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? Rhizodeposition, or the addition of C from roots to soil C pools, is expected to increase if net primary production is stimulated and some excess C is allocated below-ground. We investigated the effects of 5 yrs of elevated CO2 on below-ground C dynamics in a native, C3?C4 grassland ecosystem in Colorado, USA. ? Cylinder harvests following each growing season and monolith excavation at the end of the experiment provided data on root biomass, root C : N ratios, and root and soil ?13C values. We applied an isotopic mixing model to quantify new soil C inputs on elevated and ambient CO2 treatments. ? Root biomass increased by 23% and root C : N ratios increased by 26% after 5 yrs of elevated CO2. Species-specific...
• Theoretical and empirical research has supported the hypothesis that plant–plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems. • During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon pine, Pinus edulis ) and a common shrub (Apache plume, Fallugia paradoxa ) in replicated areas of high and low stress. • The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the...
Modern pollen assemblages from 18 small ponds and wetlands on the spatially isolated and forested Kaibab Plateau were studied to determine how the pollen assemblages recorded vegetation patterns. Vegetation of the Plateau consists of an inner core of subalpine Picea/Abies forests, surrounded by mixed Abies/Picea/Pseudotsuga/Pinus ponderosa forests, which are in turn surrounded by extensive Pinus ponderosa forests. The flanks of the Plateau are vegetated by Pinus edulis/Juniperus woodlands, with scattered Quercus populations. Arboreal pollen assemblages were dominated by Pinus (70-98%), which was most abundant in the P. ponderosa forests. Picea, Abies, Pseudotsuga and Populus pollen were abundant only at sites in...
* • Relative abundances of carbon and nitrogen isotopes in fungal sporocarps are useful in assessing mycorrhizal or saprotrophic status, and might provide insights into the evolutionary history of these traits. * • Sporocarps of known mycorrhizal or saprotrophic genera were collected at Woods Creek, OR, USA, and isotopically compared with foliage, litter, soils and wood collected from the same site. Possible trophic strategies were then isotopically assessed in archived specimens of the Pezizales of known molecular phylogeny from the western United States. * • At Woods Creek, mycorrhizal fungi were 3.5‰ ± 0.6‰ depleted in δ13C and 5.7‰ ± 0.4‰ enriched in δ15N compared with saprotrophic fungi. By...
The nitrogen-fixing lichen Pettigera aphthosa (L.) Willd. was treated under semi-natural conditions with simulated rain containing different combinations of ammonium, nitrate and sulphuric acid. Nitrogen in neutral solution had no negative effect on the nitrogen fixation rate. Sulphuric acid had a negative effect on nitrogen fixation rate, especially in combination with ammonium. The results could suggest an explanation for the sudden decline of P. aphthosa in southern Sweden. Published in New Phytologist, volume 120, issue 1, on pages 99 - 103, in 1992.
Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response...
Shifts from nitrogen (N)- to phosphorus (P)-limited growth due to high N deposition may alter the functioning of wetland vegetation. This experiment tested how N vs P deficiency affects the growth and nutrient use of wetland sedges. Five wetland Carex species were grown at nine N : P supply ratios (0.6-405) with two absolute levels of N and P. Biomass and nutrient concentrations were determined after one and two growing seasons. Shoot biomass was maximal at N : P supply ratios of 15-26 after one season but 5-15 after two seasons. Photosynthesis after the first season, second-year growth, leaf longevity, and the fraction of nutrient supply retained by plants over two seasons were all negatively related to N : P supply...
The pattern of incorporation of 14C into the thallus during incubation with H14CO3- in the light suggested that a substance in the 80% ethanol insoluble fraction, an unknown compound (probably a sugar phosphate) and glucose were intermediates in the transfer of carbon to the fungus. Results are presented which are consistent with the hypothesis that the alga releases a glucan which the fungus hydrolyses extra cellularly to form glucose. This glucose is then taken up by the fungus and converted to mannitol. Published in New Phytologist, volume 71, issue 1, on pages 31 - 39, in 1972.
Arbuscular mycorrhizal fungi (AMF) are important root symbionts that can provide benefits to plant hosts, yet we understand little about how neighboring hosts in a plant community contribute to the composition of the AMF community. We hypothesized that the composition of the plant neighborhood, including the identities of both host and neighbor, would alter AMF community composition. We tested this in a glasshouse experiment in which a native perennial grass (Nassella pulchra) and three annual grasses (Avena barbata, Bromus hordeaceaous and Vulpia microstachys) were grown in two neighborhoods: conspecific monocultures and heterospecific perennial-annual mixtures. To identify AMF taxa colonizing plant roots, we used...
To test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied...
When air-dry thalli of Hypogymnia physodes are immersed in water, four main features characterize the rewetting phase. (1) There is an immediate and substantial release of gas, the `wetting burst'. Although the released gas appears to be mostly CO2, it is non-metabolic in origin and probably the result of a previous adsorption of gas onto polysaccharide sheaths surrounding fungal hyphae. (2) There is a period of 1-2 h increased respiration, `resaturation respiration', which, unlike basal respiration, is cyanide-sensitive. The cause of resaturation respiration remains obscure, but could result from exposure of respiratory substrate to enzymes from which they are normally separated. (3) There is a brief period (1-2...
