Deterioration of cichlid habitat by increased sedimentation in the rocky littoral zone of Lake Malawi

Lake Malawi has been affected by soil erosion across the catchment area. We experimentally investigated habitat preferences of cichlids in the rocky littoral zone to examine the influence of increased sedimentation on bottom-dwelling cichlids. Numbers of individuals and species, as well as the feeding rate of some of these cichlids, increased following the clearing of sediment. The results showed that these cichlids preferred sediment-free bottom and that the sedimentation causes deterioration of their habitats. Studies on the consequences of the habitat deterioration caused by increased sedimentation are needed in the management of these most diverse freshwater fish communities.     

Etude de la réceptivité de quelques sols forestiers de Bourgogne aux maladies produites parPythium ssp. agent de fonte de semis et de nécrose racinaire

Résumé La réceptivité de 10 sols forestiers, récoltés en Bourgogne, a été déterminée à l'aide d'un test biologique permettant la mesure du potentiel infectieux de sols infestés parPythium spp. Plusieurs sols manifestent une résistance plus ou moins marquée à l'encontre des maladies causées parPythium spp. Différents processus semblent à l'origine de cette aptitude. Aucune relation entre réceptivité et facteurs écologiques du milieu n'a pu être décelée. Toutefois, les sols de la série acidophile sont globalement moins réceptifs que ceux de la série calcicole. La réceptivité d'un sol est définie comme son aptitude à accueillir plus ou moins bien un agent pathogène en agissant sur son installation, son dévelopement, sa survie, et l'expression de son pouvoir pathogène. C'est une aptitude propre au sol mais empreinte des influences antérieures et plus ou moins modifiable, notamment à la suite d'une culture.

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Study of the receptivity of Burgundian forest soils to damping off and root necrosis caused byPythium spp

Summary A standard bioassay method was used to measure soil infectivity and estimate the receptivity of ten forest soils collected in Burgundy, to disease caused byPythium spp. Several of them were more or less suppressive. These ability resulted of various processus. No single relation can be found between receptivity and ecological factors of the environment. Nevertheless the soils of the acidophilics groups were in general less conducive than the soils of the calcareous groups. The receptivity of a soil is its ability to more or less welcome a pathogen by acting on it's installation, development, survival and on the expression of its pathogenicity. The receptivity, a proper ability of the soil is influenced by previous ecological pressure, and is more or less modifiable, specially under the influence of a crop.

     

Growth and nitrogen fixation ofAeschynomene under water stressed conditions

Summary Studies on the tolerance ofAeschynomene americana L. to periods of flooding or soil moisture deficit were conducted in an attempt to elucidate nitrogen fixation as affected by soil moisture. Nitrogenase activity was not reduced significantly in pot-grown Aeschynomene plants subjected to flooding in greenhouse conditions. After 20 days of withholding water from the soil, nitrogenase activities of the drought-stressed plants were much lower than those of either the well-watered or flooded plants. Leaf water potentials were similar in flooded and control plants; however, the droughted plants had leaf water potentials that were 4 bars lower than those of the control plants. Aeschynomene plants were tolerant to long-term periods of flooding, but exhibited a reduction in nitrogenase activity and leaf water status when subjected to soil moisture deficits.     

A method to prioritize and monitor wetland restoration for water-quality improvement

Wetland restoration can improve water quality by reducing concentrations of sediment, total phosphorus, and nitrate in runoff. Managers need a simple method to choose among many possible restoration sites, particularly in large agricultural basins covering thousands of square kilometers. The purpose of this paper is to outline a method for prioritizing and monitoring wetland restoration sites in light of the factors that affect water-quality improvement by wetlands. These factors are categorized as loading factors, path factors, and process factors. The method for prioritizing wetland restoration sites depends primarily on assessing the potential effectiveness of the wetland for improving water quality. Three types of effectiveness are considered: problem effectiveness (is the site in an area with known water-quality problems?), function effectiveness (is the site likely to improve water quality more or less than other sites?), and information effectiveness (does the site fit within an overall research plan to gain information on how wetlands improve water quality?). The variables of hydraulic residence time, hydraulic flux, and wetland area, volume, and average depth are combined into a single variable termed and used as a proxy for estimating the relative function effectiveness of potential restoration sites. Monitoring restoration sites is targeted at establishing a minimum data set that can be collected consistently at different sites over time, and that can be used for inter-site comparison with simple statistical techniques. The Minnesota River Basin is used as an example throughout to demonstrate the types of data that are available to plan wetland restoration. While this paper focuses on the water-quality benefits, wetland restoration should be a multi-disciplinary effort to integrate other benefits of restoration, such as improvement of wildlife habitat and flood abatement.     

