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Nuisance blooms of heterocystous Cyanobacteria in Lake Winnipeg have nearly doubled in size since the mid 1990s. The increases are the result of a recent rapid increase in loading and concentration of phosphorus. The rapid increase in phosphorus is largely the result of two factors. The first factor is the result of rapidly increased livestock production and use of synthetic fertilizer in the Red River Valley, with smaller contributions of phosphorus from the city of Winnipeg and other human development in the Red and Winnipeg river basins. The second factor is the increased frequency and intensity of spring floods in the Red River watershed in recent years, which have greatly enhanced the transfer of phosphorus...
Phosphorus and nitrogen uptake capacities were assessed during 36–58 d drying cycles to determine whether the ability of sagebrush (Artemisia tridentata Nutt.) to absorb these nutrients changed as the roots were subjected to increasing levels of water stress. Water was withheld from mature plants in large (6 I) containers and the uptake capacity of excised roots in solution was determined as soil water potentials decreased from −0.03 MPa to −5.0 MPa. Phosphorus uptake rates of excised roots at given substrate concentrations increased as preharvest soil water potentials decreased to −5.0 MPa. Vmax and Km also increased as soil water potentials declined. Declining soil water potentials depressed nitrogen uptake...
Phosphorus (P) has been proposed to directly limit primary productivity in some temperate grassland ecosystems. Our study of a recovering prairie on Colorado's Front Range suggests that P availability, possibly via regulation of nitrogen (N) fixation, may strongly influence N availability in recovering prairie soils. Consequently, increased P availability could indirectly affect plant growth through the alleviation of N limitation. At our site, concentrations of soil inorganic N were nearly three times higher in plots fertilized with P than in control plots. Subsequent acetylene reduction analyses showed that soil N fixation rates were more than double for P fertilization plots. These results highlight potential...
Interspecific variation in polyphenol production by plants has been interpreted in terms of defense against herbivores. Several recent lines of evidence suggest that polyphenols also influence the pools and fluxes of inorganic and organic soil nutrients. Such effects could have far-ranging consequences for nutrient competition among and between plants and microbes, and for ecosystem nutrient cycling and retention. The significance of polyphenols for nutrient cycling and plant productivity is still uncertain, but it could provide an alternative or complementary explanation for the variability in polyphenol production by plants. Published in Trends in Ecology & Evolution, volume 15, issue 6, on pages 238 - 243, in...
N limitation to primary production and other ecosystem processes is widespread. To understand the causes and distribution of N limitation, we must understand the controls of biological N fixation. The physiology of this process is reasonably well characterized, but our understanding of ecological controls is sparse, except in a few cultivated ecosystems. We review information on the ecological controls of N fixation in free-living cyanobacteria, vascular plant symbioses, and heterotrophic bacteria, with a view toward developing improved conceptual and simulation models of ecological controls of biological N fixation. A model (Howarth et al. 1999) of cyanobacterial fixation in lakes (where N fixation generally increases...
The availability of nutrients is a critical determinant of ecological dynamics in grasslands, but the relationships between soil resource availability and nutrient limitation across ecosystems are not clear. To better understand how soil nutrient availability determines nutrient limitation in vegetation, we grew the same species of grass (Schizachyrium scoparium) in 98 North American grassland soils and fertilized them factorially with nitrogen (N) and phosphorus (P). On average adding N, P, and the two nutrients together increased biomass relative to unfertilized plants by 81%, 22%, and 131%, respectively. Plants grown on low-P soils were not primarily limited by P. Instead, these plants were colimited by N and...
Geostatistical analyses show that the distribution of soil N, P and K is strongly associated with the presence of shrubs in desert habitats. Shrubs concentrate the biogeochemical cycle of these elements in ?islands of fertility? that are localized beneath their canopies, while adjacent barren, intershrub spaces are comparatively devoid of biotic activity. Both physical and biological processes are involved in the formation of shrub islands. Losses of semiarid grassland in favor of invading shrubs initiate these changes in the distribution of soil nutrients, which may promote the further invasion and persistence of shrubs and cause potential feedbacks between desertification and the Earth's climate system. Published...