Loss of viability by seven species of bryophyte (Anomodon viticulosus, Racomitrium aquaticum, R. lanuginosum, Tortula intermedia, Andreaea rothii, Frullania tamarisci and Porella platyphylla) kept dry at temperatures between 20 and 100 ⚬C was followed by estimating chlorophyll content after a moist recovery period. Behaviour at different temperatures was qualitatively similar, differing in time-scale. This ranged from a few minutes or less for 50% loss of chlorophyll at 100 ⚬C to weeks or months for the more resistant species at 37 and 20 ⚬C. For most of the species investigated, the logarithms of the half-viability times gave straight lines when plotted against the reciprocal of absolute temperature, with...
The rangeland grass, Bouteloua gracilis was inoculated with its mycorrhizal symbiont, Glomus fasciculatus, to determine the influence of vesicular-arbuscular mycorrhizae on water status, stomatal behaviour and photosynthesis as well as gross plant morphology, biomass and phosphorus content. Mycorrhizal infection increased transpiration rates by over 100% with 50 to 70% lower leaf resistances to water vapour diffusion. Leaf xylem pressure was not different between mycorrhizal and non-mycorrhizal plants indicating that whole-plant resistance to water transport was reduced by more than 50%. Photosynthetic rates under saturating light conditions increased 68% with infection as a consequence of a 33% reduction in stomatal...
Root growth characteristics and water transport were compared between non-mycorrhizal Bouteloua gracilis and vesicular–arbuscular mycorrhizal Bouteloua gracilis infected with Glomus fasciculatus. Mycorrhizal plants did not have significantly different leaf area or root length from non-mycorrhizal plants, but did have significantly fewer and shorter root hairs. Mycorrhizal plants had 50% lower leaf resistance with no change in leaf or root water potentials; thus transpiration was increased 100% with a 50% reduction in whole-plant, soil-to-root and root-to-leaf resistance. Assuming that the difference in water uptake was due to transport via the mycorrhizal hyphae, estimated fungus-to-root transport was 2.8 × 10−5...
The rhizosphere differs from the bulk soil in a range of biochemical, chemical and physical processes that occur as a consequence of root growth, water and nutrient uptake, respiration and rhizodeposition. These processes also affect microbial ecology and plant physiology to a considerable extent. This review concentrates on two features of this unique environment: rhizosphere geometry and heterogeneity in both space and time. Although it is often depicted as a soil cylinder of a given radius around the root, drawing a boundary between the rhizosphere and bulk soil is an impossible task because rhizosphere processes result in gradients of different sizes. For instance, because of diffusional constraints, root uptake...
Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios ? those of plants whose growth is equally limited by N and P ? depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios <10 and >20 often (not always) correspond to N- and P-limited biomass...
Arbuscular mycorrhizal (AM) fungi form mutualistic symbioses with the root systems of most plant species. These mutualisms regulate nutrient exchange in the plant?soil interface and might influence the way in which plants respond to increasing atmospheric CO2. In other experiments, mycorrhizal responses to elevated CO2 have been variable, so in this study we test the hypothesis that different genera of AM fungi differ in their response, and in turn alter the plant's response, to elevated CO2. Four species from three genera of AM fungi were tested. Artemisia tridentata Nutt. seedlings were inoculated with either Glomus intraradices Schenck & Smith, Glomus etunicatum Becker & Gerdemann, Acaulospora sp. or Scutellospora...
? The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall (?pulses?) on carbon and water exchange in water-limited ecosystems are poorly quantified. ? In the present study, we estimated net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased (?wet?) or decreased (?dry?) monsoon-season (July?September) rainfall during 2002 and...
Studies examining plant distribution patterns across environmental gradients have generally focused on perennial-dominated systems, and we know relatively little about the processes structuring annual communities. Here, the ecological factors determining local distribution patterns of five dominant annual species distributed across micro-topographic gradients in ephemeral California wetlands are examined. Over two growing seasons in three vernal pools, patterns of inundation and above-ground biomass were characterized across the microtopographic gradient, population boundaries for five dominant species were documented and a reciprocal transplant experiment and neighbor removal treatment were conducted to test the...
The Prairie Heating and CO2 Enrichment (PHACE) experiment has been initiated at a site in southern Wyoming (USA) to simulate the impact of warming and elevated atmospheric CO2 on ecosystem dynamics for semiarid grassland ecosystems. The DAYCENT ecosystem model was parametrized to simulate the impact of elevated CO2 at the open-top chamber (OTC) experiment in north-eastern Colorado (1996-2001), and was also used to simulate the projected ecosystem impact of the PHACE experiments during the next 10 yr. Model results suggest that soil water content, plant production, soil respiration, and nutrient mineralization will increase for the high-CO2 treatment. Soil water content will decrease for all years, while nitrogen...


map background search result map search result map Rhizodeposition stimulated by elevated CO2 in a semiarid grassland Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply Rhizodeposition stimulated by elevated CO2 in a semiarid grassland