Abstract: Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO3−, OH−, H+), gaseous molecules (CO2, H2), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N2-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N2 is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N2 reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH− and HCO3− to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.

Abstract: ▪ Abstract Animals are important in nutrient cycling in freshwater ecosystems. Via excretory processes, animals can supply nutrients (nitrogen and phosphorus) at rates comparable to major nutrient sources, and nutrient cycling by animals can support a substantial proportion of the nutrient demands of primary producers. In addition, animals may exert strong impacts on the species composition of primary producers via effects on nutrient supply rates and ratios. Animals can either recycle nutrients within a habitat, or translocate nutrients across habitats or ecosystems. Nutrient translocation by relatively large animals may be particularly important for stimulating new primary production and for increasing nutrient standing stocks in recipient habitats. Animals also have numerous indirect effects on nutrient fluxes via effects on their prey or by modification of the physical environment. Future studies must quantify how the importance of animal-mediated nutrient cycling varies among taxa and along environme...

Abstract: Aquatic ecosystems respond variably to nutrient enrichment and altered nutrient ratios, along a continuum from fresh water through estuarine, coastal, and marine systems. Although phosphorus is considered the limiting nutrient for phytoplankton production in freshwater systems, the effects of atmospheric nitrogen and its contribution to acidification of fresh waters can be detrimental. Within the estuarine to coastal continuum, multiple nutrient limitations occur among nitrogen, phosphorus, and silicon along the salinity gradient and by season, but nitrogen is generally considered the primary limiting nutrient for phytoplankton biomass accumulation. There are well-established, but nonlinear, positive relationships among nitrogen and phosphorus flux, phytoplankton primary production, and fisheries yield. There are thresholds, however, where the load of nutrients to estuarine, coastal and marine systems exceeds the capacity for assimilation of nutrient-enhanced production, and water-quality degradation occurs. Impacts can include noxious and toxic algal blooms, increased turbidity with a subsequent loss of submerged aquatic vegetation, oxygen deficiency, disruption of ecosystem functioning, loss of habitat, loss of biodiversity, shifts in food webs, and loss of harvestable fisheries.

Abstract: The essential micronutrients for field crops are B, Cu, Fe, Mn, Mo, and Zn. Other mineral nutrients at low concentrations considered essential to growth of some plants are Ni and Co. The incidence of micronutrient deficiencies in crops has increased markedly in recent years due to intensive cropping, loss of top soil by erosion, losses of micronutrients through leaching, liming of acid soils, decreased proportions of farmyard manure compared to chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production. Micronutrient deficiency problems are also aggravated by the high demand of modern crop cultivars. Increases in crop yields from application of micronutrients have been reported in many parts of the world. Factors such as pH, redox potential, biological activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root hair morphology (length, density, surface area), root-induced changes (secretion of H + , OH − , HCO 3 − ), root exudation of organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial associations (enhanced CO 2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant ability to absorb and utilize micronutrients from soil. The objectives of this article are to report advances in research on the micronutrient availability and requirements for crops, in correcting deficiencies and toxicities in soils and plants, and in increasing the ability of plants to acquire needed amounts of micronutrient elements.

Abstract: The utilisation of dietary carbohydrates and their effects on fish metabolism are reviewed. Details on how dietary carbohydrates affect growth, feed utilisation and deposition of nutrients are discussed. Variations in plasma glucose concentrations emphasizing results from glucose tolerance tests, and the impact of adaptation diets are interpreted in the context of secondary carbohydrate metabolism. Our focus then shifts to selected aspects of hormonal regulation of carbohydrate metabolism and dietary carbohydrates and their variable effects on glycogen and glucose turnover. We analyse the interaction of carbohydrates with other nutrients, especially protein and protein sparing, and de novo synthesis of lipids, and finish by discussing the correlation of dietary carbohydrates with fish health.

