Evidence for effects of CO2 on soil solution chemistry in spodosols by a simple in-field extractor

Degassing of CO₂ during collection of soil solution may alter the chemical composition of the solution, especially in well-buffered soils. We used a simple syringe extractor for field sampling of soil solution along with ambient soil air in order to test the influence of degassing of CO₂ on solution pH in acid soils (Spodosol B and C horizons collected in Central Maine, USA). Soil air concentrations of CO₂ varied from 0.36 to 1.35 ml l-¹ during sampling immediately after snow melt. Degassing increased solution pH by 0.3 to 0.5 pH units. Both in-situ and degassed pH were predicted by the Reuss and Johnson soil chemical equilibrium model. The results suggest, (i) that the simple method is useful for determination of solution from wet soil under ambient soil air conditions and (ii) that degassing plays a significant role for soil solution chemistry even in Spodosol B and C horizons.     

Ozone,acidic precipitation,and soil Mg impacts on soil and loblolly pine seedling nutrient status after three growing seasons

Recent studies have suggested that the growth of loblolly pine (Pinus taeda L.) has declined in the southeastern United States, possibly due to acidic deposition and air pollutants, especially under conditions of low nutrient availability. Consequently, the potential for individual and synergistic impacts of O₃, acidic precipitation, and soil Mg status on the nutrient status of loblolly pine seedlings and soil was investigated over a 3 yr study period. Thirty-six open top chambers equipped with a rainfall exclusion/addition system were utilized to administer three levels of O₃ (subambient, ambient, or twice ambient) and two acidic precipitation treatments (pH 3.8 or 5.2) to seedlings growing in 24-L plastic pots containing soil having either 35 or 15 mg kg^?1 of exchangeable Mg. Each chamber contained 36 pots, and each treatment combination was replicated six times for a total of 1296 individual pots. After three seasons, throughfall and foliar nutrition data indicated that foliar leaching was not accelerated by increasing the acidity of precipitation from pH 5.2 to 3.8 and that increasing O₃ did not act to exacerbate foliar leaching. Further, foliar nutrient concentrations were not significantly affected by precipitation pH or O₃ treatments. Soil and soil solution data also indicate no accelerated soil leaching associated with chronic acidic precipitation. Differences in soil Mg treatments were reflected in soil solution and seedling Mg contents, but the 15 mg kg^?1 soil Mg treatment was not sufficiently low enough to induce Mg deficiency in the seedlings.     

Some plants like it warmer: Increased growth of three selected invasive plant species in soils with a history of experimental warming

Soil warming can affect plant performance by increasing soil nutrient availability through accelerating microbial activity. Here, we test the effect of experimental soil warming on the growth of the three invasive plant species Trifolium pratense (legume), Phleum pratense (grass), and Plantago lanceolata (herb) in the temperate-boreal forest ecotone of Minnesota (USA). Plants were grown from seed mixtures in microcosms of soils with three different warming histories over four years: ambient, ambient +1.7 °C, and ambient +3.4 °C. Shoot biomass of P. pratense and P. lanceolata and plant community root biomass increased significantly in soils with +3.4 °C warming history, whereas T. pratense responded positively but not significantly. Soil microbial biomass and N concentration could not explain warming effects, although the latter correlated significantly with the shoot biomass of P. lanceolata. Our results indicate that soil with a warming history may benefit some invasive plants in the temperate-boreal ecotone with potential impacts on plant community composition. Future studies should investigate the impact of warming-induced differences in soil organisms and nutrients on plant invasion.     

Soil respiration during secondary succession: Influence of temperature and moisture

The development of soil respiration was investigated from the beginning of secondary succession of a newly-exposed rubble-dump cover with and without additional management. Special interest was paid to the influence of soil temperature and soil moisture. CO₂-concentrations were measured every 2 weeks for 3 yr in the field by using an air current method with conical chambers and infrared gas analysis. No significant influence of soil moisture conditions (range of soil moisture 4–80 kPa soil water suction) on respiration rates was found. Carbon dioxide evolution generally followed soil temperature patterns in both ecosystems and showed a positive correlation with temperature T at 5 cm depth (mgCO₂m^?2h^?1 = a + b (T°C)²). Moreover both plots showed the same tendency of increasing soil respiration during the first 3 yr after reclamation, in spite of great differences in vegetation structure and coverage. This change in soil respiration rates describes the succession of soil microflora.     

