Abstract: The allocation of biomass to different plant organs depends on species, ontogeny and on the environment experienced by the plant. In this paper we first discuss some methodological tools to describe and analyse the allocation of biomass. Rather than the use of shoot:root ratios, we plead strongly for a subdivision of biomass into at least three compartments: leaves, stems and roots. Attention is drawn to some of the disadvantages of allometry as a tool to correct for size differences between plants. Second, we tested the extent to which biomass allocation of plants follows the model of a ‘functional equilibrium’. According to this model, plants respond to a decrease in above-ground resources with increased allocation to shoots (leaves), whereas they respond to a decrease in below-ground resources with increased allocation to roots. We carried out a meta-analysis of the literature, analysing the effect of various environmental variables on the fraction of total plant biomass allocated to leaves (leaf mass fraction), stem (stem mass fraction) and roots (root mass fraction). The responses to light, nutrients and water agreed with the (qualitative) prediction of the ‘functional equilibrium’ theory. The notable exception was atmospheric CO2, which did not affect allocation when the concentration was doubled. Third, we analysed the quantitative importance of the changes in allocation compared to changes in other growth parameters, such as unit leaf rate (the net difference between carbon gain and carbon losses per unit time and leaf area), and specific leaf area (leaf area: leaf biomass). The effects of light, CO2 and water on leaf mass fractions were small compared to their effects on relative growth rate. The effects of nutrients, however, were large, suggesting that only in the case of nutrients, biomass allocation is a major factor in the response of plants to limiting resource supply.

Abstract: Interspecific variation in polyphenol production by plants has been interpreted in terms of defense against herbivores. Several recent lines of evidence suggest that polyphenols also influence the pools and fluxes of inorganic and organic soil nutrients. Such effects could have far-ranging consequences for nutrient competition among and between plants and microbes, and for ecosystem nutrient cycling and retention. The significance of polyphenols for nutrient cycling and plant productivity is still uncertain, but it could provide an alternative or complementary explanation for the variability in polyphenol production by plants.

Abstract: ~~~~~~~~~~~~~Number 6 Abstract Total nitrogen (TN) and total phosphorus (TP) measurements and contemporaneous measurements of chlorophyll a (Chl a) and phytoplankton nutrient deficiency have been made across a broad range of lakes and ocean sites using common methods. The ocean environment was nutrient rich in terms of TN and TP when compared with most lakes in the study, although Lake Victoria had the highest values of TN and TP. TN concentrations in lakes rose rapidly with TP concentrations, from low values to TN concentrations that are similar to those associated with the ocean sites. In contrast, the TN concentrations in the oceans were relatively homogeneous and independent of TP concentrations. The hyperbolic shape of the TN: TP relationship created a broad range of TN: TP values for both lakes and oceans. The TN: TP ratios of the surface ocean sites were usually well in excess of the Redfield ratio that is noted in the deep ocean. Phytoplankton biomass, as indicated by Chl a, was strongly dependent upon TP in the lakes, and there was a weaker relationship with TN. Oceanic Chl a values showed a positive relationship with TP, but at much higher TP values than were observed in the lakes; there was no relation with TN. P-deficient phytoplankton growth was inferred using independent indicators when TP was 50 (molar). At intermediate TN TP ratios, either N or P can become deficient. We conclude that N or P limitation of algal growth is a product of the TN and TP concentration and the TN: TP ratio rather than a product of whether the system of study is marine or freshwater.

Abstract: Arbuscular mycorrhizal fungi (AMF) benefit plants by allowing them to grow and produce in relatively harsh mineral stress environments. This has been attributed extensively to ability of AMF to expand the volume of soil for which mineral nutrients are made available to plants compared to what roots themselves would contact. This article reviews the effects of AMF on enhancing/reducing acquisition of phosphorus (P), nitrogen (N), sulfur (S), boron (B), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), zinc (Zn), copper (Cu), manganese (Mn), iron (Fe), aluminum (Al), silicon (Si), and some trace elements in plants. The nutrients enhanced most in host plants grown in many soils (e.g., high and low soil pH) are P, N, Zn, and Cu, but K, Ca, and Mg are enhanced when plants are grown in acidic soils. Many AMF have also the ability to ameliorate Al and Mn toxicities for plants are grown in acidic soil.

