Remediation of Nitrate-Nitrogen Contaminated Groundwater by a Heterotrophic-Autotrophic Denitrification Approach in an Aerobic Environment

A novel heterotrophic-autotrophic denitrification (HAD) approach supported by mixing granulated spongy iron, methanol, and mixed bacteria was proposed for the remediation of nitrate-nitrogen (NO₃-N) contaminated groundwater in a dissolved oxygen (DO)-rich environment. The HAD process involves biological deoxygenation, chemical reduction (CR) of NO₃-N and DO, heterotrophic denitrification (HD), and autotrophic denitrification (AD). Batch experiments were performed to: (1) investigate deoxygenation capacities of HAD; (2) determine the contributions of AD, HD, and CR to the overall NO₃-N removal in the HAD; and (3) evaluate the effects of environmental parameters on the HAD. There were 174, 205, and 2,437?min needed to completely reduce DO by the HAD, spongy iron-based CR, and by the mixed bacteria, respectively. The HAD depended on abiotic and biotic effects to remove DO. CR played a dominant role in deoxygenation in the HAD. After 5?days, approximately 100, 63.0, 20.1, and 9.7?% of the initial NO₃-N was removed in the HAD, HD, AD + CR, and CR incubations, respectively. CR, HD, and AD all contributed to the overall NO₃-N removal in the HAD. HD was the most important NO₃-N degradation mechanism in the HAD. There existed symbiotic, synergistic, and promotive effects of CR, HD, and AD within the HAD. The decrease in NO₃-N and the production of nitrite-nitrogen (NO₂-N) and ammonium-nitrogen (NH₄-N) in the HAD were closely related to the C to N weight ratio. The C to N ratio of 3.75:1 was optimal for complete denitrification. Denitrification rate at 27.5°C was 1.36 times higher than at 15.0°C.     

Characterization of Groundwater Microbial Communities, Dechlorinating Bacteria, and In Situ Biodegradation of Chloroethenes Along a Vertical Gradient

The variability of hydrogeochemical conditions can affect groundwater microbial communities and the natural attenuation of organic chemicals in contaminated aquifers. It is suspected that in situ biodegradation in anoxic plumes of chloroethenes depends on the spatial location of the contaminants and the electron donors and acceptors, as well as the patchiness of bacterial populations capable of reductive dechlorination. However, knowledge about the spatial variability of bacterial communities and in situ biodegradation of chloroethenes in aquifers is limited. Here, we show that changes of the bacterial communities, the distribution of putative dechlorinating bacteria and in situ biodegradation at the border of a chloroethenes plume (Bitterfeld, Germany) are related to local hydrogeochemical conditions. Biotic reductive dechlorination occurred along a 50 m vertical gradient, although significant changes of the hydrogeochemistry and contaminant concentrations, bacterial communities and distribution of putative dechlorinating bacteria (Dehalobacter spp., Desulfitobacterium spp., Dehalococcoides spp., and Geobacter spp.) were observed. The occurrence and variability of in situ biodegradation of chloroethenes were revealed by shifts in the isotope compositions of the chloroethenes along the vertical gradient (??¹³C ranging from ?14.4?? to ?4.4??). Our results indicate that habitat characteristics were compartmentalized along the vertical gradient and in situ biodegradation occurred with specific reaction conditions at discrete depth. The polyphasic approach that combined geochemical and biomolecular methods with compound-specific analysis enabled to characterize the spatial variability of hydrochemistry, bacterial communities and in situ biodegradation of chloroethenes in a heterogeneous aquifer.     

Quantification of Oxidant Demand and Consumption for In Situ Chemical Oxidation Design: in the Case of Potassium Permanganate

Accurate estimation of oxidant consumption during in situ chemical oxidation (ISCO) is the key to determining the treatment effectiveness in contaminated sites. We established the estimation model of soil oxidant demand (SOD) and simulation equations of potassium permanganate (KMnO₄) dynamic consumption based on the reaction equation of KMnO₄ with reductive minerals and the estimation model of SOD. Model validation, model application, and simulation assessment had been accomplished. Results indicated that the simulations are in good agreement with measured data. The confidence level of the SOD estimation model of KMnO₄ was over 80%, with sensitivity in decreasing order as follows: organic matter content > initial KMnO₄ concentration > reductive minerals (RMs). Particularly, the organic matter played a dominate role in the SOD model estimation. The coefficient of determination (R²) of the SOD dynamic consumption simulation equation was above 0.9. Among the various types of soils, the overall trend of SOD value and reaction period decreased as follows: clay > loam > sand. However, the consumption rate of KMnO₄ decreased in the order of clay > sand > loam. In addition, SOD value, reaction period, and reaction rate all increased as the initial concentration of KMnO₄ went up. This work can provide a methodology and reference for selecting and estimating of the optimal oxidant doses and reaction period during field application.     