Factors Affecting on Response of Broad Bean and Corn to Air Quality and Soil CO2 Flux Rates in Egypt

In response to worldwide increases in the burning of fossil fuels to meet energy demands for electric power generation and transportation, atmospheric CO₂ concentrations are currently rising at approximately 0.5% per year and ground-level O₃ values are increasing at a rate of 0.32% per year. Some plants showed positive increases in response to elevated atmospheric CO₂ concentrations, but are depressed when exposed to enhanced O₃ air pollution. The objective of this research was to examine relationships between alterations in leaf plant characteristics in response to air quality treatments and soil CO₂ flux activities during the growing season. Field studies were conducted in 2-m diameter?×?2-m height open-top chambers (OTC’s) at Sharkia Province during 2004 and 2005 involving the growth of broad bean (Vicia faba L. cv. Giza 40) and corn (Zea mays L. cv. 30 K8) in rotations using no-till management while being subjected full-season to five air quality treatments: charcoal-filtered (CF) air; CF + 150 µL CO₂ L^?1; non-filtered (NF) air; NF + 150 µL CO₂ L^?1 and ambient air (AA). Leaf photosynthesis (P_s), leaf area index (LAI), and vegetative carbohydrate contents were determined during pre- and post-anthesis in the two crops and soil CO₂ flux rates were monitored monthly during two growing seasons (2004–2005). Multiple and stepwise regression analyses were performed to establish linkages between plant canopy characteristics and soil CO₂ flux rates with results combined over growth stages and year for each crop. Increasing the atmospheric CO₂ concentration typically stimulated leaf P_s, soluble and total leaf carbohydrate contents, LAI values, and soil CO₂ flux rates throughout the growing season in both crop; however, the elevated O₃ treatments in NF air tended to lower these values compared to CF air. Soil CO₂ flux rates were significantly correlated with LAI, soluble and total sugar contents at P?≤?0.01 and with P_s rates at P?≤?0.05 in broad bean leaves, but with soluble and total sugar contents of leaves in corns at P?≤?0.01 only. Results of this study provided solid evidences linking the impact of changing air quality on plants factors processes and possible indirect effects on soil CO₂ flux activities throughout the growing season.     

Foliar Nutrient Contents in Tree Species of the Atlantic Rain Forest as Influenced by Air Pollution from the Industrial Complex of Cubatão, SE-Brazil

Air pollutant emissions from the industrial complex of Cubatão, SE-Brazil, have caused a severe decline of the Atlantic Forest in that region. In order to test the influenceof air pollution on the mineral nutrition of trees, leaves of the native species Tibouchina pulchra Cogn. (Melastomataceae), Miconia pyrifolia Naud. (Melastomataceae) and Cecropia glazioui Snethl. (Cecropiaceae) were sampled during winter and summer seasons at four sites with different pollution characteristics. Additionally, saplings of T. pulchra were cultivated in uniform soil and exposed to ambient air at the same experimentalsites for periods of 16 weeks. In both kinds of leaf samples, the concentrations of macro-nutrients were determined and nutrient ratios calculated. In leaves of mature T. pulchraand C. glazioui growing at polluted sites N, P and S levels were significantly higher than in trees from the reference area. With respect to K contents, C. glaziouishowed an increase, T. pulchra a decrease when compared to reference trees. Mg and Ca concentrations generally did not exhibit great variations. As a consequence of the changes in mineral contents nutrient ratios (N/K, N/Ca, S/K, S/Ca) shifted to higher values. The results of the exposure experiments together with data on pollutant concentrations inambient air and rain water demonstrated that atmosphericdeposition to the plants and pollution effects on the soilchemistry are the main reasons for the observed alterations inmineral nutrition of trees.     

