Sediment-related growth limitation of Elodea nuttallii as indicated by a fertilization experiment

1. A fertilization experiment was performed to identify the limiting nutrient for the growth of submerged vegetation in ditches of a peat-grassland system in the Netherlands, in which restoration measures involved ceasing fertilization, exporting nutrients by removal of above-ground plant mass and large-scale introduction of calcium-rich, nutrient-poor artesian water.
2. Growth of Elodea was significantly enhanced by enrichment with nitrogen alone, and by fertilization with nitrogen in combination with phosphorus, and by nitrogen in combination with phosphorus and potassium.
3. Plant tissue nutrient concentrations increased significantly, for nitrogen by enrichment with nitrogen alone, and with nitrogen in combination with phosphorus and potassium; for phosphorus by enrichment with phosphorus alone and with phosphorus in combination with nitrogen and potassium; tissue concentrations of potassium were not enhanced by any treatment.
4. The elemental ratios of treated plants indicated that nitrogen, rather than phosphorus, was limiting in all treatments, except in those involving nitrogen and NK enrichment (when phosphorus was limiting).
5. The efficiency with which plants used nutrients declined with increased supply of nitrogen and phosphorus, but was unchanged when potassium was increased. Efficiencies were similar to those of other aquatic macrophytes.     

Tissue nutrient concentrations in freshwater aquatic macrophytes: high inter-taxon differences and low phenotypic response to nutrient supply

1. The elemental composition and stoichiometry of aquatic plants has often been suggested to reflect the nutrient enrichment of aquatic habitats. However, the relationship is often weak. Moreover, uncertainties remain in the relevance of laboratory derived critical plant tissue nutrient concentrations to maximum yield or growth rates in the field.
2. Aquatic vascular plants and bryophytes, overlying water and sediment samples were collected to test whether freshwater aquatic macrophytes: (i) show tissue nutrient deficiencies when growing in oligotrophic freshwater habitats, and (ii) have strict homeostatic stoichiometry.
3. Plant nutrient concentrations were significantly related to total inorganic nitrogen (or nitrate), total dissolved phosphorus and sediment total phosphorus. However, these relationships were weak. Virtually all the variance in plant tissue nutrient concentrations, however, could be explained by species (taxon) identity.
4. Critical tissue nutrient concentrations for 95% maximum yield or 95% maximum growth rate in aquatic angiosperms, determined from laboratory bioassays, suggested that nutrients should not limit yield in wild aquatic macrophytes. However, there were a substantial number of samples where potential growth rate limitation was possible, particularly due to phosphorus.
5. Strict C : N : P stoichiometric ratios were found for both vascular plants and bryophytes, suggesting little scope for plants as indicators of nutrient enrichment, but provide robust stoichiometric data for studies on ecosystem metabolism and nutrient cycling.     

Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream

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.     

Nutrient limitation of Myriophyllum spicatum growth in situ

SUMMARY. 1. The hypothesis that the submersed macrophyte biomass in natural weedbeds is nutrient limited was tested in situ by an enrichment experiment.
2. The response of Myriophyllum spicatum was significant and positive for N-enrichment, resulting in a 30–40% increase in biomass over controls. There was no response to phosphorus or to potassium enrichment.
3. Plant length and number of shoots per rephcate were also significantly increased by nitrogen additions but again showed no response to phosphorus and potassium.
4. Water depth differences were also found to affect the plant responses in some cases.
5. The macrophyte response to fertilization was similar to that recorded for emergent macrophytes and terrestrial crops but much smaller than for phytoplankton.     

Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull: the effect of fertilizer and defoliation on above- and below-ground performance

1 The role of nutrient supply and defoliation on the competitive interactions between pot-grown Calluna vulgaris and Nardus stricta plants was investigated.WP leading adjustment
2 Young plants were grown alone and together in pots under a combination of fertilizer and defoliation treatments. After 18 months, parameters reflecting both above- and below-ground performance were measured, namely: total above-ground biomass, shoot nitrogen and phosphorus content, root length and the extent of mycorrhizal infection of the roots.
3 In the pots that received fertilizer, the shoot nutrient content and above-ground biomass of Nardus plants increased to a greater extent than those of Calluna plants; this effect was more marked for Nardus plants growing with Calluna plants than for those growing with other Nardus plants. In contrast , Calluna plants growing in competition with Nardus failed to respond to the addition of nutrients. However, in unfertilized pots, Calluna gained more above-ground biomass during the experimental period than Nardus.
4 Calluna had greater root length than Nardus , but Nardus had a higher proportion of its root length infected by mycorrhizal fungi. In both plants, the addition of fertilizer reduced the mycorrhizal infection and increased the root length. Nardus root length was decreased when grown in competition with Calluna only in pots where no nutrients were added. Defoliation decreased the extent of mycorrhizal infection in Calluna roots but not in those of Nardus; defoliation decreased the shoot nutrient content in Calluna plants, but not in Nardus plants.
5 These results suggest that the competitive balance between Nardus and Calluna may be altered by the addition of nutrients, and by defoliation, which may have serious implications for the future dominance of Calluna in heathland ecosystems, particularly those where nutrient inputs are increasing significantly or where grazing pressures are high.     