Abstract: Improved understanding of crop production systems in relation to N-supply has come from a knowledge of basic plant biochemistry and physiology. Gene expression leads to protein synthesis and the formation of metabolic systems; the ensuing metabolism determines the capacity for growth, development and yield production. This constitutes the genetic potential. These processes set the requirements for the supply of resources. The interactions between carbon dioxide (CO(2)) and nitrate () assimilation and their dynamics are of key importance for crop production. In particular, an adequate supply of, its assimilation to amino acids (for which photosynthesized carbon compounds are required) and their availability for protein synthesis, are essential for metabolism. An adequate supply of stimulates leaf growth and photosynthesis, the former via cell growth and division, the latter by larger contents of components of the light reactions, and those of CO(2) assimilation and related processes. If the supply of resources exceeds the demand set by the genetic potential then production is maximal, but if it is less then potential is not reached; matching resources to potential is the aim of agriculture. However, the connection between metabolism and yield is poorly quantified. Biochemical characteristics and simulation models must be better used and combined to improve fertilizer-N application, efficiency of N-use, and yields. Increasing N-uptake at inadequate N-supply by increasing rooting volume and density is feasible, increasing affinity is less so. It would increase biomass and N/C ratio. With adequate N, at full genetic potential, more C-assimilation per unit N would increase biomass, but energy would be limiting at full canopy. Increasing C-assimilation per unit N would increase biomass but decrease N/C at both large and small N-supply. Increasing production of all biochemical components would increase biomass and demand for N, and maintain N/C ratio. Changing C- or N-assimilation requires modifications to many processes to effect improvements in the whole system; genetic engineering/molecular biological alterations to single steps in the central metabolism are unlikely to achieve this, because targets are unclear, and also because of the complex interactions between processes and environment. Achievement of the long-term objectives of improving crop N-use and yield with fewer inputs and less pollution, by agronomy, breeding or genetic engineering, requires a better understanding of the whole system, from genes via metabolism to yield.

Abstract: Degradation of oil on beaches is, in general, limited by the supply of inorganic nutrients. In order to obtain a more systematic understanding of the effects of nutrient addition on oil spill bioremediation, beach sediment microcosms contaminated with oil were treated with different levels of inorganic nutrients. Oil biodegradation was assessed respirometrically and on the basis of changes in oil composition. Bacterial communities were compared by numerical analysis of denaturing gradient gel electrophoresis (DGGE) profiles of PCR-amplified 16S rRNA genes and cloning and sequencing of PCR-amplified 16S rRNA genes. Nutrient amendment over a wide range of concentrations significantly improved oil degradation, confirming that N and P limited degradation over the concentration range tested. However, the extent and rate of oil degradation were similar for all microcosms, indicating that, in this experiment, it was the addition of inorganic nutrients rather than the precise amount that was most important operationally. Very different microbial communities were selected in all of the microcosms. Similarities between DGGE profiles of replicate samples from a single microcosm were high (95% ± 5%), but similarities between DGGE profiles from replicate microcosms receiving the same level of inorganic nutrients (68% ± 5%) were not significantly higher than those between microcosms subjected to different nutrient amendments (63% ± 7%). Therefore, it is apparent that the different communities selected cannot be attributed to the level of inorganic nutrients present in different microcosms. Bioremediation treatments dramatically reduced the diversity of the bacterial community. The decrease in diversity could be accounted for by a strong selection for bacteria belonging to the alkane-degrading Alcanivorax/Fundibacter group. On the basis of Shannon-Weaver indices, rapid recovery of the bacterial community diversity to preoiling levels of diversity occurred. However, although the overall diversity was similar, there were considerable qualitative differences in the community structure before and after the bioremediation treatments.

Abstract: Author(s): Treseder, KK; Allen, MF | Abstract: Since mycorrhizal fungi constitute an important component of the soil–plant interface, their responses to changes in nutrient availability may mediate shifts in ecosystem function. We tested the hypothesis that initial soil nutrient availability may determine effects of nitrogen (N) and phosphorus (P) additions on the growth and community of arbuscular mycorrhizal (AM) fungi. Extraradical hyphal lengths and degree of root colonization of AM fungi were measured in control and fertilized plots along a soil fertility gradient in Hawaii. Responses of individual AM genera were assessed through immunofluorescent labeling. The AM biomass was increased by N and P additions in the N- and P-limited sites, respectively, and reduced by P fertilization in the fertile site only. The abundance of Scutellospora was lower under N than under P fertilization, whereas the incidence of Glomus was higher in the fertile site than the N-limited site. Gigaspora and Acaulospora did not vary among sites or treatments. Our results indicate that a decrease in AM abundance following nutrient additions cannot be assumed to occur and the effects may differ among AM genera and ecosystems with varying soil nutrients. Limitation of N and P may be one possible explanation.