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Effects of annual additions of mineral N and P (100 kg ha-1) on plant species composition and annual aboveground net primary production (ANPP) were investigated during the first three years following disturbance in a semi-arid ecosystem. Additions of N reduced richness of perennial plant species during years 2 and 3, while P reduced the number of perennial species only in year 3. From year 1 to year 2, annual and biennial species richness declined in all treatments while ANPP of annual species increased greatly. Added N increased ANPP of annual species while it decreased ANPP of most perennial species relative to the unfertilized control treatment. Community similarities were higher for the control and native vegetation...
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In drylands of southeastern Utah, USA, the invasive exotic grass Bromus tectorum L. occurs in distinct spatial patterns suggesting soil control of ecosystem susceptibility to invasion. To improve our understanding of these patterns, we examined performance of B. tectorum in relation to additions of water, KCl, MgO, and CaO at seventeen 1600 m2 sites distributed across a calcareous soil gradient in Canyonlands National Park. Water additions resulted in a 57% increase in B. tectorum establishment. Fall establishment was significantly correlated with silt and clay content in wet plots but not in dry plots, suggesting that texture effects on B. tectorum establishment patterns may be greater in wet years than in dry...
In regions subject to strong winds, considerable amounts of soil are transported off land and deposited to nearby fields and surface water. This study investigated the nutrient supply from deposition of erodible-sized soil to surrounding soil and surface water in a controlled laboratory setting. Wind-erodible fraction (WEF) aggregates were collected from a field with no manure or fertilizer application (Treatment WEF0) and a field that had received 180 Mg ha-1 year-1 of cattle manure (WEF180) for 30 years. The WEF aggregates were applied to a loamy sand soil and to distilled water at rates equivalent to 0, 10, 50, 100, and 150 Mg ha-1 and incubated for 2 years. Deposition of carbon and nutrient-enriched WEF aggregates...
The Walker and Syers model of phosphorus (P) transformations during pedogenesis is widely accepted for the development of humid ecosystems, but long-term P dynamics of more arid ecosystems remain poorly understood. We tested the Walker and Syers model in semiarid piñon–juniper woodlands by measuring soil P fractions under tree canopies and in intercanopy spaces along a well-constrained, 3000 ka (1 ka = 1000 years) volcanic substrate age gradient in northern Arizona, USA. The various pools of soil P behaved largely as predicted; total soil P and primary mineral P declined consistently with substrate age, labile inorganic P increased early in soil development and then declined at later stages, and organic phosphorus...
A principal challenge in ecology is to integrate physiological function (e.g. photosynthesis) across a collection of individuals (e.g. plants of different species) to understand the functioning of the entire ensemble (e.g. primary productivity). The control that organism size exerts over physiological and ecological function suggests that allometry could be a powerful tool for scaling ecological processes across levels of organization. Here we use individual plant allometries to predict how nutrient content and productivity scale with total plant biomass (phytomass) in whole plant communities. As predicted by our model, net primary productivity as well as whole community nitrogen and phosphorus content all scale...
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Seasonal changes in soil phosphorus (P) availability are not yet known for many ecosystems. We report seasonal changes in several pools of soil phosphorus, including plant available P from the Mojave Desert and Colorado Plateau. In addition we show that cheatgrass changes soil P fractions in unexpected and ecologically significant ways. Monthly soil samples (0-10 cm) from four sites in Canyonlands National Park in southeast Utah, are analyzed for P with a modified Hedley P fractionation method. Labile P (plant available) peaks in spring and autumn with significant monthly variation. Surprisingly, HCl extractable P changes as well, with a pattern inverse to that of labile P. Each of these sites has considerable Bromus...
Phosphorus (P) losses from agricultural lands degrade surface waters due to anthropogenic eutrophication. Previous studies focused on plot-to-field scale P loss and reductions from best management practices (BMP's), little information in intense agricultural catchments has been gathered on the dynamics influencing P beyond the edge of the field. This study was conducted to examine the phosphorus equilibrium between the water column and sediments in three tile fed drainage ditches in Northeast Indiana. Surface water and sediment samples were collected and analyzed for organic carbon (C), particle size and P from sites along three ditches with similar soils and land use at sites within each watershed draining approximately...
