Showing papers on "Phosphorus published in 1999"

The relative influences of nitrogen and phosphorus on oceanic primary production

Abstract: A simple model has the potential to resolve the long-running debate amongst oceanographers over whether nitrogen or phosphorus exerts overall control on oceanic primary production. A representation of the competition between nitrogen-fixing and other phytoplankton is inserted into a two-box global model of the oceanic nitrogen and phosphorus cycles. Homeostatic regulation of both nitrate and phosphate concentrations results, with surface waters more deficient in nitrate than phosphate in the steady state, but with external phosphate inputs controlling longer-term primary production in the global ocean.

Growth Promotion of Plants Inoculated with Phosphate-Solubilizing Fungi

Abstract: Phosphorus is an important plant nutrient which is in short supply in many agricultural soils Because much of the soluble phosphate (P) applied to soils as fertilizer is “fixed” by the soil and rendered less available to plants, the long-term application of P fertilizers has resulted in an accumulation of total soil P, most of which is poorly soluble Many soil fungi, predominantly of the genera Aspergillus and Penicillium , have been shown to possess the ability to solubilize sparingly soluble phosphates in vitro by secreting inorganic or organic acids Growth promotion and increased uptake of P by plants inoculated with P-solubilizing fungi have also been reported by many investigators

Phosphate solubilisation in solution culture by the soil fungus Penicillium radicum

Abstract: Phosphorus is a plant nutrient which is rapidly made immobile and less available for plant use after addition to the soil as a soluble fertiliser. Phosphate-solubilising microorganisms may be able to improve the P nutrition of plants and thus stimulate plant growth. Penicillium radicum, a phosphate-solubilising fungus isolated from the rhizosphere of wheat roots, has shown promise in plant growth promotion. Its ability to solubilise inorganic phosphate was studied in vitro. The fungus was grown in liquid medium culture containing either ammonium or nitrate as the sole source of N. Insoluble, or sparingly-soluble P (1000 mg P l−1) was supplied as calcium monohydrogen phosphate (CaHPO4), calcium orthophosphate (Ca3(PO4)2), crystalline ferric phosphate (FePO4·4H2O), crystalline aluminium phosphate (AlPO4), colloidal ferric phosphate or colloidal aluminium phosphate. The titratable acidity, pH and concentrations of organic acids and soluble phosphate were determined periodically during a 20 or 31 d incubation. Phosphate solubilisation was highest from CaHPO4 (475 mg P l−1), Ca3(PO4)2 (360 mg P l−1) and colloidal aluminium phosphate (207 mg P l−1). Phosphate solubilisation was generally higher when ammonium rather than nitrate was the sole source of N. Soluble phosphate concentrations in the culture medium were directly proportional to the titratable acidity and organic acid (principally gluconic acid) concentration and inversely related to pH. The main mechanism for phosphate solubilisation was acid production leading to a decrease in pH. Evidence from an abiotic study using HCl and gluconic acid to solubilise P also indicated that chelation of Al3+ by gluconic acid may have been a factor in the solubilisation of colloidal aluminium phosphate.