Abstract: The use of nutrient concentrations in plant biomass as easily measured indicators of nutrient availability and limitation has been the subject of a controversial debate. In particular, it has been questioned whether nutrient concentrations are mainly species' traits or mainly determined by nutrient availability, and whether plant species have similar or different relative nutrient requirements. This review examines how nitrogen and phosphorus concentration and the N:P ratio in wetland plants vary among species and sites, and how they are related to nutrient availability and limitation. We analyse data from field studies in European non-forested wetlands, from fertilisation experiments in these communities and from growth experiments with wetland plants. Overall, the P concentration was more variable than the N concentration, while variation in N:P ratios was intermediate. Field data showed that the N concentration varies more among species than among sites, whereas the N:P ratio varies more among sites than among species, and the P concentration varies similarly among both. Similar patterns of variation were found in fertilisation experiments and in growth experiments under controlled nutrient supply. Nutrient concentrations and N:P ratios in the vegetation were poorly correlated with various measures of nutrient availability in soil, but they clearly responded to fertilisation in the field and to nutrient supply in growth experiments. In these experiments, biomass N:P ratios ranged from 3 to 40 and primarily reflected the relative availabilities of N and P, although N:P ratios of plants grown at the same nutrient supply could vary three-fold among species. The effects of fertilisation with N or P on the biomass production of wetland vegetation were well related to the N:P ratios of the vegetation in unfertilised plots, but not to N or P concentrations, which supports the idea that N:P ratios, rather than N or P concentrations, indicate the type of nutrient limitation. However, other limiting or stressing factors may influence N:P ratios, and the responses of individual plant species to fertilisation cannot be predicted from their N:P ratios. Therefore, N:P ratios should only be used to assess which nutrient limits the biomass production at the vegetation level and only when factors other than N or P are unlikely to be limiting.

Abstract: Rates of key soil processes involved in recycling of nutrients in forests are governed by temperature and moisture conditions and by the chemical and physical nature of the litter. The forest canopy influences all of these factors and thus has a large influence on nutrient cycling. The increased availability of nutrients in soil in clearcuts illustrates how the canopy retains nutrients (especially N) on site, both by storing nutrients in foliage and through the steady input of available C in litter. The idea that faster decomposition is responsible for the flush of nitrate in clearcuts has not been supported by experimental evidence. Soil N availability increases in canopy gaps as small as 0.1 ha, so natural disturbances or partial harvesting practices that increase the complexity of the canopy by creating gaps will similarly increase the spatial variability in soil N cycling and availability within the forest. Canopy characteristics affect the amount and composition of leaf litter produced, which largely determines the amount of nutrients to be recycled and the resulting nutrient availability. Although effects of tree species on soil nutrient availability were thought to be brought about largely through differences in the decomposition rate of their foliar litter, recent studies indicate that the effect of tree species can be better predicted from the mass and nutrient content of litter produced, hence total nutrient return, than from litter decay rate. The greater canopy complexity in mixed species forests creates similar heterogeneity in nutritional characteristics of the forest floor. Site differences in slope position, parent material and soil texture lead to variation in species composition and productivity of forests, and thus in the nature and amount of litter produced. Through this positive feedback, the canopy accentuates inherent differences in site fertility.