Nitrogen Deposition and Nitrate Leaching at Forest Edges Exposed to High Ammonia Emissions in Southern Bavaria

The atmospheric deposition of air pollutants was studied by means of monitoring canopy throughfall at six forest stands. The investigation was carried out in Norway spruce (Picea abies L. Karst.) forests in Southern Bavaria with high ambient ammonia concentrations due to either adjacent intensive agriculture or poultry housing. Five monitoring plots transected the forest edges and forest interior from the edge, at 50, 150, about 400 m and about 800m to the interior. Additionally, nutrient concentration in soil solution was sampled with suction cups at each plot, and C/N ratio of the humus layer was also determined. The variation of ambient ammonia concentration between three of the six investigated sites was estimated using diffusive samplers. In order to compare the effects of atmospheric deposition on European beech (Fagus sylvatica L.) and Norway spruce additional monitoring plotswere installed under each of these species in a mixed beech and spruce stand. Bulk deposition and soil water samples were analysed for major ions (NO₃ ^-, NH₄ ⁺, SO₄ ^2-, Cl^-, Na⁺, K⁺, Mg²⁺, Ca^2+M).The results show a substantial increase of deposition towards the forest edges for all ions. This so called 'edge effect' continued in most cases until a distance from 50 to 150 m from edge. For both ambient ammonia concentrations and nitrogen deposition, it can be concluded that increased dry deposition is the main reason for the edge effect. Over 76% of the nitrogen ratios in throughfall deposition between the edge and 50 m distance into the spruce forest exceed 1.0. Except for potassium, beech generally showed lower ratios than spruce.Due to high nitrogen deposition the forest floor, C/N ratios were lower at stand edges when compared to their interior. In contrast to the increase of nitrogen deposition at the edge, nitrate export below the main rooting zone was lower at the edge. Nitrate export was generally lower under beech than spruce. Nitrogen budgets of some plots were negative, indicating a reduction of total ecosystem nitrogen stock.The results show that forest edges, especially in areas with high air pollution, receive much more atmospheric deposition than the interior parts of closed forest stands. As many deposition studies in forests were conducted at field stations in the central parts of forests the estimated deposition for the whole forest may be underestimated. This may be important to consider in geo-statistical studies and models aiming to estimate spatial critical deposition values, especially with an increasing fragmentation of the forest cover.     

Competitive adsorption and desorption of oxytetracycline and cadmium with different input loadings on cinnamon soil

Purpose

The binary competitive effect could obviously influence the fate and transport behavior of oxytetracycline (OTC) and cadmium (Cd²⁺) in cinnamon soil. However, two pollutants loading into soil usually are different, perhaps because of the three reasons including occurrence of OTC before Cd²⁺, simultaneous occurrence of OTC and Cd²⁺, or occurrence of Cd²⁺ before OTC. The purpose of the study was to predict the competitive adsorption and desorption of OTC and Cd²⁺ as a function of above input loadings on cinnamon soil.

Materials and methods

Adsorption and desorption were determined using the batch equilibrium method in a single or binary system. The Freundlich equation was applied to describe the adsorption/desorption data of OTC and Cd²⁺ in order to obtain adsorption/desorption isotherms for each tested compound and the respective adsorption/desorption coefficients.

Results and discussion

The results indicated that cinnamon soil could strongly adsorb OTC with the adsorption affinity (K _f value) of more than 718 and Cd²⁺ with K _f value of more than 536 in the competitive and non-competitive system, and all adsorption and desorption isotherms of OTC and Cd²⁺ on cinnamon soil were well fitted by the Freundlich equation with r value of more than 0.99 (p?<?0.01). The coexistence of OTC and Cd²⁺ on cinnamon soil promoted significantly Cd²⁺ adsorption when Cd²⁺ firstly or simultaneously occurred on soil, but their coexistence did not affect adsorption of OTC when OTC firstly or simultaneously occurred on soil. Among the three input loadings, the pollutant with later occurring mode had lower K _f and hysteresis coefficient (HI) than the other two input loadings. According to the adsorption intensity parameter (1/n), the presence of Cd²⁺ or OTC with different input loadings could decrease the adsorption intensity of OTC or Cd²⁺ when compared with single occurrence of OTC or Cd²⁺.