Quantification of Shallow Groundwater Nutrient Dynamics in Septic Areas

Of all groundwater pollution sources, septic systems are the second largest source of groundwater nitrate contamination in USA. This study investigated shallow groundwater (SGW) nutrient dynamics in septic areas at the northern part of the Lower St. Johns River Basin, Florida, USA. Thirty-five SGW-monitoring wells, located at nine different urban areas served by septic systems, were used to collect the SGW samples seasonally and/or biweekly for a duration of 3?years from 2003 to 2006. Analytical results showed that there were 16 wells with nitrate concentrations exceeding the US Environmental Protection Agency's drinking water limit (10?mg?L^?1). There also were 11 and 14 wells with total Kjeldahl nitrogen (TKN) and total phosphorus (TP) concentrations, respectively, exceeding the ambient water quality criteria (0.9?mg?L^?1 for TKN and 0.04?mg?L^?1 for TP) recommended for rivers and streams in nutrient Ecoregion XII (Southeast USA). In general, site variations are much greater than seasonal variations in SGW nutrient concentrations. A negative correlation existed between nitrate/nitrite?Cnitrogen (NO_x?CN) and TKN as well as between NO_x?CN and ammonium ( $ NH_4^{ + } $ ), whereas a positive correlation occurred between TKN and $ NH_4^{ + } $ . Furthermore, a positive correlation was found between reduction and oxidation (redox) potential and water level, while no correlation was observed between potassium concentration and redox potential. This study demonstrates a need to investigate the potential adverse impacts of SGW nutrients from the septic areas upon the deeper groundwater quality due to the nutrient penetration and upon the surface water quality due to the nutrient discharge.     

Sensitive Quantification of Silver Nanoparticles by Kinetic-Spectrophotometry Method in Groundwater Samples

A novel kinetic-spectrophotometry method is developed for the determination of trace amounts of silver nanoparticles (AgNPs) in groundwater samples based on their catalytic effect on the oxidation of indigo carmine by hexacyanoferrate (III) in acetate-acetic acid medium. The absorbance was measured at 612?nm with the fixed-time method. The optimization of the operating conditions regarding concentrations of the reagents and interferences was investigated. The calibration curve is linear over the concentration range 4.0?C300???g?mL^?1 of AgNPs with good precision and accuracy. The proposed method is a highly sensitive, selective, and relatively rapid assay of AgNPs at trace level. The method was applied to the determination of AgNPs in groundwater and a synthetic sample.     

Nitrate Loss for Denitrification during High Frequency Research in Floodplain Groundwater of the Tama River

This study aims to identify the possible roles of a floodplain inbiogeochemical nitrogen cycles, based on analysis of groundwaterdynamics of inorganic nitrogen species. Groundwater samples collected from boreholes made of poly vinyl chloride located onthe floodplain in the middle reach of the Tama River, Nagata district were analyzed for dissolved organic carbon, dissolved oxygen, inorganic nitrogen compounds, major anions and the stableisotope ratio of nitrate-nitrogen.Dissolved oxygen in groundwater samples collected from marked borehole (G-5) was low (50 ± 16 μM) and nitrate decreasedfrom 63 μM to < 10 μM during our research period. Concentrations of nitrate, nitrite and nitrous oxide decreased one after the other. These decreases in concentration, combinedwith ¹⁵N-enrichment of groundwater nitrate suggest denitrification as a significant nitrate sink, yielding an isotopic fractionation of nitrate-nitrogen with an enrichment factor (ε= –17.9‰) that is comparable tothose in various soil and groundwater systems. This study provesthat floodplains can perform as nitrogen sinks for groundwaterin river catchments.     