Analysis of the Primary Sequence and Secondary Structure of the Unusually Long SSU rRNA of the Soil Bug, Armadillidium vulgare

The complete nucleotide sequence of the SSU rRNA gene from the soil bug, Armadillidium vulgare (Crustacea, Isopoda), was determined. It is 3214 bp long, with a GC content of 56.3%. It is not only the longest SSU rRNA gene among Crustacea but also longer than any other SSU rRNA gene except that of the strepsipteran insect, Xenos vesparum (3316 bp). The unusually long sequence of this species is explained by the long sequences of variable regions V4 and V7, which make up more than half of the total length. RT-PCR analysis of these two regions showed that the long sequences also exist in the mature rRNA and sequence simplicity analysis revealed the presence of slippage motifs in these two regions. The putative secondary structure of the rRNA is typical for eukaryotes except for the length and shape variations of the V2, V4, V7, and V9 regions. Each of the V2, V4, and V7 regions was elongated, while the V9 region was shortened. In V2, two bulges, located between helix 8 and helix 9 and between helix 9 and helix 10, were elongated. In V4, stem E23-3 was dramatically expanded, with several small branched stems. In V7, stem 43 was branched and expanded. Comparisons with the unusually long SSU rRNAs of other organisms imply that the increase in total length of SSU rRNA is due mainly to expansion in the V4 and V7 regions. Received: 2 March 1999 / Accepted: 22 July 1999     

Storage,patterns and environmental controls of soil organic carbon in China

Based on the data from China’s second national soil survey and field observations in northwest China, we estimated soil organic carbon (SOC) storage in China and investigated its spatial and vertical distribution. China’s SOC storage in a depth of 1 meter was estimated as 69.1 Pg (10¹⁵ g), with an average density of 7.8 kg m⁻². About 48% of the storage was concentrated in the top 30 cm. The SOC density decreased from the southeast to the northwest, and increased from arid to semi-humid zone in northern China and from tropical to cold-temperate zone in the eastern part of the country. The vertical distribution of SOC differed in various climatic zones and biomes; SOC distributed deeper in arid climate and water-limited biomes than in humid climate. An analysis of general linear model suggested that climate, vegetation, and soil texture significantly influenced spatial pattern of SOC, explaining 78.2% of the total variance, and that climate and vegetation interpreted 78.9% of the total variance in the vertical SOC distribution.     

Quantitative effects of vegetation cover on wind erosion and soil nutrient loss in a desert grassland of southern New Mexico,USA

Wind is a key abiotic factor that influences the dynamics of arid and semiarid systems. We investigated two basic relationships on vegetation manipulation (grass cover reduction) plots at the Jornada Experimental Range in southern New Mexico: (1) wind erosion rates (horizontal mass flux and dust emission) versus vegetative cover, and (2) nutrient loss versus vegetative cover. The results indicate that wind erosion rates and nutrient loss by dust emission are strongly affected by plant cover; however, the importance of shrubs and grasses in reducing dust flux may not be equal. The dramatic increase of wind erosion between 75% grass cover reduction and 100% grass cover reduction suggests that sparsely distributed mesquites are relatively ineffective at reducing wind erosion and nutrient loss compared to grasses. Comparisons of nutrients between surface soils and wind blown dust indicate that aeolian transport is a major cause for the loss of soil nutrients in susceptible environments. We found that increased aeolian flux over three windy seasons (March 2004–July 2006) removed up to 25% of total organic carbon (TOC) and total nitrogen (TN) from the top 5 cm of soil, and about 60% of TOC and TN loss occurred in the first windy season (March–July 2004). The balance between net loss of nutrients by aeolian processes and the addition of nutrients by biotic processes changed from negative (net loss) to positive (net accumulation) between 50% grass cover reduction and 25% grass cover reduction. The estimated lifetime of surface soil TOC and TN of about 10 years on the plot with 100% grass cover reduction indicates that impacts of wind erosion on soil resources can occur on very short timescales.     