Abstract: Lowland river–floodplain systems are characterized by a high degree of variability in both the frequency and period of inundation of various parts of the floodplain. Such variation should profoundly affect the processes underlying nutrient transformations in these systems. This paper explores the effect of various hydrologic regimes on nutrient cycles. Partial drying of wet (previously inundated) sediments will result in an increased sediment affinity for phosphorus and will produce a zone for nitrification coupled with denitrification. Hence, partial drying may reduce the availability of nitrogen (N) and phosphorus (P). Conversely, complete desiccation of sediments may lead to the death of bacteria (and subsequent mineralization of N and P), a decrease in the affinity of P for iron minerals, a decrease in microbial activity and a cessation of all anaerobic bacterial processes (e.g. denitrification). Colonization of exposed sediments by terrestrial plants may lead to N and P moving from the sediments to plant biomass. Re-wetting of desiccated soils and sediments will result in an initial flush of available N and P (which can be incorporated into bacterial or macrophyte biomass), coupled with an increase in bacterial activity, particularly nitrification. Inundation of floodplain soils will result in the liberation of C, N and P from leaf litter and floodplain soils. This will result in an increase in productivity, which ultimately may lead to the onset of anoxia in floodplain soils and, consequently, an increase in anoxic bacterial processes such as P release and denitrification. Copyright © 2000 John Wiley & Sons, Ltd.

Abstract: Rice: nutrient disordres & nutrient management , Rice: nutrient disordres & nutrient management , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Abstract: In Hawaiian montane forests, we assessed whether the same nutrients limit decomposition and aboveground net primary production (ANPP) along a soil chronosequence where nutrients demonstrably and predictably limit ANPP. At three sites that vary in parent material age (300, 20 000, and 4.1 × 106 yr), we used fertilization to assess whether nitrogen (N) and/or phosphorus (P) limit decomposition. Reciprocal transplants using litter bags allowed us to distinguish limitation by externally supplied nutrients vs. limitation by nutrients within litter. Nutrient limitation of decomposition was not predictable from nutrient limitation of ANPP, in that elevated litter and soil N had only small, if any, effects on decomposition, even at the young site where N limits ANPP. At the oldest site where P limits ANPP, both elevated litter P and increased availability of soil N and P increased decomposition rates. Thus, nutrients may limit decomposition more strongly in low-P than in low-N ecosystems. Fertilization affected litter nutrient dynamics more strongly than it did decomposition, and we observed uptake of both N and P by decomposers that was not always accompanied by changes in decomposition rates. Such nutrient incorporation into decomposing litter may retain nutrients within ecosystems, even when nutrients do not limit decomposition rates.

Abstract: Studies of lakes in North America reveal that concentrations of phosphate, an essential nutrient for aquatic microorganisms, are 100-1,000 times lower than estimated with conventional techniques. Among the implications are the possible need to rethink nutrient dynamics in both freshwater and marine ecosystems..

Abstract: Nutrients favouring certain types of phytoplankton over others can influence levels of CO2 in the atmosphere. Silica is one such nutrient. The case is now put that a change in its availability accounted for a rise in atmospheric CO2 at the end of the Last Glacial Maximum.

Abstract: Dissolved and particulate trace metal concentrations (dissolved Fe, Zn, Cd, Co, Cu and Ni; particulate Fe, Mn and Al) were measured along two transects in the Ross Sea during austral summer of 1990. Total Fe concentrations in southern Ross Sea and inshore waters were elevated >3.5 times that of northern waters. Dissolved Zn, Cd and Co concentrations were lower by factors of 4.5, 3.5 and 1.6 in southern surface waters relative to northern waters. Dissolved Cu and Ni concentrations were similar in both areas. Elevated Fe concentrations coincided with areas of increased productivity, phytoplankton biomass and nutrient drawdown, indicating that Fe is an important factor controlling the location of phytoplankton blooms in the Ross Sea. Particulate concentrations of Fe, Mn and Al indicate two possible sources of iron to the Ross Sea, resuspension of continental shelf sediments and iron incorporated in annual sea ice and released with meltwaters.