“In Situ” Amendments and Revegetation Reduce Trace Element Leaching in a Contaminated Soil

Various amendments and/or a plant cover (Agrostis stolonifera L.) were assessed for their potential to reduce trace element leaching in a contaminated soil under semi-arid conditions. The experiment was carried out in field containers and lasted 30 months. Five treatments with amendments (leonardite (LEO), litter (LIT), municipal waste compost (MWC), biosolid compost (BC) and sugar beet lime (SL)) and a plant cover and two controls (control without amendment but with plant (CTRP) and control without amendment and without plant (CTR)) were established. Drainage volumes were measured after each precipitation event and aliquots were analysed for pH, electrical conductivity (EC) and trace element concentrations (As, Cd, Cu, Pb and Zn). Soil pH and trace element extractability (0.01 M CaCl₂) at three different depths (0–10, 10–20 and 20–30 cm) were measured at the end of the experiment. Incorporation of amendments reduced leaching of Cd, Cu and Zn between 40–70% in comparison to untreated soil. The most effective amendments were SL, BC and MWC. At the end of the experiment, extractable concentrations of Cd, Cu and Zn were generally lower in all amended soils and CTRP compared to CTR. Soil pH decreased and extractability of metals increased in all treatments in relation to depth. Results showed that use of these amendments combined with healthy and sustainable plant cover might be a reliable option for “in situ” stabilization of trace elements in moderately contaminated soils.     

4种典型荒漠草原植物群落土壤呼吸对封育禁牧的响应

采用动态密闭气室分析法,分别研究了放牧与围封条件下赖草群落、沙蒿群落、甘草群落、冰草群落4种典型荒漠草原植物群落土壤呼吸及其与温度因子的关系,揭示了4种典型荒漠草原植物群落土壤呼吸对封育禁牧的响应过程。结果表明,放牧与围封样地的土壤呼吸速率日动态均呈现出不对称的单峰型曲线,高峰值一般出现在14:00—16:00。围封样地土壤呼吸速率值大于放牧样地,但各监测时刻差异显著性不尽相同。放牧与围封样地各植物群落中较深层土温(10,15cm)和土壤呼吸速率的相关性普遍高于浅层土温(5cm)、深层土温(20cm)和气温,其中15cm土层温度与土壤呼吸速率相关性最高。除沙蒿群落外,其他3种植物群落放牧样地Q10值均显著低于围封样地,Q10值大小依次均为:沙蒿群落>赖草群落>甘草群落>冰草群落,其中沙蒿群落Q10值远高于其他3个群落。     

Development of a Chemosensor for the In Situ Monitoring of Thallium in the Water Network

Thallium is an emerging contaminant, which can be retained in scale encrustation within the pipeline and then released to drinking water. It is included in the Priority Pollutant List of the U.S. EPA. In this study, a sample from the water pipeline of Pietrasanta (Italy), affected by the contamination of thallium, is characterized by SEM-EDS, TGA, and FT-IR. Fluorescence spectroscopy is then proposed as the optimal technique for the detection of the contaminant. The functionality of a previously reported fluorescent calix[4]arene-based chemosensor is verified first on a standard solution of thallium nitrate and then on the sample under investigation. The quenching of the fluorescence of the sensor during the complexation of thallium is confirmed, identifying an ON-OFF sensor with high sensitivity, able to detect concentrations as low as 10^?6 M and with high potential of development for the in situ and fast monitoring of the pollutant in the water network.     

Testing In Situ Sulfate Reduction by H2 injection in a Bench-Scale Column Experiment

Autotrophic microbial sulfate reduction was tested in a fixed-bed bench-scale column experiment to lower sulfate and iron loads in acid-mine-drainage-influenced groundwater. The microbial process was enhanced by injecting H₂ gas as electron donor into the silicate bed. The experiments were performed at 2.5 atm and 10°C. Complete iron removal (3.8?±?0.3 mM) and partial sulfate removal from 17 to 9 mM were achieved at rates of about 0.004–0.019 mmol SO₄ per liter per hour and at hydraulic retention times of 51.5–19.8 days. The tests showed that most microbial activity took place in immobile zones. These zones create stable environmental conditions for the microorganisms leading to constant reduction rate despite possibly unfavorable conditions prevailing in the mobile phase. Diffusion between mobile and the immobile zones was not found to be the limiting factor for sulfate reduction. Rather, low H_2(aq) concentrations due to low H₂ solubility combined with the inhomogeneous distribution of H₂ gas in the pore space limited sulfate reduction. H_2(aq) concentrations in some parts of the sediment body were insufficient to maintain H_2(aq) concentrations in the immobile zones above the level of substrate limitation. Fe and S precipitated mostly as iron monosulfide and accumulated in regions with high H_2(aq) availability. Calculations showed that the deposition of iron sulfide in the pore space does not affect the pore volume significantly.     

