Soil Characteristics and the Accumulation of Inorganic Nitrogen in an Arid Urban Ecosystem

Urbanization represents the extreme case of human influence on an ecosystem. Biogeochemical cycling of nitrogen (N) in cities is very different from that of non-urban landscapes due to the large input of reactive forms of N and the heterogeneous distribution of various land uses that alters landscape connections. To quantify the likely effects of human activities on soil N and other soil properties in urban ecosystems, we conducted a probability-based study to sample 203 plots randomly distributed over the 6,400 km² Central Arizona-Phoenix Long-Term Ecological Research (CAP LTER) area, which encompasses metropolitan Phoenix with its 3.5 million inhabitants. Soil inorganic N concentrations were significantly higher in urban residential, non-residential, agricultural, transportation, and mixed sites than in the desert sites. Soil water content and organic matter were both significantly higher under urban and agricultural land uses, whereas bulk density was lower compared to undeveloped desert. We calculated that farming and urbanization on average had caused an accumulation of 7.23 g m⁻² in soil inorganic N across the CAP study area. Average soil inorganic N of the sampled desert sites (3.23 g m⁻²) was much higher than the natural background level reported in the literature. Laboratory incubation studies showed that many urban soils exhibited net immobilization of inorganic N, whereas desert and agricultural soils showed small, but positive, net N mineralization. The large accumulation of inorganic N in soils (mostly as nitrate) was highly unusual in terrestrial ecosystems, suggesting that in this arid urban ecosystem, N is likely no longer the primary limiting resource affecting plants, but instead poses a threat to surface and groundwater contamination, and influences other N cycling processes such as denitrification.     

Responses of soil microorganisms to resource availability in urban, desert soils

Terrestrial desert ecosystems are strongly structured by the distribution of plants, which concentrate resources and create islands of fertility relative to interplant spaces. Atmospheric nitrogen (N) deposition resulting from urbanization has the potential to change those spatial patterns via resource inputs, resulting in more homogeneous soil resource availability. We sampled soils at 12 desert remnant sites around Phoenix, Arizona along a model-predicted gradient in N deposition to determine the degree to which deposition has altered spatial patterns in soil resource availability and microbial activity. Soil microbial biomass and abundance were not influenced by atmospheric N deposition. Instead, plant islands remained strong organizers of soil microbial processes. These islands of fertility exhibited elevated pools of resources, microbial abundance, and activity relative to interspaces. In both plant islands and interspaces, soil moisture and soil N concentrations predicted microbial biomass and abundance. Following experimental wetting, carbon dioxide (CO₂) flux from soil of interspaces was positively correlated with N deposition, whereas in plant islands, soil CO₂ flux was positively correlated with soil moisture content and soil organic matter. Soil CO₂ flux in both patch types showed rapid and short-lived responses to precipitation, demonstrating the brief time scales during which soil biota may process deposited materials. Although we observed patterns consistent with N limitation of microbes in interspaces, we conclude that atmospheric N deposition likely accumulates in soils because microbes are primarily limited by water and secondarily by carbon or nitrogen. Soil microbial uptake of atmospherically deposited N likely occurs only during sparse and infrequent rainfall.     

Distribution of plant species at a biome transition zone in New Mexico

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe — Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species? Location: central New Mexico, USA. Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500‐ha transitional mosaic of patches. Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis‐dominated patches and 12 species with B. eriopoda‐dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis‐dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda‐dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch‐scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes. Conclusions: Patch types at this biome transition zone have characteristic life‐form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.     

Nitrogen stable isotope ratios in surface sediments, epilithon and macrophytes from upland lakes with differing nutrient status

1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays. 2. Considerable variation was found in δ¹⁵N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ¹⁵N were positively correlated with δ¹⁵N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment δ¹⁵N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ¹⁵N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte δ¹⁵N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ¹⁵N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.     