Signatures of nutrient limitation and co‐limitation: responses of autotroph internal nutrient concentrations to nitrogen and phosphorus additions

Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and it is therefore important to understand how these nutrients, independently or in combination, influence the growth and nutrient content of primary producers. Using meta‐analysis of 118 field and laboratory experiments in freshwater, marine and terrestrial ecosystems, we tested hypotheses about co‐limitation of N and P by comparing the effects of adding N alone, P alone, and both N and P together on internal N (e.g. %N, C:N) and P (e.g. %P, C:P) concentrations in autotroph communities. In particular, we tested the following predictions. First, if only one nutrient was limiting, addition of that nutrient should decrease the concentration of the other nutrient, but addition of the non‐limiting nutrient would have no effect on the internal concentration of the limiting nutrient. If community co‐limitation was occurring then addition of either nutrient should result in a decrease in the internal concentration of the other nutrient. Community co‐limitation could also result in no change – or even an increase – in N concentrations in response to P addition if P stimulated growth of N fixers. Finally, if biochemically dependent co‐limitation was occurring, addition of a limiting nutrient would not decrease, and could even increase, the concentration of the other, co‐limited nutrient. We found no general evidence for the decrease in the internal concentration of one nutrient due to addition of another nutrient. The one exception to this overall pattern was marine systems, where N addition decreased internal P concentrations. In contrast, P addition increased internal N concentrations across all experiments, consistent with co‐limitation. These results have important implications for understanding the roles that N and P play in controlling producer growth and internal nutrient accumulation as well as for managing the effects of nutrient enrichment in ecosystems. Synthesis On a global scale, humans have doubled nitrogen (N) inputs and quadrupled phosphorus (P) inputs relative to pre‐industrial levels. N and P fertilization influences autotroph internal nutrient concentrations and ratios and thereby affects a variety of community and ecosystem processes, including decomposition and consumer population dynamics. It is therefore critical to understand the effects of nutrient additions on the growth and nutrient concentrations of primary producers. We used meta‐analysis to evaluate the responses of autotroph internal N and P concentrations to additions of N, P, and N+P and make inferences about limitation and co‐limitation of N and P across marine, terrestrial, and freshwater ecosystems. We found little evidence for single‐nutrient limitation, highlighting the fact that multiple nutrients generally limit primary production.     

Effects of delayed hay removal on the nutrient balance of roadside plant communities

1. Mass losses and nutrient losses from fresh roadside cuttings were studied in the field during a 6-week period. Large amounts (over 50%) appear to be lost from the cuttings. The losses were positively related to initial nutrient concentrations during this short-term study. Mass and nitrogen losses were best explained by the initial C:N ratio, phosphorus and potassium losses by the initial phosphorus concentration.
2. For potassium the losses were particularly large (up to 90%). For this element only, the observed relationship between loss rate and initial chemical composition could not be established significantly. It is concluded that potassium is mainly lost by leaching whereas the major nitrogen, phosphorus and mass losses are most probably caused by rapid microbial decomposition of readily soluble substances.
3. Using existing data on chemical composition of other roadside cuttings, nutrient losses after different hay removal delay times were modelled for different plant communities. All or most of the losses were assumed to return to the soil system.
4. When soil impoverishment is aimed for, cuttings should be removed within 1 or 2 weeks in most plant communities. If removal is delayed longer, the amounts of nutrients removed will often fall below the annual atmospheric input. In plant communities where annual above-ground production of nitrogen and phosphorus are lower than the annual atmospheric deposition already, rapid removal of the cuttings may be the only way to maintain at least potassium at a limiting level.
5. The main effect of hay-making on the soil nutrient status most likely consists of a reduction of the potassium availability, at least on sandy soils with a low cation exchange capacity and provided there is little delay in hay removal.     

