Factors affecting the growth of Eucalyptus delegatensis seedlings in inhibitory forest and grassland soils

In many highland forests of Eucalyptus delegatensis in Tasmania the establishment and healthy growth of eucalypts is promoted and maintained by fire. In the absence of fire, secondary succession from eucalypt forest to rainforest occurs, during which the eucalypts decline and die prematurely. On sites that are prone to radiation frost severe reduction or removal of a tree canopy allows a sward of tussock grasses to develop, in competition with which seedlings of eucalypts decline in growth and a high proportion dies.Factors of the soil that could contribute to these phenomena were investigated by means of pot experiments that used soils from:o     

The effects of ammonium sulphate deposition and root sinks on soil solution chemistry in coniferous forest soils

The effects of enhanced (NH_{4 2}SO₄deposition on soil solution cation and anion concentrations and annualionic fluxes were followed using a standardised experimental protocolin six European coniferous forests with contrasting soil types, pollutioninputs and climate. Native soil cores containing a ceramic suction cupwere installed in the field, roofed and watered every two weeks withlocal throughfall or local throughfall with added(NH₄)₂SO₄ at 75 kgNH₄ ⁺-N ha^-1 a^-1. Livingroot systems were established in half of the lysimeters.Untreated throughfall NH₄ ⁺-N deposition at thesites ranged from 3.7 to 29 kg ha^-1 a^-1Soil leachates were collected at two weekly intervalsover 12 months and analysed for volume, andconcentrations of major anions and cations. Increasesin soil solution NO₃ ^- concentrations inresponse to N additions were observed after 4–9months at three sites, whilst one sandy soil with highC:N ratio failed to nitrify under any of thetreatments. Changes in NO₃ ^- concentrationsin soil solution controlled soil solution cationconcentrations in the five nitrifying soils, withAl³⁺ being the dominant cation in the more acidsoils with low base saturation. The acidification responses ofthe soils to the (NH_{4 2}SO₄additions were primarily related to the ability of thesoils to nitrify the added NH₄ ⁺. pH and soiltexture seemed important in controllingNH₄ ⁺ leaching in response to the treatments,with two less acidic, clay/clay loam sites showingalmost total retention of added NH₄ ⁺, whilstnearly 75% of the added N was leached asNH₄ ⁺ at the acid sandy soils. The presenceof living roots significantly reduced soil solutionNO₃ ^- and associated cation concentrations attwo of the six sites. The very different responses of the sixsoils to increased (NH₄)₂SO₄deposition emphasise that the establishment of N critical loadsfor forest soils need to allow for differences in N storagecapacity and nitrification potential.     

Methane consumption in two temperate forest soils

Forest soils are thought to be an important sink for atmospheric methane. To evaluate methane consumption,¹⁴C-labeled methane was added to the headspace of intact soil cores collected from a mixed mesophytic forest and from a red spruce forest located in the central Appalachian Mountains. Both soils consumed the added methane at initially high rates that decreased as the methane mixing ratio of the air decreased. The mixed mesophytic forest soil consumed an average of 2 mg CH₄ m^–2 d^–1 versus 1 mg CH, m^–2 d^–1 for the spruce forest soil. The addition of acetylene to the headspace completely suppressed methane consumption by the soils, suggesting that an aerobic methane-consuming microorganism mediated the process. At both forest sites, methane mixing ratios in soil air spaces were greater than that in the air overlying the soil surface, indicating that these soils had the ability to produce methane. Models of methane emission from forest soils to the atmosphere must represent methane flux as the balance between production and consumption of methane, which are controlled by very different factors     