A Potential Phosphate Crisis

Abstract: Phosphate is a crucial component of DNA, RNA, ATP, and other biologically active compounds. Microbes, plants, and animals—including humans—cannot exist without it. Rocks containing phosphate have been discovered and are being mined at minimal cost. But resources are limited, and phosphate is being dissipated. Future generations ultimately will face problems in obtaining enough to exist. The current major use of phosphate is in fertilizers. Growing crops remove it and other nutrients from the soil. Long-term research at the Morrow agricultural plots of the University of Illinois at Urbana-Champaign has corroborated the fact that even the best land loses fertility unless nutrients are replenished. At the Morrow plots, there is a threefold or greater difference in yields of corn between fertilized areas and untreated ones. Most of the world's farms do not have or do not receive adequate amounts of phosphate. Feeding the world's increasing population will accelerate the rate of depletion of phosphate reserves. Corn seeds, which are a major source of food for cattle, swine, and poultry, contain substantial amounts of phosphate. About 75% of it is in the form of phytate, a water-insoluble compound. When the seeds sprout, enzymes are created that release phosphate from the phytate, making it available for biological activities. When seeds are fed to ruminants, bacteria in the rumen degrade some of the phytate, providing phosphate for use by the animals. But nonruminants such as poultry, swine, and people do not have an efficient system for making phosphate available from phytate. They excrete most of it. Ultimately, some of the phosphate excreted contributes to water pollution and eutrophication and becomes unavailable for further use. Recent scientific research has resulted in ways to diminish the loss of phosphate. One of the methods was described at the recent AAAS annual meeting in Anaheim, California. Adolphus van Loon of Hoffman-La Roche, Basel, Switzerland, reported on research results that facilitate the release of phosphate from feed prepared for chickens and hogs. One of his colleagues conducted many successful experiments to improve the stability of phytase enzymes, which catalyze the breakup of phytate. The DNA coding for phytase that is present in thermophilic bacteria was altered to produce more highly thermostable enzymes. These are incorporated in feed when it is initially being cooked. Experiments have demonstrated that as much as one-third of the phytate phosphate is made available when monogastric animals are fed the improved feed. Van Loon estimated that annual sales will total as much as $500 million. Another approach to the phytate problem has been attempts to reduce the amount of phytate in seeds. However, studies using this approach usually found that when the phytate content was decreased substantially, the seeds did not germinate, or if they did, they did not give rise to healthy plants. A breakthrough was achieved when Victor Raboy and his colleagues at the U.S. Department of Agriculture Agricultural Research Service in Aberdeen, Idaho, obtained useful mutants of corn seeds. Although the seeds contain relatively little phytate, they give rise to productive plants. Phosphate needed by the seed is stored safely by a mechanism not yet determined. Growing and feeding experiments are currently being conducted. In one study, University of Missouri scientists conducted experiments involving analysis of waste from pigs fed either unmodified or low-phytate corn. Pigs who were fed the low-phytate corn showed, on average, a 37% reduction in phosphorus excreted. In growing pigs, 64% of the phosphorus in low-phytate corn was available, as compared with 10% from genetically similar corn with normal phytate levels. The rate at which U.S. farmers will adopt low-phytate varieties of corn will depend on whether seeds also provide a combination of traits that include higher yield, increased energy for feed, and resistance to pests and herbicides. Ultimately this objective will be achieved. Accomplishments such as these that lead to the conservation of phosphorus will avert a crisis in phosphate availability in the short term, but further research is needed to avert problems in the long term.

Maize root system growth and development as influenced by phosphorus deficiency

Abstract: cept light was severely reduced so that root growth was finally reduced. Effects on leaf growth, biomass accumulation and root morphogenesis associated with the establishment of Key words: Zea mays L., maize, phosphorus, root, root phosphorus (P) deficiency were studied on maize in morphogenesis. order to test the hypothesis that the root system response can be accounted for by the effect of P

Ethylene: a regulator of root architectural responses to soil phosphorus availability

Abstract: The involvement of ethylene in root architectural responses to phosphorus availability was investigated in common bean (Phaseolus vulgaris L.) plants grown with sufficient and deficient phosphorus. Although phosphorus deficiency reduced root mass and lateral root number, main root length was unchanged by phosphorus treatment. This resulted in decreased lateral root density in phosphorusdeficient plants. The possible involvement of ethylene in growth responses to phosphorus deficiency was investigated by inhibiting endogenous ethylene production with aminoethoxyvinylglycine (AVG) and aerating the root system with various concentrations of ethylene. Phosphorus deficiency doubled the root-to-shoot ratio, an effect which was suppressed by AVG and partially restored by exogenous ethylene. AVG increased lateral root density in phosphorusdeficient plants but reduced it in phosphorus-sufficient plants. These responses could be reversed by exogenous ethylene, suggesting ethylene involvement in the regulation of main root extension and lateral root spacing. Phosphorus-deficient roots produced twice as much ethylene per g dry matter as phosphorus-sufficient roots. Enhanced ethylene production and altered ethylene sensitivity in phosphorus-deficient plants may be responsible for root responses to phosphorus deficiency.

