Fertilizer nitrogen budget of Na15NO3 and (15NH4)2SO4 split-applied to winter wheat in microplots on a loam soil

Labelled fertilizer N applied to winter wheat as Na¹⁵NO₃ and (¹⁵NH₄)₂SO₄ at a total N dressing of 100kg ha⁻¹ was used in a microplot balance study to investigate the fate of each split fraction at three growth stages: end of tillering, heading and beginning of flowering. Results indicated that while the percentage utilization of the applied N by the grain and total crop increased considerably from the first to the third split application, these values diminished steadily in the straw. Grain recovery values for the first, second and third split applications were 34.2%, 51.5% and 55.7% for the NO₃ and 32.3%, 48.4% and 52.5% for the NH₄ carrier, respectively. The corresponding recovery values for the whole plant were 54.6%, 67.8% and 69.9% for the NO₃ and 51.7%, 63.5% and 66.1% for the NH₄ carrier. A greater proportion of the fertilizer N applied at the end of tillering stage was found in the vegetative plant components as compared with the grain. The reverse occurred for the N applied at the heading and at the beginning of the flowering stages. The residual fertilizer N found in the soil amounted to 18.0%, 10.4% and 11.6% of the applied NO₃−N and to 22.5%, 12.7% and 15.2% of the applied NH₄−N for the respective split applications. No differences were found for each split application between the two carriers as far as the unaccounted fertilizer N was concerned. The losses were 26.6%, 22.3% and 18.6% of the applied N for the three split applications, respectively. The application of fertilizer N did not lead to any increase in soil N uptake by the crop.     

Accumulation and reduction of nitrate in cereal plants dependent on N supply

Summary In pot experiments the NO ₃ ^– accumulation and the occurrence of nitrate reductase (NR) capacity of wheat plants were investigated depending on late N applications at tillering, shooting and heading. NO ₃ ^– is preferentially accumulated in the stems, while NR dominates in the leaves. NO ₃ ^– accumulation is enhanced by late N treatments especially if N supply at seeding is sufficient. NR capacity of the plants is stimulated by late nitrogen supply, but its increment rates decrease with increasing NO ₃ ^– accumulation.     

Effects of parathion and malathion on transformations of urea and ammonium sulfate nitrogen in soils

Summary A study of the effects of malathion and parathion applied at 10 and 50 g/g of soil on transformations of urea and (NH₄)₂SO₄–N in a sandy loam showed that the insecticides retarded urea hydrolysis as well as nitrification of urea and (NH₄)₂SO₄–N. At 50 parts/10⁶ rate of the insecticides, inhibition of urea hydrolysis ranged from 44 to 61% after 0.5 week and from 7 to 21% after 3 weeks of application. The insecticides inhibited the conversion of NH₄ ⁺ to NO₂ ^– without appreciably affecting the subsequent oxidation of NO₂ ^– to NO₃ ^– –N. This resulted in accumulation of higher amounts of NH₄ ⁺–N in soil samples treated with ammonium sulfate or urea N.The results suggest that transformations of urea and NH₄ ⁺ fertilizers in soils may be influenced by the amount of organophosphorus insecticide present and this may affect plant nutrition and fertilizer use.     

The role of ammonia volatilization in controlling the natural 15N abundance of a grazed grassland

Although the variation in natural ¹⁵N abundance in plants and soils is well characterized, mechanisms controlling N isotopic composition of organic matter are still poorly understood. The primary goal of this study was to examine the role of NH₃ volatilization from ungulate urine patches in determining ¹⁵N abundance in grassland plants and soil in Yellowstone National Park. We additionally used isotopic measurements to explore the pathways that plants in urine patches take up N. Plant, soil, and volatilized NH₃¹⁵N were measured on grassland plots for 10 days following the addition of simulated urine. Simulated urine increased ¹⁵N of roots and soil and reduced ¹⁵N of shoots. Soil enrichment was due to the volatilization of isotopically light NH₃. Acid-trapped NH₃¹⁵N increased from –28 (day 1) to –0.3 (day 10), and was lighter than the original urea-N added (1.2). A mass balance analysis of urea-derived N assimilated by plants indicated that most of the N taken up by plants was in the form of ammonium through roots. However, isotope data also showed that shoots directly absorbed ¹⁵N – depleted NH₃-N that was volatilized from simulated urine patches. These results indicate that NH₃ volatilization from urine patches enriches grassland soil with ¹⁵N and shoots are a sink for volatilized NH₃, which likely leads to accelerated cycling of excreted N back to herbivores.     