Contemporary carbon stocks of mineral forest soils in the Swiss Alps

Soil organic carbon (SOC) has been identified as the main globalterrestrial carbon reservoir, but considerable uncertainty remains as toregional SOC variability and the distribution of C between vegetationand soil. We used gridded forest soil data (8–km × 8–km)representative of Swiss forests in terms of climate and forest typedistribution to analyse spatial patterns of mineral SOC stocks alonggradients in the European Alps for the year 1993. At stand level, meanSOC stocks of 98 t C ha^–1 (N = 168,coefficient of variation: 70%) were obtained for the entiremineral soil profile, 76 t C ha^–1 (N =137, CV: 50%) in 0–30 cm topsoil, and 62 t Cha^–1 (N = 156, CV: 46%) in0–20 cm topsoil. Extrapolating to national scale, we calculatedcontemporary SOC stocks of 110 Tg C (entire mineral soil, standarderror: 6 Tg C), 87 Tg C (0–30 cm topsoil, standarderror: 3.5 Tg C) and 70 Tg C (0–20 cm topsoil, standarderror: 2.5 Tg C) for mineral soils of accessible Swiss forests(1.1399 Mha). According to our estimate, the 0–20 cm layers ofmineral forest soils in Switzerland store about half of the Csequestered by forest trees (136 Tg C) and more than five times morethan organic horizons (13.2 Tg C).At stand level, regression analyses on the entire data set yielded nostrong climatic or topographic signature for forest SOC stocks in top(0–20 cm) and entire mineral soils across the Alps, despite thewide range of values of site parameters. Similarly, geostatisticalanalyses revealed no clear spatial trends for SOC in Switzerland at thescale of sampling. Using subsets, biotic, abiotic controls andcategorial variables (forest type, region) explained nearly 60%of the SOC variability in topsoil mineral layers (0–20 cm) forbroadleaf stands (N = 56), but only little of thevariability in needleleaf stands (N = 91,R ² = 0.23 for topsoil layers).Considerable uncertainties remain in assessments of SOC stocks, due tounquantified errors in soil density and rock fraction, lack of data onwithin-site SOC variability and missing or poorly quantifiedenvironmental control parameters. Considering further spatial SOCvariability, replicate pointwise soil sampling at 8–km × 8–kmresolution without organic horizons will thus hardly allow to detectchanges in SOC stocks in strongly heterogeneous mountain landscapes.     

Rhizospheric Processes Influencing the Biogeochemistry of Forest Ecosystems

In the rhizosphere, biotic and abiotic processes interact to create a zone distinct from the bulk soil that may strongly influence the biogeochemistry of forest ecosystems. This paper presents a conceptual model based upon three operationally defined soil-root compartments (bulk soil, rhizosphere and soil-root interface) to assess nutrient availability in the mineral soil-root system. The model is supported by chemical and mineralogical analyses from bulk and rhizosphere soils collected from a Norway spruce forest. The rhizosphere was more intensively weathered and had accumulated more acidity, base cations and phosphorus than the bulk soil. The quantity and quality of organic matter regulate the reciprocal relationships between soil and roots with their associated biota. However, the biogeochemical role of organic matter in the rhizosphere still remains as an area in which more future research is needed. The mechanisms that may regulate nutrient availability in the rhizosphere are also discussed and related to nutrient cycling and adaptation of forests growing under nutrient poor or perturbed conditions. We suggest that the rhizosphere is not an ephemeral environment in the soil, but persists over time and is resilient against perturbation as evinced by consistent differences between rhizosphere and bulk chemistry and mineralogy over wide range of field treatments.     

火对森林土壤的影响

林火是林地上自由蔓延的火,主要有计划火烧(控制火烧)和野火2种类型.火烧的严重程度决定了火灾的效果及其影响的持续时间.火能够通过燃烧有机质和改变粘土矿物来影响土壤结构,导致土壤容重增加和持水能力降低.长期来看,火烧引起土壤有机质减少.火烧之后土壤温度和pH值的升高会促进土壤呼吸,而微生物数量的减少、根系的死亡和可分解物质的减少则减弱土壤呼吸.林火消耗了森林的枯落物层,通过氧化、挥发、灰分颗粒对流、淋溶等途径减少了森林土壤的养分含量.火灾对土壤生物有重要影响,火灾引起的高温可直接杀死土壤微生物,同时通过改变土壤的各种理化性质及林地微环境,间接影响土壤微生物生存和群落的组成.火对土壤动物的影响显著小于对土壤微生物的影响,原因是前者具有更高的灵活性来逃避火灾.中低程度火烧主要通过土壤环境变化间接影响土壤酶,而严重火烧则通过高温直接使其变性.今后应开展跨学科的森林火灾长期研究,加强对火烧后土壤动物、微生物和生态系统之间相互关系的研究,注重计划用火、火灾对土壤有机碳、土壤养分库的稳定、土壤斥水性、水土流失强度和生物多样性影响方面的研究.     