Internal phosphorus loading in shallow Danish lakes

Abstract: High phosphorus concentrations due to internal loading from the sediment with a strongly negative impact on lake water quality, is often seen in shallow lakes after a reduction of external loading. To analyse the nature of internal loading we studied 1. the seasonal phosphorus concentrations of 265 Danish shallow, mainly eutrophic lakes; 2. seasonal phosphorus mass balances and retention for eight years in 16 eutrophic lakes, and 3. phosphorus mass balances and changing sediment phosphorus profiles for 15 years in one hypertrophic lake. Lake water, inlets and outlets were routinely sampled 10–26 times annually. Total phosphorus (TP) concentrations during summer were two–four times higher than winter values in lakes with a mean summer total phosphorus concentration (TPsum) above 0.2 mg P 1−1. Annual phosphorus retention decreased with increasing TPsum and was lower than predicted from the Vollenweider model, particularly in lakes with TPsum above 0.2 mg P1−1. The seasonal phosphorus retention in lakes with TPsum below 0.1 mg P 1−1 was positive during the whole season, except July and August when mean retention ranged from −10 to −30% of inlet loading. In lakes with TPsum above 0.1 mg P 1−1, the retention was positive during winter, but negative from April to September. The negative retention was most pronounced in lakes with the highest TPsum, particularly in May and July when mean retention ranged from −50 to −68% in lakes with TPsum above 0.2 mg P 1−1. The retention was generally less negative in June, when a clearwater phase typically occurs and close to 0 also in lakes with a high TPsum. Mass balances from the hypertrophic lake have now shown a 15-yr net annual negative retention following reduced external loading. Sediment profiles suggest phosphorus release from depths down to 25 cm and that net internal phosphorus loading may persist for another 15 yrs. It is concluded that internal loading of shallow eutrophic lakes may have a considerable and persistent impact on summer TP after reduced external loading.

Psr1, a nuclear localized protein that regulates phosphorus metabolism in Chlamydomonas

Abstract: Understanding the ways in which phosphorus metabolism is regulated in photosynthetic eukaryotes is critical for optimizing crop productivity and managing aquatic ecosystems in which phosphorus can be a major source of pollution. Here we describe a gene encoding a regulator of phosphorus metabolism, designated Psr1 (phosphorus starvation response), from a photosynthetic eukaryote. The Psr1 protein is critical for acclimation of the unicellular green alga Chlamydomonas reinhardtii to phosphorus starvation. The N-terminal half of Psr1 contains a region similar to myb DNA-binding domains and the C-terminal half possesses glutamine-rich sequences characteristic of transcriptional activators. The level of Psr1 increases at least 10-fold upon phosphate starvation, and immunocytochemical studies demonstrate that this protein is nuclear-localized under both nutrient-replete and phosphorus-starvation conditions. Finally, Psr1 and angiosperm proteins have domains that are similar, suggesting a possible role for Psr1 homologs in the control of phosphorus metabolism in vascular plants. With the identification of regulators such as Psr1 it may become possible to engineer photosynthetic organisms for more efficient utilization of phosphorus and to establish better practices for the management of agricultural lands and natural ecosystems.

Why Recover Phosphorus for Recycling, and How?

Abstract: The removal of phosphorus from municipal waste waters prior to their discharge into the receiving waters is practised widely and a number of effective and reliable techniques can be employed. In almost all of these, the phosphorus is removed from the aqueous phase into the sludge phase (either as a precipitated metal salt or as a component of the sludge biomass) along with other solids. There are, however, a growing number of techniques, some at a laboratory stage, a few in full scale operation where the phosphorus is removed in the form of a simple inorganic phosphate salt which may have utility for recycling in industrial applications. The potential for recovery and recycling of phosphorus is reviewed from both a historical and a modern industrial perspective.

