Retrospective assessment of dryland soil stability in relation to grazing and climate change

Accelerated soil erosion is an aspect of dryland degradation that is affected by repeated intense drought events and land management activities such as commercial livestock grazing. A soil stability index (SSI) that detects the erosion status and susceptibility of a landscape at the pixel level, i.e., stable, erosional, or depositional pixels, was derived from the spectral properties of an archived time series (from 1972 to 1997) of Landsat satellite data of a commercial ranch in northeastern Utah. The SSI was retrospectively validated with contemporary field measures of soil organic matter and erosion status that was surveyed by US federal land management agencies. Catastrophe theory provided the conceptual framework for retrospective assessment of the impact of commercial grazing and soil water availability on the SSI. The overall SSI trend was from an eroding landscape in the early drier 1970s towards stable conditions in the wetter mid-1980s and late 1990s. The landscape catastrophically shifted towards an extreme eroding state that was coincident with the “The Great North American Drought of 1988”. Periods of landscape stability and trajectories toward stability were coincident with extremely wet El Niño events. Commercial grazing had less correlation with soil stability than drought conditions. However, the landscape became more susceptible to erosion events under multiple droughts and grazing. Land managers now have nearly a year warning of El Niño and La Niña events and can adjust their management decisions according to predicted landscape erosion conditions.

Soil Moisture Deficit as a predictor of the trend in soil water status of grass fields

Nutrient loss from agricultural land following organic fertilizer spreading can lead to eutrophication and poor water quality. The risk of pollution is partly related to the soil water status during and after spreading. In response to these issues, a decision support system (DSS) for nutrient management has been developed to predict when soil and weather conditions are suitable for slurry spreading. At the core of the DSS, the Hybrid Soil Moisture Deficit (HSMD) model estimates soil water status relative to field capacity (FC) for three soil classes (well, moderately and poorly drained) and has potential to predict the occurrence of a transport vector when the soil is wetter than FC. Three years of field observation of volumetric water content was used to validate HSMD model predictions of water status and to ensure correct use and interpretation of the drainage classes. Point HSMD model predictions were validated with respect to the temporal and spatial variations in volumetric water content and soil strength properties. It was found that the HSMD model predictions were well related to topsoil water content through time, but a new class intermediate between poor and moderate, perhaps ‘imperfectly drained’, was needed. With correct allocations of a field into a drainage class, the HSMD model predictions reflect field scale trends in water status and therefore the model is suitable for use at the core of a DSS.

Manipulation of intracellular glycerol and erythritol enhances germination of conidia at low water availability

The insect pathogen Beauveria bassiana, Metarhizium anisopliae and Paecilomyces farinosos can be effective biocontrol agents when relative humidity (RH) is close to 100%. At reduced water availability, germination of propagules, and therefore host infection, cannot occur. Cultures of B. bassiana, M. anisopliae and P. farinosus were grown under different conditions to obtain conidia with a modified polyol and trehalose content. Conidia with higher intracellular concentrations of glycerol and erythritol germinated both more quickly and at lower water activity (a(w)) than those from other treatments. In contrast, conidia containing up to 235.7 mg trehalose g-1 germinated significantly (P < 0 05) more slowly than those with an equivalent polyol content but less trehalose, regardless of water availability. Conidia from control treatments did not germinate below 0.951 - 0.935 a(w) (≡ 95.1 - 93.5% RH). In contrast, conidia containing up to 164.6 mg glycerol plus erythritol g-1 germinated down to 0.887 a(w) (≡ 88.7% RH). These conidia germinated below the water availability at which mycelial growth ceases (0.930 - 0.920 a(w)). Germ tube extension rates reflected the percentage germination of conidia, so the most rapid germ tube growth occurred after treatments which produced conidia containing the most glycerol and erythritol. This study shows for the first time that manipulating polyol content can extend the range of water availability over which fungal propagules can germinate. Physiological manipulation of conidia may improve biological control of insect pests in the field.