Conclusions

The binary competitive effect influenced the adsorption/desorption of OTC and Cd²⁺ differently. The presence of OTC had a stronger influence on the adsorption/desorption of Cd²⁺ than the presence of Cd²⁺ on the adsorption of OTC. The later occurring pollutant on soil had stronger ecological risk than the former occurring pollutant in the binary competitive system. The physical adsorption in the single or binary system could be identified as the dominant mechanisms of OTC and Cd²⁺ adsorption.     

Methane emissions from stems of Fraxinus mandshurica var. japonica trees in a floodplain forest

We measured methane (CH₄) emissions from the stem surfaces of mature Fraxinus mandshurica var. japonica trees in a floodplain forest. Flux measurements were conducted almost monthly from May to October 2005, and positive CH₄ fluxes were detected throughout the study period, including the leafless season. The mean CH₄ flux was 176 and 97 μg CH₄ m⁻² h⁻¹ at the lower (15 cm above the ground) and upper (70 cm above the ground) stem positions, respectively. The CH₄ concentration was lower in soil gas than in ambient air to a depth of at least 40 cm. One possible source of CH₄ emitted from the stems might be the dissolved CH₄ in groundwater; maximum concentrations were 10,000 times higher than atmospheric CH₄ concentrations. Our results suggest that CH₄ transport from the submerged soil layer to the atmosphere may occur through internal air spaces in tree bodies.     

Fate of prions in soil: Degradation of recombinant prion in aqueous extracts from soil and casts of two earthworm species

Prions represent the active agent in transmissible spongiform encephalopathy (TSE) diseases and can remain infective to mammals even after prolonged periods in soil. The influence of mesofauna on prion dispersal and degradation in soil, however, remains unknown. In this study the effect of earthworms on the retention/dissemination of TSEs in soil was evaluated using a model recombinant prion protein (recPrP) and aqueous extracts from soil and fresh casts of two earthworm species, Lumbricus terrestris and Aporrectodea caliginosa. Our results showed that earthworm gut-derived enzymes did not enhance the degradation of recPrP in comparison to soil, even though non-prion related proteolytic activity was higher in fresh worm excrements than in soil samples. Complete degradation of recPrP occurred in the aqueous extracts from all samples within up to 6 days at +15 °C. The proteolytic enzymes responsible for degrading recPrP were inhibited by aprotinin and leupeptin and studies in pure cultures suggested these were most probably of soil microbial origin.     

A model to simulate response of plant stomata to environmental conditions

In order to simulate plant transpiration under different field climatic conditions we have developed and verified a semi-empirical model for predicting the stomatal response to solar global radiation, leaf temperature, vapour pressure difference between the leaf and ambient air, ambient air CO₂ concentration and soil water potential. The transpiration and the stomatal relative conductance of a Nicotania Tabaccum var “samsun” plant leaves were measured in a laboratory apparatus and compared to those predicted by the model: good agreement was obtained between them for the different investigated cases. The model was incorporated in a numerical greenhouse microclimate model and its effects on the canopy microclimate are discussed here.     

Microbial responses of forest soil to moderate anthropogenic air pollution

There is a need to introduce soil microbiological methods into long term ecological monitoring programs. For this purpose we studied the impact of moderate anthropogenic air pollution in polluted and less polluted area districts, forest site types Calluna (CT), Vaccinium (VT) and Myrtillus (MT) and the amount of organic matter, measured as carbon content on the soil respiration activity and the ATP content. The main sources of local air pollutants (SO₂ and NO_x) in the polluted area district were from the capital' region and an oil refinery. Humus (F/H-layer) and the underlying 0 to 5 cm mineral soil samples were collected from 193 study plots located in the 5300 km² study area. We found that the soil respiration rate in humus layer samples was lower in the polluted area district compared to the less polluted one (16.0 and 19.5 μL CO₂ h^?1g^?1 dw, respectively), but the difference occurred only in the dry, coarse-textured CT forest site type. The mineral soil respiration rate and the mineral soil and humus layer ATP content were not affected, by the air pollution. Most of the variations of the biological variables were explained primarily by the soil carbon content, secondly by the forest site type and thirdly by the area division.     