祁连山高山植物根际土放线菌生物多样性

从祁连山老虎沟不同海拔位点的15种植物根际土中培养得到78株特异表型放线菌,并结合菌体形态、生理代谢特征、抗菌活性及16S rDNA序列对其生理及系统发育多样性进行了研究。结果表明,分离菌株分属于链霉菌属(Streptomyces spp.)(73株)、诺卡氏菌属(Nocardia spp.)(4株),另有1株与GenBank中同源性最高的菌株Micromonospora saelicesensis相似性达92%,为1潜在新种。链霉菌属为主要类群,占分离菌株的93.6%,该属菌株在5个海拔位点的15种植物根际土中均有分布,但存在海拔位点、植物种类的差异性和特异性;诺卡氏菌属的菌株仅见于海拔2200 m的猪毛菜、海拔2800 m的钉柱萎陵菜和3800 m处的甘肃蚤缀根际土中;1潜在新种分离自海拔2200 m处的沙生针茅根际土。次级代谢物产生和拮抗性筛选研究结果表明:H2O2酶、脂酶2(Tween-40)、脲酶、蛋白酶、脂酶3(Tween-80)、淀粉酶、H2S、脂酶1(Tween-20)、可溶性色素及有机酸这10类次级代谢物产生菌分别占供试菌株的89.7%、82.1%、70.5%、62.8%、53.8%、52.6%、48.7%、44.9%、32.1%和17.9%,其中,淀粉酶、脂酶1、色素和有机酸仅由链霉菌产生;有29株放线菌对参试人类病原菌具有抑制作用,占供试菌株的37.2%,分布于5个海拔位点的12种植物根际土,其中,从药用植物甘肃黄芪和四裂红景天根际土中分离到的抗性菌株占拮抗性放线菌总数的60%。研究表明,高山地区植物根际土放线菌资源丰富,菌株生理功能多样,是新放线菌种和生物活性物质的重要资源库。     

Ambient ozone effects on forest trees of the eastern United States: a review

Tropospheric ozone can affect crop yield and has been reported to cause reductions in growth and biomass of forest tree species in laboratory and glasshouse studies. However, linkages between growth and ambient ozone concentrations in the field are not well established for forest trees. Ambient ozone concentrations have been shown to cause foliar injury on a number of tree species throughout much of the eastern USA. Symptom expression is influenced by endogenous and exogenous factors and, therefore, ozone-exposure/tree-response relationships have been difficult to confirm. Clearly defined, cause-effect relationships between visible injury and growth losses due to ozone have not been validated. Generalizations of sensitivity of forest trees to ozone are complicated by tree development stage, microclimate, leaf phenology, compensatory processes, within-species variation and other interacting stresses. In general, decreases in above-ground growth at ambient ozone levels in the eastern USA appear to be in the range of 0–10% per year. However, these conclusions are based on a small number of tree species, with the vast majority of studies involving individual tree seedlings in a non-competitive environment. Comparative studies of small and large trees indicate that seedlings are not suitable surrogates for predicting responses of mature trees to ozone. Process-level modelling is a promising methodology that has been recently utilized to assess ozone effects on a stand to regional scale, indicating that ozone is affecting forest growth in the eastern USA. The extent and magnitude of the response is variable and depends on many edaphic and climatic factors. It is imperative when conducting assessment exercises, however, that forest biologists constantly keep in mind the tremendous variability that exists within natural systems. Scaling of single site/physiological response phenomena from an individual tree to an ecosystem and/or region necessitates further research.     

