over $170/tCO2 for high (95–99% CO2) and low CO2 concentration (4% CO2) applications, respectively. Re-purposing the existing coal-fired power plant fleet into BECCS displaces CO2 emissions from coal-use and enables a just transition, i.e., avoiding job loss, providing a supportive economic framework that does not rely on government subsidies. Negative emissions generated from capturing and storing atmospheric CO2 can be converted into negative emission credits (NECs) and auctioned to hard-to-decarbonise sectors, thus providing another revenue stream to the power plant. A levelised cost of electricity (LCOE) between $70 and $100 per MWh can be achieved through auctioning NECs at $90–$135 per tCO2. Offsetting the global industrial CO2 emissions of 9 GtCO2 would require 3000 BECCS plants under this framework. This approach could jumpstart industrial decarbonisation whilst giving this sector more time to develop new CCS technologies.

(Keynote) Electroreduction of CO2 on Pb and Bi Nanoneedles

The rise in atmospheric CO2 concentration since the beginning of the Industrial Age has raised concerns on its potential effect on global temperatures. While several factors such as natural climate changes, variations in solar activity, and volcanic eruptions may also contribute to observed variations in CO2 concentrations, it has been well established that anthropogenic emissions contribute to the annual emission of over 30 billion tonnes of CO2, only about half of which is recycled through natural pathways. CO2 conversion into value-added products is an avenue explored to mitigate environmental issues. Conversion methods appear promising, as they offer the potential to produce liquid and gaseous fuels for use either in the aeronautics industry, or as a chemical storage method for the intermittent energy produced by windmills and photovoltaic panels. Fuel synthesis may be achieved via either the production of a syngas mixture (H2 and CO) through the water gas shift reaction and subsequent conversion into hydrocarbon fuel through processes such as Fisher-Tropsch, or by direct electrochemical reduction of CO2. While all methods may result in similar conversion yields, depending on the catalyst used, direct electrochemical conversion of CO2 into value-added products is a low-temperature process, with the further advantage of requiring relatively simple equipment. Formic acid, or formate salts, are used in a variety of chemical processes such as electrowinning, leather tanning, and aircraft de-icing. Alternatively, formic acid and formate salts may be considered a hydrogen storage medium. Direct formic acid, and more recently, direct formate fuel cells, have been investigated in the literature as they demonstrate significant benefits over methanol fuel cells, including higher open circuit voltage and lower crossover. Among earth-abundant CO2 electrocatalysts, Sn, Pb and Bi are known to be highly selective for formate production, with faradic efficiency (FE) > 90%. However, several challenges need to be addressed for these materials to become viable alternatives for industrial applications based on an ERC process to become economically viable. In particular, issues related with large overpotential and low current densities need to be addressed, along with the long term stability of electrodes. In this study, we compared the activity and stability for CO2 electroreduction of high surface area metallic Bi and Pb nanoneedles (see Figure 1). In both cases, the materials films were prepared through a direct electrodeposition (potentiostatic method). Both types of films were extensively characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and x-ray diffraction (XRD), while electrochemical activities were characterized by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and potentiostatic measurements. Figure 1

Density and Viscosity Variations in an Amine Based Absorption Column

In this study, the variations of physicochemical properties like density and viscosity in CO2 loaded aqueous monoethanol amine were studied in a laboratory rig. Liquid samples were collected from the top and the bottom of the absorber column and temperature and liquid flow rate were recorded for further analysis. CO2 concentration in the solution was determined by using a precipitation and titration method. Density and viscosity were measured from the collected samples under the recorded column temperatures. The measured physicochemical properties were compared with correlations given by Weiland and Hartono. The study showed that both correlations could predict density and viscosity with acceptable deviations. Further, these correlations can be used in equipment design and process simulations. pH of the liquid was also measured to relate it with the CO2 concentration in the top and bottom of the absorber column. The variation of the measured pH is a possible measure of the variations of CO2 concentration.