Root allocation and multiple nutrient limitation in the New Jersey Pinelands

Abstract Current understanding of the effects of resource stress on plant communities emphasizes the adaptive integration of multiple limiting factors, but it has been difficult to directly demonstrate the fundamental assumption of the adaptive control of limitation. One model predicts a positive correlation between optimal allocation to the uptake of each resource and growth response to enrichment of that resource. Here we report a test of this prediction by a fertilization experiment in the New Jersey Pinelands Biosphere Reserve where limitation was measured by growth response to nutrient enrichments, and allocation was measured by root proliferation in microsites enriched by those same nutrients. Results suggest that a stand of regenerating Pinus echinata Mill. (shortleaf pine) was jointly limited by nitrogen and potassium, with a possible small effect of phosphorus. Root allocation was proportional to growth response, which supports the assumption of adaptive control of limitation.     

Seedling growth of four fire-following Mediterranean plant species deprived of single mineral nutrients

1. Anthyllis vulneraria, Cistus creticus, Hippocrepis unisiliquosa and Pinus brutia are frequent post-fire colonizers whose seedlings are subjected to the mineral conditions imposed by ash in the early stages of growth. We test the hypothesis that the effective internal supply of mineral elements in the seeds of these species may complement the external availability of specific nutrients.
2. Newly germinated seedlings were grown in nutrient solutions, each deficient in one of the following elements: nitrogen, phosphorus or potassium. Control treatments consisted of full nutrients and distilled water. Seedling growth was monitored over 12weeks. The final dry masses of the seedlings were taken as a measure of the availability of each element from the seeds' own stored reserves.
3. For the two legume species nitrogen was the most limiting element followed by phosphorus. Potassium deficiency had no effect on the final legume dry biomass. For Cistus, the order was nitrogen followed by potassium, while phosphorus deprivation had no effect on growth during the 12weeks of the experiment. Deprivation of single minerals had no significant effect on seedling growth of Pinus over the same period. The two legumes and Cistus when denied nitrogen had relatively greater relative root ratios than control plants.
4. The relative seedling requirements of each species for early external supplies of the various nutrients for establishment in the field are discussed with reference to the adaptation of the four species to post-fire regeneration.     

Nutrient Relations of Groundsel (Senecio vulgaris) Infected by Rust (Puccinia lagenophorae) at a Range of Nutrient Concentrations I. Concentrations, Contents and Distribution of N, P and K

Groundsel (Senecio vulgaris L.), healthy or infected with therust fungus Puccinia lagenophorae was grown in sand and fedwith a complete nutrient medium diluted to give a range of concentrations.Analysis of bulked, dried tissues of the plant showed that undernutrient-rich conditions rust infection resulted in increasedconcentrations of total (Kjeldahl) nitrogen and potassium buthad little effect on phosphorus concentration. Thus, despitereduced dry weight growth, total plant nitrogen contents wereno less in rusted than control plants. Although total contentsof phosphorus and potassium were reduced by rust, effects wereprobably related to loss of these nutrients in fungal spores. Interactions between rust infection and nutrient supply weresignificant but differed between nutrients: rust caused increasednitrogen concentrations only under nutrient-rich conditionsbut increased phosphorus concentrations only when nutrient supplywas limited. Increased concentrations were not confined to infectedtissues. Mechanisms underlying rust-nutrient interactions appearto be complex and to depend inter alia on the partitioning andrecycling of the particular nutrient within the plant. Rust-inducedincreases in potassium concentration occurred under both highand low nutrient conditions but were confined to infected tissues.Potassium accumulation in nutrient deficient conditions wasprobably due to increased transpirational flux into infectedtissues, but under nutrient-rich conditions reduced potassiumexport appeared to assume greater significance. The possible implications of the changed nutrient relationsfor the wider interactions of rust-infected plants in naturalvegetation are discussed. Senecio vulgaris, Puccinia lagenophorae, rust infection, nutrient deficiency, nutrient content, nutrient concentration, nutrient distribution     

Nutrient availability and the carnivorous habit in Utricularia vulgaris

1. Carnivory in plants is thought to enhance growth through an increased supply of nutrients, although there are considerable costs involved. It has been assumed that the relative investment of biomass in traps is inversely proportional to the availability of nutrients from non-carnivorous sources. Our aim was to test the effect of increasing nutrient concentration on investment in carnivory by Utricularia vulgaris .
2. Plants were grown under controlled conditions and nitrogen and phosphorus added at three loadings in a crossed design. Investment in carnivory was assessed as the proportion of (i) leaf biomass and (ii) leaf area comprising traps.
3. There was no effect of nutrient additions on plant growth or periphyton abundance. Investment in carnivory declined with increasing phosphorus loading. There was no effect of nitrogen, despite this being the nutrient commonly thought to be sought by carnivorous plants. Analysis of previously published data also indicated a decline in investment with increasing P availability.
4. Investment in carnivory in U. vulgaris is inversely proportional to the availability of phosphorus from non-carnivorous sources.     