Relevance of Rhizosphere Research to the Ecological Risk Assessment of Trace Metals in Soils

The chemical, mineralogical, and microbial properties of the rhizosphere of a range of forested ecosystems were studied to identify the key processes controlling the distribution and fate of trace metals at the soil–root interface. The results of our research indicate that: (1) the rhizosphere is a soil microenvironment where properties (e.g., pH, organic matter, microbes) and processes (nutrient and water absorption, exudation) differ markedly from those of the adjacent bulk soil; (2) the rhizosphere is a corrosive medium where the weathering and neoformation of soil solid phases are enhanced; (3) the concentrations of solid-phase and water-soluble trace metals like Cd, Cu, Ni, Pb, and Zn are generally higher in the rhizosphere as shown by both macroscopic and microscopic approaches; (4) a larger fraction of water-soluble metals is complexed by dissolved organic substances in the rhizosphere; and (5) soil microorganisms play, either directly or indirectly, a distinct role on metal speciation, in particular Cu and Zn, in the rhizosphere. These results improve our capacity to estimate metal speciation and bioavailability at the soil–root interface. Furthermore, the research emphasizes the crucial physical position occupied by the rhizosphere with respect to the process of elemental uptake by plants and its key functional role in the transfer of trace metals along the food chain. We conclude that the properties and processes of the rhizosphere should be viewed as key components of assessments of the ecological risks associated with the presence of trace metals in soils.     

Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, differing from that of the bulk soil found only a few mm from the root. The aim of this study was to investigate adaptation mechanisms of herbs growing in acid soils through studying their rhizosphere chemistry in a greenhouse experiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 in the soil solution) in the greenhouse. The concentrations of NO₃ ^-, SO₄ ^2-, phosphates, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, NH₄ ⁺ and pH were analysed in soil solutions obtained by centrifugation. The general pattern found was a depletion of nutrients in the rhizosphere compared with their concentrations in the bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appeared to be caused by plant uptake of NO₃ ^- (r²=0.88). The ion concentrations in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher growth rates showed a higher degree of ion depletion (except for Na⁺, SO₄ ^2-) in the rhizosphere, there were also species specific responses. A field study of five herbs at five oak forest sites in Southern Sweden (Scania) was also carried out. In addition to the soil solution concentrations, the loss on ignition (LOI) and the concentrations of 0.1 M BaCl₂ extractable K⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Al ions were measured. The amount of soil solution Al was determined as free ionic (quickly reacting) Al. For all species and sites, the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measured in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO₃ ^- and quickly reacting Al. The lower concentrations of quickly reacting Al in the rhizosphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were only small and mostly non-significant. Plant-soil interactions and the ability of plants to utilise heterogeneity of the soil appear to be more important for plant growth in acid soils than recognised heretofore. Rhizosphere studies provide an important means of understanding plant strategies in acid soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantification of ammonium sorption in acid forest soils by sorption isotherms

Quantity/Intensity relations of NH₄ ⁺ were established to evaluate the role of NH₄ ⁺ sorption as a N-sink as well as for the transport of NH₄ ⁺ within the soil profile. Two haplic podzols (originating from Phyllite and Granite) from the German Fichtelgebirge were used. Exchange isotherms were obtained from batch experiments with NH₄ ⁺ concentrations between 0.01 to 0.6 mmol NH₄ ⁺ L^-1. Background solutions had a composition close to the natural soil solutions to identify the cations displaced by NH₄ ⁺. Organic layers sorbed a maximum amount of 0.3–0.5 mmol_c NH₄ ⁺ 100 g^-1, A horizons 0.1–0.2 mmol_c NH₄ ⁺ 100 g^-1 and B horizons 0.09 mmol_c 100 g^-1, corresponding to less than 1% of the cation exchange capacities. NH₄ ⁺ displaced K⁺ and Ca²⁺ throughout the whole profile, and additionally Al in the subsoil horizons and H₃O⁺ in the organic and A horizons. Initial mass isotherms (Quantity/Quantity isotherms) were established with distribution coefficients ranging from 0.17 to 0.35 for NH₄ ⁺ with highest values in the O horizons. While exchangeable NH₄ ⁺ in acid forest soils is considered to be only a minor long-term sink for deposited NH₄ ⁺, it has its relevance in the seasonal dynamics and transport of NH₄ ⁺.     