Real-time multi-level CO2 concentration monitoring in vadose zone wells and the implication for detecting leakage events

Multi-level wells screened at different depths in the vadose zone were installed and used for CO2 and carbon isotope monitoring. Well CO2 time series data were collected along with subsurface and atmospheric parameters such as air pressure, temperature, wind speed, and moisture content. Our aim was to determine the natural factors affecting the variation of CO2 concentration and how the influence of these factors varies with time of day and seasons of the year. We were motivated to understand the cause and extent of CO2 natural fluctuations in vadose zone wells in order to separate natural variation from signals due to anthropogenic CO2 leaks anticipating future monitoring using these wells. Variations of seasonal mean and variance of CO2 concentrations at different depths seem to follow the diurnal trend of subsurface temperature changes that reflect the atmospheric temperature but with time delay and amplitude damping due to heat transport considerations. The temperature in the ground lags behind the change in the atmospheric temperature, thus, the deeper the depth, the longer the time delay and the smaller the amplitude of the change. Monitored seasonal variation as shown in Appendix A shows the temperature-dependent depth-dependent CO2 production in the soil zone indicating higher CO2 concentrations in the summer and fall seasons with high concentrations ranging between 10,990 and 51,600 ppm from spring to summer, and 40,100 and 17,760 ppm from fall to winter. As the temperature in the organic-rich topsoil layer changes from daytime to nighttime, the concentration of CO2 in the soils also changes dynamically in response to chemical and biological reactions. When a screened well is installed in the vadose zone the dynamic temporal and depth difference in CO2 production is further complicated by upward (out of the subsurface) or downward (into the subsurface) gas flow, which will amplify or attenuate the temporal and vertical biochemically produced differences. Nested wells screened at different depths in the vadose zone and wells fully screened through the vadose zone were used for comparison. In addition, experiments changing the well from open to surface air to sealed at the top were conducted. The flow rates of inhaled (downward) and exhaled (upward) gas were estimated based on multi-level monitoring data. Based on time-series monitoring data, we proposed a time-dependent conceptual model to explain the changes of CO2 concentration in wells. The conceptual model was tested through analytical model computations. This conceptual model of natural variation of CO2 will be helpful in utilizing the vadose zone well as a method for monitoring CO2 leakage from subsurface storage or anthropogenic CO2 -producing activities.

Plot-scale spatiotemporal variations of CO2 concentration and flux across water-air interfaces at aquaculture shrimp ponds in a subtropical estuary.

Human activities have increased anthropogenic CO2 emissions, which are believed to play important roles in global warming. The spatiotemporal variations of CO2 concentration and flux at fine spatial scales in aquaculture ponds remain unclear, particularly in China, the country with the largest aquaculture. In this study, the plot-scale spatiotemporal variations of water CO2 concentration and flux, both within and among ponds, were researched in shrimp ponds in Shanyutan Wetland, Min River Estuary, Southeast China. The average water CO2 concentration and flux across the water-air interface in the shrimp ponds over the shrimp farming period varied from 22.79 ± 0.54 to 186.66 ± 8.71μmolL-1 and from - 0.50 ± 0.04 to 2.87 ± 0.78molm-2day-1, respectively. There was no remarkable difference in CO2 concentration and flux within the ponds, but significantly spatiotemporal differences in CO2 flux were observed between shrimp ponds. Chlorophyll a, pH, salinity, air temperature, and morphometry were the important factors driving the spatiotemporal patterns of CO2 flux in the shrimp ponds. Our findings highlighted the importance and spatiotemporal variations of CO2 flux in the important coastal ecosystems.

Diurnal and Seasonal Variations of Carbon Dioxide (CO2) Concentration in Urban, Suburban, and Rural Areas around Tokyo

Site environments and instrumental characteristics of carbon dioxide (CO2) measurements operated by local governments in the Kanto Plain, the center of which is Tokyo, were summarized for this study. The observation sites were classified into environments of three types: urban, suburban, and woodland. Based on a few decades of accumulated hourly data, the diurnal and seasonal variations of CO2 concentrations were analyzed as a composite of anomalies from annual means recorded for each site. In urban areas, the highest concentrations appear before midnight in winter. The second peak corresponds to the morning rush hour and the strengthening of the inversion layer. Suburban areas can be characterized as having the highest concentration before dawn and the lowest concentration during the daytime in summer in association with the activation of respiration and photosynthesis of vegetation. In these areas, concentration peaks also appear during the morning rush hour. Woodland areas show background features, with the highest concentration in early spring, which are higher than the global background by about 5 ppmv.

