Abstract: Invariably, many agricultural soils of the world are deficient in one or more of the essential nutrients needed to support healthy plants. Acidity, alkalinity, salinity, anthropogenic processes, nature of farming, and erosion can lead to soil degradation. Additions of fertilizers and/or amendments are essential for a proper nutrient supply and maximum yields. Estimates of overall efficiency of applied fertilizer have been reported to be about or lower than 50% for N, less than 10% for P, and about 40% for K. Plants that are efficient in absorption and utilization of nutrients greatly enhance the efficiency of applied fertilizers, reducing cost of inputs, and preventing losses of nutrients to ecosystems. Inter- and intra-specific variation for plant growth and mineral nutrient use efficiency(NUE) are known to be under genetic and physiological control and are modified by plant interactions with environmental variables. There is need for breeding programs to focus on developing cultivars with high NUE. Iden...

Abstract: To understand the importance of plants in structuring the vertical distributions of soil nutrients, we explored nutrient distributions in the top meter of soil for more than 10,000 profiles across a range of ecological conditions. Hypothesizing that vertical nutrient distributions are dominated by plant cycling relative to leaching, weathering dissolution, and atmospheric deposition, we examined three predictions: (1) that the nutrients that are most limiting for plants would have the shallowest average distributions across ecosystems, (2) that the vertical distribution of a limiting nutrient would be shallower as the nutrient became more scarce, and (3) that along a gradient of soil types with increasing weathering-leaching intensity, limiting nutrients would be relatively more abundant due to preferential cycling by plants. Globally, the ranking of vertical distributions among nutrients was shallowest to deepest in the following order: P > K > Ca > Mg > Na = Cl = SO4. Nutrients strongly cycled by plants, such as P and K, were more concentrated in the topsoil (upper 20 cm) than were nutrients usually less limiting for plants such as Na and Cl. The topsoil concentrations of all nutrients except Na were higher in the soil profiles where the elements were more scarce. Along a gradient of weathering-leaching intensity (Aridisols to Mollisols to Ultisols), total base saturation decreased but the relative contribution of exchangeable K C to base saturation increased. These patterns are difficult to explain without considering the upward transport of nutrients by plant uptake and cycling. Shallower distributions for P and K, together with negative associations between abundance and topsoil accumulation, support the idea that plant cycling exerts a dominant control on the vertical distribution of the most limiting elements for plants (those required in high amounts in relation to soil supply). Plant characteristics like tissue stoichiometry, biomass cycling rates, above- and belowground allocation, root distribu- tions, and maximum rooting depth may all play an important role in shaping nutrient profiles. Such vertical patterns yield insight into the patterns and processes of nutrient cycling through time.

Abstract: Low phosphorus availability is a primary constraint to plant productivity in many natural and agricultural ecosystems. Plants display a wide array of adaptive responses to low phosphorus availability that generally serve to enhance phosphorus mobility in the soil and increase its uptake. One set of adaptive responses is the alteration of root architecture to increase phosphorus acquisition from the soil at minimum metabolic cost. In a series of studies with the common bean, work in our laboratory has shown that architectural traits that enhance topsoil foraging appear to be particularly important for genotypic adaptation to low phosphorus soils (‘phosphorus efficiency’). In particular, the gravitropic trajectory of basal roots, adventitious rooting, the dispersion of lateral roots, and the plasticity of these processes in response to phosphorus availability contribute to phosphorus efficiency in this species. These traits enhance the exploration and exploitation of shallow soil horizons, where phosphorus availability is greatest in many soils. Studies with computer models of root architecture show that root systems with enhanced topsoil foraging acquire phosphorus more efficiently than others of equivalent size. Comparisons of contrasting genotypes in controlled environments and in the field show that plants with better topsoil foraging have superior phosphorus acquisition and growth in low phosphorus soils. It appears that many architectural responses to phosphorus stress may be mediated by the plant hormone ethylene. Genetic mapping of these traits shows that they are quantitatively inherited but can be tagged with QTLs that can be used in plant breeding programs. New crop genotypes incorporating these traits have substantially improved yield in low phosphorus soils, and are being deployed in Africa and Latin America.