Mineral control of organic carbon mineralization in a range of temperate conifer forest soils

Coupled climate–ecosystem models predict significant alteration of temperate forest biome distribution in response to climate warming. Temperate forest biomes contain approximately 10% of global soil carbon (C) stocks and therefore any change in their distribution may have significant impacts on terrestrial C budgets. Using the Sierra Nevada as a model system for temperate forest soils, we examined the effects of temperature and soil mineralogy on soil C mineralization. We incubated soils from three conifer biomes dominated by ponderosa pine (PP), white fir (WF), and red fir (RF) tree species, on granite (GR), basalt (BS), and andesite (AN) parent materials, at three temperatures (12.5°C, 7.5°C, 5.0°C). AN soils were dominated by noncrystalline materials (allophane, Al‐humus complexes), GR soils by crystalline minerals (kaolinite, vermiculite), and BS soils by a mix of crystalline and noncrystalline materials. Soil C mineralization (ranging from 1.9 to 34.6 [mg C (g soil C)^?1] or 0.1 to 2.3 [mg C (g soil)^?1]) differed significantly between parent materials in all biomes with a general pattern of ANδ¹³C values of respired CO₂ suggest greater decomposition of recalcitrant soil C compounds with increasing temperature, indicating a shift in primary C source utilization with temperature. Our results demonstrate that soil mineralogy moderates soil C mineralization and that soil C response to temperature includes shifts in decomposition rates, mineralizable pool size, and primary C source utilization.     

Physicochemical and biological factors affecting atmospheric methane oxidation in gray forest soils

The decline of methane oxidizing activities in gray forest soil upon its conversion into arable land was shown to be caused by major changes in biotic and physicochemical properties of soil. Using the method of immune serums, methane-oxidizing bacteria were detected in both forest and agricultural soils, but their populations differed significantly in both abundance and composition. In the forest soil, the number of methanotrophs was an order of magnitude higher than in arable soil, amounting to 3.5 × 10⁸ and 0.24 × 10⁸ cells/g soil, respectively. All methane-oxidizing bacteria identified in the forest soil belonged to the genus Methylocystis, and 94% of these were represented by a single species, M. parvus. The arable soil was dominated by type I methanotrophs (Methylobacter and Methylomonas, 67.6%), occurring along with bacteria of the genus Methylocystis. In addition, arable soil is characterized by a low content of microbial biomass, lower porosity and water resistance of soil aggregates, and the predominance of nitrogen mineralization processes over those of nitrogen immobilization. These factors can also contribute to lower rates of methane oxidation in arable soil as compared to forest soil.__________Translated from Mikrobiologiya, Vol. 74, No. 2, 2005, pp. 255–260.Original Russian Text Copyright © 2005 by Kravchenko, Semenov, Kuznetsova, Bykova, Dulov, Pardini, Gispert, Boeckx, Cleemput, Galchenko.     

Mobilization of aluminium in the rhizosphere soil solution of growing tree roots in an acidic soil

Chemical conditions in the rhizosphere in many respects are different from the bulk soil. Especially in acid forest soils aluminium chemistry at the soil root interface is of particular interest because of its importance for evaluating the risk of rhizotoxicity. In the present study we have used micro suction cups to collect soil solution from the rhizosphere of oak seedlings (Quercus robur L.) in high spatial resolution and capillary electrophoresis for the determination of major ions and Al³⁺. While the concentrations of nutrient cations, especially Ca²⁺ and Mg²⁺, decreased in the vicinity of growing roots the concentrations of Al³⁺ significantly increased. Al³⁺-ions were probably released when root-exuded protons were buffered by the soil. Their occurrence indicates, that the oak roots in our experiments had only limited capabilities to detoxify Al in their rhizosphere. The restriction of this effect to the very small soil compartment close to the roots suggests, that common soil analysis which neglect rhizosphere processes might greatly underestimate the in situ concentration of Al³⁺ near tree roots. Our experiments furthermore indicate, that also suberized roots have a significant influence on rhizosphere soil solution chemistry. This revised version was published online in June 2006 with corrections to the Cover Date.     