Effects of elevated atmospheric CO2 and N fertilization on abundance,diversity and C-isotopic signature of collembolan communities in arable soil

Rising atmospheric CO₂ concentrations are expected to have marked impacts on the carbon (C) turnover in agro-ecosystems through increased plant photosynthetic rates, leading to an enhanced biomass, and wider plant C/N ratios. Through increased carbon allocation below-ground, as well as through changed litter quality, CO₂ enrichment will indirectly affect soil faunal communities. In the present study we investigated how elevated atmospheric CO₂ and two different levels of N fertilization may affect abundance and diversity of collembolans, as important catalysts in decomposition processes, within an agro-ecosystem under winter wheat cultivation. The investigations were carried out in 2002 within a field experiment using the “Free Air CO₂ Enrichment” technique (FACE) at the Federal Agricultural Research Centre (Braunschweig, Germany). Stable C-isotopic analysis of collembolans, soil, and crops gave insight into C translocation. During our investigations δ¹³C values of all components analysed were significantly more negative under FACE compared to ambient air conditions. Stable C-isotopic signatures of collembolans were similar to those of soil under ambient air, but in between those of soil and roots under elevated CO₂ conditions. Our results revealed significant effects of both treatments (CO₂ enrichment and N fertilization) on density and species diversity of collembolans. Overall, collembolans were stimulated under elevated CO₂ conditions, showing an increased abundance of more than 50% (11 240 ind m⁻²) as well as a higher biodiversity (Shannon Weaver index = 2.5; evenness = 0.75) compared to ambient air conditions (7520 ind m⁻²; Shannon Weaver index = 2.2; evenness = 0.72). With regard to N supply, a decrease of about 20–30% under CO₂ enrichment and 45–55% under ambient air conditions in collembolan abundance with no alteration in diversity was recorded under reduced N fertilization. The observed impacts were species-specific.     

Increased snow depth affects microbial activity and nitrogen mineralization in two Arctic tundra communities

Microbial activity in Arctic tundra ecosystems continues through the winter and is an important component of the annual C budget. This activity is sensitive to climatic variation, particularly snow depth because that regulates soil temperature. The influence of winter conditions on soil N cycling is poorly understood. In this study, we used intact core incubations sampled periodically through the winter and following growing season to measure net N mineralization and nitrification in dry heath and in moist tussock tundra under ambient and experimentally increased snow depths (by use of a snowfence). In dry heath, we sampled soils under Dryas octopetela or Arctostaphylos alpine, while in tussock tundra, we sampled Eriophorum vaginatum tussocks and Sphagnum dominated areas between tussocks. Our objectives were to: (1) examine how different winter snow regimes influenced year-round N dynamics in the two tundra types, and (2) evaluate how these responses are affected by dominant species present in each system. In tussock tundra, soils with increased winter snow cover had high net N mineralization rates during the fall and winter, followed by immobilization during thaw. In contrast, N mineralization only occurred during the autumn in soils with ambient snow cover. During the growing season when N immobilization dominated in areas with ambient snow cover, soils with increased winter snow cover had positive net mineralization and nitrification rates. In dry heath tundra, soils with increased snow depth had high late winter net N mineralization rates, but these rates were: (a) comparable to early winter rates in soils under Arctostaphylos plants with ambient snow cover; (b) greater in soils under Arctostaphylos plants than in soils under Dryas plants; and (c) less than the rates found in tussock tundra. Our findings suggest under ambient snow conditions, low soil temperatures limit soil N mineralization, but that deeper snow conditions with the associated warmer winter soil temperatures dramatically increase over-winter N mineralization and thereby alter the amount and timing of plant-available N in tundra ecosystems.     