Nitrogen-fixers <Emphasis Type="Italic">Alnus</Emphasis> and <Emphasis Type="Italic">Lupinus</Emphasis> influence soil characteristics but not colonization by later successional species in primary succession on Mount St. Helens

Changes to the primary successional environment caused by colonizing plants that present symbiotic associations with nitrogen-fixing bacteria were investigated at two areas on Mount St. Helens. One area was occupied by alder (Alnus viridis) thickets and old lupine (Lupinus lepidus) patches and the other area by young lupine patches and pumice barrens. Alder thicket soils had higher levels for a few soil nutrients and had greater cover by other pioneer species as compared to old lupine patches. Many soil nutrients, including nitrogen and soil organic matter, were below detection limits in old lupine patches but not in alder thicket soils. Young lupine patch soils were generally not different from barren site soils but had greater cover by other pioneer species. Below detection nitrogen and soil organic matter levels also occurred in many barren soil samples but not in young lupine patch soils. Barren soils were moister than were the other sites. The apparent increase in soil fertility has not led to invasion by later successional species, perhaps due to dry conditions or to other inhibitory factors. Seedbanks, composed of early successional species, appear to be developing in these areas.     

Soil CO2 efflux measurements in a mixed forest: impact of chamber disturbances,spatial variability and seasonal evolution

A closed‐dynamic‐chamber system (CDCS) was used to measure the spatial and seasonal variability of the soil CO₂ efflux (F_s) in beech and in Douglas fir patches of the Vielsalm forest (Belgium). First the difference between natural and measured soil CO₂ efflux induced by the presence of the CDCS was studied. The impact on the measurements of the pressure difference between the outside (natural condition) and the inside of the chamber was found to be small (0.4%). The influence of wind disturbance in the closed chamber was tested by comparison with an open‐chamber system characterized by a different wind distribution. A very good correlation between the two systems was found (r² = 0.99) but the open system yielded slightly lower fluxes than the closed one (slope = 0.88 ± 0.05). A measurement procedure has been developed to minimize the effect of the other sources of perturbation. The spatial and seasonal evolution of the soil CO₂ efflux was obtained by performing regular measurements on 29 spots in the beech patch over a period of 12 months and on 24 spots in the Douglas fir patch over 8 months. For each spot, the experimental relationship between Fs and soil temperature was compared with the fitted line for an Arrhenius relationship with a soil temperature‐dependent activation energy. Soil temperature explains 73% of the seasonal variation for all the data. The spatial average of the soil CO₂ efflux at 10 °C (Fs₁₀) in the beech patch is 2.57 ± 0.41 μmol m^?2 s^?1, approximately twice the average in the Douglas fir patch recorded at 1.42 ± 0.22 μmol m^?2 s^?1. The litter fall analysis seems to indicate that soil organic matter quality and quantity may be one the reasons for this difference. Finally the annual soil CO₂ efflux was calculated for the beech and Douglas fir patches (870 ± 140 and 438 ± 68 gC m^?2 y^?1, respectively). The beech value would represent 92 ± 15% of the annual ecosystem respiration estimated from the eddy covariance measurements.     

Short-term variation in the isotopic composition of organic matter allocated from the leaves to the stem of Pinus sylvestris: effects of photosynthetic and postphotosynthetic carbon isotope fractionation

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ¹³C of the fast‐turn‐over carbon pool (water soluble organic carbon and CO₂ emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem. For that purpose, we determined δ¹³C in total and water‐soluble organic matter of the foliage plus δ¹³C and δ¹⁸O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine‐day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the δ¹³C of stem‐emitted CO₂. In the canopy (foliage and twigs), the δ¹³C of soluble organic matter varied diurnally with amplitudes of up to 1.9‰. The greatest ¹³C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. ¹³C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO₂ emitted from the stem was enriched by 2.3–5.2‰ compared with phloem organic matter. When day‐to‐day variation was addressed, water‐soluble leaf δ¹³C and twig phloem δ¹⁸O were strongly influenced by c_i/c_a and stomatal conductance (G_s), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence δ¹³C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ¹³C of respired CO₂ must be considered when using the latter for partitioning of ecosystem CO₂ fluxes or when the assessment of δ¹³C in organic matter is applied to estimate environmental effects in c_i/c_a.     