Fertilizer nitrogen budgets of two doses of Na15NO3 dressings split-applied to winter wheat in microplots on a loam soil

In a field experiment performed in microplots, winter wheat was fertilized at two different total N dressings (135 and 180 kg ha^–1) split-applied as Na¹⁵NO₃ in three equal applications at tillering, stem elongation, and flag leaf.No significant differences were found in the percentage recovery values for the entire plant at the three split applications between the two N dressings. The total percentage recovery of fertilizer N by the plant was high and practically equal at both fertilization levels (76.65% and 75.84% for 135 and 180 kg N ha^–1, respectively); crop yields were also similar. In contrast, gaseous losses calculated after drawing up the balance sheet were, in absolute values, higher for the tillering and stem elongation split applications when using the 180 kg N ha^–1 dressing (7.67 and 4.84 kg N ha^–1, respectively) than for the 135 kg N ha^–1 dressing (3.45 and 1.26 kg N ha^–1, respectively). They were found to be zero at flag leaf at both fertilization levels. The amount of applied fertilizer N did not influence the amount of N taken up from the soil which was about 143 kg ha^–1.     

Spatial variability of symbiotic N2 fixation in grass-white clover pastures estimated by the 15N isotope dilution method and the natural 15N abundance method

Aiming at estimating the spatial variability in N₂ fixation, and to evaluate the appropriateness of the ¹⁵N isotope dilution (ID) method and the natural¹⁵N abundance (NA) method in reflecting spatial variability under the influence of cattle grazing, the symbiotic N₂ fixation in grass–white clover mixture was studied. At the Foulum site, where the ID method was used, differences in the climatic conditions between the two years of investigations caused a considerable difference in plant growth rates and proportion of clover. Consequently, the total N₂ fixation in ungrazed reference plots was significantly less in 1998 than in 1997, being 5.9 and 12.5 g N m^–2, respectively. In both years there was a wide range in concentration of inorganic N in the soil with coefficients of variance of approximately 60–190% for ammonium and 70–340% for nitrate. Significant negative correlations between pNdfa, determined by the ID method, and the log-transformed values of inorganic N and total N in grass were found. The NA method was applied on three nearby commercial dairy farms. They also showed high coefficients of variation. The coefficient of variance for NO₃ ^–-N ranged from 37 to 282% and for NH₄ ⁺-N from 29 to 237%. Average estimates of pNdfa values, which in the NA method were calculated using apparent B values ranging from –2.10 to –2.59, were generally lower (0.7–0.87) for these farms than for the Foulum site (0.89–0.95) using the ID method. For the NA method the ¹⁵N values, i.e. deviation in ¹⁵N concentration from atmospheric N₂, ranged from –7.0 to 5.7 for the grass N, which in several cases was lower than for clover N. Due to this high variability of the ¹⁵N values, probably caused by deposition and plant assimilation of ¹⁵N depleted urinary N in the pastures, the NA method was marginal for accurate determination of pNdfa. Consequently no significant correlation between the pNdfa determined by this method, and the log-transformed values of inorganic N in soil or total N in grass were found.     