Abstract: Objectives: To survey existing literature comparing nutrient content of organic and conventional crops using statistical methods to identify significant differences and trends in the data. Design: Published comparative measurements of organic and conventional nutrient content were entered into a database for calculation. For each organic-to-conventional comparison, a percent difference was calculated: (organic-conventional)/conventional × 100. For nutrients where there was adequate data, the Wilcoxon signed-rank test was used to identify significant differences in nutrient content as represented by the percent difference. Mean percent difference values were also calculated for each significant nutrient by study and by vegetable for the most frequently studied vegetables. The nutrient content of the daily vegetable intake was calculated for both an organic and conventional diet. Results: Organic crops contained significantly more vitamin C, iron, magnesium, and phosphorus and significantly less nitrates th...

Abstract: Plant nutrients, with the exception of nitrogen, are ultimately derived from weathering of primary minerals. Traditional theories about the role of ectomycorrhizal fungi in plant nutrition have emphasized quantitative effects on uptake and transport of dissolved nutrients. Qualitative effects of the symbiosis on the ability of plants to access organic nitrogen and phosphorus sources have also become increasingly apparent. Recent research suggests that ectomycorrhizal fungi mobilize other essential plant nutrients directly from minerals through excretion of organic acids. This enables ectomycorrhizal plants to utilize essential nutrients from insoluble mineral sources and affects nutrient cycling in forest systems.

Abstract: Balanced supply of of essential nutrients is one of the most important factors in increasing crop yields. The objective of this review is to discuss interactions among major and minor nutrients in crop plants. In crop plants, the nutrient interactions are generaly measured in terms of growth response and change in concentration of nutrients. Upon addition of two nutrients, a increase in crop yield that is more than adding only one, the interaction is positive (synergistic). Similarly, if adding the two nutrients together produced less yield as compared to individual ones, the interactions is negative (antagonistic).When there is no change, there is no interaction. All the three interactions among essential plant nutrients have been reported. However, most interactions are complex. A nutrient interacting simultaneously with more than one nutrients. This may induced deficiencies, toxicities, modified growth responses, and/or modified nutrient composition. Better understanding of nutrient interactions may be...

Abstract: The primary source of mineral nutrients for plants is the decomposition of organic matter by soil microbes. Plants are traditionally viewed as largely passive participants in the decomposition process, incapable of directly affecting rates of decomposition significantly and primarily assimilating nutrients unused by the microbial pool. We performed a 13C pulse-chase experiment on a common grazing tolerant grass, Poa pratensis L., of Yellowstone National Park, to follow carbon flow into the soil rhizosphere and microbial biomass and the associated effects on soil N availability and plant N dynamics. Grazing promoted root exudation of carbon, which was quickly assimilated into a burgeoning microbial population in the rhizosphere of clipped plants. Moreover, these facilitating effects of defoliation on rhizospheric processes positively fed back on soil inorganic N pools, plant N uptake, leaf N content, and photosynthesis. Such findings are the first evidence, to our knowledge, that suggest (1) plants are cap...

Abstract: We determined the influence of nutrient availability on the mechanisms used by plants to acquire nitrogen and phosphorus from the soil. Extracellular acid phosphatase production, mycorrhizal colonization, and N and P uptake capacities were measured in control, N-, and P-fertilized forests in three sites that varied in nutrient status from N limited to relatively fertile to P limited. Nitrogen fertilization increased extracellular phosphatase activity in all sites. Phosphorus additions consistently reduced phosphatase activity, mycorrhizal colonization, and P uptake capacity across sites. Our results indicate that these plants efficiently allocate resources to nutrient acquisition as suggested by an economic model. Investment in acquisition of a nutrient was greatest when that nutrient was limiting to growth, and plants appeared to allocate excess N to construction of extracellular phosphatases to acquire P. This increase in phosphatase production with N fertilization implies that even P-limited systems might respond to N deposition with greater productivity.