Interactions of Nitrogen, Phosphorus, and Potassium supplied in Leaf Sprays or in Fertilizer added to the Soil

When sugar-beet plants grown in pots were sprayed daily withnutrient solutions supplying nitrogen, phosphorus, and potassiumseparately or in all combinations, with precautions to preventspray falling on the soil in which the plants were grown, allthree nutrients were absorbed through the leaves. In one experimentnitrogen and potassium, and in another only nitrogen, causedincreases in plant dry weight and leaf area. Swedes absorbedphosphorus from leaf sprays and from fertilizer applied to thesoil, but only the fertilizer caused an increase in dry weight. Absorption of any of the nutrients tested from a spray containingmore than one nutrient was unaffected by the presence of othersin the spray, but spraying with nitrogen-containing solutionsincreased the absorption of phosphorus and potassium from thesoil, and potassium in sprays increased the uptake of phosphorusfrom the soil. Nitrogenous fertilizer applied to the soil increased the leafarea of sugar-beet plants, and hence it also increased the amountsof nitrogen, phosphorus, and potassium deposited on the leaveswhen they were sprayed with solutions of these nutrients, andthe amounts absorbed from the spray into the plants. Phosphaticfertilizer had no effect on uptake from leaf sprays. Potassicfertilizer did not affect leaf area or the estimated volumeof spray solution retained on the leaves, but it appeared toreduce uptake of potassium from the spray. Dry weight per plant was increased by all three nutrients infertilizer, and sugar yield of the roots was increased by nitrogenand potassium in fertilizer, and by nitrogen in spray. Applicationof a nutrient in leaf spray reduced the responses in dry weightand sugar yield to the same nutrient applied in fertilizer tothe soil. Less nitrogen, but more phosphorus, was taken up from the leafsprays than from fertilizer. Nutrients from sprays producedsmaller increases in total dry weight and in dry weight perunit of absorbed nutrient than the same nutrient from fertilizer. The apparent percentage recovery of nitrogen applied in spray,based on estimates of the volumes of solution retained on theleaves, was unaffected by fertilizer treatment, that of phosphoruswas increased by nitrogen fertilizer, and that of potassiumwas increased by nitrogen fertilizer and reduced by potassiumfertilizer. The volume of spray solution held on the leaveswas probably overestimated, so that the highest apparent recovery,about 60 per cent., may represent an almost complete true recovery,because only trivial amounts of the nutrients that had beenapplied in spray remained on the leaf surface to be removedby washing before harvest. Lower apparent recoveries may bedue to reduced uptake from the soil of the nutrient suppliedin spray.     

The influence of mesozooplankton on phytoplankton nutrient limitation: a mesocosm study with northeast Atlantic plankton

We used marine phytoplankton from mesocosms seeded with different zooplankton densities to study the impact of mesozooplankton on phytoplankton nutrient limitation. After 7 d of grazing (copepod mesocosms) or 9 d (appendicularian mesocosms) phytoplankton nutrient limitation was studied by enrichment bioassays. After removal of mesozooplankton, bioassay bottles received either no nutrients, phosphorus or nitrogen alone, or a combination of nitrogen and phosphorus and were incubated for 2 d. Phytoplankton reproductive rates in the bottles without nutrient addition were calculated after correction for grazing by ciliates and indicated increasing nitrogen limitation with increasing copepod abundance. No nutrient limitation was found in the appendicularian mesocosms. The increase of nutrient limitation with increasing copepod density seems to be mainly the result of a trophic cascade effect: Copepods released nanoplankton from ciliate grazing pressure, and thereby enhanced nitrogen exhaustion by nanophytoplankton and reduced nitrogen excretion by ciliates. Nitrogen sequestration in copepod biomass, the mechanism predicted by the ecological stoichiometry theory, seems to have been a weaker effect because there was only little copepod growth during the experiment.     