Abstract: Our study quantified the nutrients and energy introduced into the nesting beach at Melbourne Beach, Florida, from distant foraging grounds by loggerhead sea turtles, Caretta caretta. The fate of eggs deposited into 97 nests was determined by monitoring nests throughout incubation. The organic matter, energy, lipid, nitrogen, and phosphorus content of fresh eggs, eggs at successive stages of development, hatchlings, and hatching remains were determined. From these analyses, we estimated the flow of energy and nutrients introduced into the 14305 nests (-1.6 X 106 eggs) deposited in a 21-km stretch of Mel- bourne Beach in 1996. We quantified the amount of energy and nutrients incorporated into each of four pathways: ingested by nest predators; consumed by detritivores, decomposers, and plants; lost as metabolic heat or gases during embryological development and hatching; or returned to the ocean as hatchlings. Each nest introduced a mean of 688 g of organic matter, 18 724 kJ of energy, 151 g of lipids, 72 g of nitrogen, and 6.5 g of phosphorus into the beach. Twenty-five percent of the organic matter, 27% of the energy, 34% of the lipids, 29% of the nitrogen, and 39% of the phosphorus introduced into the nests returned to the ocean as hatchlings. Quantities of energy and nutrients transported by the turtles are com- parable to quantities moved by other important biological transporters. Human activities have substantially altered the quantity of energy and nutrients transported, and the distri- bution of those nutrients on the beach. By introducing nutrients into beach ecosystems, sea turtles may help maintain stable dune systems that are critical to their reproductive success.

Abstract: The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses.

Abstract: By factorial field experiments we analyzed the relative effects of increased nutrient (N1P) loading and natural grazing pressure on species composition, carbon storage, and nitrogen retention in the Baltic Sea littoral food web, composed of macroalgae, grazers (snails, isopods, amphipods), and predators (shrimps, crabs, fish). Nitrogen was depleted relative to phosphorus throughout most of the year. Increasing nitrogen (6‐200% over ambient concentrations) enhanced algal productivity and cover of fast-growing annual algae, grazer, and predator densities, suggesting a three-level bottom-up effect. With increasing nitrogen loading, annual algae increasingly blocked perennial algal recruitment (65‐98% decrease) and growth. Grazers counteracted the effects of nutrient enrichment on algal species composition through selective consumption of annual algae. Grazer exclusion had equivalent negative effects on perennial recruitment as a 85% increase in nitrogen loading. Nutrient enrichment increased algal nitrogen content and decreased tissue C : N ratios in spring and summer but not in fall. Carbon storage and nitrogen retention, measured as C and N retained in plant biomass at the end of the growth season, were increased by grazers (C: 39%, N: 24%) but decreased with increasing nitrogen loading (C: 271%, N: 274%). Our results emphasize the important role of grazers in buffering moderate eutrophication effects and illustrate how food web interactions and shifts in species composition are tightly linked to coastal ecosystem function. Currently, humans are more than doubling the rate at which nitrogen and phosphorus enter the global biogeochem

Abstract: In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, phytoplankton biomass, and production almost doubled at the end of the 1970s and remained high ever since. Principal component analysis of 21-yr (1974‐ 1994) high-resolution time series of the 32 most numerous marine algal species revealed that the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988, and that it was relatively stable in-between (1974‐1976, 1978‐1987) and thereafter (1988‐1994). These major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN : TP) nutrient concentrations. During the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-controlled system to an even more eutrophic but nitrogen-controlled environment. The system reshifted towards P-control between 1987 and 1988. The coincidence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN : TP ratios and phytoplankton community structure. Among diatoms, the observed increase in phytoplankton biomass under eutrophic N-controlled conditions was particularly due to an increase of the abundance of larger algae. Our results indicate that the N budget of the area is correlated with the community structure, suggesting enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells. In shallow coastal marine waters, the total primary production and biomass of phytoplankton is generally assumed to be hyperbolically related to nutrient loadings from land and subsequent availability of these nutrients in the water column (e.g., Borum and Sand-Jensen 1996). However, understanding the effect of nutrient enrichment on living resources requires detailed knowledge of how nutrients enter and leave these waters. Dissolved and particulate materials as well as living organisms are exchanged between the coastal waters and the open sea, and net material fluxes appear to depend on physical and biological responses within these systems to changes in nutrient loadings from land (e.g., Dame and Allen 1996).

Abstract: Nutrient enrichment of marine sediments or the water column has been used to study plant nutrient limitation and its cascading effects on community structure, Here we develop methodological recommendations for in situ enrichment. We review 18 published enrichment methods. Nutrient concentrations varied through time and among sites, with sediment depth, distance from the source, fertilizer type and load. Combining available data, we could predict an increase in sediment porewater phosphate (r 2 = 0.48) but not ammonium (r 2 = 0.07) concentrations in a multiple regression model. In three comparative field experiments we applied a coated slow-release fertilizer in the sediment and the water column and followed nutrient concentrations over time. We recommend coated fertilizer pellets, because they provide gradual nutrient release, allow for realistic nutrient gradients, and even application but we emphasize that nutrient concentrations need to be monitored through time.