Abstract: Several biogeochemical processes that regulate the removal of nutrients in wetlands are affected by temperature, thus influencing the overall treatment efficiency. In this paper, the effects of temperature on carbon, nitrogen, and phosphorus cycling processes in treatment wetlands and their implications to water quality are discussed. Many environmental factors display annual cycles that mediate whole system performance. Water temperature is one of the important cyclic stimuli, but inlet flow rates and concentrations, and several features of the annual biogeochemical cycle, also can contribute to the observed patterns of nutrient and pollutant removal. Atmospheric influences, including rain, evapotranspiration, and water reaeration, also follow seasonal patterns. Processes regulating storages in wetlands are active throughout the year and can act as seasonal reservoirs of nutrients, carbon, and pollutants. Many individual wetland processes, such as microbially mediated reactions, are affected by temperature. Response was much greater to changes at the lower end of the temperature scale (< 15 degrees C) than at the optimal range (20 to 35 degrees C). Processes regulating organic matter decomposition are affected by temperature. Similarly, all nitrogen cycling reactions (mineralization, nitrification, and denitrification) are affected by temperature. The temperature coefficient (theta) varied from 1.05 to 1.37 for carbon and nitrogen cycling processes during isolated conditions. Phosphorus sorption reactions are least affected by temperature, with theta values of 1.03 to 1.12. Physical processes involved in the removal of particulate carbon, nitrogen, and phosphorus are not affected much by temperature. In contrast, observed wetland removals may have different temperature dependence. Design models are oversimplified because of limitations of data for calibration. The result of complex system behavior and the simple model is the need to interpret whole ecosystem data to determine temperature coefficients. Temperature seems to have minimal effect on biochemical oxygen demand (0.900 < theta < 1.015) and phosphorus (0.995 < theta < 1.020) removal, and more significant effect on nitrogen removal (0.988 < theta < 1.16). In colder climates, there may be seasonal slowdown of treatment, which can decrease the overall treatment efficiency of constructed wetlands.

Abstract: The effect of nutrients on plant growth and development has been studied for over 350 years since the experiments of van Helmont in 1648 (6). Recent studies on nutrient effects in plants have involved separating their role as building blocks of organic matter or cofactors from their potential role

Abstract: The rice plant (Oryza sativa L. cv Oochikara) is known to be a Si accumulator, but the mechanism responsible for the high uptake of Si by the roots is not well understood. We investigated the role of root hairs and lateral roots in the Si uptake using two mutants of rice, one defective in the formation of root hairs (RH2) and another in that of lateral roots (RM109). Uptake experiments with nutrient solution during both a short term (up to 12 h) and relatively long term (26 d) showed that there was no significant difference in Si uptake between RH2 and the wild type (WT), whereas the Si uptake of RM109 was much less than that of WT. The number of silica bodies formed on the third leaf in RH2 was similar to that in WT, but the number of silica bodies in RM109 was only 40% of that in WT, when grown in soil amended with Si under flooded conditions. There was also no difference in the shoot Si concentration between WT and RH2 when grown in soil under upland conditions. Using a multi-compartment transport box, the Si uptake at the root tip (0–1 cm, without lateral roots and root hairs) was found to be similar in WT, RH2, and RM109. However, the Si uptake in the mature zone (1–4 cm from root tip) was significantly lower in RM109 than in WT, whereas no difference was found in Si uptake between WT and RH2. All these results clearly indicate that lateral roots contribute to the Si uptake in rice plant, whereas root hairs do not. Analysis of F2 populations between RM109 and WT showed that Si uptake was correlated with the presence of lateral roots and that the gene controlling formation of lateral roots and Si uptake is a dominant gene.

Abstract: Aquatic primary productivity is frequently limited by the availability of nutrients. The ability to identify factors limiting algal growth is of considerable importance to our understanding of the ecology of aquatic plants and to water management practices. Methods used to identify limiting resources in the past have included a) analysis of nutrient availability, b) elemental composition and cell quotas for various nutrients, c) bio-assays monitoring growth of test species or of natural populations following nutrient enrichment and d) measurements of various physiological parameters, such as enhancement of respiration and dark carbon fixation rates and perturbation of photosynthetic rate following re-supply of nutrients.¶In this paper we briefly review the merits and methodological limitations of these approaches for the assessment of the nutrient status of algal populations. We discuss how an understanding of biochemical and metabolic changes induced by nutrient limitation has led to the development of rapid and simple tools to monitor the nutrient status of aquatic plants and algae. In particular, we describe the use of transient changes in chlorophyll a fluorescence as a potential tool for rapid assessment of algal nutrient status and the development of molecular probes specific to nutrient limited cells, such as flavodoxin as a diagnostic tool for Fe-limitation.