Modeling changes in soil organic matter in Amazon forest to pasture conversion with the Century model

Land use and land cover changes in the Brazilian Amazon region have major implications for regional and even global carbon cycling. We analyzed the effects of the predominant land use change, conversion of tropical forest to pasture, on total soil C and N, using the Century ecosystem model and data collected from the Nova Vida ranch, Western Brazilian Amazon. We estimated equilibrium organic matter levels, plant productivity and residue carbon inputs under native forest conditions, then simulated deforestation following the slash and burn procedure. Soil organic matter dynamics were simulated for pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911. Using input data from the Nova Vida ranch, the Century model predicted that forest clearance and conversion to pasture would cause an initial decline in soil C and N stocks, followed by a slow rise to levels exceeding those under native forest. Simulated soil total C and N levels (2500 g C m^?2 and 245 g N m^?2 in the 0–20 cm layer) prior to conversion to pasture were close to those measured in the native forest. Simulated above‐ and below‐ground biomass for the forest and pasture were comparable with literature values from this region. The model predicted the long‐term changes in soil C and N under pasture inferred from the pasture chronosequence, but there was considerable variation in soil C stocks for pastures <20 years in age. Differences in soil texture between pastures were relatively small and could not account for much of the variability between different pastures of similar ages, in either the measured or simulated data. It is likely that much of the variability in C stocks between pastures of similar ages is related to initial C stocks immediately following deforestation and that this was the largest source of variability in the chronosequence. Internal C cycling processes in Century were evaluated using measurements of microbial biomass and soil δ¹³C. The relative magnitude and long‐term trend in microbial biomass simulated by the model were consistent with measurements. The close fit of simulated to measured values of δ¹³C over time suggests that the relative loss of forest‐derived C and its replacement by pasture‐derived C was accurately predicted by the model. After 80 years, almost 90% of the organic matter in the top 20 cm was pasture derived. While our analysis represents a single ‘case study’ of pasture conversion, our results suggest that modeling studies in these pasture systems can help to evaluate the magnitude of impacts on C and N cycling, and determine the effect of management strategies on pasture sustainability.     

Arthropod communities on creosote bush (<Emphasis Type="Italic">Larrea tridentata</Emphasis>) in desert patches of varying degrees of urbanization

The goal of the present study was to investigate the influence of urbanization on the richness, abundance and composition of arthropod communities associated with creosote bush, Larrea tridentata [DC] Cov., in Phoenix, Arizona. Arthropod communities were sampled in two desert types varying in degree of urbanization including fringe deserts (relatively undisturbed expanses of desert outside of Phoenix) and urban deserts (patches of desert within the urban core of Phoenix). Two studies were conducted including (1) a seasonal study (conducted at two fringe desert and two urban desert sites over a nine-month period), and (2) a snapshot study (conducted at multiple fringe desert and urban desert sites over an eight-day period). Results from both studies demonstrated that overall richness and abundance of creosote bush arthropod communities were lower in urban deserts than in fringe deserts. Additionally, creosote bush arthropod community composition varied greatly, both temporally and spatially. These differences in richness and abundance between fringe deserts and urban deserts suggest that the creosote bush arthropod community may be a useful focal biotic community to monitor when assaying for environmental change due to urbanization in arid habitats.     