Bioavailability of 2,4-dichlorophenol associated with soil water-soluble humic material

Interaction of organic xenobiotics with soil water-soluble humic material (WSHM) may influence their environmental fate and bioavailability. We utilized bacterial assays (lux-based toxicity and mineralization by Burkholderia sp. RASC) to assess temporal changes in the bioavailability of [¹⁴C]-2,4-dichlorophenol (2,4-DCP) in soil water extracts (29.5 μg mL^-1 2,4-DCP; 840.2 μg mL^-1 organic carbon). HPLC determined and bioavailable concentrations were compared. Gel permeation chromatography (GPC) was used to confirm the association of a fraction (>50%) of [¹⁴C]-2,4-DCP with WSHM. Subtle differences in parameters describing 2,4-DCP mineralization curves were recorded for different soil-2,4-DCP contact times. Problems regarding the interpretation of mineralization data when assessing the bioavailability of toxic compounds are discussed. The lux-bioassay revealed a time-dependent reduction in 2,4-DCP bioavailability: after 7 d, less than 20% was bioavailable. However, GPC showed no quantitative difference in the amount of WSHM-associated 2,4-DCP over this time. These data suggest qualitative changes in the nature of the 2,4-DCP-WSHM association and that associated 2,4-DCP may exert a toxic effect. Although GPC distinguished between free- and WSHM-associated 2,4-DCP, it did not resolve the temporal shift in bioavailability revealed by the lux biosensor. These results stress that assessment of risk posed by chemicals must be considered using appropriate biological assays.

Photosynthetic pigments in soils from northern Victoria Land (continental Antarctica) as proxies for soil algal community structure and function

Although soil algae are among the main primary producers in most terrestrial ecosystems of continental Antarctica, there are very few quantitative studies on their relative proportion in the main algal groups and on how their distribution is affected by biotic and abiotic factors. Such knowledge is essential for understanding the functioning of Antarctic terrestrial ecosystems. We therefore analyzed biological soil crusts from northern Victoria Land to determine their pH, electrical conductivity (EC) water content (W), total and organic C (TC and TOC) and total N (TN) contents, and the presence and abundance of photosynthetic pigments. In particular, the latter were tested as proxies for biomass and coarse-resolution community structure. Soil samples were collected from five sites with known soil algal communities and the distribution of pigments was shown to reflect differences in the relative proportions of Chlorophyta, Cyanophyta and Bacillariophyta in these sites. Multivariate and univariate models strongly indicated that almost all soil variables (EC, W, TOC and TN) were important environmental correlates of pigment distribution. However, a significant amount of variation is independent of these soil variables and may be ascribed to local variability such as changes in microclimate at varying spatial and temporal scales. There are at least five possible sources of local variation: pigment preservation, temporal variations in water availability, temporal and spatial interactions among environmental and biological components, the local-scale patchiness of organism distribution, and biotic interactions. (C) 2009 Elsevier Ltd. All rights reserved.

The dynamics of arsenic in four paddy fields in the Bengal delta

Irrigation with arsenic contaminated groundwater in the Bengal Delta may lead to As accumulation in the soil and rice grain. The dynamics of As concentration and speciation in paddy fields during dry season (boro) rice cultivation were investigated at 4 sites in Bangladesh and West Bengal, India. Three sites which were irrigated with high As groundwater had elevated As concentrations in the soils, showing a significant gradient from the irrigation inlet across the field. Arsenic concentration and speciation in soil pore water varied temporally and spatially; higher As concentrations were associated with an increasing percentage of arsenite, indicating a reductive mobilization. Concentrations of As in rice grain varied by 2-7 fold within individual fields and were poorly related with the soil As concentration. A field site employing alternating flooded-dry irrigation produced the lowest range of grain As concentration, suggesting a lower soil As availability caused by periodic aerobic conditions.

