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Shifts in plant community structure in shrub and grass-dominated ecosystems are occurring over large land areas in the western US. It is not clear what effect this vegetative change will have on rates of carbon and nitrogen cycling, and thus long-term ecosystem productivity. To study the effect of different plant species on the decomposability of soil organic substrates and rates of C- and N-cycling, we conducted laboratory incubations of soils from a 15-yr-old experimental plot where big sagebrush (Artemisia tridentata Nutt.) and crested wheatgrass (Agropyron desertorum [Fisch.] Schult.) plants had been planted in a grid pattern. Soil samples collected from beneath crested wheatgrass had significantly greater total...
We report the first simultaneous measurements of ?15N and ?13C of DNA extracted from surface soils. The isotopic composition of DNA differed significantly among nine different soils. The ?13C and ?15N of DNA was correlated with ?13C and ?15N of soil, respectively, suggesting that the isotopic composition of DNA is strongly influenced by the isotopic composition of soil organic matter. However, in all samples DNA was enriched in 13C relative to soil, indicating microorganisms fractionated C during assimilation or preferentially used 13C enriched substrates. Enrichment of DNA in 15N relative to soil was not consistently observed, but there were significant differences between ?15N of DNA and ?15N of soil for three...
The field of soil ecology has relatively few fundamental unifying principles that can be used to explain and predict patterns and processes in belowground ecosystems. Here we propose that a first step towards developing a more comprehensive set of unifying principles in soil ecology is to identify and understand the characteristics shared by a wide range of soils, the common mechanisms driving soil biogeochemical processes, and the biogeochemical constraints imposed on soil biota regardless of soil type. Very often, soil ecologists focus on the differences between soils when, in fact, many soils share a common set of ecological mechanisms that govern biogeochemical processes. Here we explore evidence for the existence...
Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ15N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial 15N enrichment relative to soil extractable and total N was low at the...
We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250?2000 ?m) formation in a three months' incubation experiment and determined the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250?2000 ?m, 53?250 ?m and <53 ?m) using 13C and 15N data. A silty loam soil (sieved <250 ?m) was incubated with and without addition of 15N labelled maize leaves (C/N = 27.4) and roots (C/N = 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and...
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Changes in nutrient inputs due to aboveground herbivory may influence the litter and soil microbial community responsible for processes such as decomposition. The mesophyll-feeding scale insect (Matsucoccus acalyptus) found near Sunset Crater National Monument in northern Arizona, USA significantly increases pi�on (Pinus edulis) needle litter nitrogen (N) and phosphorus (P) concentrations by 50%, as well as litter inputs to soil by 21%. Because increases in needle litter quality and quantity of this magnitude should affect the microbial communities responsible for decomposition, we tested the hypothesis that insect herbivory causes a shift in soil microbial and litter microarthropod function. Four major findings...
Soil organic matter (SOM) biomarker methods were utilized in this study to investigate the responses of fungi and bacteria to freeze–thaw cycles (FTCs) and to examine freeze–thaw-induced changes in SOM composition and substrate availability. Unamended, grass-amended, and lignin-amended soil samples were subject to 10 laboratory FTCs. Three SOM fractions (free lipids, bound lipids, and lignin-derived phenols) with distinct composition, stability and source were examined with chemolysis and biomarker Gas Chromatography/Mass Spectrometry methods and the soil microbial community composition was monitored by phospholipid fatty acid (PLFA) analysis. Soil microbial respiration was also measured before and during freezing...
Recent research suggests that micronutrients such as Mn may limit growth of slow-growing biological soil crusts (BSCs) in some of the drylands of the world. These soil surface communities contribute strongly to arid ecosystem function and are easily degraded, creating a need for new restoration tools. The possibility that Mn fertilization could be used as a restoration tool for BSCs has not been tested previously. We used microcosms in a controlled greenhouse setting to investigate the hypothesis that Mn may limit photosynthesis and consequently growth in Collema tenax, a dominant N-fixing lichen found in BSCs worldwide. We found no evidence to support our hypothesis; furthermore, addition of other nutrients (primarily...
Archaea are common and abundant members of biological soil crust communities across large-scale biogeographic provinces of arid North America. Regardless of microbial community development, archaeal populations averaged 2 � 107 16S rRNA gene copies per gram of soil, representing around 5% of the prokaryotic (total calculated bacterial and archaeal) numbers assessed by quantitative-PCR. In contrast, archaeal diversity, determined by denaturing gradient gel electrophoresis fingerprinting and clone libraries of 16S rRNA genes, was very restricted. Only six different phylotypes (all Crenarchaea) were detected, three of which were very dominant. Some phylotypes were widespread, while others were typical of Southern desert...
We report the first simultaneous measurements of ?15N and ?13C of DNA extracted from surface soils. The isotopic composition of DNA differed significantly among nine different soils. The ?13C and ?15N of DNA was correlated with ?13C and ?15N of soil, respectively, suggesting that the isotopic composition of DNA is strongly influenced by the isotopic composition of soil organic matter. However, in all samples DNA was enriched in 13C relative to soil, indicating microorganisms fractionated C during assimilation or preferentially used 13C enriched substrates. Enrichment of DNA in 15N relative to soil was not consistently observed, but there were significant differences between ?15N of DNA and ?15N of soil for three...