Accumulation of Zinc and Copper in an Arable Field after Animal Manure Application

An experiment was conducted to examine the accumulation and mobility of heavy metals (Zn and Cu) at different depths in three types of arable soils (Brown Lowland soil, Andosol, and Brown Forest soil) amended with cattle and pig farmyard manures for 5 years. Nitric-perchloric acid digestion was performed for the determination of the total amounts of heavy metals, and 0.1 M hydrochloric acid extraction was performed for the determination of the amounts of soluble heavy metals. Results of the soil analysis indicated that pig farmyard manure application resulted in serious contamination of arable soils with Zn and potentially Cu. Especially, the Brown Forest soil displayed a high ability to accumulate heavy metals on the soil surface. Total-Zn concentration in surface soils was considerably affected by the holding capacity of soluble-Zn traction. Although the Andosol amended with pig farmyard manure showed higher concentrations of heavy metals related to the higher ability of retention on a weight basis, the soil did not contribute to high heavy metal accumulation because of its low bulk density. Heavy metals were easily leached in sandy soils such as Brown Lowland soil, and Cu was potentially stable compared with Zn. We suggest that long-term pig farmyard manure application to the Brown Lowland soil and Andosol with a light soil texture is associated with a higher risk of groundwater pollution than the application to the Brown Forest soil.     

Principles, Developments and Design Criteria of In Situ Chemical Oxidation

Sustainable remediation comprises soil and groundwater risk-management actions that are selected, designed, and operated to maximize net environmental, social, and economic benefit (while assuring protection of human health and safety). This paper describes a benchmarking exercise to comparatively assess potential differences in environmental management decision making resulting from application of different sustainability appraisal tools ranging from simple (qualitative) to more quantitative (multi-criteria and fully monetized cost-benefit analysis), as outlined in the SuRF-UK framework. The appraisal tools were used to rank remedial options for risk management of a subsurface petroleum release that occurred at a petrol filling station in central England. The remediation options were benchmarked using a consistent set of soil and groundwater data for each tier of sustainability appraisal. The ranking of remedial options was very similar in all three tiers, and an environmental management decision to select the most sustainable options at tier 1 would have been the same decision at tiers 2 and 3. The exercise showed that, for relatively simple remediation projects, a simple sustainability appraisal led to the same remediation option selection as more complex appraisal, and can be used to reliably inform environmental management decisions on other relatively simple land contamination projects.     

典型草原区退耕及封育草地土壤水分物理性质研究

对典型草原云雾山保护区退耕6a，11a（放牧8a）和16a的3个退耕地和4个封育草地（百里香群落、铁杆蒿群落、大针茅群落和长芒草群落）对照坡耕地表层土壤的水分特征曲线数学模型、持水参数和比水容量进行了测定。结果表明，封育草地的持水和供水性能要大于退耕地和坡耕地，放牧对退耕11a的持水和供水性能有较大影响。云雾山自然保护区坡地退耕和草地封育后，样地土壤的持水能力大小顺序为封育长芒草群落〉大针茅群落〉百里香群落〉铁杆蒿群落〉退耕16a〉退耕6a〉坡耕地〉退耕11a（放牧8a）。供水能力大小顺序为封育长芒草群落〉百里香群落〉铁杆蒿群落〉大针茅群落〉退耕16a〉退耕6a〉退耕11a（放牧8a）〉坡耕地。这说明土壤持水和供水性能随着退耕年限的增加和封育草地的正向植被演替均逐渐增强的趋势。坡地退耕和草地封育能够通过改善土壤的水分物理性质提高土壤的蓄水持水性能和抗侵蚀能力。     

Simulation of Groundwater Flow in a Sedimentary Aquifer System Subjected to Overexploitation

Groundwater flow models are applied to a large variety of hydrogeological conditions in different aquifer types, in order to simulate the groundwater flow of the investigated system. This paper aims to present the application of a groundwater flow model for the simulation of a sedimentary aquifer, located in Northern Greece. The simulation involves the period between April 2003 and April 2004, and the model is divided into two distinct stress periods, each containing two different time steps. The simulation of the aquifer is found to be satisfactory, conclusion which is based on both graphical as well as arithmetical verifications. The groundwater flow simulation was achieved by the application of the MODFLOW code.     