Abstract: The flavonol content of Arabidopsis thaliana and tomato seedlings was assessed in conditions of reduced nitrogen or phosphorus availability. In both systems, a significant inverse relationship was observed between nutrient availability and flavonol accumulation, with nitrogen limitation promoting the greatest increase in flavonols. A trial was established to determine the effects of decreased nitrogen and phosphorus availability on the flavonol content of leaf and fruit tissues of tomato plants (Lycopersicon esculentum cv. Chaser) in a commercial situation. Nutrients were supplied by a hydroponic system with nutrient regimes designed to provide the highest and lowest nitrogen and phosphorus levels with which it is possible to support plant growth and fruit set. Fruiting was abundant and tomato fruits were harvested at mature green, breaker and red stages of ripening; leaves were also harvested from the tops of the plants. All tissues were analysed for flavonol content using reversed-phase high-performance liquid chromatography. Flavonol accumulation in the leaves of mature tomato plants was found to increase significantly in response to nitrogen stress, whereas phosphorus deficiency did not elicit this response. Reduced nitrogen availability had no consistent effect on the flavonol content of tomato fruits. Phosphorus deficiency elicited an increase in flavonol content in early stages of ripening. Effects of nutrient stress on the flavonol content of tomato fruits were lost as ripening progressed. The findings suggest that nutrient status may be employed to manipulate the flavonol content of vegetative tissues but cannot be used to elevate the flavonol content of tomato fruit.

Abstract: Root hairs are presumably important in the acquisition of immobile soil resources such as phosphorus. The density and length of root hairs vary substantially within and between species, and are highly regulated by soil phosphorus availability, which suggests that at high nutrient availability, root hairs may have a neutral or negative impact on fitness. We used a root-hairless mutant of the small herbaceous dicot Arabidopsis thaliana to assess the effect of root hairs on plant competition under contrasting phosphorus regimes. Wildtype plants were grown with hairless plants in a replacement series design at high (60 μm phosphate in soil solution) and low (1 μm phosphate in soil solution) phosphorus availability. At high phosphorus availability, wildtype and mutant plants were equal in growth, phosphorus acquisition, fecundity and relative crowding coefficient (RCC). At low phosphorus availability, hairless plants accumulated less biomass and phosphorus, and produced less seed when planted with wildtype plants. Wildtype plants were unaffected by the presence of hairless plants in mixed genotype plantings. Wildtype plants had RCC values greater than one while hairless plants had RCC values less than one. We conclude that root hairs increase the competitiveness of plants under low phosphorus availability but do not reduce growth or competitiveness under high phosphorus availability.

Abstract: Root nodule bacteria require access to adequate concentrations of mineral nutrients for metabolic processes to enable their survival and growth as free-living soil saprophytes, and in their symbiotic relationship with legumes. Essential nutrients, with a direct requirement in metabolism of rhizobia are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, iron, manganese, copper, zinc, molybdenum, nickel, cobalt and selenium. Boron does not seem to be required by rhizobia, but is essential for the establishment of effective legume symbioses. Nutrient constraints can affect both free-living and symbiotic forms of root nodule bacteria, but whether they do is a function of a complex series of events and interactions. Important physiological characteristics of rhizobia involved in, or affected by, their mineral nutrition include nutrient uptake, growth rate, gene regulation, nutrient storage, survival, genetic exchange and the viable non-culturable state. There is considerable variation between genera, species and strains of rhizobia in their response to nutrient deficiency. The effects of nutrient deficiencies on free-living rhizobia in the soil are poorly understood. Competition between strains of rhizobia for limiting phosphorus and iron in the rhizosphere may affect their ability to nodulate legumes. Processes in the development of some legume symbioses specifically require calcium, cobalt, copper, iron, potassium, molybdenum, nickel, phosphorus, selenium, zinc and boron. Limitations of phosphorus, calcium, iron and molybdenum in particular, can reduce legume productivity by affecting nodule development and function. The effects of nutrient deficiencies on rhizobia-legume signalling are not understood. The supply of essential inorganic nutrients to bacteroids in relation to nutrient partitioning in nodule tissues and nutrient transport to the symbiosome may affect effectiveness of nitrogen fixation. An integration of molecular approaches with more traditional biochemical, physiological and field-based studies is needed to improve understanding of the agricultural importance of rhizobia response to nutrient stress.