Thyme travels: 15N isoscapes of Thymus vulgaris L. invasion in lightly grazed pastoral communities

Alterations to ecosystem nitrogen (N) cycling by introduced plant species may increase the invasibility of habitat providing a positive feedback for the introduced species to become invasive. Spatial patterns of foliar and soil δ¹⁵N ratios reflect variation in rates and process of N‐cycling across invaded landscapes and provide insight into N‐source uptake and utilization strategies of invasive plant species. To evaluate invasion‐associated changes in soil and foliar δ¹⁵N at different scales: regional (among different sites), local (between north‐ and south‐facing aspect at the same site), and microsite (within populations in the same community), we measured foliar and soil δ¹⁵ N, animal faeces cover (as a proxy for grazing intensity) and N₂‐fixing species cover from inside to outside Thymus vulgaris L. (thyme)‐invaded lightly grazed pastoral communities in Central Otago, southern South Island, New Zealand. Mean thyme foliar δ¹⁵N were near‐zero across the invaded landscape, and did not change across the advancing edge of invasion or with aspect. There was no evidence that associations with N₂‐fixing species provide a potential N source. Soil δ¹⁵N was lower inside of thyme compared to at the edge or outside of thyme and was varied between aspects at some sites. Animal faeces cover as a proxy for grazing intensity explained only 23% of this observed variation of soil δ¹⁵N. Thyme invasion may result in lowered soil δ¹⁵N reflecting alterations to N dynamics. Associated invasion‐related impacts of animal grazing may also impact soil δ¹⁵N. Further studies are required to distinguish the underlying mechanism responsible for the observed patterns of foliar and soil δ¹⁵N values across thyme‐invaded Central Otago landscapes.     

A distinctive latitudinal trend of nitrogen isotope signature across urban forests in eastern China

Rapid urbanization has greatly altered nitrogen (N) cycling from regional to global scales. Compared to natural forests, urban forests receive much more external N inputs with distinctive abundances of stable N isotope (δ¹⁵N). However, the large-scale pattern of soil δ¹⁵N and its imprint on plant δ¹⁵N remain less well understood in urban forests. By collecting topsoil (0–20 cm) and leaf samples from urban forest patches in nine large cities across a north–south transect in eastern China, we analyzed the latitudinal trends of topsoil C:N ratio and δ¹⁵N as well as the correlations between tree leaf δ¹⁵N and topsoil δ¹⁵N. We further explored the spatial variation of topsoil δ¹⁵N explained by corresponding climatic, edaphic, vegetation-associated, and anthropogenic drivers. Our results showed a significant increase of topsoil C:N ratio towards higher latitudes, suggesting lower N availability at higher latitudes. Topsoil δ¹⁵N also increased significantly at higher latitudes, being opposite to the latitudinal trend of soil N availability. The latitudinal trend of topsoil δ¹⁵N was mainly explained by mean annual temperature, mean annual precipitation, and atmospheric deposition of both ammonium and nitrate. Consequently, tree leaf δ¹⁵N showed significant positive correlations with topsoil δ¹⁵N across all sampled plant species and functional types. Our findings reveal a distinctive latitudinal trend of δ¹⁵N in urban forests and highlight an important role of anthropogenic N sources in shaping the large-scale pattern of urban forest ¹⁵N signature.     

Water chemistry,landscape, and spatial controls of δ13C and δ15N of zooplankton taxa in boreal lakes: One size does not fit all

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Ectomycorrhizas associated with a relict population of Dryas octopetala in the Burren, western Ireland. I. Distribution of ectomycorrhizas in relation to vegetation and soil characteristics