Five centuries of changing forest vegetation in the Northeastern United States

Maps of physiognomic classes of vegetation in the northeastern UnitedStates before European human impact and for the late 20th century, based onpollen data, closely resemble each other, indicating a robust pattern in theface of the novel and extensive disturbances of the last 500 years. On the otherhand, species abundances have changed considerably, with today's vegetationhaving less Fagus L. and TsugaCarrière and more Betula L.,Picea A. Dietr., and Abies Miller.Picea and Castanea Miller increasedfrom 1700 to 1900, but have decreased in this century. The difference betweenthe main physiognomic classes has remained strong, however, with continuouslymore Picea, Abies andBetula in the north and more QuercusL. and Carya Nutt. in the south, indicating the dominanceof climate in regulating the relative abundances of these genera. Thevegetational patterning within these broad classes has changed in some placesand remained stable in others, suggesting differences in local factors thatdetermine relative species abundances. The influence of slight altitudinalgradients, for example, may influence forest composition only after severalgenerations of trees, while substrate differences may be apparent in the firstforest that regenerates after agricultural abandonment or logging. Forests oftoday, therefore, reflect complex interactions of disturbance and environment,and may be in equilibrium with some but not all features of their currentenvironments.     

自然林下与田间根腐三七根际微生物群落特征及比较分析

【背景】三七根际微生物群落特征与其土传根腐病害密切相关,而针对自然林下根腐三七的相关研究鲜见报道。【目的】比较分析自然林下与田间根腐三七根际土壤微生物群落的组成特征,结合土壤理化性质与酶活性分析,为三七根腐病害防治与仿野生栽培提供科学依据。【方法】采集自然林下与田间根腐三七根际土壤,利用高通量测序技术,分析土壤细菌与真菌群落的物种组成与多样性,并测定土壤理化性质和酶活性。【结果】自然林下与田间根腐三七根际土壤中细菌和真菌群落组成具有明显差异,自然林下根腐三七根际土壤中担子菌门(Basidiomycota)、酸杆菌门(Acidobacteria)和疣微菌门(Verrucomicrobia)的相对丰度较高,而田间根腐三七根际土壤中子囊菌门(Ascomycota)、变形菌门(Proteobacteria)和绿弯菌门(Chloroflexi)的相对丰度较高。在属分类水平,镰刀菌属(Fusarium)是自然林下根腐三七根际土壤中的优势菌群,相对丰度为17.30%,而癣囊腔菌属(Plectosphaerella)是田间根腐三七根际土壤中的优势菌群,相对丰度为22.55%;Candidatus Ba...     

Assessment of soil calcium status in red spruce forests in the northeastern United States

Long-term changes in concentrations of available Ca in soils of redspruce forests have been documented, but remaining questions aboutthe magnitude and regional extent of these changes have precluded anassessment of the current and future status of soil Ca. To addressthis problem, soil samples were collected in 1992—93from 12 sites in New York, Vermont, New Hampshire, and Maine toprovide additional data necessary to synthesize all availableresearch results on soil Ca in red spruce forests. Sites werechosen to encompass the range of environmental conditionsexperienced by red spruce. Concentrations of exchangeableCa ranged from 2.13 to 21.6 cmol_ckg^–1 in the Oa horizon, and from 0.11 to 0.68cmol_c kg^–1 in the upper 10 cm of theB horizon. These measurements expanded the range of exchangeable Ca reported in the literature for both horizons in northeastern redspruce forests. Exchangeable Ca was the largest Ca fraction in theforest floor at most sites (92% ofacid-extractableCa), but mineral Ca was the largest fraction at the three sites that also had the highest mineral-matter concentrations. Theprimary factor causing variability in Ca concentrations among siteswas the mineralogy of parent material, but exchangeable concentrationsin the B horizon of all sites were probably reduced by acidicdeposition. Because the majority of Ca in the forest floor isin a readily leachable form, and Ca inputs to the forest floor from the mineral soil and atmospheric deposition have beendecreasing in recent decades, the previously documented decreasesin Ca concentrations in the forest floor over previous decades mayextend into the future.     