Natural 15N abundances of maize and soil amended with urea and composted pig manure

To investigate the effect of inorganic fertilizer and composted manure amendments on the N isotope composition (delta ¹⁵N) of crop and soil, maize (Zea mays L.) was cultivated under greenhouse conditions for 30, 40, 50, 60, and 70 days. Composted pig manure (delta ¹⁵N= +13.9) and urea (-2.3) were applied at 0 and 0 kg N ha^–1 (C0U0), 0 and 150 kg N ha^–1 (C0U2), 150 and 0 kg N ha^–1 (C2U0), and 75 and 75 kg N ha^–1 (C1U1), respectively. The delta ¹⁵N of total soil-N was not affected by both amendments, but delta ¹⁵N of NH⁺ ₄ and NO^– ₃ provided some information on the N isotope fractionation in soil. During the early growth stage, significant differences (P < 0.05) in delta ¹⁵N among maize subjected to different treatments were observed. After 30 days of growth, the delta ¹⁵N values of maize were +6.6 for C0U0, +1.1 for C0U2, +7.7 for C2U0, and +4.5 for C1U1. However, effects of urea and composted manure application on maize delta ¹⁵N progressively decreased with increasing growth period, probably due to isotope fractionation accompanying N losses and increased uptake of soil-derived N by maize. After 70 days of growth, delta ¹⁵N of leaves and grains of maize amended with composted pig manure were significantly (P < 0.05) higher than those with urea. The temporal variations in delta ¹⁵N of maize amended with urea and composted manure indicate that plant delta ¹⁵N is generally not a good tracer for N sources applied to field. Our data can be used in validation of delta ¹⁵N fractionation models in relation to N source inputs.     

The fate of15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

The stable isotope¹⁵N was added as (¹⁵NH₄)₂SO₄ to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two N saturated coniferous forests ecosystems in the Netherlands. The fate of the¹⁵N was followed at high-N (44–55 kg N ha^–1 yr^–1) 1) and low-N (4–6 kg N ha^–1 yr^–1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest.The applied¹⁵N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total¹⁵N recovery in major ecosystem compartments was 71–100% during two successive growing seasons after the start of a year-round¹⁵N application to throughfall-N. Nine months after the year-round¹⁵N application, the¹⁵N assimilated into tree biomass was 29–33% of the¹⁵N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total¹⁵N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited¹⁵NH₄-N, whereas 46% and 65% of the¹⁵N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the¹⁵N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total¹⁵N leaching losses within a year (below 90 cm) were 10–20% in the high-N deposition plots in comparison to 2–6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels.Species differences in uptake and tree health seem to contribute to lower¹⁵N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting N saturation lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the¹⁵N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.     

Fate of 15N labelled urine on four soil types

A field lysimeter experiment was conducted over a 406 day period to determine the effect of different soil types on the fate of synthetic urinary nitrogen (N). Soil types included a sandy loam, silty loam, clay and peat. Synthetic urine was applied at 1000 kg N ha^-1, during a winter season, to intact soil cores in lysimeters. Leaching losses, nitrous oxide (N₂O) emissions, and plant uptake of N were monitored, with soil ¹⁵N content determined upon destructive sampling of the lysimeters. Plant uptake of urine-N ranged from 21.6 to 31.4%. Soil type influenced timing and form of inorganic-N leaching. Macropore flow occurred in the structured silt and clay soils resulting in the leaching of urea. Ammonium (NH ₄ ⁺ –N), nitrite (NO ₂ ^- –N) and nitrate (NO₃ ^-–N) all occurred in the leachates with maximum concentrations, varying with soil type and ranging from 2.3–31.4 g NH ₄ ⁺ –N mL^-1, 2.4–35.6 g NO ₂ ^- –N mL^-1, and 62–102 g NO ₃ ^- –N mL^-1, respectively. Leachates from the peat and clay soils contained high concentrations of NO ₂ ^- –N. Gaseous losses of N₂O were low (<2% of N applied) over a 112 day measurement period. An associated experiment showed the ratio of N₂–N:N₂O–N ranged from 6.2 to 33.2. Unrecovered ¹⁵N was presumed to have been lost predominantly as gaseous N₂. It is postulated that the high levels of NO ₂ ^- –N could have contributed to chemodenitrification mechanisms in the peat soil.     