A tripartite microbial reporter gene system for real-time assays of soil nutrient status

Plant-derived carbon is the substrate which drives the rate of microbial assimilation and turnover of nutrients, in particular N and P, within the rhizosphere. To develop a better understanding of rhizosphere dynamics, a tripartite reporter gene system has been developed. We used three lux-marked Pseudomonas fluorescens strains to report on soil (1) assimilable carbon, (2) N-status, and (3) P-status. In vivo studies using soil water, spiked with C, N and P to simulate rhizosphere conditions, showed that the tripartite reporter system can provide real-time assessment of carbon and nutrient status. Good quantitative agreement for bioluminescence output between reference material and soil water samples was found for the C and P reporters. With regard to soil nitrate, the minimum bioavailable concentration was found to be greater than that analytically detectable in soil water. This is the first time that bioavailable soil C, N and P have been quantified using a tripartite reporter gene system.

Water and Temperature Relations of Growth of the Entomogenous Fungi Beauveria bassiana, Metarhizium anisopliae, and Paecilomyces farinosus

The effects of temperature (5-50°C), water availability (0.998-0.88 water activity, aw), and aw × temperature interactions (15-45°C) on growth of three entomogenous fungi, Beauveria bassiana, Metarhizium anisopliae, and Paecilomyces farinosus, were evaluated on a Sabouraud dextrose-based medium modified with the ionic solute KCl, the non-ionic solute glycerol, and an inert solute, polyethylene glycol (PEG) 600. The temperature ranges for growth of B. bassiana, M. anisopliae, and P. farinosus were 5-30, 5-40, and 5-30°C, and optimum growth temperatures were 25, 30, and 20°C, respectively. All three species grew over a similar aw range (0.90-0.998) at optimum temperatures for growth. However, there were significant interspecies variations in growth rates on media modified with each of the three aw-modifying solutes. Growth aw optima ranged between 0.99 and 0.97 on KCl-, glycerol-, and PEG 600-modified media for M. anisopliae and P. farinosus. B. bassiana grew optimally at 0.998 aw, regardless of aw. Comprehensive two-dimensional profiles of aw × temperature relations for growth of these three species were constructed for the first time. The results are discussed in relation to the environmental limits that determine efficacy of entomogenous fungi as biocontrol agents in nature. © 1999 Academic Press.

Biosensing 2,4-dichlorophenol toxicity during biodegradation by Burkholderia sp. RASC c2 in soil

Biodegradation of the model pollutant, 2,4-dichlorophenol (2,4-DCP) by Burkholderia sp. RASC c2, in contaminated soil was assessed by combining chemical analysis with a toxicity test using Escherichia coli HB101 pUCD607. E. coli HB101 pUCD607 was previously marked with luxCDABE genes, encoding bacterial bioluminescence and was used as an alternative to Microtox. Mineralization of ¹⁴C-2,4-DCP (196.2 μg g^-1 dry wt) in soil occurred rapidly after a 24 h lag. Correspondingly, 2,4-DCP concentrations in soil and soil water extracts decreased with time and concentrations in the latter were at background levels (<0.12 μg mL^-1) after day 2. Toxicity of soil water extracts to the lux-based biosensor also decreased with time. Mean light output of E. coli was stimulated by ~1.5 X control values in soil water extracts when concentrations of 2,4-DCP were approaching the limit of detection by HPLC but returned to values equivalent to those of controls when soil water 2,4-DCP concentrations were below the detection limit. No mineralization or microbial growth was detected in noninoculated microcosms. 2,4-DCP concentration in sterile controls decreased significantly with time as did toxicity to E. coli Lux-based E. coli was a sensitive biosensor of 2,4-DCP toxicity during biodegradation and results complemented chemical analysis.