Changes in carbon storage in temperate humic loamy soils after forest clearing and continuous corn cropping in France

Soil samples from forest and agricultural sites in three areas of southwest France were collected to determine the effect of forest conversion to continuous intensive corn cropping with no organic matter management on soil organic carbon (C) content. Soils were humic loamy soils and site characteristics that may affect soil C were as uniform as possible (slope, elevation, texture, soil type, vegetation). Three areas were selected, with adjacent sites of various ages of cultivation (3 to 35 yr), and paired control forest sites. The ploughed horizon (0-Dt cm) and the Dt-50 cm layer were collected at each agricultural site. In forest sites, each 10 cm layer was collected systematically down to 1 meter depth. Carbon concentrations were converted to total content to a given depth as the product of concentration, depth of sample and bulk density, and expressed in units of kg m^-2. For each site and each sampled layer, the mineral mass of soil was calculated, in order to base comparisons on the same soil mass rather than the same depth. The pattern of C accumulation in forest soils showed an exponential decrease with depth. Results suggested that soil organic carbon declined rapidly during the first years of cultivation, and at a slower rate thereafter. This pattern of decrease can be fitted by a bi-exponential model assuming that initial soil organic carbon can be separated into two parts, a very labile pool reduced during the first rapid decline and more refractory fractions oxidizing at a slower rate. Sampling to shallow depths (0-Dt cm) resulted in over-estimation of the rate of carbon release in proportion to the initial amount of C, and in under-estimation of the total loss of C with age. The results for the 0–50 cm horizon indicated that losses of total carbon average about 50% in these soils, ranging in initial carbon content from 19 to 32.5 kg m^-2. Carbon release to the atmosphere averaged 0.8 kg m^-2 yr^-1 to 50 cm depth during the first 10 years of cultivation. The results demonstrate that temperate soils may also be an important source of atmospheric carbon, when they are initially high in carbon content and then cultivated intensively with no organic matter management.     

Functional diversity and community structure of microorganisms in rhizosphere and non-rhizosphere Canadian arctic soils

Functional diversities of microorganisms in arctic soil samples at three incubation temperatures were assessed using sole-carbon-source-utilization (SCSU). Soil samples from four sites were collected from the rhizosphere and non-rhizosphere soils. Microorganisms were extracted from samples and inoculated into ECO-Biolog plates and incubated at 4, 10 and 28 °C. Calculations of Shannon–Weaver diversity and Shannon–Weaver evenness were based on the substrate utilization in the Biolog plates. Shannon–Weaver diversities (H) in rhizosphere samples were significantly greater ( _H = 3.023 ± 0.197; P < 0.005) than in non-rhizosphere samples ( _H = 2.770 ± 0.154). Similarly, the evenness (E) of the inoculated microbial cells exhibited significant differences (P < 0.005) between the rhizosphere and non-rhizosphere soil samples ( _E = 0.880 ± 0.057 for soils with rhizosphere; _E = 0.807 ± 0.044 for non-rhizosphere samples). Higher microbial diversity and evenness were observed in samples incubated at 4 °C than at 28 °C [least significant difference (lsd) = 0.29], and evenness indices were higher in rhizosphere samples than in non-rhizosphere soils incubated at all three temperatures (lsd = 0.02). Principal component analysis (PCA) of the multivariate data set differentiated the soil samples on the relatively gross scale of microbial communities isolated from rhizosphere and non-rhizosphere soils at all three temperatures.