Abstract: Effects of arbuscular mycorrhizal fungi (AMF) and salt stress on nutrient acquisition and growth of two tomato cultivars exhibiting differences in salt tolerance were investigated. Plants were grown in a sterilized, low-P (silty clay) soil-sand mix. Salt was applied at saturation extract (ECe) values of 1.4 (control), 4.9 (medium) and 7.1 dS m–1 (high salt stress). Mycorrhizal colonization occurred irrespective of salt stress in both cultivars, but AMF colonization was higher under control than under saline soil conditions. The salt-tolerant cultivar Pello showed higher mycorrhizal colonization than the salt-sensitive cultivar Marriha. Shoot dry matter (DM) yield and leaf area were higher in mycorrhizal than nonmycorrhizal plants of both cultivars. Shoot DM and leaf area but not root DM were higher in Pello than Marriha. The enhancement in shoot DM due to AMF inoculation was 22% and 21% under control, 31% and 58% under medium, and 18% and 59% under high salinity for Pello and Marriha, respectively. For both cultivars, the contents of P, K, Zn, Cu, and Fe were higher in mycorrhizal than nonmycorrhizal plants under control and medium saline soil conditions. The enhancement in P, K, Zn, Cu, and Fe acquisition due to AMF inoculation was more pronounced in Marriha than in the Pello cultivar under saline conditions. The results suggest that Marriha benefited more from AMF colonization than Pello under saline soil conditions, despite the fact that Pello roots were highly infected with the AMF. Thus, it appears that Marriha is more dependent on AMF symbiosis than Pello.

Abstract: In the mangrove of the Caete Estuary (North Brazil) a detailed sampling was carried out in order to gain insight into the dynamics of inorganic nutrients and organic matter in one of the world's largest mangroves and to identify the driving forces behind these processes. Throughout 36 tidal cycles in the course of one year, concentrations of dissolved and particulate organic carbon and nitrogen (DOC, POC, DON, PON) and dissolved inorganic nutrients (N, P, Si compounds) were determined in a mangrove tidal creek. Annual average concentrations (in μM) were DOC 360, POC 240, DON 20, PON 29, dissolved inorganic nitrogen (DIN) 11, silicate 170 and phosphate 2·4. Ammonium and nitrite were about 80 and 17% of DIN, respectively. Nutrient dynamics in the creek were significantly influenced by porewater input from the upper forest sediment layer. This led to characteristic tidal signatures of nutrient and organic matter concentrations with maximum values during low tide. Annual phosphate and DOC oscillations were caused mainly by tidally dependent variations of porewater input. Their concentrations in the creek were directly proportional to the hydraulic gradient between creek and sediment water table. Due to autotrophic activity, dissolved oxygen, pH and DOC were higher and DIN was lower during daytime than at night. Annual oscillations of DIN and DON could also be attributed to varying phytoplanktonic activity. Silicate and phosphate showed only weak response to aquatic photosynthesis with slightly lower concentrations during the day. Assuming equilibrated fluxes, nitrogen fixation in the forest could be estimated to be 2·3 mmol N m−2 d−1. Based on these findings, tidal range and porewater concentrations were identified as driving forces behind coastal outwelling of nutrients and organic matter from mangroves. Outwelling probably occurs only from mangroves where the nutrient concentration in porewater exceeds the demands of the benthic community and trees, caused by positive sedimentation balances and high nitrogen fixation rates, and only in macrotidal regions where porewater can flow in considerable amounts to the tidal creeks and the ocean.

Abstract: We present the first data that comprehensively quantify the interactive effects on an herbivore of a plant allelochemical and dietary macronutrients. Locusts (Locusta migratoria) were reared across the fifth larval stadium on one of 20 foods with fixed total macronutrient content (42%), but varying in protein:carbohydrate (P:C) ratio (7:35, 14:28, 21:21, 28:14, or 35:7) and tannic acid (TA) content (0, 3.3, 6.7, or 10%). The effects of the allelochemical on consumption, nutrient uptake, body composition, development rate, and survivorship were highly dependent on the balance of macronutrients in the food. Mortality was low and independent of TA on the near-optimally balanced 21:21 food, but rose markedly with increasing TA levels as dietary P:C ratio became more unbalanced. The time course of deaths differed for excess P and excess C foods, suggesting different underlying causes. The major effect of TA in foods of low P:C ratio was to reduce the rate of intake, while TA in foods containing excess protein ...