Review of Phosphorus Control Measures in the United States and Their Effects on Water Quality

Abstract: Historical information on phosphorus load­ ings to the environment and the effect on water quality are summarized in this report, which was produced as part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program. Phosphorus is a water-quality constit­ uent of concern because it is often the limiting nutrient responsible for accelerated eutrophica­ tion in water bodies. Phosphorus inputs to the environment have increased since 1950 as the use of phosphate fertilizer, manure, and phosphate laundry deter­ gent increased; however, the manufacture of phos­ phate detergent for household laundry was ended voluntarily by the industry in about 1994 after many States had established phosphate detergent bans. Total phosphorus concentrations in raw wastewater effluent contained about 3 milligrams per liter of total phosphorus during the 1940’s, increased to about 11 milligrams per liter at the height of phosphate detergent use (1970), and have currently declined to about 5 milligrams per liter. However, in some cases, tertiary wastewater treatment still is needed to effectively improve water quality of streams. Downward trends in phosphorus concentra­ tions since 1970 have been identified in many streams, but median total phosphorus concentra­ tions still exceed the recommended limit of 0.1 milligram per liter across much of the Nation. Data from the NAWQA Program are representa­ tive of a variety of phosphorus-control measures and, therefore, may be used to evaluate the effects of various control strategies. Current areas of concern include: evaluation of the effects of increased manure loadings of phosphorus on soil phosphorus and, subsequently, on ground water and subsurface runoff; determination of pointsource and nonpoint-source components of phos­ phorus loads by geographic modeling and hydro­ logic separation techniques; and development of methods or indices to evaluate nutrient impair­ ment in streams and rivers to serve as a basis for developing phosphorus criteria or standards.

Effectiveness and Longevity of Phosphorus Inactivation with Alum

Abstract: Effectiveness and longevity of alum treatments were evaluated in 21 lakes (or lake basins) across the United States; 9 were polymictic and 12 were dimictic. Effectiveness was judged from reductions in lake TP (total phosphorus) and internal loading rate, as well as chlorophyll a (chl a), both initially and over periods ranging from 4 to 20 years following treatment. Internal loading rate was reduced in six of nine polymictic lakes/basins by an average of two-thirds, and lake TP was reduced by about one-half, which persisted for 5–11 years. Internal loading rate in dimictic lakes (7 of 7 with adequate data) remained reduced by an average of 80% for 4 to 21 years (average 13 yrs). For the six polymictic lakes, in which treatment was effective, chl a decreased by an average of two-thirds initially, but was about 40% less than die pre-treatment level after 5 to 11 years. Chl a decreased in seven dimictic lakes by an average of 57% initially and 42% after 5–18 years. In some cases, response was indepe...

Osteoclastic responses to various calcium phosphates in cell cultures

Abstract: Disks made of hydroxyapatite, beta-tricalcium phosphate, carbonate apatite, tetracalcium phosphate, alpha-tricalcium phosphate, dicalcium phosphate dihydrate, and octacalcium phosphate were incubated in osteoclastic cell cultures for 2 days. The first five salts were sintered and the last two were compressed before incubation. Osteoclasts resorbed only the sintered carbonate apatite disks. However, osteoclasts were able to resorb octacalcium phosphate disks that were preincubated for 1 day in medium without cells, indicating that surface conditioning was important for osteoclastic resorption of this calcium phosphate. Although resorption did not occur, medium calcium and phosphorus changed to an appreciable extent after a 2-day incubation of beta-tricalcium phosphate, tetracalcium phosphate, alpha-tricalcium phosphate, and dicalcium phosphate dihydrate. These changes in the medium calcium and phosphate concentrations could explain why osteoclasts appeared to have lost their activity on these calcium phosphate disks and were not capable of resorbing them. With hydroxyapatite disks no changes were observed in the medium calcium and phosphorus before and after incubation. Moreover, the osteoclasts appeared to be essentially the same as with the sintered carbonate apatite disks and with bone slices used as a control. Nevertheless, no pits or lacunae were observed on the hydroxyapatite disks, indicating that sintered carbonate apatite should be superior to sintered hydroxyapatite as a bioresorbable bone substitute.