Abstract: In order to apply manure or compost to fulfill the nutrient requirements of a crop, knowledge of the amount of nutrients mineralized following application is needed. Nutrient mineralization from applied manure depends on temperature, soil moisture, soil properties, manure characteristics, and microbial activity. Since these factors cannot be accurately predicted, nutrient mineralization from applied manure can only be approximated. Nitrogen (N) availability from applied manure includes the inorganic N (NO3-N and NH4-N) in manure plus the amount of organic N mineralized following application. Nitrogen mineralization differs for different manure types since the inorganic/organic fraction and quality of organic N varies among manure types. Mineralization of organic N is expected to be low for composted manure (∼ 18%) and high for swine or poultry (hens) manure (∼ 55%). Phosphorus (P) availability from all animal production sources of manure is high (> 70%), as most of the manure P is inorganic and becomes plant-available after application. Potassium (K) availability from manure is nearly 100%; therefore, manure can be used similar to K fertilizer. When manure was analyzed for plant-available nutrients, greater than 55% of calcium (Ca) and magnesium (Mg) and less than 40% of zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), sulfur (S), and boron (B) were plant-available. To effectively utilize the nutrients in manure, their mineralization potential should be considered when determining application rates.

Abstract: This book contains articles on the nutrient requirements and feeding of finfish for aquaculture. Topics include the following: introduction to fish nutrition; marine fishes (European sea bass, Asian sea bass, red sea bream, gilt-head sea bream, Atlantic salmon, Atlantic halibut, Japanese flounder, North American flounder, yellowtail, red drum, southern bluefin tuna, and milkfish); and freshwater fishes (rainbow trout, Arctic char, percids, coregonids, common carp, Indian major carps, tilapia, channel catfish, eel, hybrid striped bass, sturgeon, silver perch, largemouth bass, hybrid bluegill, Brazilian species, snakehead and Pangasius catfish, and baitfish).

Abstract: Phosphorus (P) is an essential macronutrient for all living organisms. It serves various basic biological functions as a structural element in nucleic acids and phospholipids, in energy metabolism, in the activation of metabolic intermediates, as a component in signal transduction cascades, and the regulation of enzymes. Of the major nutrients, P is the most dilute and the least mobile in soil. High sorbing capacity for P in the soil (e.g. sorbtion to metal oxides), P mineralization (e.g. calcium phosphates such as apatite), and/or fixation of P in organic soil matter (by converting soluble P into organic molecules) result in low availability of this macronutrient for uptake into plants (Marschner, 1995). P is absorbed by plants as orthophosphate (Pi, inorganic phosphate). Pi concentration in the soil solution hardly reaches 10 µM and may even drop to submicromolar levels at the root/soil interface, where Pi uptake by plants and root surface-colonizing microorganisms leads to the generation of a zone of Pi depletion around the root cylinder that is maintained due to slow diffusion of Pi from regions distant to the root surface (Figure 1). Fig. 1. A transverse section through the tip of a primary root. The dotted line indicates the outer border of the P depletion zone. The arrow indicates the direction of growth. In industrialized countries, low P availability in agricultural soils is compensated by a high input of P fertilizer to guarantee high crop productivity and yield. Water run-off, soil erosion and leakage in highly fertilized agricultural soils may cause environmental problems such as eutrophication of lakes and rivers. As forecasted by Tilman et al. (2001), during the next 50 years, which is likely to be the final period of rapid agricultural expansion, demand for food by global population will be a major driver of global environmental change. Conversion of natural ecosystems to agriculture by 2050 will be accompanied by an approximate 2.5-fold increase in nitrogen- and P- driven eutrophication of terrestrial, freshwater, and near-shore marine ecosystems. Modern agricultural soils are almost universally maintained at high fertilization. Selection of new cultivars is usually made under such conditions and will not normally distinguish between plants varying in nutrient efficiency (Stevens and Rick, 1986). To alleviate the forecasted adverse negative effects of agricultural expansion, scientists have started to use classical breeding strategies and biotechnology to improve crop plants, based on the current knowledge and aiming at an improved crop yield with a lower input of fertilizer, thus protecting the environment. In contrast, in many developing tropical countries, subsistence farmers can not buy enough fertilizer due to limited financial capacities or poor infrastructure (Sanchez et al., 1997). As a consequence, P deprivation dramatically limits crop yield and is one of the reasons for poverty and malnutrition. In the future, agriculture from both developed as well as developing countries could thus benefit from modern crop varieties with enhanced P efficiency, thus leading to improved fertilizer management and increased crop yield on low-P soils. Thorough knowledge of the plant's response to P deprivation stress will contribute to the rational and targeted breeding of P efficient crop plants. Therefore, the authors of this chapter focus on summarizing the current state of research covering physiology, biochemistry, and molecular genetics of P acquisition and allocation, and P homeostasis within the plant. Although this review will mainly focus on knowledge acquired on Arabidopsis thaliana, some specific results obtained with other plant species will also be included in this work. For example, formation of mycorrhizae, which is observed in most vascular plants and strongly contributes to plant P nutrition, does not occur in Brassicaceae and therefore Arabidopsis is not a suitable model for mycorrhizae studies.