Abstract: The development of short-rotation intensive cultural (SRIC) willow systems as a source of bioenergy and bioproducts is growing in the northeastern and midwestern United States. Important data for sustainable management such as nutrient removal and nutrient use e8ciency in willow bioenergy plantations is lacking. This study reports wood biomass production, annual removal of nutrients, and nutrient use e8ciency in experimental plantings of SRIC willow and poplar at Tully, New York. E9ects of clone, fertilization, irrigation, planting density, and harvest cycle were analyzed. Annual biomass production of 15 –22 dry Mg=ha removed 75 –86, 10 –11, 27–32, 52–79 and 4 –5 kg=ha=year of N, P, K, Ca and Mg, respectively. For all the variables studied, the responses depended on clone. Fertilization and irrigation increased rates of nutrient removal by means of increased biomass production. Unlike planting density, harvest cycle signiBcantly a9ected rates of nutrient removal and nutrient use e8ciency. For clone SV1 ( Salix dasyclados), an irrigated and fertilized planting with a density of 36,960 trees=ha harvested on a 3-year rotation had the highest biomass production and nutrient use e8ciency, and the lowest rates of nutrient removal. The annual harvest cycle had the lowest nutrient use e8ciency and the highest annual removal of nutrients suggesting that this choice would be most appropriate for biomass crops that are to be used as bu9er strips to manage nutrient runo9 from agricultural Belds. An appropriate choice of clone, planting density, and harvest cycle could tailor the rates of nutrient removal and nutrient use e8ciency to match the objective of the planting. c 2001 Elsevier Science Ltd. All rights reserved.

Abstract: Anthropogenic loading of nutrients in rivers often increases disproportionally among N, P, and Si, and thus may shift the type of phytoplankton nutrient limitation in the coastal receiving waters. The effect of anthropogenic nutrient loading has rarely been addressed in the Pearl River estuary along the southern coast of China, even though it is one of the largest rivers in the world. We conducted a cruise along the Pearl River estuary and adjacent coastal waters south of Hong Kong during July 17 to 18, 1999. Samples were taken for salinity and nutrients (NO3, SiO4, PO4, NH4 and urea) and nutrient addition experiments were conducted on board. Vertical profiles of salinity showed a salt-wedge estuary and the coastal plume covering the waters south of Hong Kong. Con- centrations of NO3 were very high (ca 90 µM) upstream of the Pearl River estuary, and much of the riverine NO3 was not utilized in the estuary until depletion at the edge of the coastal plume on the east side of Hong Kong. SiO4 was 120 µM upstream and its utilization was similar to that of NO3. PO4 was low in surface waters (<0.5 µM) and higher below the halocline in the estuary. NH4 and urea were generally < 4 and 1.5 µM, respectively. In the estuary, N:P ratio was 200:1, indicating potential P limitation, while N:Si was below 1:1. Beyond the coastal plume to the east of Hong Kong, N:P and N:Si ratios were <5:1 and 1:0.3, respectively, indicating potential N limitation. Nutrient limitation was shown in nutrient addition experiments and was consistent with the ratios of nutrients. There- fore, nutrient limitation shifted across the coastal plume from P limitation in the estuary to N limita- tion in the oceanic waters. Potential P limitation was observed in the estuary; P and Si co-limiting occurred at the edge of the coastal plume, and N was limiting in the oceanic side. This spatial shift in nutrient limitation has great implications for nutrient pollution control and coastal management of Hong Kong waters.

Abstract: Differences in root foraging behavior between species have been well documented, but the effects of these differences on belowground competitive ability are only beginning to be studied. Here we report the results of a competition experiment in homogeneous and heterogeneous soils between two species that differ in their ability to acquire nutrients from patchy environments. The perennial grasses Festuca rubra and Anthoxanthum odoratum have comparable growth rates, but results of previous studies with isolated plants designated the latter species as the more effective forager, probably due to its higher physiological plasticity (stronger increase in nutrient uptake rates per unit root mass in enriched nutrient patches). We introduced nutrient soil heterogeneity at two spatial scales. In a fine-grained heterogeneous treatment, the nutrient-rich patches were smaller and more concentrated than in a coarse-grained heterogeneous (checkerboard) treatment. Overall, the level of nutrient availability in these hete...