Fate of nitrogen fertilizer applied on two main arables crops,winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris) in the loam region of Belgium

Since 1986, the fate of fertilizer N (NH₄NO₃ or NaNO₃) applied in field conditions on two main arable crops, winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris), has been studied using ¹⁵N. Up to a rate of 200 kg ha^-1 of N, mean recovery of fertilizer by winter wheat was 70%, provided it had been split applied. Single application (with or without dicyandiamid) was less effective. For sugar beet, in 1990, 1991 and 1992, 40% of fertilizer N was found in the crop at harvest when NH₄NO₃ had been broadcast at 100 to 160 kg N ha^-1 at sowing time. For the same N rate, recovery was 50% when row applied near the seeds and 60% for 80 kg N ha^-1. For the two experimental crops, residual fertilizer N in soil was exclusively organic. It ranged from 15 to 30% of applied N and was located in the 30 cm upper layer. Losses were generally lower with winter wheat (12%) than with sugar beet (20–40%) and could be ascribed to volatilization and denitrification. Soil derived N taken up by the plant was site and year dependent.     

Influence of nitrogen on the behaviour of nickel in wheat

A glasshouse experiment was conducted to study the effect of Ni on the growth and nutrients concentration in wheat (Triticum aestivum Cv. WH 291) in the presence and absence of applied N as urea. Responses to N application were observed up to 120 g N g^–1 soil. No response to Ni was observed in the dry matter yield of wheat tops (leaves + stem) in the absence of applied N while in the presence of applied N, significant yield increases were obtained at 12.5g Ni g^–1 soil. Nickel was not toxic to wheat up to 50g Ni g^–1 soil in the presence of 120g N g^–1 soil. Nitrogen and Ni concentration in wheat tops and roots increased with increasing levels of applied N and Ni, respectively. Applied Ni had an antagonistic effect on N concentration. Similarly, N reduced the Ni concentration in the wheat tissues. Positive growth responses to Ni were associated with 22 and 15g Ni g^–1 in wheat tops, in the presence of applied N at 60 and 120g N g^–1 soil, while Ni toxicity was associated with 63, 92.5 and 112.5g Ni g^–1 in wheat tops, in the absence and presence of applied N at 60 and 120g N g^–1 soil, respectively.     

Growth and ion uptake by wheat supplied nitrogen as nitrate,or ammonium,or both

Summary The effects of concentration and source (NH₄, NO₃, and NO₃ plus NH₄) of added N on the rate of growth, final yield, and content and rate of intake of N, P, K, Ca, Mg and S by wheat seedlings were evaluated. Rate of growth in dilute liquid cultures differed among the N sources giving yields relative to those of the all-NO₃ system of 92 per cent for the all-NH₄ system, and of 154 per cent for the NO₃ plus NH₄ system. At low rates of NH₄ intake in the all-NH₄ systems growth rates were equal to or slightly better than those of plants supplied equivalent concentrations of NO₃. Rates of NH₄ intake exceeding 100 mole g^–1 h^–1 resulted in reduced growth rates and incipient NH₄ toxicity. Yields at 95 per cent of maximum resulted with steady-state N concentrations of 80 M in all NO₃ systems, 30 M NH₄ in all-NH₄ systems, and in combined source systems when 200M NO₃ plus 30 M NH₄ were supplied. The rate of N intake and plant protein content, were maximal when both NO₃ and NH₄ were supplied. Increasing rates of NO₃ intake were associated with increases in the rates of Ca, Mg, and K intake; but with increasing rates of NH₄ absorption, intake of Ca and Mg decreased. The yield and growth rate enhancement observed from the addition of low concentrations of NH₄ to cultures supplying adequate NO₃ is suggested to result from the reduced energy requirement for utilization of NH₄, as compared to NO₃ in protein synthesis and from the increased photosynthetic capacity of the higher-protein NH₄-fed plants. In the all-NH₄ systems the maximum attainable growth rate was limited by NH₄ toxicity; whereas in the all-NO₃ systems the rate of NO₃ reduction was limiting.Contribution from the Department of Soils and Plant Nutrition, University of California, Davis, California 95616.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