Long-term effects of forestry managements on water quality and loading in brooks

Abstract: The concentration of suspended solids in a meso-eutrophic basin remained unchanged in the years following clear felling (1983-85). The amount of suspended solids increased upon ditching, ploughing and mounting to a mean level of 81.8 mg l -1 in 1986-88, decreased to a level of 7.8 mg l -1 for 1989-91 and 4.5 mg l -1 for 1992-94. Total phosphorus concentration increased four-fold (142 μg l -1 ) in 1983-85 following clear felling, and remained at three times the reference concentration (95.4 μg l -1 ) in 1986-88 following ditching and site preparation. Phosphate phosphorus concentration increased more than five-fold after clear felling (97.8 μg l -1 ) in 1983-85 and was more than two-fold the reference (39.5 μg l -1 ) following ditching and site preparation in 1986-88. The concentrations of total phosphorus and phosphate phosphorus returned to the level recorded in a natural slate by 1989 onwards. Total nitrogen concentrations doubled (965 μg l -1 ) upon clear felling, and remained the same after ditching and site preparation in 1986-88. Where a protective strip of forest was left between the felling site and the brook, suspended solid did no increase in response to either clear felling or ploughing of the felled area, nor were any changes observed in total phosphorus, phosphate phosphorus or nitrogen componds. The ditching of peatland down to the level of the mineral soil led to an increase in the suspended solid concentration (17.8 mg l -1 ) by a factor of more than 12 in the first three years (1983-85) and by a factor of more than four (5.7 mg l -1 ) in the following three years (1986-88) compared with the reference period. Total nitrogen, nitrate and nitrite concentrations of ground water were markedly low all the time the forests remained in their natural state, but began to rise gradually following clear felling. A high increase was recorded as a consequence of ditching and site preparation.

Variability of inorganic and organic phosphorus turnover rates in the coastal ocean

Abstract: Phosphorus is an essential nutrient in pelagic marine ecosystems. Phosphorus cycling in the upper ocean is, however, poorly understood, and few studies have directly investigated the biological utilization of this essential element1,2,3,4. Here, we have determined in situ phosphorus-turnover rates in a coastal marine environment by measuring the activities of two cosmogenic radionuclides (32P and 33P, with half lives of 14.3 and 25.3 days, respectively) in dissolved inorganic, dissolved organic and total particulate phosphorus pools over a seasonal cycle. Phosphorus turnover rates within dissolved and particulate pools are rapid and vary over seasonal timescales, suggesting that low phosphorus concentrations can support relatively high primary production. Furthermore, picoplankton, such as bacteria, appear preferentially to utilize certain dissolved organic phosphorus compounds to obtain other associated nutrients, such as carbon and nitrogen. It seems that the significance of the roles of both dissolved inorganic and organic phosphorus in supporting primary production—and, hence, CO2 uptake and particulate organic carbon export—has been hitherto underestimated.

XAFS study of adsorbed and mineral forms of phosphate

Abstract: The solubility of inorganic phosphorus in soils is regulated by surface-adsorbed phosphate or phosphate minerals. The objective of this study was to determine whether different phosphate species of relevance to soils showed distinguishing XAFS spectral features. Phosphorus KXANES spectra for Fe-phosphates were characterized by a unique pre-edge feature near -3 eV (relative energy) that increased in intensity with increasing mineral crystallinity and was very weak for phosphate adsorbed on goethite. Spectra of Ca-phosphates and a soil sample exhibited a distinct shoulder on the high-energy side of the absorption edge. Spectra of Al-phosphates were characterized by a weak pre-edge feature at -1 eV.