Abstract: Nitrogen (N) is the only nutrient that promotes forest growth when given individually. An extra stem growth of 15 m 3 ha -1 is obtained during a 10 yr period following an application of 150 kg N ha -1 . Larger growth increases have often been the result of more intensive N fertilization. Lime or wood ash give a minor growth stimulation on sites with a carbon (C) to N ratio below 30 in the humus layer, while the opposite effect prevails on N-poor sites. Nutrients given as soluble fertilizers are readily taken up by trees. Boron deficiency may be induced in northern Sweden after N fertilization or liming. The ground vegetation may be altered by single-shot N fertilization, but long-term effects occur only for intensive regimes. Lime or wood ash may modify the flora if soil pH is significantly altered: the change will be in response to N availability. Fruit-body production of mycorrhizal fungi is disfavoured by chronic N input, but also by lime or ash. However, the mycorrhizal structures on root tips are les...

Abstract: It is well known that secondary chemicals produced at one trophic level may affect organisms at subsequent levels of the food chain. Effects of nutrient supplements may also propagate through trophic levels, but the mechanisms here are less clear. We tested the hypothesis that predators can be affected by the nutrient composition of the prey's food. Wolf spider (Pardosa amentata) hatchlings were raised ad libitum on fruit flies (Drosophila melanogaster) that were cultured in poor basic medium with additions of different nutrients. These additions strongly affected the performance of the spiders. Growth rates increased when additions consisted of 19 different amino acids or fatty acids+cholesterol or commercial dogfood. Survival increased in spiders reared on fruit flies from cultures containing 19 amino acids or methionine or dogfood. The addition of dogfood increased spider growth and survival more significantly than the addition of any single nutrient group alone. Adult female flies from the dogfood culture were significantly heavier than females from the basic culture. The nutrients added to the fruit fly media were thus able to create biological effects at both the second and the third trophic levels. To test whether nutrients passed to the predators through the gut content of the prey, we included a treatment where the spiders were fed flies that had been starved for 48 h in order to empty their guts. Gut emptying of the flies did not reduce the positive effects of the enriched fruit fly media, i.e. the nutritional benefits were not due to nutrients that passed directly through the guts of the flies. Since the nutrients added to the fruit fly media were separated from the spiders that benefited from them by two trophic transformations, this phenomenon was a true tritrophic interaction.

Abstract: Savanna trees have a multitude of positive and negative effects on understorey grass production. but little is known about how these effects interact. We report on a fertilization and shading experiment carried out in a Tanzanian tropical city savanna around Acacia tortilis trees. In two years of study there was no difference in grass production under tree canopies or in open grassland, Fertilization, however, indicate that trees do affect the nutrient limitation of the grass layer with an N-limited system in open grassland to a P-limited system under the trees. The NT ratios of grass gave a reliable indication of the nature of nutrient limitation, but only when assessed at the end of the wet season, Mid-wet season nutrient concentrations of grasses were higher under than outside the tree canopy, suggesting that factors other than nutrients limit grass production. A shading experiment indicated that light may be such a limiting factor during the wet season when water and nutrients are sufficiently available. However, in the dry season when water is scarce, the effect of shade on plant production became positive. We conclude that whether trees increase or decrease production of the herbaceous layer depends on how positive effects (increased soil fertility) and negative effects (shade and soil water availability) interact and that these interactions may significantly change between wet and dry seasons.

Abstract: 1. We used artificial substrata in forested and open streams in South-East Queensland, Australia, to determine the relative importance of shading from riparian vegetation and of nutrients on periphyton growth, and whether nitrogen and/or phosphorus limited algal productivity. 2. Nutrient-diffusing substrata consisting of agar enriched with N, P and N + P, and controls without nutrients, were deployed in duplicate at 15 sites in headwater streams with riparian canopy cover ranging from 0 to 88%. 3. Shading was the over-riding factor controlling periphyton biomass accrual on the artificial substrata, with nutrients playing a relatively minor role. Microscopic examination of periphyton scrapings taken after 7 weeks revealed that diatoms dominated on the artificial substrata in shaded streams, whereas filamentous green algae dominated the algal assemblage in the more open canopy streams. 4. Whilst nutrients had little effect on the accrual of algal biomass compared with riparian shading, there was evidence that nitrogen, and not phosphorus, stimulated periphyton production in streams with sufficient light.