Exploration of a long-term measurement approach for air change rate

Ventilation using outdoor air can have both favorable and unfavorable impacts on indoor air pollution. It also can be an important contributor to energy use in buildings. Outdoor ventilation air change rate (ACR), the rate at which outdoor air enters a building divided by its volume, is a temporally dynamic metric that can be used to characterize ventilation performance of buildings. Conventional measurement techniques for ACR have either complex or invasive experimental procedures, or present a temporal snapshot of ACR. In this study, we further developed and explored a novel signal processing approach to measure yearlong time-resolved ACR in a residence using the variations in indoor and outdoor CO2 concentrations. Results showed that ACR varied considerably over the year [geometric mean (GM) = 0.47/h, geometric standard deviation (GSD) = 3.44] and that the air change rates calculated from the signal processing approach were in good agreement (on average, within 13%) with those measured simultaneously from 15 hour-long decay periods. In addition, estimates of ACR were largely insensitive to the occupancy status of the building. This behavior may be because the indoor CO2 concentration variations introduced by changes in occupancy status were not large enough to impact long-term ACR values, but they may be sufficient to impact short-term ACR values. Moreover, we anticipate that cut-off frequency and filter order, two parameters needed for the signal processing approach, may be building-specific and can influence calculations of air change rates in a given building. The results suggest that this approach has promise for assessing time-varying ACR in buildings with time-resolved indoor and outdoor concentration measurements.

Analysis of the Seasonal Variation of CO2 Concentration in China Based on GOSAT Satellite Data

Carbon dioxide (CO2) is one of the major gases that contribute to the global warming. Therefore, studying the distribution of CO2 can help people understand the carbon cycle. Based on the GOSAT retrieved CO2 products, the temporal and spatial distribution and seasonal variation of CO2 concentration were analyzed from 2011 to 2015. CO2 concentration has obvious seasonal variation. It was low in summer, and was high in spring, and the annual increase was about 2 ppm. Nevertheless, the annual growth rate of CO2 concentration in summer was higher than that in spring, it was 0.5425% in summer and was 0.46% in spring. CO2 concentration was low in the northwest and was high in the southeast. The growth rate of CO2 was 2.8 ppm in the northwest and was 3.42 ppm in the southeast. More human’s activities made CO2 concentration higher in the southeast than that in other regions.

Characteristics of and Factors Affecting Atmospheric CO2 Concentration in Hangzhou

In situ measurement of CO2 concentration(volume fraction) was carried out in both urban and rural areas of Hangzhou from August 2015 to September 2016. The characteristics of CO2 concentration at the urban site were compared to those at the rural site, and the factors affecting CO2 concentration in Hangzhou were analyzed via wind direction, weekday-weekend difference in CO2 concentration, and evolution of CO2 concentration during the G20 summit. The results revealed that the diurnal variation of CO2 concentration in both the urban and rural areas presented a single peak curve most of the time, which resulted from the daily evolution of plant photosynthesis/respiration and atmospheric transport conditions. The diurnal variation of the difference in CO2 concentration observed at the urban and rural sites showed a bimodal peak curve, because anthropogenic emissions played a more important role. The diurnal amplitude of CO2 concentration in rural area was higher than that in urban area in spring and summer, but lower in autumn. The seasonal variation of CO2 concentration in both the urban and rural areas showed the same trend, with higher values appearing in winter and spring and lower values in summer. The difference in CO2 concentration observed at the urban and rural sites reached its highest level in winter, and dropped to its lowest in summer. The wind direction induction of high CO2 concentration was consistent with the location of the surrounding urban areas. A weekday-weekend difference in CO2 concentration was observed in Hangzhou, especially in urban area, as traffic emissions had an impact on the weekday-weekend difference in diurnal distribution of CO2 concentration. The average volume fraction of CO2 in urban area of Hangzhou was 9.3×10-6 higher than that in rural area, and the reduction of anthropogenic emissions during the G20 summit reduced the atmospheric CO2 concentration effectively, especially in urban area.