Abstract: affect decomposition, and values less than five can inhibit decomposition of leaf litter (Qualls and Haines, Like many wetland ecosystems, areas of the northern Everglades 1990). While many bogs exhibit very acid conditions of Florida, USA, have been influenced by P eutrophication. Our objective was to determine if P enrichment of water influences the that are potentially inhibitory to decomposition, fens litter decomposition rate and nutrient immobilization by litter and, such as the Everglades often have soil pH values near further, to determine the quantitative relationship of these responses neutrality and near the optimum for decomposition. across a range of P concentrations in surface water. In addition, we High concentrations of dissolved organic acids are a determined whether P additions rapidly elevated microbial biomass common characteristic of many wetlands and can inhibit P in the soil. In order to isolate the effects of P enrichment, we placed decomposition by creating highly acid water. Qualls and bags containing cattail (Typha domengensis Crantz) and sawgrass Haines (1990) showed that this affected decomposition (Cladium jamaicense Pers.) litter into two sets of experimental chan- only by controlling H 1 concentration, not by any other nels into which controlled inputs of five different phosphate concentrainhibitory properties of the organic acid molecules. tions were added continuously. After 1 yr of incubation, litter was The decomposition of plant litter can also be limited analyzed for C, P, N, Cu, Ca, and K content. Loss of C at the end of 1 yr increased linearly with increasing average PO4 content in the by the concentration of inorganic N and P (Alexander, channels with a similar slope for both species of litter. Immobilization 1977) in soil or water surrounding the decomposer micaused an absolute increase in P content of the litter up to approxi- croflora. Suberkropp and Chauvet (1995) found that mately ninefold across the range of water P concentrations, while NO3 concentration was the only variable correlated with immobilization of N, Ca, and K did not vary with water P concentra- differences in litter decomposition among six hardwater tions. During decomposition, litter exhibited a net uptake of Cu (a streams; however, Triska and Sedell (1976) found no nutrient potentially limiting plant growth on peat soils). The microbial decomposition response to NO