Pilot-Scale Demonstration of In Situ Chemical Reduction Technology at a Formerly Used Defense Site

This paper describes results of the first field demonstration conducted to evaluate an innovative in situ chemical reduction technology to rapidly degrade 81?m² of high concentrations of explosives contaminated soils at Ravenna Army Ammunition Plant, Ravenna, Ohio, USA. The primary explosives of concern and associated baseline levels of contamination were 2,4,6-trinitrotoluene (TNT) (383?mg/kg), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (180?mg/kg), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) (15?mg/kg). The in situ chemical treatment process involved selected reductant of sodium hydrosulfite (Na₂S₂O₄) at optimal pH (9?C10) with appropriate buffers (e.g., sodium bicarbonate and potassium carbonate) that, when mixed with water, generate free radicals that react very rapidly to degrade nitroaromatics to carbon dioxide (CO₂), formate (HCOO?C), and gaseous nitrogen compounds (NOx). Significant reductions in TNT, RDX, and HMX (>95?%) were observed within 24?h and achieved response complete in under 2?weeks after in situ application of the buffered sodium hydrosulfite.     

两种温度制备生物质炭在榉树人工林土壤的原位稳定性

研究生物质炭的稳定性是理解其土壤固碳功能的基础。以水稻为原料在两种温度（300 ℃和500 ℃）制备生物质炭,利用13C稳定同位素技术,通过182 d的野外原位实验,比较两种生物质炭在榉树人工林土壤中的稳定性。整个培养期间,300 ℃和500 ℃生物质炭的累积分解量分别为10.5和3.65 g·m-2。双指数模型拟合的结果显示,300 ℃和500 ℃生物质炭的难分解碳库的平均停留时间分别为99.8 a和302 a。300 ℃生物质炭的分解速率与土壤表层温度显著正相关（P = 0.034,r = 0.417,n = 26）,而500 ℃生物质炭的相关性不显著（P = 0.549,r = 0.123,n =26）,表明土壤温度对生物质炭稳定性的影响因制备温度而异。野外原位实验表明500 ℃生物质炭具有更高的稳定性,为深入探讨生物质炭的稳定性提供了基础数据。     

Effects of High Rates of Coal Fly Ash on Soil,Turfgrass, and Groundwater Quality

A field study (1993–1996) assessed the effects of applying unusually high rates of coal fly ash as a soil additive forthe turf culture of centipedegrass (Eremochloa ophiroides).In addition, the quality of the soil and the underlying groundwater was evaluated. A Latin Square plot design was employed to include 0 (control, no ash applied), 280, 560, and 1120 Mg ha^-1 (mega gram ha^-1, i.e., tonne ha^-1)application rates of unweathered precipitator fly ash. The onceapplied fly ash was rototilled and allowed to weather for 8 months before seeding. Ash application significantly increasedthe concentrations in plant tissue of B, Mo, As, Be, Se, and Bawhile also significantly reducing the concentrations of Mg, Mn,and Zn. The other elements measured (i.e., N, K, Ca, Cu, Fe, Ag,Cd, Cr, Hg, Ni, Pb, Sb, Tl, Na, and Al) were not affected. Of these elements Mg, Cu, and Mo concentrations in plant tissue increased with time while B and Se decreased temporally. The diminution of B and Na appears to be related to the leaching ofsoluble salts from ash-treated soils. Of all the elements measured, only Mn produced significant correlation (p = 0.0001) between the tissue and soil extractable concentrations. Ash treatment elevated the soil pH to as high as 6.45 with theenhanced effect occurring primarily in the 0–15 cm depth. Soilsalinity increased with the application rate with the largestincreases occurring in the initial year of application. However,by the second year, most of the soluble salts had already leachedfrom the treatment zone into deeper depths, and by the fourthyear, these salts had completely disappeared from the profile.The chemical composition of the underlying groundwater was notadversely impacted by the ash application. Plant tissue and groundwater data however, indicate that much higher rates of fly ash can be used on this type of land use where the plant species is tolerant of soil salinity and does not appear tobioaccumulate potentially toxic trace elements.     

