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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...
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...
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...
C and N released in root exudates throughout a growing season were estimated in Bouteloua gracilis and Agropyron smithii (dominant species in the shortgrass steppe ecosystem) and A. cristatum (dominant species in a 40-yr-old crested wheatgrass ecosystem). The release of C and N exudate was measured with or without the presence of a rhizosphere microflora. These values were used to estimate the potential contribution of root exudates to the N cycle of both ecosystems. Total C released through root exudates by A. cristatum, A. smithii and vB. gracilis was estimated at 8, 17 and 15%, respectively, of C fixed. The contributions of root exudates to the N cycle were estimated to be 11 g N m?2 in the shortgrass steppe...
In desert ecosystems, belowground characteristics are influenced chiefly by the formation and persistence of ?shrub-islands of fertility? in contrast to barren plant interspaces. If soil microbial communities are exclusively compared between these two biogeochemically distinct soil types, the impact of characteristics altered by shrub species, especially soil C and N, are likely to be overemphasized and overshadow the role of other characteristics in structuring microbial composition. To determine how belowground characteristics influence microbial community composition, and if the relative importance of these characteristics shifts across the landscape (i.e., between and within shrub and interspace soils), changes...
Although metabolic activity of soil organisms is determined by water accessibility, little attention was given to rewetting with different water potentials. Rapid water potential increase induced a respiration pulse in organic layers in laboratory experiments and significant effects could be observed when soil below −6300 hPa was rewetted. Published in Soil Biology and Biochemistry, volume 41, issue 7, on pages 1577 - 1579, in 2009.
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...
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...
Dryland ecosystems have long been considered to have a highly heterogeneous distribution of nutrients and soil biota, with greater concentrations of both in soils under plants relative to interspace soils. We examined the distribution of soil resources in two plant communities (dominated by either the shrub Coleogyne ramosissima or the grass Stipa hymenoides) at two locations. Interspace soils were covered either by early successional biological soil crusts (BSCs) or by later successional BSCs (dominated by nitrogen (N)-fixing cyanobacteria and lichens). For each of the 8 plant type�crust type�locations, we sampled the stem, dripline, and 3 interspace distances around each of 3 plants. Soil analyses revealed that...
Phenol oxidase and peroxidase activities in desert grassland soils at the Sevilleta Long Term Ecological Research site in central New Mexico (USA) are far greater than those of temperate soils. Activity is uniformly distributed across particles ranging from >1 mm to <38 ?m and is unaffected by autoclaving, in contrast to hydrolase activities. The sorbed enzymes are readily extractable and inactivated by boiling. High soil pH, high stabilized oxidative enzyme activity, and carbonates create optimal conditions for degradation of phenols which increase decomposition potentials and limit soil organic matter accumulation. Published in Soil Biology and Biochemistry, volume 40, issue 2, on pages 550 - 553, in 2008.
Soil respiration in semiarid ecosystems responds positively to temperature, but temperature is just one of many factors controlling soil respiration. Soil moisture can have an overriding influence, particularly during the dry/warm portions of the year. The purpose of this project was to evaluate the influence of soil moisture on the relationship between temperature and soil respiration. Soil samples collected from a range of sites arrayed across a climatic gradient were incubated under varying temperature and moisture conditions. Additionally, we evaluated the impact of substrate quality on short-term soil respiration responses by carrying out substrate-induced respiration assessments for each soil at nine different...