Abstract: Since the mid 1970s substrate growing has become popular in the greenhouse industry in The Netherlands. Because of the small rooting volumes that are used in substrate growing, such systems require an accurate fertilization, but at the same time they offer possibilities for precise control and management of the conditions in the root environment.The osmotic potential of the substrate solution in the root environment is often used for improvement of the quality of the produce. For adequate management of the osmotic potential, firstly information about the absorption of water and ions by the crop is essential. Secondly, the effect of the osmotic potential and its interaction with climatic conditions in the greenhouse on crop development must be known. Thirdly, information on the spatial distribution of water and ions in the root environment should be available, because this may strongly affect salinity effects on plants.In studies on effects of low osmotic potentials on crops, both osmotic and specific ion effects should be distinguished. The osmotic effects predominate for most crops and growing conditions. Osmotic effects can be described according to the model developed by Maas and Hoffman. This model is characterized by two parameters, the salinity threshold value and the salinity yield decrease value. In this simple model the EC caused by nutrients is not taken into account separately, though nutrients have a significant effect on the EC of the substrate solution in greenhouse cultivation. So the model needs adjustment for the contribution of nutrients to the EC. Furthermore, effects of EC variations in time and space have been described.Fruit vegetables and cut flowers were used as test crops in experiments with different EC values in the root environment. Comparisons were made between EC effects caused by NaCl and by nutrients. Yield of tomato, cucumber, and sweet pepper were reduced at increasing EC, but most fruit quality characteristics were favourably affected. Blossom-end rot, however, increased with increasing EC. For sweet pepper this was especially the case after NaCl addition. Salinity threshold values for the vegetable crops varied between 2.3 and 3.5 dS m -1 and relative salinity yield decrease values between 2.3 and 7.6 % per dS m -1 . The flower weigths of gerbera, carnation, rose, aster, bouvardia and lily were negatively affected by increasing EC. Salinity threshold values ranged from 1.1 to 4.3 dS m -1 and salinity yield decrease values varied between 2.1 and 16.8% per dS m -1 . For aster such parameters could not be obtained, because the highest EC of 4.2 dS m -1 did not affect production. However, the regrowth of this crop after the first harvest was specifically strongly hindered by NaCl. Bouvardia also exhibited a specific sensitivity to NaCl. This effect was studied in more detail to obtain information about which ion, either Na or Cl, was responsible for this effect. The results showed that bouvardia was specifically sensitive to Na.The response of tomato and cucumber to an unequal distribution of nutrients and NaCl in the root environment was studied with plants grown in a split-root system. Tomato yield was determined by the EC value considered optimal for production if present in one of the rockwool cubes, despite the fact that the EC in the other cube was up to 10 dS m -1 . Tomato absorbed water preferably from the root part with the lowest EC and nutrients from the root part with the highest EC. When the EC in the root parts was raised by nutrients from low to standard values, the nutrient uptake by cucumber was highest from the parts with the highest concentration. In root parts with concentrations of nutrients > 4 dS m -1 the uptake decreased strongly. Nutrient uptake from one root part with high NaCl was also reduced when the NaCl concentration in the other part was low. When both root parts had high NaCl concentrations the plant was able to take up adequate amounts of nutrients. Like tomato, cucumber absorbed water preferably from the root part with the lowest EC. In case no nutrients were supplied in one root part, the water uptake from that root part was reduced.Interactions between salinity effects and climatic conditions and effects of temporal variation of salinity were studied with tomato as the test crop. High EC under low light conditions did not affect yields. In spring and summer yield reductions between 5 and 7 % per dS m -1 were found. In one experiment at very high humidity the yield reduction was about 10 % per dS m -1 . This was in contradiction with the nature of the interaction between salinity and climate in other studies. Obviously the calcium status of the plant had played a dominant role in this experiment. From the experiments with temporal variation of EC it could be concluded that for estimation of the yield reduction not only the lengths of the EC-intervals and the EC-level during the interval but also the light intensity during the interval has to be taken into account.The management of salinity in relation to nutrient supply was discussed. Nutrient absorption of greenhouse crops was studied by determining the total nutrient uptake and the nutrient uptake in relation to the water absorption (the so-called uptake concentrations). The widely published very low external concentrations to achieve optimal yields, are not realistic because of the high flow rate necessary to adequately supply crops with nutrients. External nutrient concentrations corresponding with 1.5 dS m -1 are required for sufficient nutrient supply to greenhouse crops.Required and acceptable external concentrations were defined considering the following aspects. Required external concentrations should not exclusively be related to a sufficient supply of nutrients in order to attain maximum growth or yield, but also to quality demands of the market. Acceptable concentrations should be considered with respect to maximum accumulation of residual ions to a level that does not negatively affect crop production and quality. In this way leaching and thus environmental pollution is minimized.In the assessment of required and acceptable concentrations osmotic and specific ion effects should be clearly distinguished. When no specific ion effects occur, the "space" between the nutrient concentration required for maximal production and the required concentration with respect to the produce quality or the acceptable concentration with respect to maximum salt accumulation can be filled up with any ion available in the system. With specific ion sensitivities the accumulation is restricted by the critical non toxic level to the crop.Required and acceptable concentrations of ions strongly depend on crop and growing conditions. Under cool and humid growing conditions, use of drip irrigation, and CO 2 supply, EC-values in the substrate solution between 3 and 6 dS m -1 seem to be realistic. Such conditions can be realised in greenhouses in North-West Europe from autumn until early spring. For summer conditions the EC-values suggested in this study between 1.5 and 3.0 dS m -1 are more realistic. In the interpretation of EC-values more credit should be given to the consequences of spatial distribution of ions in the substrate. The stable equilibrium established between low and high concentrated spots in the systems, offers excellent possibilities for an osmotic escape by plants. The discussion is concluded with some calculations of environmental pollution as a consequence of different management strategies of irrigation and drain-off.