Denitrification potential of intermittently saturated floodplain soils from a subtropical perennial stream and an ephemeral tributary

Denitrification has the potential to remove excess nitrogen from groundwater passing through riparian buffers, thus improving water quality downstream. In regions with markedly seasonal precipitation, transient stream flow events may be important in saturating adjacent floodplain soils and intermittently providing the anaerobic conditions necessary for denitrification to occur. In two experiments we characterised the denitrification potential of soils from two contrasting floodplains that experience intermittent saturation. We quantified under controlled laboratory conditions: 1) potential rates of denitrification in these soils with depth and over time, for a typical period of saturation; and 2) the influences on rates of nitrate and organic carbon. Treatments differed between experiments, but in each case soil-water slurries were incubated anaerobically with differing amendments of organic carbon and nitrate; denitrification rates were measured at selected time intervals by the acetylene-block technique; and slurry filtrates were analysed for various chemical constituents. In the first experiment (ephemeral tributary), denitrification was evident in soils from both depths (0-0.3 m; 0.3-1.1 m) within hours of saturation. Before Day 2, mean denitrification rates at each depth were generally comparable, irrespective of added substrates; mean rates (Days 0 and 1) were 5.2 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m) and 1.6 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.1 m). Rates generally peaked on Days 2 or 3. The availability of labile organic carbon was a major constraint on denitrification in these soils. Acetate addition greatly increased rates, reaching a maximum in ephemeral floodplain soils of 17.4 ± 1.8 mg N kg dry soil⁻¹ day⁻¹ on Day 2: in one deep-soil treatment (low nitrate) this overcame differences in rates observed with depth when acetate was not added, although the rate increase in the other deep-soil treatment (high nitrate) was significantly less (P ≤ 0.01). Without acetate, peak denitrification rates in this experiment were 6.9 ± 0.4 and 2.8 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ in surface and deep soils, respectively. Differences in rates were observed with depth on all occasions, despite similar initial concentrations of dissolved organic carbon (DOC) at both depths. Levels of substrate addition in the second experiment (perennial stream) more closely reflected natural conditions at the site. Mean denitrification rates were consistently much higher in surface soil (P ≤ 0.001), while the source of water used in the slurries (surface water or groundwater from the site) had little effect on rates at any depth. Mean rates when all treatments retained nitrate were: 4.5 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m depth); 0.8 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.0 m); and 0.6 ± 0.1 mg N kg dry soil⁻¹ day⁻¹ (1.8-3.5 m). For comparable treatments and soil depths, denitrification potentials at both sites were similar, apart from higher initial rates in the ephemeral floodplain soils, probably associated with their higher DOC content and possibly also their history of more frequent saturation. The rapid onset of denitrification and the rates measured in these soils suggest there may be considerable potential for nitrate removal from groundwater in these floodplain environments during relatively short periods of saturation.     

Below ground carbon turnover and greenhouse gas exchanges in a sub-arctic wetland

Here we present results from a field experiment in a sub-arctic wetland near Abisko, northern Sweden, where the permafrost is currently disintegrating with significant vegetation changes as a result. During one growing season we investigated the fluxes of CO₂ and CH₄ and how they were affected by ecosystem properties, i.e., composition of species that are currently expanding in the area (Carex rotundata, Eriophorum vaginatum and Eriophorum angustifolium), dissolved CH₄ in the pore water, substrate availability for methane producing bacteria, water table depth, active layer, temperature, etc. We found that the measured gas fluxes over the season ranged between: CH₄ 0.2 and 36.1 mg CH₄ m⁻² h⁻¹, Net Ecosystem Exchange (NEE) −1000 and 1250 mg CO₂ m⁻² h⁻¹ (negative values meaning a sink of atmospheric CO₂) and dark respiration 110 and 1700 mg CO₂ m⁻² h⁻¹. We found that NEE, photosynthetic rate and CH₄ emission were affected by the species composition. Multiple stepwise regressions indicated that the primary explanatory variables for NEE was photosynthetic rate and for respiration and photosynthesis biomass of green leaves. The primary explanatory variables for CH₄ emissions were depth of the water table, concentration of organic acid carbon and biomass of green leaves. The negative correlations between pore water concentration and emission of CH₄ and the concentrations of organic acid, amino acid and carbohydrate carbon indicated that these compounds or their fermentation by-products were substrates for CH₄ formation. Furthermore, calculation of the radiative forcing of the species expanding in the area as a direct result of permafrost degradation and a change in hydrology indicate that the studied mire may act as an increasing source of radiative forcing in future.     

我国草地退化及其生态安全评价指标体系的探索

草地是生态环境的载体,对改善生态环境,维护生态平衡,保护人类生存和发展起着重要的作用.然而,我国草地退化情况十分严重,令人担忧.因而正确评价我国草地利用状况势在必行.本文通过介绍我国草地概况和国内外生态安全的研究进展,提出了我国草地系统生态安全的评价体系.