Particle-size distribution of PAH in the air of a remote Norway spruce forest in northern Bavaria

The aim of the present study was to elucidate thedistribution of particulate polycyclic aromatichydrocarbons (PAH) in the air of a remote Norwayspruce (Picea abies (L.) karst.) stand.The study encompassed a total of twenty differentcongeners. Particles in ambient air were alternativelycollected by two different Berner cascade impactors atthe field site in Northern Bavaria over a 14-weeksperiod from the end of April through to the end ofJuly 1994 and subsequently analyzed for their PAHcontent. The concentrations of total suspendedparticulate matter (TSP) ranged from 9–31 μg m^-3,suggesting that the field site is an area with a lowlevel of airborne particles. There was a negativerelationship of total particle concentrations with theamount of canopy precipitation due to particle washout at precipitation events. The distribution of theparticle masses on the different size fractions witha maximum in the 1-μm range is typical for remoteareas and indicates a medium-range transport. Theaerosol-bound PAH load decreased from spring sampleswith 2–4 ng m^-3 to values <1 ng m^-3 in the summersamples. Concurrently, the proportion of low molecularweight congeners in the total PAH load declined.Frequently, the highest PAH concentrations (referringto the air) were found in the 0.1–3 μm sizeseparates (accumulation mode). The results suggestthat apart from PAH input to the soil with litter, dryand wet deposition of aerosol particles is animportant pathway of PAH contamination of acid forestsoils in the Fichtelgebirge mountain range.     

Availability of soil nitrogen and phosphorus under elevated [CO2] and temperature in the Taihu Lake region,China

Global climate change affects the availability of soil nutrients, thereby influencing crop productivity. This study examined the effects of elevated [CO₂] and temperature on the availability of soil N and P in a paddy field in the Taihu Lake region, China. Winter wheat (Triticum aestivum L.) was planted at two levels of atmospheric [CO₂] (375 μmol L^–1 ambient; 575 μmol L^–1 elevated) and two temperature levels (ambient; ambient + 2°C). The results were as follows: Compared to ambient, the interaction effects of elevated [CO₂] and temperature significantly decreased soil NH$ _4^+ $ ‐N contents by 20.3%, 20.6%, and 18.7% in the jointing, heading, and ripening stages (p < 0.05), respectively, while soil NO₃^–‐N content had no clear variation trend under different [CO₂] and temperature conditions. Elevated [CO₂] significantly increased soil available P content by 14.3% in the jointing stage, and elevated temperature significantly decreased soil available P content by 18.8% in the jointing stage. Compared with ambient [CO₂], elevated [CO₂] significantly increased wheat biomass in jointing and heading stages (p < 0.05). The positive effect of elevated [CO₂] on wheat biomass was more significant at ambient temperature (AT) than at elevated temperature (ET) in the middle and late plant growth stages. These results explained that the availability of soil N and P varied under elevated [CO₂] and temperature conditions. The application of N and P should be adjusted to meet the need of wheat plants after the wintering stage.     

Enrichment of N2 and Ar in the atmosphere of CO2-consuming soils

The phenomenon of unexplained N₂Ar-enrichment in soil air is quite frequently to be encountered in soil air studies on anthropogenically influenced sites. In the present study two anthropogenic deposits and a calcareous fluvisol were investigated for their soil air composition. While in the alkaline deposits extreme enrichments of N₂ and Ar (N₂ + Ar: up to 99%, v/v) were found as persistent site characteristics, the fluvisol showed only slight (about 1%, v/v) transient N₂/Ar-enrichments in summer. All sites, which did not show substantial vertical seepage percolation, exhibited enhanced CO₂-solubility-either due to strong calcite precipitation or dissolution. So, it was concluded that intensive continuous depletion of CO₂ was responsible for the subsequent convective influx of atmospheric air. From the results obtained it was concluded that an encasement of the concerned soil volume rather impermeable to gas transport as well as intense dissolution of CO₂ in the pore water are prerequisites for substantial N₂/Ar-enrichments in soil air.     