Abstract: Iron toxicity is a widespread nutrient disorder of lowland rice in West Africa. Soluble iron present in the soil solution under waterlogged conditions is absorbed by roots and accumulates in leaves. It causes poor growth and tillering and severe yield reductions, associated with leaf discoloration. Field experiments were conducted during 1994–1996 at an iron‐toxic lowland site at Korhogo (Ivory Coast) to study the interactions between iron toxicity and the addition of various plant nutrients. Nine nutrient element treatments (combinations of N, P, K and Zn, including no fertilizer) were tested on one iron‐susceptible (Bouake 189) and one iron‐tolerant (CK 4) cultivar. The application of P, K and Zn with N reduced iron toxicity symptoms and increased yield in both cultivars. Strong correlations were observed between grain yield and scored leaf iron toxicity symptoms across seasons and treatments. The iron‐tolerant rice cultivar absorbed less iron or transported less from roots to leaves, indicatin...

Abstract: 1. Despite the widespread release of effective biocontrol agents, water hyacinth remains the world's most problematic aquatic weed, particularly in eutrophic waterbodies. However, understanding of the interaction between control and trophic status is still incomplete. 2. Growth of water hyacinth plants was measured at two water nutrient concentrations (high and medium) and in the presence and absence of two insect biocontrol agents in a large circulating hydroponic system in a glasshouse. 3. At the high nutrient concentration (1.6mg l N and 1.0 mg l P), plants multiplied more quickly, attaining greater biomass. Both insect species reduced plant growth at both nutrient concentrations. Neochetina bruchi, however, performed better than N. eichhorniae at the high nutrient concentration by inflicting more damage on the plants and reducing biomass by a greater extent. 4. Insect damage reduced the concentrations of nitrogen and phosphorus found in plants growing in high nutrient water but not in medium nutrient water (0.4 mg l N and 0.025 mg l P). 5. The developmental and reproductive performance of N. bruchi was determined at both nutrient concentrations. Water hyacinth plants grown at higher nutrient concentration were superior hosts to N. bruchi than plants grown at medium concentrations. Net reproductive rate and intrinsic rate of increase were significantly greater at the high concentrations. Greater damage by N. bruchi to water hyacinth at the high nutrient concentration was due to the greater production of offspring, and hence greater larval damage. 6. We predict that water hyacinth problems will be greater in eutrophic waterbodies, where N. bruchi will be a superior biocontrol agent to N. eichhorniae. In low-nutrient waterbodies, local nutrient enrichment of water may assist the establishment of control agents. These results illustrate the importance of wider ecological factors on the success of biological control.

Abstract: An experiment was conducted in an 11-year-old black walnut (Juglans nigra L.), red oak (Quercus rubra L.), maize (Zea mays L.) alley cropping system in the midwestern USA to examine the extent of tree-crop competition for nitrogen and decomposition dynamics of tree leaves and fine roots. A below-ground polyethylene root barrier (1.2 m deep) isolated black walnut roots from maize alleys in half the number of plots providing two treatments viz. ‘barrier’ and ‘no barrier’. The percentage of N derived from fertilizer (%NDF) and fertilizer use efficiency (%UFN) were determined using 15N enriched fertilizer. Further, maize grain and stover biomass, tree leaf biomass, tissue N concentration, and N content were quantified in both treatments. The ‘barrier’ treatment resulted in a significantly greater grain (67.3% more) and stover (37.2% more) biomass than the ‘no barrier’ treatment. The %NDF in both grain and stover was higher in the ‘no barrier’ treatment as a result of competition from tree roots for water and mineralized N in soil. Maize plants growing in the ‘no barrier’ treatment had a lower %UFN than those in the ‘barrier’ treatment due to their smaller size and inability to take up fertilizer. Analysis of tree leaf and fine root decomposition patterns revealed faster release of N (39% over 15 days for black walnut and 17.7% for red oak) and P (30% over 15 days for both species) from roots compared to the leaves of both species. Following an early release of P (11.3% over 45 days), red oak leaves exhibited significant immobilization for the rest of the incubation period. The data indicate that competition for N from fertilizer is minimal since nutrient acquisition is not simultaneous among black walnut and maize. However, competition for mineralized N in soil can exist between black walnut and maize depending on water availability and competition. Tree leaves and fine roots can enhance soil nutrient pools through the addition of soil carbon and nutrients. Tree fine roots seem to play a more significant role in nutrient cycling within the alley cropping system because of their faster release of both N and P as compared to leaves. Selection of tree species and their phenology will impact the magnitude and rate of nutrient cycling.