Denitrification measured by a direct N2 flux method in sediments of Waquoit Bay,MA

Denitrification was directly estimated in estuarine sediments of Waquoit Bay, Cape Cod, MA by detection of N₂ increases above ambient in the water overlying sediment cores. Denitrification rates (–9 to 712 mol N₂ m^–2 h^–1 ) were high compared to previous studies, but compared well with estimates of N loss from mass balance studies. The precision of the estimate depended on the N₂/0₂ flux ratio. The N₂/0₂ flux ratio was lower in Waquoit Bay than previously studied estuaries, and estuaries had lower N₂/0₂ flux ratios than shelf sites. The contribution of temperature-driven solubility changes to estuarine fluxes was estimated by modeling sediment temperature variations and found to be potentially important (43 mol N₂ m^–2 h^–1); however, control incubations indicate the temperature model overestimates solubility driven fluxes. The relatively low fluxes under anaerobic conditions and the low rate of N0₃ ^–/N0₂ ^– removal from the overlying water indicates coupled nitrification/denitrification produced the observed N₂ fluxes.     

Effects of atmospheric CO2 enrichment, water status and applied nitrogen on water- and nitrogen-use efficiencies of wheat

Atmospheric CO₂ levels are expected to exceed 700 mol mol^–1 by the end of the 21st century. The influence of increased CO₂ concentration on crop plants is of major concern. This study investigated water- and nitrogen-use efficiency (WUE and NUE, respectively, were defined by the amount of biomass accumulated per unit water or N uptake) of spring wheat (Triticum aestivumL.) grown under two atmospheric CO₂ concentrations (350 and 700 mol mol^–1), two soil moisture treatments (well-watered and drought) and five nitrogen amendment treatments. Results showed that enriched CO₂ concentration increased canopy WUE, and more N supply led to higher WUE under the increased CO₂. Canopy WUE was significantly lower in well-watered treatments than in drought treatment, but increased with the increased N supply. Elevated CO₂ reduced the apparent recovery fraction of applied N by the plant root system (N_r, defined as the ratio of the increased N uptake to N applied), but increased the NUE and agronomic N efficiency (NAE, defined as the ratio of the increased biomass to N applied). Water limitation and high N application reduced the N_r, NUE and NAE, indicating a poor N efficiency. In addition, there was a close relationship between the root mass ratio and NUE. Canopy WUE was negatively related to the root mass ratio and NUE. Our results indicated that CO₂ enrichment enhanced WUE more at high N application, but increased NUE more when N application was less.     

Biosphere-atmosphere interactions of ammonia with grasslands: Experimental strategy and results from a new European initiative