Phosphorus biogeochemistry in subtropical ecosystems

Abstract: Global Issues: Sources and Sinks Global Issues of Phosphorus in Terrestrial Ecosystems - A.N. Sharpley The Role of Wetlands in Storage, Release, and Cycling of Phosphorus on the Landscape: A 25 Year Retrospective - C.J. Richardson Global Issues of Phosphorus in Aquatic Systems - P. Brezonik Phosphorus in Florida's Environment: Analysis of Current Issues - K.R. Reddy, E. Lowe, and T. Fontaine Introduction to Soils of Subtropical Florida - W.G. Harris and W. Hurt Biogeochemical Transformations Inorganic Forms of Phosphorus in Water, Soil and Sediments - D.A. Graetz Phosphorus Sorption/Desorption Reactions on Soils and Sediments - D. Rhue and W.G. Harris Forms of Organic Phosphorus in Water, soils, and Sediments - S. Newman and J.S. Robinson Organic Phosphorus Mineralization in Soils and Sediments - R.G. Wetzal Biogeochemical Responses Influence of Phosphorus Loading on Microbial Processes in Soil and Water Column of Wetlands - K.R. Reddy and J.R. White Effects of Phosphorus Enrichment on Structure and Function of Plant Communities in Florida Wetlands - S. Miao and W.F. DeBusk Influence of Phosphorus Loading on Wetland Periphyton Assemblages: A Case Study from the Everglades - P. McCormick and L. Scinto Assessing Nutrient Limitation and Trophic State in Florida Lakes - C.L. Schelske, F.J. Aldridge and W.F. Kenney Transport Processes Hydrologic Processes Influencing Phosphorus Transformations and Transport - K.L. Campbell and J.C. Capece Transport of Phosphorus in Wetlands - R.H. Kadlec Effects of Hydrodynamics on Phosphorus Dynamics - P. Sheng Phosphorus Management Phosphorus Management in Flatwood (Spodosols) Soils - A.B. Bottcher, T.K. Tremwel and K.L Campbell Phosphorus Management in Organic (Histosols) Soils - F. Izuno and P.J. Whalen Long-Term Water Quality Trends in the Everglades - W.W. Walker Techniques for Optimizing Phosphorus Removal in Treatment Wetlands - T.A. DeBusk and F.E. Dierberg Phosphorus Retention by the Everglades Nutrient Removal Project: An Everglades Stormwater Treatment Area - M.Z. Moustafa, S, Newman, T.D. Fontaine, M.J. Chimney, and T.C. Kosier The Role of Phosphorus Reduction and Export in the Restoration of Lake Apopka - L.E. Battoe, M.F. Coveney, E.F. Lowe, and D.F. Stites Phosphorus on Lake Okeechobee: Sources, sinks, and Strategies - A.D. Steinman, K.E. Havens, N.G. Aumen, R.T. James, K.R. Jin, J. Zhang, and B.H. Rosen Phosphorus Loading and Surface Water Quality in Florida Bay - J.N. Boyer and R.D. Jones Synthesis Management Models to Evaluate Phosphorus Transport Form Watersheds - K.L. Campbell Ecosystem Analysis of Phosphorus Impacts in South Florida - F. Sklar Management Models to Evaluate Phosphorus Impacts on Wetlands - R.H. Kadlec and W.W. Walker Management Models to Evaluate Phosphorus Loads In Lakes - S. Chapra and M. Auer Policy Implications to Phosphorus Management in Florida Ecosystems - J. Brooks

Phosphorus sorption characteristics of intertidal marsh sediments along an estuarine salinity gradient