Abstract: Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg 1−1 02) in 1985–1992 averaged 8000 to 9000 km2 but increased to up to 16000 to 20 700 km2 in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions.

Abstract: The influence of marine plants representing different stages of eutrophication on carbon decomposition and production, benthic nutrient fluxes and denitrification was examined in 4 shallow warm-temperate Australian lagoons. Differences in carbon production and decomposition across the lagoons were the main regulators of the quantity and quality of benthic nutrient fluxes and the rela- tive proportion of nitrogen lost through denitrification. For example, the efficiency with which the lagoon sediments recycled nitrogen as N2, (i.e. denitrification efficiency: N2-N/(N2-N + DIN), decreased as carbon decomposition rates increased. C:N ratios of the remineralised organic matter in some of the plant-sediment systems were much higher than expected from the stoichiometry of the dominant carbon supply. Dark DON fluxes were also very high in all the plant-sediment systems (30 to 80% of the total nitrogen flux). We offer 2 alternative explanations for the observed sediment and benthic flux characteristics: (1) The low dark C:N ratios of the remineralised organic matter may have been due to dark uptake by benthic microalgae and possibly other plants. The large DON effluxes were either the hydrolysis product of freshly produced in situ organic material or/and associated with the grazing of benthic microalgae. This explanation has important implications regarding the impor- tance of benthic microalage as a sink for nitrogen. (2) Alternatively, the high C:N ratios of the rem- ineralised organic matter may have been directly related to the large dissolved organic nitrogen (DON) effluxes; large DON effluxes with a low C:N ratio increase the C:N ratio of organic matter in the surface sediments, which in turn causes an uptake and accumulation of nitrogen by bacteria due to N-limitation of the microbial decomposition. Production by all the plant groups had a significant influence on benthic nutrient fluxes, with a typical pattern of an efflux during the dark cycle and an uptake during the light cycle. As such, the sediment productivity/respiration (p/r) ratio was one of the major controls on (best indicators of) net benthic inorganic and organic nutrient fluxes and appears to be one of the key changes which occur in shallow coastal lagoons as these become eutrophic. This has important management implications, demonstrating the need to maintain the balance of benthic autotrophy and heterotrophy. The robustness of the denitrification efficiency and sediment p/r rela- tionships across such a diverse range of plant-sediment systems that represent the different stages of eutrophication suggests that these may be useful in synthesising denitrification and benthic flux data across shallow coastal systems and in defining suitable carbon loading rates.

Abstract: 1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams. 2. Four submerged macrophyte species were tested, Elodea canadensis, Callitriche cophocarpa, Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants. 3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa, indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment. 4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.

Abstract: All higher plants require water to be freely available for their establishment and survival. On the other hand, water can be a very effective barrier to gas exchange. In conjunction with oxygen-consuming processes it can, to varying degrees, help create oxygen deficiency in soils and roots and be detrimental to many plant processes, disturbing growth, nutrient and water uptake, and hormonal balances. Further to this, the total disappearance of oxygen from soils is often the prelude to microbially mediated anaerobic transformations of mineral and organic compounds with the creation of pools of often highly phytotoxic materials such as sulfides and the lower monocarboxylic acids. Excess water in the soil environment can thus prove harmful or even lethal for land plants, and in many parts of the world agricultural production is adversely affected by heavy seasonal rainfall and poorly draining soils. In Western Australia alone it has been estimated that winter waterlogging costs farmers tens of millions of dollars each year in reduced cereal yields and pasture production (Ayling, 1990). Indeed, much practical agriculture is associated, directly or indirectly, with ensuring adequate drainage and optimizing soil pore size distribution for plant roots (Russell, 1977).