A new study to address the biosphere-atmosphere exchange of ammonia (NH₃) with grasslands is applying a European transect to interpret NH₃ fluxes in relation to atmospheric conditions, grassland management and soil chemistry. Micrometeorological measurements using the aerodynamic gradient method (AGM) with continuous NH₃ detectors are supported by bioassays of the NH₃ `stomatal compensation point' (<sub>s</sub>). Relaxed eddy accumulation (REA) is also applied to enable flux measurements at one height; this is relevant to help address flux divergence due to gas-particle inter-conversion or the presence of local sources in a landscape.Continuous measurements that contrast intensively managed grasslands with semi-natural grasslands allow a scaling up from 15 min values to seasonal means. The measurements demonstrate the bi-directional nature of NH<sub>3</sub> fluxes, with typically daytime emission and small nocturnal deposition. They confirm the existence of enhanced NH<sub>3</sub> emissions (e.g. 30 g N ha<sup>–1</sup> d<sup>–1</sup>) following cutting of intensively managed swards. Further increased emissions follow fertilization with NH<sub>4</sub>NO<sub>3</sub> (typically 70 g N ha<sup>–1</sup> d<sup>–1</sup>). Measurements using REA support these patterns, but require a greater analytical precision than with the AGM.The results are being used to develop models of NH<sub>3</sub> exchange. `Canopy compensation point' resistance models reproduce bi-directional diurnal patterns, but currently lack a mechanistic basis to predict changes in relation to grassland phenology. An advance proposal here is the coupling of _s to dynamic models of grassland C–N cycling, and a relationship with modelled plant substrate-N is shown. Applications of the work include incorporation of the resistance models in NH₃ dispersion modelling and assessment of global change scenarios.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

15N natural abundance studies in Australian commercial sugarcane

The measurement of natural ¹⁵N abundance is a well-established technique for the identification and quantification of biological N₂ fixation in plants. Associative N₂ fixing bacteria have been isolated from sugarcane and reported to contribute potentially significant amounts of N to plant growth and development. It has not been established whether Australian commercial sugarcane receives significant input from biological N₂ fixation, even though high populations of N₂ fixing bacteria have been isolated from Australian commercial sugarcane fields and plants. In this study, ¹⁵N measurements were used as a primary measure to identify whether Australian commercial sugarcane was obtaining significant inputs of N via biological N₂ fixation. Quantification of N input, via biological N₂ fixation, was not possible since suitable non-N₂ fixing reference plants were not present in commercial cane fields. The survey of Australian commercially grown sugarcane crops showed the majority had positive leaf ¹⁵N values (73% >3.00, 63% of which were >5.00), which was not indicative of biological N₂ fixation being the major source of N for these crops. However, a small number of sites had low or negative leaf ¹⁵N values. These crops had received high N fertiliser applications in the weeks prior to sampling. Two possible pathways that could result in low ¹⁵N values for sugarcane leaves (other than N₂ fixation) are proposed; high external N concentrations and foliar uptake of volatilised NH₃. The leaf ¹⁵N value of sugarcane grown in aerated solution culture was shown to decrease by approximately 5 with increasing external N concentration (0.5–8.0 mM), with both NO₃ ^– and NH₄ ⁺ nitrogen forms. Foliar uptake of atmospheric NH₃ has been shown to result in depleted leaf ¹⁵N values in many plant species. Acid traps collected atmospheric N with negative ¹⁵N value (–24.45±0.90) from above a field recently surface fertilised with urea. The ¹⁵N of leaves of sugarcane plants either growing directly in the soil or isolated from soil in pots dropped by 3.00 in the same field after the fertiliser application. Both the high concentration of external N in the root zone (following the application of N-fertilisers) and/or subsequent foliar uptake of volatilised NH₃ could have caused the depleted leaf ¹⁵N values measured in the sugarcane crops at these sites.     

Metabolism measurements ofAurelia aurita planulae larvae,and calculation of maximal survival period of the free swimming stage

Respiration, ammonia and phosphate excretion experiments were performed with planula larvae ofAurelia aurita (Scyphozoa) from Kiel Fjord, Baltic Sea, in summer 1983. The mean respiration measured was 3.22 nl O₂ ind^–1 h^–1 (at 20 °C). Excretion experiments revealed average values of 11.41 pM NH₄-N ind^–1, and 0.92 pM PO₄-P ind^–1h^–1, respectively. The atomic C:N:P ratio of excretion products was 133:10:1. The O:N ratio of 25:1 and O:P ratio of 313:1 point to a lipid-carbohydrate-oriented catabolism of theAurelia larvae. On the basis of experimental results and of biomass determinations, the maximal survival period of the non-feeding free swimming planula stage was calculated. Typically, the value lies in the range of some days to one week.