Abstract: The phosphate sorption capacity of intertidal vegetated marsh sediments was measured along a salinity gradient in the Cooper River estuary, South Carolina. The phosphate sorption capacity of the surface sediments (0‐10 cm) of a freshwater marsh was higher than the sorption capacity of sediments from brackish and saline marshes, and surface sediments had greater sorption capacity than subsurface (10‐20 cm) sediments. These trends were opposite that of available phosphorus, which increased downstream and with depth. Freshwater marsh sediments trap phosphorus in a less-bioavailable form as evidenced by the low zero equilibrium phosphorus concentration (ZEPC) of the ambient sediment and low exchangeable phosphorus found there. Soil ZEPC values were similar to the in situ mean pore-water phosphate concentrations, which shows that sorption has a major effect on the spatial distribution of pore-water phosphorus along the estuarine salinity gradient. The magnitude of phosphorus sorption by the freshwater marsh sediments greatly reduced the pore-water phosphate concentration, while the phosphorus sorption properties of brackish and salt marsh sediments maintained in situ equilibrium pore-water phosphorus concentrations at surplus levels (with respect to its availability to plants). These differences in P sorption properties of the sediments can be explained on the basis of their physical and chemical characteristics. For instance, approaching the sea, the surface area of sediments declined, with freshwater marsh sediments (0‐10 cm) supporting 8.5 3 higher surface area than the salt marsh sediments. However, the sorption capacity of freshwater sediments was 333 greater than that of salt marsh sediments, which indicates that other properties such as sediment mineral composition are important. The concentrations of important elements such as Al and Fe in sediments also declined downstream. The results suggest that the differences in phosphorus exchange properties among these marshes are a function of sediment type and sedimentary concentrations of Fe and Al. These in turn are related to the changes in ionic strength and associated parameters (e.g., pH) and physical sorting mechanisms.

Water quality of a lowland stream in a New Zealand dairy farming catchment

Abstract: A small stream in a predominantly dairying catchment in the Waikato region of New Zealand was monitored for 2 years at three sites. Total nitrogen (TN) concentrations were up to 7.09 g m‐3 in winter, with the bulk comprising nitrate nitrogen (NO‐ 3‐N). During summer NO‐ 3‐N was near zero and TN mostly comprised organic nitrogen. Maximum concentrations of total phosphorus (TP) and dissolved reactive phosphorus (DRP) were 1.64 and 0.555 g m‐3, respectively, and peaks coincided with spring and autumn applications of phosphorus fertiliser. Ammoniacal nitrogen concentrations exceeded 1 g m‐3 on several occasions and mean concentrations at the three sites were 0.165–0.272 g m‐3. Faecal coliform and enterococci bacteria concentrations were 64–26000 and 7–23000 cfu per 100 ml, respectively. Specific yields of TN and NO‐ 3‐N (35.3 and 30.7 kg ha yr‐1, respectively) were much greater than any previously reported for New Zealand pasture catchments, whereas TP and DRP yields (1.16 and 0.54 kg ha yr‐1, respec...

Phosphate solubilization by organic anion excretion from rice growing in aerobic soil: rates of excretion and decomposition, effects on rhizosphere pH and effects on phosphate solubility and uptake