The distribution of ectomycorrhizas on Dryas octopetala L in grass heaths of the 450 km² karst region known as the Burren in Western Ireland was examined in relation to soil factors and vegetation type. Ectomycorrhizas were identified or characterised from 56 soil cores from 30 sites, and the occurrence of each ectomycorrhizal (EM) type was quantified by estimating the total length of mycorrhizal tips of each type. Soil organic matter, total nitrogen, extractable phosphorus, pH and depth were the soil factors determined. In total, 24 EM types were recorded. The EM community of Dryas roots was significantly more species-rich in one vegetation type—Hyperico-Dryadetum—than in others (Arctostaphylo-Dryadetum or Asperulo-Seslerietum). Multiple linear regression analyses indicated that soil organic matter and soil depth explained a significant portion of the variation in EM abundance, while soil organic matter and extractable phosphorus explained a significant portion of the variation in EM diversity. Canonical correspondence analysis showed that some individual EM types (e.g. Craterellus lutescens, Cenococcum geophilum, Tomentella sp., Boletus sp.) exhibited distinct soil preferences, most markedly in relation to soil organic matter, which, in this analysis, was the main significant soil variable distinguishing the three vegetation types.     

Comparing urbanization patterns in Guangzhou of China and Phoenix of the USA: The influences of roads and rivers

Examining the similarities and differences in urbanization pattern between cities in developed and developing countries may enhance our understanding of urbanization processes and mechanisms. Using a buffer analysis approach, we compared the long-term spatiotemporal patterns of urbanization between Guangzhou of China and Phoenix of the United States during the 20th century, with a particular emphasis on the impacts of major roads and rivers. The development of Guangzhou was relatively compact, whereas the development of Phoenix was much more dispersed. The two metropolitan areas were characterized mainly by two types of urban expansion processes. Guangzhou experienced a diffusion–coalescence–diffusion–coalescence process while Phoenix experienced a diffusion–coalescence process. The buffer analysis indicated that the impacts of roads and rivers on the urbanization of Guangzhou and Phoenix were similar in terms of urban area, the number of urban patches, the mean patch size, but different in terms of the complexity of urban patches. The urban area declined with increasing distances to roads, but it increased when the distance was close to the rivers and then declined with the increment of distances to rivers. In general, the impacts of roads are stronger in Phoenix than in Guangzhou, while the impacts of rivers are greater in Guangzhou than in Phoenix. These similarities and differences between the two metropolitan regions are reflective of those in both physical conditions and land use policies in the two countries. By comparing these similarities and differences, we can improve our understanding of the urbanization processes in both developed and developing countries, which is necessary for achieving global urban sustainability.     

古尔班通古特沙漠梭梭冠下裸斑形成土壤学机制

古尔班通古特沙漠梭梭与齿肋赤藓共生区域中,梭梭冠下常形成藓类结皮裸露斑块。为研究梭梭冠下裸斑形成机理,以裸斑内土壤、裸斑外藓类结皮土壤、背景裸地土壤以及背景藓类结皮土壤为研究对象,测定四类土壤营养物质含量和理化因子,并测定土壤代谢组以及分析其之间差异。结果表明,裸斑内土壤养分及生态化学计量比与裸斑外藓类结皮和背景藓类结皮土壤并无显著性差异,土壤养分及生态化学计量并非造成梭梭冠下藓类裸斑的原因。裸斑内土壤Na⁺、K⁺、Mg²⁺、SO^2-₄、CO^2-₃、HCO^-₃含量显著高于裸斑外土壤、背景裸地土壤和背景结皮土壤,总盐含量最高为1.705 g/kg。这个含量不足以对齿肋赤藓正常生长造成影响,因此裸斑内土壤中较高的离子浓度也并非藓类裸斑产生的原因。土壤代谢组结果显示不同土壤代谢物具有极显著差异,油酰胺等酰胺类化合物相对丰度最高,占总代谢物的72.68%,且在裸斑内土壤中相对丰度显著高于裸斑藓类结皮土壤、裸地土壤和背景结皮土壤,因此推测裸斑内土壤中油酰胺等酰胺类化合物可能是抑制齿肋赤藓生长的主要原因,是造成古尔班通古特沙漠梭梭冠下藓类裸斑产生的主要土壤学机制。     

Vegetation Change and Soil Nutrient Distribution along an Oasis-Desert Transitional Zone in Northwestern China