The Effect of Air Pollution on Yield and Quality of Mung Bean Grown in Peri-Urban Areas of Varanasi

There is growing concern that air pollution may have adverse impacts on crops in developing countries, yet this has been little studied. This paper addresses this issue, for a major leguminous crop of the Indian sub continent, examining the effect of air pollution in and around an Indian city. A field study was conducted using a gradient approach to elucidate the impact of air pollutants on selected production characteristics of Vigna radiata L. cv. Malviya Jyoti (mung bean) plants grown from germination to maturity at locations with differing concentrations of air pollutants around peri-urban and rural areas of Varanasi. The 6 -h daily mean SO₂, NO₂ and O₃ concentrations varied from 8.05 to 32.2 ppb, 11.7 to 80.1 ppb and 9.7 to 58.5 ppb, respectively, between the sites. Microclimatic conditions did not vary significantly between the sites. Changes in plant performance at different sites were evaluated with reference to ambient air quality status. Reductions in biomass accumulation and seed yields were highest at the site experiencing highest concentrations of all three gaseous pollutants. The magnitude of response indicated that at peri-urban sites SO₂, NO₂ and O₃ were all contributing to these effects, whereas at rural sites NO₂ and O₃ combinations appeared to have more influence. The quality of seed was also found to be negatively influenced by the ambient levels of pollutants. It is concluded that the air pollution regime of Varanasi City causes a major threat to mung bean plants, both in terms of yield and crop quality, with serious implications for the nutrition of the urban poor.     

Principal reactions of airborne mercury,ozone and some other microcomponent in pure water or very dilute sulfuric acid

Numerous bubblings of pure H₂O or very dilute H₂SO₄-solutions with ambient air have been undertaken. All of them have shown the formation of HgIIa and HgIIb in the liquid phase (nomenclature according to Brosset, 1987). With increasing air volume such systems clearly approach a steady state. Earlier it has been shown that Hg° in water is oxidized by O₃ primarily to Hg²⁺, (Brosset, l.c., Iverfeldt and Lindqvist, 1986, Munthe, Thesis 1991). However, the attainment of a steady state indicates that a reaction involving a removal of Hg from the solution must exist as well. It has now been anticipated that a substance ‘X’, present in the ambient air, should be responsible, at least partly, for this removal. Starting from this assumption a model has been developed, which seems to be supported by all experimental tests hitherto performed by us. Further it has been tried to establish the chemical nature of ‘X’. So far, some experiments show that ‘X’ may be (CH₃)₂S. Finally it has been shown that HgIIb observed in precipitation is not identical with the corresponding compound formed by bubbling of ambient air at the ground level through it.     

The interactions between soil type and earthworm species determine the properties of earthworm casts

Earthworms are recognized to increase soil porosity, reorganize soil structure, and stimulate soil microflora and nutrient mineralization. The properties of earthworm casts should depend both on earthworm species or ecological group and on soil properties. Interactions between earthworm species and soil types have been suggested, but only poorly demonstrated. In order to better understand those interactions, two hypotheses led our study: (1) Soil type has a greater influence on cast properties than earthworm; (2) Earthworms from different species influence cast properties differently; (3) The intensity and direction of the impact of each earthworm species on cast properties vary with soil properties. Fifteen physical and chemical variables (N–NH₄⁺, N–NO₃⁻, total organic C and N, C/N ratio, CaCO₃, pH, P, K⁺, Mg²⁺, Mn²⁺, Na⁺, CEC, moisture, wettability) were measured in casts of three earthworm species (Lumbricus terrestris, Allolobophora chlorotica and Aporrectodea rosea) produced in three temperate soils. Univariate and multivariate analyses showed that earthworm species and soil types significantly impacted cast properties. pH, N_t, K and Mg contents were interactively altered by both factors. Multivariate analysis showed that a difference of soil type had a major impact on casts properties (62%) compared to the impact of a difference of earthworm species (10%). Cast properties were most impacted by L. terrestris, then by A. chlorotica and last by A. rosea. The response ratio (ratio of the properties of the casts to the properties of the bulk soil) was used to quantify the effect of earthworm species compared to the control soil. It showed a higher response of variables in casts in nutrient-rich soils, especially in casts of L. terrestris. The interactions between earthworm species and soil types on cast properties were discussed with regards to earthworm ecology, properties of the soil, and earthworm modifications of cast microflora.