Phenotypic diversity amongst strains of Pleurotus sajor-caju: implications for cultivation in and environments

In arid regions, biodiversity and biomass are limited by water availability, and this problem has been compounded by desertification associated with global climate change. The saprotrophic macrofungi that are indigenous to hot subtropical and tropical regions, such as Pleurotus spp., can play key roles in water sequestration, nutrient cycling, human nutrition, and bioremediation of waste materials. We studied 15 strains of Pleurotus sajor-caju, a widespread and phenotypically-diverse species, to establish variability in growth response and primordium development over a range of stress parameters: osmotic potential (-0.5 to -5 MPa), temperature (5-40 degrees C) and pH (2-12). The initiation of primordia precedes basidiome production and therefore represents a key stage in bioremediation strategies and fungi-driven nutrient cycles. Primordia were produced at low pH (4-6), at suboptimal growth temperatures (<or =25 degrees C), and under moderate water stress (-0.5 to -3.5 MPa). Although the growth windows for different strains were similar, their maximum growth rates and the optimum conditions for growth varied. We discuss the phenotypic diversity of Pleurotus strains and discuss their potential for cultivation, bioremediation and ecological regeneration.

A high resolution 15,600-year pollen and microcharcoal record from the Cederberg Mountains, South Africa

The Cederberg Mountains (Western Cape Province, South Africa) are located within the Fynbos Biome, which exhibits some of the highest levels of species richness and endemism in the world. The region's post-glacial vegetation history, however, remains largely unknown. Presented here are high resolution pollen and microcharcoal records spanning the last 15,600 years obtained from the De Rif rock hyrax midden from the Driehoek Valley of the central Cederberg. In this region, previous pollen studies have shown muted variability in vegetation community composition during periods of globally marked climatic variability (e.g. the last glacial-interglacial transition). In our record, however, significant changes in vegetation composition are apparent. Most notably, they indicate a shift from ericaceous/restioid fynbos (present from 15,600 to 13,300 cal yr BP) to a brief, but prominent, development of proteoid fynbos at the beginning of the Holocene around 11,200 cal yr BP. This vegetation shift is associated with increased moisture at the site, and coincides with reduced fire frequency as indicated by the microcharcoal record. At 10,400 cal yr BP, there is a marked reduction in Protea-type pollen, which is replaced by thicket, characterised by Dodonaea, which became the dominant arboreal pollen type. This shift was likely the result of a long relatively fire-free period coupled with warmer and wetter climates spanning much of the early Holocene. A brief but marked decrease in water availability around 8500-8000 cal yr BP resulted in the strong decrease of Dodonaea pollen. The vegetation of the mid- to late Holocene is characterised by the increased occurrence of Asteraceae and succulent taxa, suggesting substantially drier conditions. These data give unprecedented insight into the vegetation dynamics across a period of substantial, rapid climate change, and while they confirm the presence of fynbos elements throughout the last 15,600 years, the results highlight significant fluctuations in the vegetation that were triggered by changes in both climate and fire regimes. (C) 2013 Elsevier B.V. All rights reserved.

Arbuscular mycorrhizal fungal communities are phylogenetically clustered at small scales

Next-generation sequencing technologies with markers covering the full Glomeromycota phylum were used to uncover phylogenetic community structure of arbuscular mycorrhizal fungi (AMF) associated with Festuca brevipila. The study system was a semi-arid grassland with high plant diversity and a steep environmental gradient in pH, C, N, P and soil water content. The AMF community in roots and rhizosphere soil were analyzed separately and consisted of 74 distinct operational taxonomic units (OTUs) in total. Community-level variance partitioning showed that the role of environmental factors in determining AM species composition was marginal when controlling for spatial autocorrelation at multiple scales. Instead, phylogenetic distance and spatial distance were major correlates of AMF communities: OTUs that were more closely related (and which therefore may have similar traits) were more likely to co-occur. This pattern was insensitive to phylogenetic sampling breadth. Given the minor effects of the environment, we propose that at small scales closely related AMF positively associate through biotic factors such as plant-AMF filtering and interactions within the soil biota.