Abstract: Growing interest in possible global climate change has underlined the need for better information concerning the way in which carbon partitioning between ecosystem components is influenced by constraints on nutrient availability. Micro-organisms play a fundamental role in the cycling of carbon and nutrients in all ecosystems but the role of fungi in particular is pivotal in boreal forest ecosystems. Traditional models of nutrient cycling are based on methods and concepts developed in agricultural systems where microorganisms are considered primarily as nutrient processors providing plants with inorganic nutrients. The filamentous nature of fungi, their ability to translocate carbon and nutrients between different substrates and the capacity of ectomycorrhizal fungi to utilise organic nutrients have all been largely ignored. In this article, a new model is suggested which emphasises competition for organic nutrients between decomposer organisms and plants, with the plants depending on their associated mycorrhizal fungi for nutrient acquisition. Antagonistic interactions involving nutrient transfer between decomposer and mycorrhizal fungi are proposed as important pathways in nutrient cycling. Due to the nutrient conservative features of decomposer fungi, inorganic nutrients are considered less important for plant nutrition. The implications of the new nutrient cycling model on the carbon balance of boreal forests are discussed.

Abstract: The exudation of organic acids into the rhizosphere by plant roots has been hypothesized to be one potential mechanism by which plants can enhance the mobilization of poorly soluble nutrients in the soil. The experiments undertaken in this study were aimed at determining whether the organic acids, citrate and oxalate, could enhance the uptake of P-33 from a calcareous soil with a high P fixation capacity (Typic rendoll). Soil-filled rhizosphere microcosms were constructed which allowed the growth of a single maize root axis through a (KH2PO4-)-P-33 labelled patch of soil. After passage of the root through the P-33-labelled soil, organic acids or distilled water (control) were added to the patch at concentrations of 1 and 10 mM over a subsequent 4-day period. While oxalate resulted in an approximately two-fold enhancement in shoot P-33 accumulation, citrate did not result in a significant enhancement of P-33 uptake above controls to which only distilled water were added. No synergistic effect on shoot P-33 accumulation was observed when both oxalate and citrate were added to the soil simultaneously. We hypothesize that the observed differences in shoot P-33 accumulation by the two organic acids were due primarily to the differences in their biodegradation rate and P mobilization reactions. This study demonstrates that in vivo, organic acids can cause a significant enhancement of plant P uptake, however, the magnitude of the P mobilization response is likely to be highly context dependent. (C) 2002 Published by Elsevier Science Ltd. (Less)

Abstract: A manipulative mesocosm experiment in Danish coastal waters tested the effect on plankton biodiversity and function of adding nitrate, phosphate and glucose. A comprehensive set of measurements was made over a 6 d period; these included phytoplankton biomass and production in 3 size fractions (>10, 10-2 and <2 µm), bacterial biomass and production, nitrate and ammonium uptake, and pigment taxonomy. Addition of nitrate and phosphate resulted in increases of biomass and production of all size fractions of phytoplankton. Inorganic nutrients alone had only a minor effect on bacterial abundance and production, with slight increases relative to the control. The largest changes occurred in mesocosms to which glucose was added in excess with nitrate and phosphate. Pigment composition indicated little change in phytoplankton assemblage composition in any treatment. A large increase in bacterial activity in the presence of added glucose had a negative effect on the phytoplankton assemblage and resulted in a decline in phytoplankton biomass. Data on nutrient uptake and size-fractionated carbon fixation suggest that the mechanism of this phyto- plankton suppression was the ability of heterotrophic bacteria to out-compete for available inorganic nutrients, resulting in nutrient limitation of the phytoplankton assemblage.

Abstract: Supplying plants with inorganic nutrients is one of the major functions of roots. The ability of the roots to fulfil this function depends on two complex phenomena: nutrient availability in soil and nutrient acquisition by plants. The term nutrient availability summarizes the soil properties affecting nutrient supply to the plants, and it comprises two aspects: a chemical one and a positional one. The chemical aspect depends on the chemical bonds between the element and other ions or the soil matrix, and the concentration of the element in the soil. The positional aspect is related to both the distribution of the element in the rooting volume and its mobility in the soil. Mobility determines the rate of ion transport, and thus the amount of and the distance from which the ion can move through the soil toward the surface of a root. The term nutrient acquisition encompasses the plant properties that take part in nutrient supply to the plants. This phenomenon includes the physiological processes that are responsible for nutrient entry into the plant (see also Chapters 34 by Glass and 37 by Silberbush in this volume). Nutrient acquisition from a given soil may vary according to genus, species, or even variety. In many soils nutrient availability is inadequate for crop growth unless fertilized. Because of their importance for plant production, both phenomena have been studied intensively (for overview see Barber, 1995; Marschner, 1995; Tinker and Nye, 2000).

Abstract: Summary In tropical montane forests nutrients released from the organic layers of the soil can supply a large part of the vegetation's requirements. We have examined concentrations, storage, and turnover times of nutrients in the organic layer and the fluxes of nutrients by the fall of small litter (leaves, seeds, flowers, small twigs, and plant debris that passed an opening of 0.3 m × 0.3 m) in such a forest in Ecuador. The times taken for litter to turn over were estimated by relating nutrient storage in the organic layer to rate of litterfall and by incubating samples in the laboratory. The organic layer had a thickness of 2–43 cm, a mass of 30–713 t ha−1, and a nutrient storage of 0.87–21 t N, 0.03–0.70 t P, 0.12–2.5 t K, 0.09–3.2 t Ca, and 0.07–1.0 t Mg ha−1. The pH (in H2O) ranged between 3.1 and 7.4 and was correlated with the concentrations of Ca and Mg (r= 0.83 and 0.84, respectively). The quantity of small litter (8.5–9.7 t year−1) and mean concentrations of nutrients in litter (19–22 g N, 0.9–1.6 g P, 6.1–9.1 g K, 12–18 g Ca, and 3.5–5.8 g Mg kg−1) were larger than in many other tropical montane forests. The mean turnover times of elements in the organic layer increased in the order, Mg (7.0 years) < Ca (7.9) < K (8.5) < P (11) < N (14) < S (15) when calculated as the quotient of storage in the organic layer to flux by litterfall; they were < 12 years for N, P, and S in the incubation experiment. Under optimum conditions in the laboratory, the mineralization of S was just as large as the S deposition by litterfall. In weakly acid soils Mn and Zn and in strongly acid soils Ca added in a nutrient solution were immobilized during incubation. Thus, lack of S, Mn, Zn, and Ca might limit plant growth on some soils.

Abstract: 1. We studied the growth of a submerged aquatic plant in relation to periphytic and planktonic algae over a range of nutrient and dissolved inorganic carbon (DIC) availabilities. 2. In consecutive years two factorial experiments were conducted in 48 artificial ponds (each 3.14 m3), comprising four concentrations of DIC (1.5, 2.5, 3.5 or 4.5 mM) each crossed with three separate nutrient loadings (10 microgram L(-1) P and 0.2 mg L N, 50 microgram L(-1) P and 1 mg L N, or 200 microgram L(-1) P and 4 mg L N). The second experiment differed by the inclusion of fish in the ponds. 3. In the first year DIC had no effect on plant growth, but nutrient loading did. Plants failed to grow in treatments where phytoplankton density was high (> 100 microgram L(-1)). Where phytoplankton was low, high numbers of invertebrates colonized the ponds, and periphyton abundance on the plants was low. In the second year, where phytoplankton never achieved the densities of the previous year, there was a significant effect of DIC concentration on plant growth but not of nutrients. Invertebrate abundance was lower and periphyton on the plants correspondingly higher. 4. In both years increased nutrient loading had no effect on the abundance of periphyton growing on the surface of the plants. Periphyton abundance was determined by the density of grazing invertebrates in the ponds. 5. There was a negative relationship between periphyton density and final plant density, which became significantly less steep with increasing DIC, indicating that periphyton and plants were competing for carbon. 6. DIC concentration has the potential to influence community structure in shallow lakes, altering competitive interactions between periphyton and plants and rendering low DIC lakes more prone to loss of plants when nutrient loading increases. However, the expression of this competition between periphyton and plants will depend on the density of grazing invertebrates present, which is itself influenced by the intensity of fish predation on those invertebrates.