Fractionation processes of δ13C in a soil-shallow groundwater system of a potential CO2 sequestration site: Wandoan, QLD, Australia

Near-surface leakage detection is often a crucial part of verifying the success of CO2 sequestration projects, and it cannot be achieved without a detailed description of the natural state of an operational site prior to injection. Without baseline studies, it is difficult to define CO2 anomalies outside natural variation in the subsurface that may be related to leakage. CO2 concentrations and δ13C-CO2 values (of DIC, dissolved gas, and soil gas) were investigated to establish a baseline modelling of natural chemical processes leading to CO2 occurrence and δ13C-CO2 fractionation in the soil-groundwater interface on an example at the Glenhaven site in Queensland, Australia. This was analysed within the context of the hydrogeology and hydrogeochemistry of a shallow aquifer system (<20 m depth). The Glenhaven site and its five monitoring station are located in an area which is a candidate for a future small scale carbon storage pilot project. We combine major ion, stable isotope data of groundwater and soil gas as well as physical hydrogeology data with extensive isotope models to explain the pathways and origins of groundwater and CO2. The results indicate shallow aquitard claystone units at the site restrict vertical groundwater leakage and inter-aquifer mixing. Groundwater chemistry is controlled by transpiration of plants, cation exchange, weathering and meteoric recharge to produce Na-Cl and Na-HCO3-Cl type waters. δ18O and δ2H values suggest a shallow meteoric recharge source largely unaffected by evaporation. CO2 concentrations in soil and dissolved in groundwater range between 631 and 47,516 ppm and δ13C values average −17 and −21‰. The mixing of three potential CO2 sources is discussed: 1) the aerobic decay of C3 plant matter, 2) the aerobic decay of C4 plant matter and, 3) shallow coal related CO2. Despite this, the variability of CO2 concentrations and equivocal sources is not likely to hinder leakage detection in the future of site monitoring.

Carbon biogeochemistry of a flooded Pantanal forest over three annual flood cycles

The Pantanal is the largest wetland in the world and yet little is known about the variability in carbon (C) dynamics across its flood seasons. We examined the effect of inundation on the C cycle in the 2013–2015 flood cycles illustrated by dissolved CO2, CH4, organic C (DOC) concentration measurements, and optical properties of dissolved organic matter (DOM) evaluated by absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). During the 2015 flood cycle, pCO2 varied between 5973 and 14,292 μatm, with pCH4 concentrations ranging between 2956 and 51,675 μatm respectively, with high temporal variability for both gases. The supersaturation of CO2 and CH4 in relation to the atmospheric equilibrium caused the system to behave as a net source of CO2 and CH4 to the atmosphere with evasion rates of 320 mg CO2 m−2 d−1 and 20 mg CH4 m−2 d−1, respectively. Mean DOC concentration was 7.0 ± 0.4 mg L−1 and did not differ between flood cycles. Higher concentrations of DOC were measured at the start (rising floodwaters) and at the end (receding floodwaters) of flood cycles, while lower DOC concentrations were observed during the peak flood. The PARAFAC analysis indicated the presence of five DOM components: humic (C1 and C2) and fulvic type material (C3) showed the highest relative abundance (68.5% of the total PARAFAC component fluorescence), as well as protein-like material (C4 and C5) derived from microorganisms. Our measured diffusive flux levels were below the range of emissions found for wetlands and floodplains for CO2, but were slightly higher for CH4 relative to other studies in lakes and seasonally flooded areas of the Pantanal. The large variations in concentrations of CO2, CH4 and DOC and the optical properties of DOM during the course of each flood cycle suggest a close relationship between carbon and water cycles in this tropical wetland.