Lignotuber reserves support regrowth following clipping of two Mediterranean shrubs

1. We investigated whether reserves stored in the lignotubers of two Mediterranean shrubs, Arbutus unedo and Erica arborea , were significantly mobilized to support the demands of regrowth and respiration after clipping the tops at different frequencies.
2. After a single clipping, Arbutus showed a 29% decrease of phosphorus concentration by the end of the first growing season. Two years after recovery from clipping, the starch levels remained lower than those of unclipped plants. Similarly, Erica showed depletion of starch, but no nutrient reserves were depleted significantly.
3. Regrowth after multiple clippings mobilized a large fraction of the starch and nutrients stored in the lignotuber. Mean starch concentrations were depleted by 87–93% after multiple clippings and concentrations of nitrogen, phosphorus, potassium and magnesium were depleted by 10–45%, 27–41%, 19–39% and 23–31%, respectively.
4. An average-sized lignotuber produced 288 resprouts for Arbutus and 1990 resprouts for Erica during a 27 month period of multiple clippings, at the end of which the first plants died.
5. Plant mortality after multiple clipping was 10% for Arbutus and 30% for Erica , and was primarily attributed to exhaustion of carbon reserves because starch concentrations decreased by 96% in dead plants.     

丛枝菌根真菌群落对白三叶草生长的影响

不同施肥处理影响AMF(Arbuscular mycorrhizal fungi)群体结构,然而不同AMF群体结构对植物的生长以及养分吸收的影响尚未见报道,试验利用盆栽实验研究了7种不同来源的丛枝菌根真菌(AMF)群落对白三叶草生长和N、P、K以及微量元素Cu、Zn、Mn的吸收的影响。7种AMF群落分离自长期定位施肥试验地,分别为NPK、OM、CK、1/2OM、NP、NK和PK。每年施肥量是300kg N/hm2,135kg P2O5/hm2,300kg K2O/hm2。有机肥处理的N、P、K养分量与试验地NPK处理含量相同,原料以粉碎的麦秆为主,加上适量的大豆饼和棉仁饼,有机肥经堆制发酵后施用。试验土壤采用封丘试验地土壤,经灭菌处理。试验结果表明,接种不同AMF群落均能促进三叶草的生长,对养分吸收则表现不同。分离自CK试验地的AMF群落对三叶草侵染率显著低于其它6种AMF群落。分离自1/2OM和OM试验地的AMF群落较分离自NPK、CK、NP和NK的AMF群落显著促进了三叶草对P的吸收;各种AMF群落都促进了对N和K的吸收;分离自OM、CK、1/2OM、NP、NK试验地的降低了三叶草植株N含量;分离自NPK试验地的AMF群落提高了三叶草植物K含量;对于Cu、Zn、Mn元素的吸收,不同处理存在较大的差异。AMF群落对三叶草生长以及养分吸收贡献不同,这与不同施肥管理下不同AMF群落的优势种属的侵染率、养分转化以及菌丝发育及分布有关。     

Effects of Pyrenochaeta lycopersici infection on nutrient uptake by tomato plants

The growth of young tomato plants in nutrient solution or in soil and infected with Pyrenochaeta lycopersici Schneider & Gerlach, the cause of tomato brown root rot, was decreased relative to that of uninfected plants. The roots of plants grown in nutrient solution and infected with a mycelial mat of the pathogen contained lower concentrations of potassium and higher concentrations of calcium than roots of uninfected plants. These changes occurred largely in the visibly affected tissue, as opposed to the root system as a whole. The concentrations of magnesium, total nitrogen and phosphorus in the roots of infected plants were not significantly different from those of control plants. Magnesium, nitrogen and phosphorus concentrations in the tops of infected plants were also not significantly different from those of healthy plants, but no consistent changes were found in the concentrations of calcium and potassium. Young tomato plants grown in soil infested with P. lycopersici contained lower concentrations of phosphorus and potassium in the tops than plants grown in sterilized soil. It was not possible to separate intact damaged root systems of infected plants from soil. The changes in composition found in infected plants are discussed in relation to possible methods of manipulating the nutrition of the plant to offset the effects of the disease on crop yield.     