Many studies have focused on soil nutrient heterogeneity and islands of fertility in arid ecosystems. However, few have been conducted on an oasis-desert transitional zone where there is a vegetation pattern changing from shrubs to annual herbs. The goal of the present study was to understand vegetation and soil nutrient heterogenity along an oasis-desert transitional zone in northwestern China. Three replicated sampling belts were selected at 200 m intervals along the transitional zone. Twenty-one quadrats （10 x 10m） at 50m intervals were located along each sampling belt. The vegetation cover was estimated through the quadrats, where both the soil under the canopy and the open soil were sampled simultaneously. The dominated shrub was Haloxylon ammodendron in the areas close to the oasis and Nitraria tangutorum dominated the areas close to the desert. In general, along the transitional zone the vegetation cover decreased within 660 m, increased above 660 m and decreased again above 1 020 m （close to the desert）. The soil nutrients （organic matter, total N, NO3^- and NH4^＋） showed significant differences along the zone. The soil nutrients except the soil NH4^＋ under the canopy were higher than those in open soil, confirming ＂islands of fertility＂ or nutrient enrichment. Only a slight downward trend of the level of ＂islands of fertility＂ for soil organic matter appeared in the area within 900 m. Soil organic matter both under canopy and in interspace showed a positive correlation with the total vegetation cover, however, there was no significant correlation between the other soil nutrients and the total vegetation cover. We also analyzed the relationship between the shrubs and annuals and the soil nutrients along the zone. Similarly, there was no significant correlation between them, except soil organic matter with the annuals. The results implied that annual plants played an important role in soil nutrient enrichment in arid ecosystem.     

Nests of subterranean fungus-growing termites (Isoptera, Macrotermitinae) as nutrient patches for grasses in savannah ecosystems

Savannah are ecosystems in which mineral nitrogen is considered as a limiting factor for plant productivity. They are heterogeneous and spatially structured in patches, or islands, where mineral nitrogen content is concentrated. Among the soil macrofauna, termites of the Macrotermitinae subfamily are major determinants of soil heterogeneity through the biogenic underground nest structures (fungus‐comb chambers) they produce. To study the role of the heterogeneity created by termites on Pennisetum pedicellatum, an herbaceous grass species was grown in greenhouse. This was carried out using an homogeneous soil poor in mineral nitrogen, and an heterogeneous soil with patch, made of (i) Ancistrotermes cavithorax fungus‐comb chamber wall and (ii) soil with the same mineral nitrogen content as the termite handled soil. Plants exhibited a better growth on patch of termite‐modified soil whereas no significant differences were shown with the supply of mineral nitrogen. The presence of fungus‐comb chamber wall material resulted in an increase of fine root biomass and root/shoot ratio. We conclude that termites, through their building activities, may create nutrient patches available to grasses. Concurrently, our data illustrate that the higher mineral nitrogen content in termite‐built structures is not the only factor responsible for plant growth.     

宁夏荒漠草原不同植物群落微斑块内土壤微生物区系特征

植物群落斑块化是天然放牧草地最基本的特征之一.为探索植物群落斑块化对土壤微生物群落组成及多样性的影响,本研究以宁夏荒漠草原为研究对象,采用磷脂脂肪酸(PLFA)法对比分析了不同植物群落微斑块内土壤微生物生物量和群落结构特征的变化.结果表明: 1) 4种植物群落微斑块内土壤微生物种类丰富,且都表现为细菌含量最高,真菌和放线菌含量较少,革兰氏阳性菌含量高于革兰氏阴性菌;2)4种植物群落中,甘草微斑块的土壤微生物总量显著高于猪毛蒿、苦豆子和黄芪;3)冗余分析表明,磷脂脂肪酸总量、革兰氏阳性菌、革兰氏阴性菌、真菌、厌氧菌、真菌/细菌均与土壤有机碳呈显著正相关,与pH呈显著负相关,表明土壤有机碳、pH是荒漠草原土壤微生物生长和发育的重要影响因素.     