Nutrient inputs generally are increased by human-induced land use changes and can lead to eutrophication and impairment of surface waters. Understanding the scale at which land use influences nutrient loading is necessary for the development of management practices and policies that improve water quality. The authors assessed the relationships between land use and stream nutrients in a prairie watershed dominated by intermittent stream flow in the first-order higher elevation reaches. Total nitrogen, nitrate, and phosphorus concentrations were greater in tributaries occupying the lower portions of the watershed, closely mirroring the increased density of row crop agriculture from headwaters to lower-elevation alluvial...
Increased nutrient availability reduces vesicular?arbuscular mycorrhizal (VAM) associations with plants, but whether increased nutrients in small volumes of soil affect local VAM colonization is not known. In a field experiment we investigated VAM colonization at different times following fertilization of small soil patches. Soil volumes of ~ 1000 cm3 were treated with a nutrient solution (enriched patch) or distilled water (control patch) on opposite sides of individual plants of the tussock grass Agropyron desertorum and the shrub Artemisia tridentata. Agropyron had significantly lower (p = 0.03) arbuscular infection in the locally enriched patches compared to control patches (32 and 40%, respectively). This reduced...
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Many soils in southeastern Utah are protected from surface disturbance by biological soil crusts that stabilize soils and reduce erosion by wind and water. When these crusts are disturbed by land use, soils become susceptible to erosion. In this study, we compare a never-grazed grassland in Canyonlands National Park with two historically grazed sites with similar geologic, geomorphic, and geochemical characteristics that were grazed from the late 1800s until 1974. We show that, despite almost 30 years without livestock grazing, surface soils in the historically grazed sites have 38-43% less silt, as well as 14-51% less total elemental soil Mg, Na, P, and Mn content relative to soils never exposed to livestock disturbances....
In order to better elucidate fixed-C partitioning, nutrient acquisition and water relations of prairie grasses under elevated [CO2], we grew the C4 grass Bouteloua gracilis (H.B.K.) lag ex Steud. from seed in soil-packed, column-lysimeters in two growth chambers maintained at current ambient [CO2] (350 ?L L?1) and twice enriched [CO2] (700 ?L L?1). Once established, plants were deficit irrigated; growth chamber conditions were maintained at day/night temperatures of 25/16�C, relative humidities of 35%/90% and a 14-hour photoperiod to simulate summer conditions on the shortgrass steppe in eastern Colorado. After 11 weeks of growth, plants grown under CO2 enrichment had produced 35% and 65% greater total and root...
In an effort to develop indicators for Great Lakes near-shore conditions, diatom-based transfer functions to infer water quality variables were developed from 155 samples collected from coastal Great Lakes wetlands, embayments and high-energy shoreline sites. Over 2,000 diatom taxa were identified, and 352 taxa were sufficiently abundant to include in transfer function development. Multivariate data exploration revealed strong responses of the diatom assemblages to stressor variables, including total phosphorus (TP). Spatial variables such as lake, latitude and longitude also had notable relationships with assemblage characteristics. A diatom inference transfer function for TP provided a robust reconstructive relationship...
This review provides a critical overview of conservation practices that are aimed at improving water quality by retaining phosphorus (P) downstream of runoff genesis. The review is structured around specific downstream practices that are prevalent in various parts of the United States. Specific practices that we discuss include the use of controlled drainage, chemical treatment of waters and soils, receiving ditch management, and wetlands. The review also focuses on the specific hydrology and biogeochemistry associated with each of those practices. The practices are structured sequentially along flowpaths as you move through the landscape, from the edge-of-field, to adjacent aquatic systems, and ultimately to downstream...
map background search result map search result map Multi-Decadal Impacts of Grazing on Soil Physical and Biogeochemical Properties in Southeast Utah Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance Local reduction of mycorrhizal arbuscule frequency in enriched soil microsites Cheatgrass (Bromus tectorum L.) and seasonal patterns of soil phosphorus availability in arid ecosystems Performance of Bromus tectorum L. in relation to soil properties, water additions, and chemical amendments in calcareous soils of southeastern Utah, USA Multi-Decadal Impacts of Grazing on Soil Physical and Biogeochemical Properties in Southeast Utah Cheatgrass (Bromus tectorum L.) and seasonal patterns of soil phosphorus availability in arid ecosystems Performance of Bromus tectorum L. in relation to soil properties, water additions, and chemical amendments in calcareous soils of southeastern Utah, USA Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance