Abstract: Mineral nutrients taken up by the roots are, as a rule, transported in the xylem to the shoot, and photoassimilates transported in the phloem to the roots. According to the Thornley model of photosynthate partitioning, nutrient deficiencies should favour photosynthate partitioning to the roots. Examples are cited to show that this preferential partitioning is dependent on phloem mobility and hence on nutrient cycling from shoot to roots. Thus, root growth is enhanced under nitrogen and phosphorus deficiencies, but not under deficiencies of nutrients of low mobility in the phloem, such as calcium and boron. Enhanced root growth under nutrient deficiency relies on the import of both photosynthates and mineral nutrients. Cycling of mineral nutrients serves a number of other functions. These include the root supply of nutrients assimilated in the shoot (nitrate and sulphate reduction), maintenance of cation-anion balance in the shoot, providing an additional driving force for solute volume flow in the phloem and xylem, and acting as a shoot signal to convey nutrient demand to the root. Cycling of certain mineral nutrients through source leaves has a considerable impact on photosynthate export as demonstrated in impaired export under magnesium, potassium, or zinc deficiencies. Mineral nutrient deficiency can, therefore, affect photosynthate partitioning either directly via phloem loading and transport or indirectly by depressing sink demand.

Abstract: Nutrient enrichment of shallow coastal waters changes the composition of plant communities so that slow-growing, benthic macrophytes are replaced by fast-growing algae such as phytoplankton and ephemeral macroalgae. This scenario suggests that fast-growing algae suffer more from nutrient limitation than slow-growing algae at low nutrient availability. We tested this hypothesis by comparing the effect of in situ nutrient enrichment on the phytoplankton community, 4 ephemeral macroalgae (Ulva lactuca, Cladophora serica, Chaetomorpha linum and Ceramiurn rubrum) and 1 perennial macroalga (Fucus vesiculosus). Nitrogen was the main limiting nutrient to algal growth and fast-growing algae were N limited for a longer period during summer than slower-growing species. Differences in the temporal extent of N limitation were related to species-specific variations in N requirements for growth and in N storage pools. The N requirements per unit biomass and time were up to 30-fold higher for fast-growing algae compared to slow-growing species due to 10-fold faster growth and 3-fold higher demands for the internal N concentration needed to sustain maximum growth (i.e. critical concentrations). The pools of N reserves only varied 2-fold among algal species and could support maximum growth for 0.5 d in the phytoplankton community and for 12 d in E vesiculosus. Growth of phytoplankton and E vesiculosus could proceed at reduced rates for another 2.6 and 34 d. respectively, based on other internal pools of N. The results suggest that the species-specific differences in growth rate and critical N concentrations account for a substantial part of the variation in the duration of nutrient limitation among different algal types and, therefore, provide further clarification of the reasons why fast-growing algae are stimulated by increased nutrient availability while slowgrowing algae remain unaffected or are hampered due to shading.

Abstract: Under nutrient-poor conditions initially fast-growing species will in the long term be competitively inferior to slow-growing species. Here, we ask whether this phenomenon can be explained by constraints caused by tissue density. The hypothesis is that low tissue density is necessary for fast growth but has as a consequence short organ life span. This leads to a rapid loss of nutrients that cannot be sustained under nutrient-poor conditions. Biomass accumulation, turnover rate of leaves and roots, and tissue density were studied for five ecologically contrasting grass species. Plants were grown in a garden experiment over two growing seasons on sand with a low nutrient supply level. Species that were characteristic of nutrient-rich sites had a low leaf and root tissue density and were larger after one growing season than species of nutrient-poor sites. However, after two growing seasons the species of nutrient-poor sites were larger. These species had a high tissue density. Life span of both leaves and roots was also correlated with tissue density. Species with low tissue density had a faster turnover of leaves and roots. It is concluded that tissue structure is an inherent constraint that prevents simultaneous maximization of both nutrient acquisition and nutrient conservation. The short life span of fast-growing organs explains the long-term disadvantage of a high growth rate for plants in low nutrient conditions.

Abstract: The soil microbial carbon (C), nitrogen (N) and phosphorus (P) pools were quantified in the organic horizon of soils from an arctic/alpine low-altitude heath and a high-altitude fellfield by the fumigation-extraction method before and after factorial addition of sugar, NPK fertilizer and benomyl, a fungicide. In unamended soil, microbial C, N and P made up 3.3-3.6%, 6.1-7.3% and 34.7% of the total soil C, N and P content, respectively. The inorganic extractable N pool was below 0.1% and the inorganic extractable P content slightly less than 1% of the total soil pool sizes. Benomyl addition in spring and summer did not affect microbial C or nutrient content analysed in the autumn. Sugar amendments increased microbial C by 15 and 37% in the two soils, respectively, but did not affect the microbial nutrient content, whereas inorganic N and P either declined significantly or tended to decline. The increased microbial C indicates that the microbial biomass also increased but without a proportional enhancement of N and P uptake. NPK addition did not affect the amount of microbial C but almost doubled the microbial N pool and more than doubled the P pool. A separate study has shown that CO2 evolution increased by more than 50% after sugar amendment and by about 30% after NPK and NK additions to one of the soils. Hence, the microbial biomass did not increase in response to NPK addition, but the microbes immobilized large amounts of the added nutrients and, judging by the increased CO2 evolution, their activity increased. We conclude: (1) that microbial biomass production in these soils is stimulated by labile carbon and that the microbial activity is stimulated by both labile C and by nutrients (N); (2) that the microbial biomass is a strong sink for nutrients and that the microbial community probably can withdraw substantial amounts of nutrients from the inorganic, plant-available pool, at least periodically; (3) that temporary declines in microbial populations are likely to release a flush of inorganic nutrients to the soil, particularly P of which the microbial biomass contained more than one third of the total soil pool; and (4) that the mobilization-immobilization cycles of nutrients coupled to the population dynamics of soil organisms can be a significant regulating factor for the nutrient supply to the primary producers, which are usually strongly nutrient-limited in arctic ecosystems.

Abstract: The nutrition of animals involves procurement of food, digestion and absorption of component nutrients, metabolism and (or) assimilation of absorbed nutrients, and elimination of resulting waste products. Adequate nutrition is essential to an organism’s survival and reproductive success (i.e., fitness). Determining what constitutes adequate nutrition for free-living animals poses a challenge. An animal’s nutritional status depends on (1) its nutrient needs, (2) nutrient accessibility, and (3) the metabolic, physiological, morphological, and behavioral plasticity that the animal can invoke either to avert or to minimize discrepancies between demand and accessibility, or to resolve conflicting demands through the course of its annual or life cycle (e.g., adaptive anorexias; Mrosovsky and Sherry 1980). This chapter focuses on these three elements of the nutritional budgets of birds. An exhaustive review of the literature germane to this topic is neither practical nor possible. This chapter is intended only to provide an overview and some direction for future research. For the sake of brevity, reviews and recent reports have been preferentially cited here. Two sources of notable value, because of their scope and attention to avian nutrition, are Scott et al. (1982) and Robbins (1993).

Abstract: Based on comprehensive observations since 1983, this study summarizes major features of nutrient elements (nitrogen, phosphorus and silicon) in large Chinese river/estuary systems. Elevated nutrient element levels were observed in Chinese rivers, when compared to large and less disturbed aquatic systems (e.g. the Amazon, Zaire and Orinoco). Data from this study are similar to those obtained from the polluted and/or eutrophic rivers in Europe and North America (e.g. the Rho´ne and Loire). Nutrient elements may have either conservative or active distributions, or both, in the mixing zone, depending on the element and the estuary. For example, non-conservative behaviors were observed in the upper estuary, where nutrient elements may be remobilized due to the strong desorption and variations of the fresh water end-member, but conservative distributions were found afterwards in the lower estuary. Outside the riverine effluent plumes, nutrient elements may be depleted in surface waters relative to elevated bioproduction, whereas the regeneration with respect to decomposition of organic material and/or nitrification/denitrification offshore, may sustain high levels of nutrient elements in near-bottom waters. Laboratory experiment data generally compares well with field observations. The high fluxes and area] yields of nutrient elements from large Chinese rivers, indicate the extensive use of chemical fertilizers and domestic waste drainage over watersheds in China.

Abstract: Improved salt tolerance of mycorrhizal plants is commonly attributed to their better mineral nutrition, particularly phosphorus. However, the effect of arbuscular-mycorrhizal (AM) fungi on salt tolerance may not be limited to this mechanism. We investigated the possibility that non-nutritional effects of AM fungi, based on proline accumulation or increased photosynthesis and related parameters, can influence the tolerance of lettuce (Lactuca sativa L.) to salinity. Three levels of salt (3, 4 and 5 g NaCl kg -1 dry soil) were applied and plants were maintained under these conditions for 7 weeks. The salt-treated AM plants produced greater root and shoot dry weights than unfertilized or P-fertilized non-AM controls. With increasing salinity, both shoot and root dry weights were reduced, but this decrease was greater in uninoculated plants. In particular, shoot dry weight was not reduced in G. fasciculatum-colonized plants as a consequence of salt, whereas in uninoculated plants it was reduced by about 35% at the highest salt level. Proline accumulation was considerably lower for P-amended non-AM and for AM plants except for G. mosseae-colonized plants than was the case for unamended plants. Transpiration, carbon dioxide exchange rate (CER), stomatal conductance and water use efficiency (WUE) were higher in mycorrhizal plants. At 5 g NaCl kg -1 , both photosynthesis and WUE increased by more than 100% in mycorrhizal treatment relative to uninoculated plants. The contents of phosphorus of P-fertilized non-AM plants was similar to or higher than those of G. mosseae- and G. fasciculatum-colonized plants. Plants colonized by G. deserticola had the highest P-content regardless of salt level. Hence, the effect of G. mosseae and G. fasciculatum on salt tolerance in this experiment could not be attributed to a difference in the P content. The mechanisms by which these two fungi alleviated salt stress appeared to be based on physiological processes (increased CER, transpiration, stomatal conductance and WUE) rather than on nutrient uptake (N or P).

Abstract: Bottom sediments in shallow lakes can play a major role in releasing nutrients to the overlying water column during wind induced sediment resuspension or by constant flux due to diffusion. Internal nutrient loads due to these processes may be equal to or higher than external loads. Laboratory and field experiments were conducted on Lake Apopka, a shallow, hypereutrophic subtropical lake located in central Florida. Ammonium (NH⁺₄) and soluble reactive P (SRP) flux during sediment resuspension were measured under laboratory conditions using intact sediment cores. Ammonium N and SRP flux due soley to diffusion were assessed using in situ porewater concentrations. Average diffusive flux from sediment to the overlying water was estimated to be 25 mg NH₄-N m⁻² d⁻¹ and 1 mg P m⁻¹ d⁻². Resuspension fluxes of NH⁺₄ and SRP were higher than diffusive flux. Soluble reactive P profiles of porewater showed distinct profile differentiation, with the surface 0 to 8 cm sediment depth acting as a P-depletion zone, and the underlying sediment displaying steep gradients in porewater SRP. These results suggest that dissolved NH⁺₄ and SRP transport from the surface 8 cm of sediment was due to sediment resuspension, while below this depth, upward mobility of NH⁺₄ and SRP was regulated by diffusion. Although dissolved N and P flux is upwards (from sediment to water column), during extended periods (annual cycle) the lake is functioning as a net sink for N and P by transforming inorganic pools of nutrients into organic forms and depositing them on the sediment surface. Florida Agric. Exp. Stn. J. Ser. R-04707.

Abstract: Phytoplankton biomass and primary productivity were assessed on the continental shelf in the plume of the Amazon River during a series of cruises conducted within periods of minimum, maximum, rising and falling river discharge. Chlorophyll concentrations were greatest (up to 25.5 μg l −1 ) in a zone located outside the turbid, high nutrient, low salinity riverine waters but shoreward of the clear, high salinity, low nutrient waters. Vertical distributions of chlorophyll further delineated the influences of these environmental regimes, with maximum chlorophyll concentrations occurring in the upper 5 m of water columns characterized by reduced salinities and elevated nutrients at the surface and low nutrient, high salinity water below. Fluorescence was elevated in the transition zone as a result of the phytoplankton standing stocks, and was also elevated in low-salinity waters influenced by the Amazon outflow. The residual fluorescence was coupled to salinity but not to chlorophyll, which suggested that it was related to dissolved organic matter which originated in the Amazon. Primary productivity on the continental shelf was greatest in the transition zone and occasionally exceeded 8 g C m −2 d −1 . Productivity in the turbid, nutrient-rich waters and the clear, offshore regions averaged 2.18 and 0.81 g C m −1 d −1 , respectively. Phytoplankton photosynthesis in waters influenced by the Amazon River appeared to be limited by low levels of available irradiance inshore, whereas offshore it was nutrient-limited. The narrow zone of high production was supported by the riverine input of nutrients and the dynamics of sediment flocculation. Removal of the inorganic sediment load was necessary to allow for adequate irradiance penetration to support photosynthesis.

Abstract: We used the terrestrial ecosystem model “Century” to evaluate the relative roles of water and nitrogen limitation of net primary productivity, spatially and in response to climate variability. Within ecology, there has been considerable confusion and controversy over the large-scale significance of limitation of net primary production (NPP) by nutrients versus biophysical quantities (e.g., heat, water, and sunlight) with considerable evidence supporting both views. The Century model, run to a quasi-steady state condition, predicts “equilibration” of water with nutrient limitation, because carbon fixation and nitrogen fluxes (inputs and losses) are controlled by water fluxes, and the capture of nitrogen into organic matter is governed by carbon fixation. Patterns in the coupled water, nitrogen, and carbon cycles are modified substantially by ecosystem type or species-specific controls over resource use efficiency (water and nitrogen used per unit NPP), detrital chemistry, and soil water holding capacity. We also examined the coupling between water and nutrients during several temperature perturbation experiments. Model experiments forced by satellite-observed temperatures suggest that climate anomalies can result in significant changes to terrestrial carbon dynamics. The cooling associated with the Mount Pinatubo eruption aerosol injection may have transiently increased terrestrial carbon storage. However, because processes in the water, carbon, and nitrogen cycles have different response times, model behavior during the return to steady state following perturbation was complex and extended for decades after 1- to 5-year perturbations. Thus consequences of climate anomalies are influenced by the climatic conditions of the preceding years, and climate-carbon correlations may not be simple to interpret.

Abstract: The kinetics of ammonium and phosphate uptake by leaves and roots of the tropical seagrass Thalassia hemprichii were investigated in laboratory experiments. Uptake in leaves of plants from 3 different locations, covering the range from coastal to oceanic conditions in the region of investigation (Spermonde Archipelago, South Sulawesi, Indonesia), was compared. The leaves from all plant samples showed a clear capacity for both ammonium and phosphate uptake. This uptake could be described by Michaelis-Menten kinetics. v(max) ranged between 32 and 37 mu mol g(-1) leaf dry weight h(-1) for ammonium and between 2.2 and 3.2 mu mol g(-1) leaf dry weight h(-1) for phosphate. K-m ranged between 21 and 60 mu M for ammonium and between 7.7 and 15 mu M for phosphate. There was no significant site difference in uptake characteristics (v(max) and K-m) of ammonium and phosphate. Uptake of ammonium and phosphate by roots was investigated with plants from the intermediate location, Barang Lompo, using an approach which allowed only calculation of uptake rates at natural pore water concentrations. Uptake rates were 22 and 1.0 mu mol g(-1) root dry weight h(-1) for ammonium and phosphate, respectively Calculations suggest that at all 3 locations uptake of ammonium and phosphate by roots was probably limited by the diffusion of nutrients in the sediment rather than by their uptake capacity. Evidence was found that the availability of nutrients in the root zone relative to the leaf zone affects the uptake affinity of the leaves. The role of roots versus leaves in supplying plant nutrients is discussed. We concluded that even in the tropics, where water column nutrient concentrations are often very low, leaves clearly have a significant ability for ammonium or phosphate uptake and that in some situations nutrient uptake by the leaves may even be essential in meeting plant nutrient demands. [KEYWORDS: ammonium; phosphate; nutrient uptake kinetics; leaves; roots; seagrass; Thalassia hemprichii; Indonesia Zostera-marina-l; south sulawesi indonesia; phosphorus; epiphytes; release; phosphate; kinetics; biomass; nitrate; ammonia]

Abstract: The influence of mineral nutrients, light, carbon dioxide, ozone, ammonia, water, temperature, soil texture and soil acidity on carbon allocation in trees is reviewed. The growth rhythms of the different plant parts on a seasonal basis are examined as well as the change in source-sink balance caused by plant age and genetic constitution. The exact outcome of all these factors on plant growth and carbon allocation is difficult to predict. However, one distinct pattern with regard to carbon allocation, and important for plant survival, becomes evident from this evaluation. Root growth is always decreased when plants become carbon limited, independently of whether this situation is caused by reduced photosynthesis (O3, low light, or shortage of K, Mg or Mn) or competition between root growth and NH4+ as sinks for carbon skeletons (atmospheric NH3 and root uptake of NH4+). Low soil temperatures as well as competition from intensive shoot growth affect root development in a similar way. Inhibition of mycorrhizal development after exposure to O3, NH3, and low availability of Mg can also be explained by the same mechanisms.

Abstract: 1. Soil microbes are fed primarily by root-derived substrates, fulfil functions such as mineralization, immobilization, decomposition, pathogeneity and improvement of plant nutrition, and form the basis of the below-ground food web. Hitherto, below-ground processes have generally been monitored using a 'black-box' approach, thereby ignoring effects of global change at a finer level of resolution. We describe shifts in the activity between microbial functional groups associated with roots of Artemisia tridentata, and the influence of this change on higher trophic levels. 2. We tested the hypothesis that elevated atmospheric CO 2 causes the soil community to change qualitatively. We measured the responses of several soil microbe and soil microfaunal parameters to a double-ambient CO 2 concentration and nutrient additions. The soil community, as measured by those parameters, showed great changes in response to the treatments. There was a very strong interaction between elevated CO 2 and the nutrient addition. 3. Under low nutrient conditions, total microbial biomass did not change under elevated atmospheric CO 2 , but doubled under conditions of elevated CO 2 and added nutrients. As we increased the resolution of our analysis, however, results shifted. Under low nutrient conditions, mycorrhizal fungi responded positively to elevated CO 2 , whereas with added soil nutrients they responded negatively to the same elevated CO 2 concentration. Bacteria and non-mycorrhizal fungi did not respond under the former conditions but more than doubled in biomass under conditions of elevated CO 2 and added nutrients. Soil fauna was also affected by the treatments. Overall, elevated CO 2 shifted carbon flow in the plant-soil system to a more mutualistic-closed, mycorrhizal-dominated system, whereas the combination of elevated CO 2 and nutrient addition shifted carbon flow to a more opportunistic-open, saprobe/pathogen-dominated one. 4. This indicates that elevated atmospheric CO 2 may lead to far less predictable feedback patterns than previously thought and that qualitative shifts in the soil community may be far more important than mere changes in total C sink strength.

Abstract: Nutrient budgeting strategies focus primarily on recycling manure to land as fertilizer for crop production. Critical elements for determining environmental balance and accountability require knowledge of nutrients excreted, potential nutrient removal by plants, acceptable losses of nutrients within the manure management and crop production systems, and alternatives that permit export of nutrients off-farm, if necessary. Nutrient excretions are closely related to nutrient intake and can be predicted by subtracting predicted nutrients in food animal products exported from the farm from total nutrients consumed. Intensifying crop production with double- or triple-cropping often is necessary for high-density food animal production units to use manure without being forced to export manure or fertilizer coproducts to other farms. Most manures are P-rich relative to N largely because of 1) relatively large losses of volatilized NH3, most of it converted from urea in urine, 2) denitrification losses in soil under wet, anaerobic conditions, and 3) ability of many crops to luxury-consume much more N than P. Most soils bind P effectively and P usually is permitted to accumulate, allowing for budgets to be based on N. However, P budgeting may be required in regions where surface runoff of P contributes to algae growth and eutrophication of surface waters or where soil P increases to levels of concern. Research is needed to determine whether dietary P allowances can be lowered without detriment to animal production or health in order to lower P intake and improve N:P ratios in manure relative to fertilization needs.

Abstract: The chemical composition of common marine macroalgae from Hong Kong was determined both in the winter (cool and dry) and the summer (hot and wet) of 1994. During the winter, macroalgal diversity was high and variation in chemical composition between the species was great. In contrast, during the summer, algal diversity was reduced and the shores were dominated by only a small number of comparatively nutrient rich encrusting algae. Multivariate analysis indicated that the nutritional value of algal species (in terms of ash, lipid and soluble carbohydrate content) was related to their systematic position and that, overall, the Phaeophyta had the highest levels of nutrients, while the Chlorophyta and the Corallinaceae were of comparatively low nutritional value. The chemical composition of the species varied temporally as did the abundance and diversity of algae on the shore. Overall, however, seasonal differences in algal availability appeared to be more important in determining total nutrient availability than temporal changes in the nutritional value of the species. Local macroalgae are the main food source for a large number of marine herbivores and variation in algal availability and chemical composition are likely to effect the ecology of these grazers and subsequently community organization.

Abstract: Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO2 than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO2 enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 μl l-1) and elevated (680 μl l-1) CO2 concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha-1 year-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO2 enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19–27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO2 response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO2 enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO2. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO2 enrichment of the currently dominating species are unlikely.

Abstract: 1. The results of a restoration experiment carried out on a permanent grassland on peaty, heavy clay in the Netherlands are described. The experiment started in 1985, 7 years after fertilizer application had ceased, and was designed to provide insight into ecologically significant processes accompanying restoration. An analysis was made of the effect of management regime and of raising the water table on nutrient availability, dry matter production, tissue nutrient concentration, dynamics of species numbers and plant species replacement. Three management practices were compared: cutting and removal (RR), cutting and mulching (MM), sod removal in 1985, and thereafter cutting and removal of the hay (RS). Data are presented on changes during a 5-year period. 2. No trend was discernible in soil pH, total C, N and P in the RR treatment; extractable P and K decreased sharply in the field with the raised groundwater level. 3. Nine years after fertilizer application ceased, dry matter production had fallen from 10-11 to 6-7 t ha-1 year-1. In the subsequent 5 years of the experiment it declined to 5-6 t ha-1 year-1 when all cut biomass was removed, and to about 4 t ha-1 year-1 after sod removal. Mulching caused an increase to 11 t ha-1 year-1. No effect was seen of the raised water level. 4. The dry matter yield of the first June cut in the RR treatment decreased. The tissue K concentration also decreased, but no increase of the tissue P concentration was detected. It was concluded that the availability of K and to some extent of P was more important than N availability in explaining the decrease in dry matter production. The tissue nutrient concentrations were not influenced by the water table. 5. Sod removal to a depth of 5 cm resulted in the lowest productivity and the lowest tissue concentrations of P, while tissue concentrations of N and K were not affected. 6. Raising the water level resulted in a more rapid establishment of species indicative of wet conditions, some of which invaded from nearby ditches. 7. The trends of dominant species are described with a set of response models. The species were ranked from disappearing to colonizing species. The relationship between rank order of replacement and indicator values of species was investigated. Raising the water table resulted in species indicative of wet conditions becoming dominant, independently of vegetation management. The removal of nutrients resulted in the appearance of species with a lower maximal height, indicative of lower P and K availability.

Abstract: We tested the hypothesis that species characteristic of nutrient rich sites always grow faster than species from nutrient poor sites, but have a shorter organ life span. It has been suggested that this fast tissue turnover may limit their performance at nutrient poor sites. Three ecologically contrasting grasses were studied: Bromus erectus, characteristic of nutrient poor meadows, and Arrhenatherum elatius and Dactylis glomerata, characteristic of more nutrient rich grasslands. These species were grown at three meadows differing in nutrient availability and harvested 3, 7, 9.5 and 11 months after germination. D. glomerata produced at all sites the largest total biomass. B. erectus showed the lowest growth response to site conditions. D. glomerata and A. elatius had a faster leaf turnover than B. erectus. D. glomerata had a faster root turnover than B. erectus. Root turnover of A. elatius was probably also faster than that of B. erectus, but this was partially obscured by decomposition of dead roots. D. glomerata and A. elatius had a lower tissue density of leaves and roots than B. erectus. We conclude that there is a negative correlation between the plant's ability to increase its growth with increasing nutrient availability and the tissue life span. Nutrient losses due to fast tissue turnover may prevent the dominance of fast-growing species at nutrient poor sites, although these species are in general better at nutrient acquisition. The negative correlation between growth rate and tissue longevity is to a large extent a result of constraints caused by plant tissue structure.

Abstract: Various aspects of forage intake regulation are discussed with the objective of providing a basis on which assessments could be made of (i) the scope for forage intake manipulation, and (ii) priority areas for further research. A simple conceptual model of the regulation is presented which permits the linking of rumen function and energy metabolism. It takes cognizance of upper physiological limits for (i) energy disposal, (ii) the clearance of digesta organic matter from the rumen, and (iii) muscular fatigue, as well as a range of dietary and environmental constraints. The transmission to the brain of signals relating to amount of digesta in the rumen and the ruminant's energy deficit are considered to be important in the intake regulation. An alternative conceptual model which recognises the amount of energy in the circulating energy pool, rather than the energy deficit, as the origin of signals relating to energy metabolism, is also discussed. It is considered that over a range of forage qualities neither the rumen digesta load ceiling nor the capacity to use energy limit intake; in this range both the resistance of the forage organic matter to removal from the rumen and the net energy value of the forage act as constraints. A method to calculate forage intake constraint is presented, and theoretical relationships between rumen digesta load, net energy intake, energy deficit and forage intake constraint have been formulated to facilitate interpretation of data obtained in forage intake studies. Forage intake in the reproduction cycle is discussed in the context of an optimum nutritional strategy for ensuring species survival. It is considered that the intake changes at mating and immediately prior to parturition, together with the decrease in rate of nutrient storage in maternal reserve tissues in late pregnancy and the use of these tissue stores in early lactation, are consistent with such a strategy. In this context it is suggested that (i) the relevant reproduction hormones affect intake via modulation of the metabolism of the maternal tissue stores and (ii) this type of regulation and its accompanying production losses need not be necessary in those production systems permitting some control of nutrition.

Abstract: summary Effects of arbuscular mycorrhizas (AM) on plant exploitation of soil nutrient heterogeneity were studied with non-mycorrhizal and mycorrhizal Agropyron desertorum (Fisch. ex Link) Schult. in two-compartment containers. A central cylindrical plant compartment was separated from an outer hyphal compartment by two layers of stainless-steel screen with a 2 mm air gap between the screen layers. Patchy or uniform nitrate (NO2−) and phosphate (P) distribution patterns were created in the outer compartment. Only AM hyphae could cross the double-screen barrier to access those nutrients. Mycorrhizal plants acquired significantly more labelled P in both the patchy- and the uniform-nutrient treatments than did non-mycorrhizal plants. Mycelia in root-free soil delivered similar amounts of P from the more distant rich patches to mycorrhizal plants as from the uniform and more proximate labelling. The uptake of a more mobile and abundant element, nitrate, was not affected significantly by either mycorrhizal infection or by nutrient distribution patterns in the root-free soil. Despite a lower root:shoot mass ratio, mycorrhizal plants had significantly greater shoot phosphorus concentration than did nonmycorrhizal plants. There was no significant difference in P uptake or phosphorus concentration between the two nutrient distribution patterns for mycorrhizal plants, indicating that AM hyphae can explore the root-free soil for available P and transport it to host plants equally well when P was distributed in either patchy or uniform patterns in the root-free soil.

Abstract: Iron toxicity is a nutrient disorder associated with high concentrations of iron in soil solutions. Deficiencies of other nutrients, such as P, K, Ca, Mg and Zn, have been implicated in its occurrence in rice plants. Field experiments were carried out in 1992 and 1993 in Ivory Coast to evaluate the iron toxicity tolerance of promising rice cultivars available in West Africa, and to provide additional information for selecting breeding materials. Two sites, differing in their potential to cause iron toxicity, were used. Glasshouse and field studies were also conducted to test the role of other nutrients in the occurrence of iron toxicity. The results showed that genetic tolerance to iron toxicity can significantly improve rice production in iron-toxic soils, with some cultivars producing yields in excess of 5 t/ha. The application of N, P, K and Zn in the field decreased the uptake of iron in rice tops, and this can be a significant factor in the iron-toxicity tolerance of the cultivars.

Abstract: As water quality in the Chesapeake Bay has declined over recent decades, formely healthy submersed plant communities have disappeared from littoral areas of the mesohaline estuary. A dynamic simulation model of shallow regions of bay tributaries (<1 m) was developed to investigate growth responses of submersed vascular plants to eutrophication and habitat degradation. Our objectives were to elucidate mechanisms responsible for the decline and to evaluate conditions required for plant restoration and survival. State varibles in the model are plant leaves, roots, phytoplankton, epiphytes, and detrital material. The model calculates biomass pools and biogeochemical rate processes over annual cycles with a time step of 6 h. Simulations were performed to investigate the influence of phytoplankton and epiphytes on the underwater light environment, how the balance of limiting resources (light and nutrients) controls growth and productivity of submersed plants, and conditions necessary, for the restoration of submersed vegetation. Model output for submersed plants was calibrated to baseline data from the mid 1970s (r2=0.86); simulations reproduced declines in plant biomass with increasing nutrient enrichment. Model experiments showed, that by increasing nutrient inputs 40% above levels observed in the 1960s, submersed plants disappeared within 1–2 yr due to enhanced growth of phytoplankton and epiphytes, which reduced light below required levels. Epiphytes were more important than were phytoplankton in attenuating light. The relationship between nutrient enrichment and plant loss rate was complex, as epiphyte density on leaf surfaces was not linearly related to nutrient levels. Relatively small nutrient increases could have a large effect on submersed plants because epiphyte density on leaves increased exponentially as leaf surface area decreased. Exchanges of organic carbon and nutrients between leaf and root compartments were seasonally variable and were critical for survival of submersed plants. The amount of root-rhizome material available for regrowth could control the outcome of nutrient reduction strategies. Consequently, model predictions of plant restoration success were highly dependent on initial conditions. The model is being used successfully as a research tool to interpret ecological relationships in the ongoing re-evaluation of management alternatives for submersed plant restoration.

Abstract: 1. The breakdown of oak (Quercus robur L.), chestnut (Castanea sativa Miller) and eucalypt (Eucalyptus globulus Labill.) litter enclosed in 5-mm mesh bags was compared between first-order headwaters (two with native riparian forest and two with eucalypt plantations) and a third-order reach of Aguera stream. Weight loss and dynamics of phosphorus and nitrogen in litter were studied for a period of 155 days. 2. Among the different sites, processing rates ranged from 0.0045 to 0.0080 day–1 for chestnut leaf litter, from 0.0036 to 0.0051 day–1 for oak, and from 0.0027 to 0.0158 day–1 for eucalypt. 3. The availability of nutrients in water clearly influenced nitrogen and phosphorus dynamics in litter. In headwater reaches, net immobilization was not observed and losses of phosphorus and nitrogen followed mass loss. However, there was an enrichment of litter at the low reach, where influence of human settlements—located upstream—could lead to a greater availability of phosphorus in water. 4. The enhancement of litter decay by the exogenous nutrient supply depended on leaf quality, as only the processing rate of eucalypt increased at the nutrient-rich site. 5. The processing rates differed little among headwaters, suggesting that riparian forest type, i.e. deciduous forest v eucalypt plantations, did not affect litter decay in the stream.