Analysis on the variations of atmospheric CO2 concentrations along the urban–rural gradients of Chinese cities based on the OCO-2 XCO2 data

ABSTRACTAnthropogenic CO2 emissions contribute the most to the growth of atmospheric CO2 concentrations. These emissions are largely concentrated in urban areas where human activities are intense. Studies have been conducted to explore the urban and rural difference in CO2 concentrations based on ground-based measurements. The launch of NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite provides a new opportunity to monitor CO2 concentrations and their spatial and temporal variations at city scale. The objective of this study is to analyse the spatial and temporal distributions of CO2 concentrations along the urban–rural gradients of Chinese cities, using the column averaged CO2 dry air mole fraction (XCO2) data derived from OCO-2. We used both conceptual and physical urban–rural gradients to analyse variations in CO2 concentrations over space. The results show that the urban and rural difference in CO2 concentrations of these cities can be monitored. And, the seasonal variations of CO2 concentrations in these cities can also be detected using the XCO2 data. Moreover, the variations in CO2 concentrations along the urban–rural gradients have four main types with significant enhancements of CO2 concentrations were observed in urban areas, urban–rural transitional areas, rural areas, and without regular patterns, respectively. The results are generally different from the common assumption that CO2 concentrations peak in central urban areas and decline in rural areas. In conclusion, the XCO2 data can be used to analyse the spatial-temporal variations of CO2 concentrations along the urban–rural gradients of Chinese cities, and the results have important policy implications for mitigating CO2 emissions.

Comparative analyses of hydrological responses of two adjacent watersheds to climate variability and change using the SWAT model

Abstract. Water quality problems in the Chesapeake Bay Watershed (CBW) are expected to be exacerbated by climate variability and change. However, climate impacts on agricultural lands and resultant nutrient loads into surface water resources are largely unknown. This study evaluated the impacts of climate variability and change on two adjacent watersheds in the Coastal Plain of the CBW, using the Soil and Water Assessment Tool (SWAT) model. We prepared six climate sensitivity scenarios to assess the individual impacts of variations in CO2 concentration (590 and 850 ppm), precipitation increase (11 and 21 %), and temperature increase (2.9 and 5.0 ∘C), based on regional general circulation model (GCM) projections. Further, we considered the ensemble of five GCM projections (2085–2098) under the Representative Concentration Pathway (RCP) 8.5 scenario to evaluate simultaneous changes in CO2, precipitation, and temperature. Using SWAT model simulations from 2001 to 2014 as a baseline scenario, predicted hydrologic outputs (water and nitrate budgets) and crop growth were analyzed. Compared to the baseline scenario, a precipitation increase of 21 % and elevated CO2 concentration of 850 ppm significantly increased streamflow and nitrate loads by 50 and 52 %, respectively, while a temperature increase of 5.0 ∘C reduced streamflow and nitrate loads by 12 and 13 %, respectively. Crop biomass increased with elevated CO2 concentrations due to enhanced radiation- and water-use efficiency, while it decreased with precipitation and temperature increases. Over the GCM ensemble mean, annual streamflow and nitrate loads showed an increase of ∼ 70 % relative to the baseline scenario, due to elevated CO2 concentrations and precipitation increase. Different hydrological responses to climate change were observed from the two watersheds, due to contrasting land use and soil characteristics. The watershed with a larger percent of croplands demonstrated a greater increased rate of 5.2 kg N ha−1 in nitrate yield relative to the watershed with a lower percent of croplands as a result of increased export of nitrate derived from fertilizer. The watershed dominated by poorly drained soils showed increased nitrate removal due do enhanced denitrification compared to the watershed dominated by well-drained soils. Our findings suggest that increased implementation of conservation practices would be necessary for this region to mitigate increased nitrate loads associated with predicted changes in future climate.