Twospotted spider mite population level, distribution, and damage on ivy geranium in response to different nitrogen and phosphorus fertilization regimes

The influence of plant nutrition on arthropod pests has often been studied by comparing plants provided suboptimal nutrients with those provided sufficient or luxurious nutrients, but such results have limited applicability to commercially produced crops because nitrogen (N) and phosphorus (P) are almost never limiting in greenhouse production. We conducted a series of experiments with ivy geranium, Pelargonium peltatum (L.) L'H?. ex Aiton 'Amethyst 96' to determine the response of twospotted spider mite, Tetranychus urticae Koch (Acarina: Tetranychidae), to six combinations of N (8 or 24 mM) and P (0.32, 0.64, or 1.28 mM) that reflected commercial production practices. All six combinations resulted in saleable plants when plants were free of spider mites, but tissue N and P concentrations among fertilizer combinations were different. On mite-infested plants, no difference in mite numbers or plant damage was found in response to N fertilization rates. Phosphorus had no effect on mite population level until week 8, at which time plants fertilized with 0.64 mM P had slightly more mites than plants fertilized with 0.32 mM. However, overall quality and dry weight of plants fertilized by 0.32 mM P was lower than that of 0.64 and 1.28 mM, which suggests that ivy geranium plants fertilized with the higher P rates may better compensate for mite feeding damage. Positive correlations were found between within-plant distribution of mites and the corresponding tissue N and P concentrations in three foliage strata, suggesting that tissue nutrient content may influence mite selection of feeding sites.     

Maize growth and ion absorption in Richter's solution at different flow rates

By comparing maize plants cultivated in standing nutrient solution with those from solutions flowing at different flow rates it has been established that absorption of nitrogen, potassium and especially of phosphorus was increased owing to the flow. There was likewise a relative rise in the distribution of nutrients to the overground parts of the plants. The content expressed per unit dry matter was increased only in the case of phosphorus; with nitrogen and potassium it was slightly lower than in the standing solution. Increasing amounts of iron were required under the conditions of flowing nutrient solutions to prevent chlorosis of the plants. The production of dry matter,NAR andRGR was also increased because of the flow. The flow considerably changed the habitus of the primary roots of the maize plants. The roots were longer, thinner and on the whole they contained relatively less dry matter (RWR). The lengthening of the roots is explained as a response to stimulation by the solution flow—the rheotropism.     

Alternative criteria for assessing nutrient limitation of a wetland macrophyte (Peltandra virginica (L.) Kunth)

Various nutrient incorporation and plant production parameters were measured to assess their relative usefulness in determining possible nutrient limitation of the wetland plant Peltandra virginica (L.) Kunth. From four stations located along a transect in a tidal freshwater marsh, we documented spatial differences in peak standing biomass of plants. Plant biomass was positively correlated with porewater concentrations of both ammonium and phosphate, but not with sediment concentrations of total nitrogen and phosphorus. Tissue nitrogen and phosphorus concentrations decreased significantly over the growing season, but there were no differences among plants from the four stations, and correlations between plant biomass and ratios of carbon to nitrogen and carbon to phosphorus were weak. Because in situ fertilization of plants had no effect on either peak biomass or tissue concentrations of nitrogen and phosphorus, growth of Peltandra was probably not nutrient limited. Other criteria did predict nitrogen or nitrogen and phosphorus limitation, however, demonstrating that application of parameters used by ecologists to support contentions of nutrient limitation can yield conflicting results. Assessment of nutrient limitation of primary producers may be an ambiguous and unnecessary task in some environments where these criteria are utilized.     

The effect of nitrogen fertilization and rust fungus infection, singly and combined, on the leaf chemical composition of Rumex obtusifolius

1. The chrysomelid beetle Gastrophysa viridula occurs on Rumex obtusifolius growing in a range of nutrient conditions and also on plants infected with the foliar fungus Uromyces rumicis . In a controlled environment, we investigated the effect of fertilizing plants with eight nitrate and four ammonium concentrations, with or without infection, on leaf nutritional quality.
2. Increasing nitrate fertilization increased leaf oxalate, total nitrogen and nitrate concentrations and water content, and decreased total non-structural carbohydrate (NSC) concentrations. Increasing ammonium fertilization increased leaf nitrogen concentration and water content, decreased nitrate and NSC concentrations, and had no effect on oxalate concentrations.
3. Infection produced a mainly additive effect to fertilization, increasing NSC and oxalate, and decreasing nitrate and nitrogen concentration in whole plants fed nitrate, and increasing nitrate and NSC in whole plants fed ammonium.
4. Young leaves on infected plants remained uninfected and had greater nitrogen and NSC concentrations, and lower oxalate and nitrate concentrations, than infected leaves on the same plant.
5. These results are discussed in relation to changes in C:N and NSC:organic nitrogen ratios, the effect of nitrate and oxalate, and the known feeding and oviposition preferences of the beetle. The results suggest that there is an optimum nitrogen fertilization level for G. viridula development.