In arid ecosystems, abiotic processes facilitate the physical and chemical degradation of plant litter to the extent that decomposition models that use climatic and litter composition variables as surrogates for microbial activity are not predictive. The purpose of this study was to estimate the potential contribution of photodegradation to the decomposition of plant litters that varies in architecture and chemical composition. Litter of Pinus edulis, Juniperus monosperma and Populus deltoides were exposed to ambient and attenuated sunlight, with and without supplemental water additions, at a riparian forest site along the Middle Rio Grande (New Mexico, USA). Mass loss, elemental composition, and microbial extracellular...
Although freeze–thaw cycles can alter soil physical properties and microbial activity, their overall impact on soil functioning remains unclear. This review addresses the effects of freeze–thaw cycles on soil physical properties, microorganisms, carbon and nutrient dynamics, trace gas losses and higher organisms associated with soil. I discuss how the controlled manipulation of freeze–thaw cycles has varied widely among studies and propose that, despite their value in demonstrating the mechanisms of freeze–thaw action in soils, many studies of soil freeze–thaw cycles have used cycle amplitudes, freezing rates and minimum temperatures that are not relevant to temperature changes across much of the soil...
Although freeze?thaw cycles can alter soil physical properties and microbial activity, their overall impact on soil functioning remains unclear. This review addresses the effects of freeze?thaw cycles on soil physical properties, microorganisms, carbon and nutrient dynamics, trace gas losses and higher organisms associated with soil. I discuss how the controlled manipulation of freeze?thaw cycles has varied widely among studies and propose that, despite their value in demonstrating the mechanisms of freeze?thaw action in soils, many studies of soil freeze?thaw cycles have used cycle amplitudes, freezing rates and minimum temperatures that are not relevant to temperature changes across much of the soil profile in...
Soil organic matter (SOM) biomarker methods were utilized in this study to investigate the responses of fungi and bacteria to freeze?thaw cycles (FTCs) and to examine freeze?thaw-induced changes in SOM composition and substrate availability. Unamended, grass-amended, and lignin-amended soil samples were subject to 10 laboratory FTCs. Three SOM fractions (free lipids, bound lipids, and lignin-derived phenols) with distinct composition, stability and source were examined with chemolysis and biomarker Gas Chromatography/Mass Spectrometry methods and the soil microbial community composition was monitored by phospholipid fatty acid (PLFA) analysis. Soil microbial respiration was also measured before and during freezing...
Plant detritus is an important source of labile C that drives soil microbial growth and regulates the balance of N mineralization and immobilization. In semiarid ecosystems, timing of plant detrital inputs may be especially important in regulating microbial C and N cycling because of the relatively short window of time when moisture is available. Low soil moisture in early-summer may inhibit microbial colonization of recently released detritus, resulting in C-limitations to microbial growth, and this may explain the NO3? accumulation commonly observed in semiarid, arid, and Mediterranean ecosystems. We examined linkages between soil C availability and gross N cycling rates during summer in three common semiarid...
Current soil enzyme methods measure potential enzyme activities, which are indicative of overall enzyme concentrations. However, they do not provide insight in the actual rates of enzymatically catalyzed reactions under natural in situ conditions. The objectives of this review are to (1) clarify what is being measured by current standard soil enzymology methods; (2) present an overview of the factors that control in situ activities of soil enzymes; and (3) evaluate how emerging technologies and modeling approaches could enhance our understanding of in situ extracellular enzyme activity (EEA). Genomic studies targeting functional genes coding for extracellular enzymes can identify the genetic potential of microbial...
Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 Î&frac14;g chlorophyll a g−1 soil or 3.0 Î&frac14;g chlorophyll a g−1 soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them. Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH4+-producers (7.4×109 g−1 crust), starch-mineralizing microbes (1.7×108 g−1 crust), cellulose-mineralizing microbes (1.4×106 g−1 crust) and NO2− and NO3− producers...
The effect of inoculation of crested wheatgrass (Agropyron cristatum L.), perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) with the soil diazotroph Bacillus polymyxa was studied. Plant growth responses to inoculation varied from slightly negative (perennial ryegrass) to highly positive (white clover and crested wheatgrass) when root, shoot and plant dry weights were measured. Root-to-shoot ratios were also increased in the latter two species. Seedling emergence in crested wheatgrass was shown to be enhanced by inoculation with the bacterium. Possible mechanisms of the growth response include suppression of pathogenic organisms in the rhizosphere by the inoculant strain, root-associated...
Differences in rhizosphere microbial community function in response to Gutierrezia sarothrae plants grown in elevated CO2 (750 ?l 1?1) and fertilized with nitrogen were studied using the Biolog microplate analysis of sole C substrate utilization. Compared to ambient CO2, under elevated CO2, polymers were more slowly oxidized by the microbial community, amides showed no change in usage, and all other substrate groups were more rapidly utilized, although there was no significant change in the number of viable bacteria. No microbial community responses to N fertilization were detected. The results indicate that potential functional changes in the soil microbial community in response to elevated CO2 have to be taken...
We used dual labelled stable isotope (13C and 15N) techniques to examine how grassland plant species with different growth strategies vary in their ability to compete with soil microbes for different chemical forms of nitrogen (N), both inorganic and organic. We also tested whether some plant species might avoid competition by preferentially using different chemical forms of N than microbes. This was tested in a pot experiment where monocultures of five co-existing grassland species, namely the grasses Agrostis capillaris, Anthoxanthum odoratum, Nardus stricta, Deschampsia flexuosa and the herb Rumex acetosella, were grown in field soil from an acid semi-natural temperate grassland. Our data show that grassland...
map background search result map search result map Impacts of herbivorous insects on decomposer communities during the early stages of primary succession in a semi-arid woodland Impacts of herbivorous insects on decomposer communities during the early stages of primary succession in a semi-arid woodland