Pan-European δ13C values of air and organic matter from forest ecosystems

We present carbon stable isotope, δ¹³C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 (14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that δ¹³C values derived from large‐scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem‐scale δ¹³C values. In this framework, we also tested the potential of δ¹³C in canopy air and plant organic matter to record regional‐scale ecophysiological patterns. Our network estimates for the mean δ¹³C of ecosystem respired CO₂ and the related ‘discrimination’ of ecosystem respiration, δ_er and Δ_er, respectively, were ?25.6±1.9‰ and 17.8 ±2.0‰ in 2001 and ?26.6±1.5‰ and 19.0±1.6‰ in 2002. The results were in close agreement with δ¹³C values derived from regional‐scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional‐scale atmospheric sampling programs generally capture ecosystem δ¹³C signals over Europe, but may be limited in capturing some of the interannual variations. In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in δ_er. From west to east, across the network, there was a general enrichment in ¹³C (～3‰ and ～1‰ for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal ¹³C enrichment between July and September, followed by depletion in November (from about ?26.0‰ to ?24.5‰ to ?30.0‰), was also observed. In 2001, July and August δ_er values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (T_a), but not significantly with monthly average precipitation (P_m). In contrast, in 2002 (a much wetter peak season), δ_er was significantly related with T_a, but not significantly with VPD and RH. The important role of plant physiological processes on δ_er in 2001 was emphasized by a relatively rapid turnover (between 1 and 6 days) of assimilated carbon inferred from time‐lag analyses of δ_er vs. meteorological parameters. However, this was not evident in 2002. These analyses also noted corresponding diurnal cycles of δ_er and meteorological parameters in 2001, indicating a rapid transmission of daytime meteorology, via physiological responses, to the δ_er signal during this season. Organic matter δ¹³C results showed progressive ¹³C enrichment from leaves, through stems and roots to soil organic matter, which may be explained by ¹³C fractionation during respiration. This enrichment was species dependent and was prominent in angiosperms but not in gymnosperms. δ¹³C values of organic matter of any of the plant components did not well represent short‐term δ_er values during the seasonal cycle, and could not be used to partition ecosystem respiration into autotrophic and heterotrophic components.     

Within-lake variability in carbon and nitrogen stable isotope signatures

1. We assessed spatial and temporal variation in carbon and nitrogen isotopic signatures in different compartments of a single lake ecosystem. Stable isotope analyses were made on samples of particulate organic matter (POM), zooplankton, periphyton, macrophytes, macroinvertebrates and fish collected from several locations throughout the ice‐free period. 2. No spatial variation in δ¹³C or δ¹⁵N values was found for pelagic samples of POM and zooplankton. However, pelagic δ¹⁵N signatures increased steadily through the summer resulting in an almost 6‰ average increase in POM and zooplankton. A concurrent decrease in epilimnetic nitrate concentrations suggested that the increase in δ¹⁵N of POM and zooplankton could have resulted from a progressive ¹⁵N‐enrichment of the available inorganic nitrogen pool as the size of this pool was reduced. 3. Significant spatial variation in isotopic ratios was observed within littoral and profundal communities. Some spatial differences were likely related to lake‐specific characteristics, such as a major inlet and a small harbour area and some were interconnected with temporal events. 4. Marked differences between spring and autumn δ¹⁵N and δ¹³C values of fish at one site probably reflected a spring spawning immigration from a larger downstream lake and also indicated limited dispersal of these immigrants. 5. Our results indicate that restricted sampling of ecosystem components from lakes may provide misleading single values for the isotope end members needed for quantitative uses of stable isotopes in mixing models and for estimating trophic position. Hence we strongly advise that studies of individual lakes, or multiple lake comparisons, that utilise stable isotope analyses should pay more attention to potential within lake spatial and temporal variability of isotope ratios.