Abstract: Rice (Oryza sativa) plants were grown with their roots sandwiched between thin layers of phosphorus-deficient soil from which they were separated by fine mesh, and root-induced changes in the soil affecting phosphate solubility were measured. The concentrations of low molecular weight organic anions in the thin layers, particularly citrate, increased in the presence of the plants. Apparent rates of citrate excretion from the roots, calculated from the quantities in the soil and rates of decomposition calculated with a first order rate constant measured independently, varied from 337–155 nmol g−1 root f. wt h−1 over the course of plant growth, equivalent to 2–3% of plant d. wt. Rates of excretion were similar for NH4+ and NO3−-fed plants. The soil pH decreased from its initial value by up to 0.6 units for the NH4+-fed plants and increased by up to 0.4 units for the NO3−-fed ones. The contribution of organic anion excretion to the pH changes was small compared with that of the inorganic cation-anion balance in the plants. The extent to which the observed excretion of citrate and root-induced pH changes could account for the observed phosphate solubilization and uptake was assessed using a mathematical model. Previous work had shown that phosphate solubilization by rice in this soil could not be explained by enhanced phosphatase activity in the rhizosphere, and the roots were not infected with mycorrhizas. The model allows for the diffusion of the solubilizing agent (citrate or H+) away from the roots, its decomposition by soil microbes (citrate only); its reaction with the soil in solubilizing phosphate and diffusion of the solubilized phosphate to the roots. The model contains no arbitrary assumptions and uses only independently measured parameter values. The agreement between the measured time course of phosphorus uptake and that predicted for solubilization by citrate was good. Root-induced acidification by NH4+-fed plants resulted in additional solubilization, the acidification enhancing the solubilizing effect of citrate. However, the final phosphorus uptake by NH4+-fed plants was no greater than that of NO3−-fed plants, presumably because the acidification inhibited plant growth. The mechanism of solubilization by citrate involved formation of soluble metal-citrate chelates rather than displacement of phosphate from adsorption sites.

Simulation of the effect of citrate exudation from roots on the plant availability of phosphate adsorbed on goethite

Abstract: Rhizosphere processes strongly influence the availability of phosphorus (P) to plants. Organic ligands that are exuded from the root surface mobilize phosphorus by dissolution of P minerals or by desorption of adsorbed phosphate. We developed a mechanistic model to study the mobilization of phosphate sorbed on goethite by the exudation of citrate and consequent uptake of phosphate by the root. The use of a model allows the effects of the organic anion and pH on P desorption to be separated. The model is also used to predict concentration profiles developing around the root for phosphate, citrate (with or without accounting for degradation) and pH, providing insight into the processes that occur in the rhizosphere. Results of model calculations show that with larger rates of citrate exudation, greater P availability is predicted. Exudation at a rate of 0.5 mol citrate m1 root day1, which is in the range found for P-deficient plants, increased P availability almost 2-fold at fairly large phosphate loading of goethite (1.9 mol m2) and almost 30-fold at small phosphate loading (1.3 mol m2). Competitive adsorption causes a much greater relative increase in the phosphate concentration in solution at small than at large phosphate loading, which explains this result. Simultaneous acidification of the rhizosphere results in a smaller P mobilization than at a fixed pH of 5, as a result of the pH dependence of phosphate adsorption in the presence of citrate. Sorption of citrate increases its persistence against microbial decay, and hence has a positive effect on the mobilization of adsorbed phosphate.

Nitrogen and phosphorus limitation in a coastal barrier salt marsh : The implications for vegetation succession

Abstract: 1 A factorial fertilizer experiment was conducted in a 15-year-old coastal barrier salt marsh with a low soil nitrogen content, and in an older 100-year-old marsh with a higher nitrogen content. Plots were fertilized at high and low marsh elevations in both marshes. Nitrogen and phosphorus were applied at low and high concentrations both separately and in combination in each of 3 successive years. 2 Nitrogen limited above-ground plant growth in both young and old salt marshes in all years. Phosphorus limitation of plant growth was apparent in the first year in the young marsh and in the last year in both marshes. In young marshes with low soil organic matter, phosphorus limitation may occur. In addition, phosphorus limitation occurs at both successional stages when a marsh is saturated with nitrogen. 3 Plant species that are typical of nitrogen-rich habitats and late successional stages significantly increased in biomass after fertilization. Limonium vulgare, a low stature species of early and intermediate successional stages, decreased in biomass, whereas the taller Elymus pycnanthus and Artemisia maritima increased. After 3 years of fertilization, plant species composition in a young marsh was similar to the species composition in an unfertilized older marsh. Fertilization of a 100-year-old marsh, however, still resulted in a change in plant species composition, suggesting that succession was still occurring and that, overall, plants in marshes of different age are similar in their response to fertilization.