Emission characteristics of atmospheric carbon dioxide in Xi'an, China based on the measurements of CO2 concentration, △14C and δ13C

Given that cities contributed most of China's CO2 emissions, understanding the emission characteristics of urban atmospheric CO2 is critical for regulating CO2 emissions. Regular observations of atmospheric CO2 concentration, △14C and δ13C values were performed at four different sites in Xi'an, China in 2016 to illustrate the temporal and spatial variations of CO2 emissions and recognize their sources and sinks in urban carbon cycles. We found seasonal variations in CO2 concentration and δ13C values, the peak to peak amplitude of which was 80.8ppm for CO2 concentration and 4.0‰ for its δ13C. With regard to the spatial variations, the urban CO2 “dome” effect was the most pronounced during the winter season. The use of △14C combines with δ13C measurements aid in understanding the emission patterns. The results show that in the winter season, emissions from fossil fuel derived CO2 (CO2ff) contributed 61.8±10.6% and 57.4±9.7% of the excess CO2 (CO2ex) in urban and suburban areas respectively. Combining with the result of estimated δ13C value of fossil fuel (δ13Cff=−24‰), which suggest coal burning was the dominant source of fossil fuel emissions. In contrast, the proportions of CO2ff in CO2ex varied more in the summer season than that in the winter season, ranging from 42.3% to >100% with the average contributions of 82.5±23.8% and 90.0±24.8%. Given the estimation of δ13C value of local sources (δ13Cs) was −21.9‰ indicates that the intensively biogenic activities, such as soil respiration and corn growth have significantly impacted urban carbon cycles, and occasionally played a role of carbon sink.

Initial recovery of compacted soil—planting and technical treatments decrease CO2 concentrations in soil and promote root growth

The combination of technical treatments and planting of alder trees in a compacted forest soil improves the circulation of air and water through the pore system. This leads to decreases in CO 2 concentrations and increases in root growth in the soil. Both are indicative of an initial recovery of soil structure. The compaction of forest soils, caused by forest machinery, has as a principal consequence: the destruction of soil structure and thus the reduction of the soil aeration status. Thus, the gas exchange between soil and atmosphere is reduced and the depth propagation of roots is limited, resulting in the shortage of water and nutrient supplies for trees. This research aimed at detecting the first stages of recovery of soil structure in a compacted forest soil, which was treated with a combination of techniques (i.e., planting tree species, mulching, addition of lime), which could presumably accelerate the regeneration of soil structure. The variation of CO2 concentrations and the dynamics of root growth were repeatedly measured. Linear mixed models were developed in order to test the effects of the treatments and the planting of trees on soil aeration, as well as to identify the influence of the different environmental effects on CO2 concentration in soil. The planting of root-active trees showed significant effects on decreases in CO2 concentrations. However, during the short-term observation, some negative effects occurred especially for the mulched sites. Nevertheless, all applied technical treatments promoted an improved soil aeration and a higher root growth compared to untreated sites which points to an initial enhanced recovery of soil structure. Pronounced seasonal and interannual variations of soil respiration were highly influenced by soil temperature and soil water content variations. An initial regeneration of soil structure is indicated by distinct changes of the soil aeration status. This regeneration is partially enhanced by the applied treatments. The quantitative potential of the regeneration techniques needs a longer observation period for mid- and long-term soil recoveries.

Estimating CO2 gas exchange in mixed age vegetable plant communities grown on soil-like substrates for life support systems

If soil-like substrate (SLS) is to be used in human life support systems with a high degree of mass closure, the rate of its gas exchange as a compartment for mineralization of plant biomass should be understood. The purpose of this study was to compare variations in CO2 gas exchange of vegetable plant communities grown on the soil-like substrate using a number of plant age groups, which determined the so-called conveyor interval. Two experimental plant communities were grown as plant conveyors with different conveyor intervals. The first plant community consisted of conveyors with intervals of 7 days for carrot and beet and 14 days for chufa sedge. The conveyor intervals in the second plant community were 14 days for carrot and beet and 28 days for chufa sedge. This study showed that increasing the number of age groups in the conveyor and, thus, increasing the frequency of adding plant waste to the SLS, decreased the range of variations in CO2 concentration in the “plant–soil-like substrate” system. However, the resultant CO2 gas exchange was shifted towards CO2 release to the atmosphere of the plant community with short conveyor intervals. The duration of the conveyor interval did not significantly affect productivity and mineral composition of plants grown on the SLS.