Abstract: We measured DOM fluxes from the O horizon of Hawaiiansoils that varied in nutrient availability and mineralcontent to examine what regulates the flux ofdissolved organic carbon (DOC), nitrogen (DON) andphosphorus (DOP) from the surface layer of tropicalsoils. We examined DOM fluxes in a laboratory study from N, P and N+Pfertilized and unfertilized sites on soils that rangedin age from 300 to 4 million years old. The fluxesof DOC and DON were generally related to the % Cand % N content of the soils across the sites. Ingeneral, CO2 and DOC fluxes were not correlatedsuggesting that physical desorption, dissolution andsorption reactions primarily control DOM release fromthese surface horizons. The one exception to thispattern was at the oldest site where there was asignificant relationship between DOC and CO2flux. The oldest site also contained the lowestmineral and allophane content of the three sites andthe DOC-respiration correlation indicates arelationship between microbial activity and DOC fluxat this site. N Fertilization increased DON fluxes by50% and decreased DOC:DON ratios in the youngest,most N poor site. In the older, more N rich sites, Nfertilization neither increased DON fluxes nordecreased DOM C:N ratios. Similarly, short termchanges in N availability in laboratory-based soil Nand P fertilization experiments did not affect the DOMC:N ratios of leachate. DOM C:N ratios were similar tosoil organic matter C:N ratios, and changes in DOM C:Nratios with fertilization appeared to have beenmediated through long term effects on SOM C:N ratiosrather than through changes in microbial demand for Cand N. There was no relationship between DON andinorganic N flux during these incubations suggestingthat the organic and inorganic components of N fluxfrom soils are regulated by different factors and thatDON fluxes are not coupled to immediate microbialdemand for N. In contrast to the behavior of DON, thenet flux of dissolved organic phosphorus (DOP) and DOMC:P ratios responded to both long-term P fertilizationand natural variation in reactive P availability. There was lower DOP flux and higher DOM C:P ratiosfrom soils characterized by low P availability andhigh DOP flux and narrow DOM C:P ratios in sites withhigh P availability. DOP fluxes were also closelycorrelated with dissolved inorganic P fluxes. PFertilization increased DOP fluxes by 73% in theyoungest site, 31% in the P rich intermediate agesite and 444% in the old, P poor site indicating thatDOP fluxes closely track P availability in soils.

Abstract: Summary 1 We tested the hypothesis that competitive hierarchies are invariant with respect to changing nutrient supply. 2 The competitive performance of 26 shoreline plant species was determined experimentally as the relative ability to suppress the growth of a common indicator (phytometer) species, Penthorum sedoides. Each species was grown with the phytometer under each of two nutrient treatments created with different concentrations of a modified Hoagland’s solution (n = 5 replicates per species/treatment), for two growing seasons. 3 Although shifts in ranking of relative competitive performance were apparent between nutrient levels, competitive performance under high and low nutrient conditions was significantly correlated in both year 1 (r = 0.65) and year 2 (r = 0.76), when all species were considered. 4 At the broad community scale, the outcome of competitive interactions thus appears to be relatively predictable and independent of the environment, and therefore provides a useful tool for exploring and understanding community pattern. These results cannot address questions related to the outcome of competitive interactions between similar species or the effects of fine-scale pattern.

Abstract: The ability of rhizosphere diazotrophs to remain competitive during increased nitrogen availability in situ was tested in a salt marsh grass stand. Nitrogen (16.3 g m(-2)) or nitrogen (16.3 g m(-2)) and phosphorus (18.0 g m(-2)) were added to plots of short form Spartina alterniflora for either 2 weeks or 8 weeks. The diazotroph assemblage composition was monitored via the polymerase chain reaction using nifH specific primers followed by denaturing gradient gel electrophoresis (DGGE) analysis. DGGE profiles from the short-term experiments (2 and 8 weeks) were compared to profiles from control (no additions) and from long-term (>10 y) nutrient addition plots. Nitrogen fixation activity was assayed in each short-term treatment and control plot using an acetylene reduction technique. The control and nutrient addition DGGE profiles were very similar throughout the short-term experiments. One DGGE band that was prominent in the control plots was not found in the long-term nutrient addition plots. Diazotrophy may provide a competitive advantage for some species in this system, as indicated by results from the long-term nutrient amended plots. However, the rhizosphere environment in situ appears to limit the immediate impacts of increased nutrient availability on the diazotroph assemblage composition. Results from the short-term nutrient amended plots showed no measurable effect on the diazotroph assemblage. These results indicate substantial short-term stability of the diazotroph assemblage composition, but the potential for change in the face of long-term changes in nutrient availability.