The change of soil carbon stocks and fine root dynamics after land use change from a native pasture to a pine plantation

A published meta-analysis of worldwide data showed soil carbon decreasing following land use change from pasture to conifer plantation. A paired site (a native pasture with Themeda triandra dominant, and an adjacent Pinus radiata plantation planted onto the pasture 16 years ago) was set up as a case study to assess the soil carbon reduction and the possible reason for the reduction under pine, including the change in fine root (diameter <2 mm) dynamics (production and mortality). Soil analysis confirmed that soil carbon and nitrogen stocks to 100 cm under the plantation were significantly less than under the pasture by 20 and 15%, respectively. A 36% greater mass of fine root was found in the soil under the pasture than under the plantation and the length of fine root was about nine times greater in the pasture. Much less fine root length was produced and roots died more slowly under the plantation than under the pasture based on observations of fine root dynamics in minirhizotrons. The annual inputs of fine root litter to the top 100 cm soil, estimated from soil coring and minirhizotron observations, were 6.3 Mg dry matter ha⁻¹ year⁻¹ (containing 2.7 Mg C and 38.9 kg N) under the plantation, and 9.7 Mg ha⁻¹ year⁻¹ (containing 3.6 Mg C and 81.4 kg N) under the pasture. The reduced amount of carbon, following afforestation of the pasture, in each depth-layer of the soil profile correlated with the lower length of dead fine roots in the layer under the plantation compared with the pasture. This correlation was consistent with the hypothesis that the soil carbon reduction after land use change from pasture to conifer plantation might be related to change of fine root dynamics, at least in part.     

Vertical distribution of fine roots in relation to soil factors in Pinus tabulaeformis Carr. forest of the Loess Plateau of China

Growth and vertical distribution of fine root closely depend on soil resource availability. Better understanding of relationships of root profile with vertical distribution of available soil resource and soil characteristics can allow ecologists to predict the fine root distribution on the scales ranging from individual plants to vegetation communities. The objective of the study was to understand the fine root mass density (FRMD), fine root length density (FRLD), fine root area density (FRAD), mean root diameter and specific root length (SRL), vertical distribution in soil profile and their relation with soil environment factors in semiarid and arid Loess Plateau of China. The vertical fine root distribution and soil bulk density, soil moisture and soil inorganic N in 0-60 cm soil profile (0–15, 15–30, 30–45 and 45–60 cm intervals) were investigated by soil coring methods in three Pinus tabulaeformis Carr. forests chosen at three locations. The fine root density parameters (FRMD, FRLD and FRAD) and SRL peaked in the most upper soil layer (0–15 cm interval) and decreased with increased soil depth. The results provided a strong support that soil water rather than soil inorganic N is a key control on fine root distribution in the Loess Plateau. With increased soil moisture, the root mass, length and SRL increased and the mean root diameter decreased. The effects of soil bulk density on the fine root parameters were consistent with those of the soil water. An unexpected result was obtained about the relationships between soil organic N and the root distributions and occurrences because of no differences among the soil depth intervals in soil inorganic N content. It might be associated with severe soil water deficit limiting soil nitrogen utilization efficiency in arid Loess Plateau.     

Hormonal Interactions and Stomatal Responses

Both environmental and hormonal factors and their interactions affect stomatal behavior. Methodologies for identifying hormonal interactions affecting stomatal function are reviewed. Although there is abundant evidence that abscisic acid (ABA) closes stomata, evidence that the other classical plant hormones (auxins, cytokinins, ethylene, gibberellins) in isolation alter stomatal response often comes from exogenous applications to detached epidermes and leaves, rather than correlation of endogenous concentrations with stomatal conductance (g_s). Evidence for hormonal interactions comes from isolated tissues with exogenous hormones supplied at nonphysiological concentrations, or from variation in stomatal response to xylem ABA concentration in planta. The roles of hormonal changes in causing stomatal closure following changes in soil environment are considered. Although soil drying induces multiple changes in xylem sap composition, analysis of stomatal responses suggests a dominant role for increased endogenous ABA concentrations and relatively little evidence of roles for other hormones. A similar picture emerges from studies of soil compaction. Although soil flooding decreases ABA export from the root system, there is some evidence that apoplastic ABA accumulation elicits stomatal closure. Stomatal closure following nitrogen deprivation does not appear to involve ABA and may provide a suitable experimental system to investigate roles for other hormones. The availability of mutant or transgenic lines with altered hormone homeostasis or sensitivity provides opportunities to screen for altered stomatal behavior in response to different environments, and may provide new evidence that hormonal interactions are important in the control of stomatal behavior.