Responses of alfalfa and Barley to foliar application of N and P on a coal mine spoil

Summary To ensure adequate growth of plants on the highly impoverished and erodable surface mined lands, the application of N and P fertilizers by suitable methods is essential. In the present study, five growth chamber experiments were conducted to evaluate the relative efficacy of foliar and spoil application of N and P using alfalfa (Medicago sativa L. var. Erand) and barley (Hordeum vulgare L. var. Manker) as test crops on a freshly exposed coal mine spoil collected from western North Dakota. In general, barley responded to both N and P, but alfalfa mainly to P. Growth responses of barley to foliar or spoil-applied N+P were substantial and similar in magnitude. However, the yields were much higher when the plants received 3–4 sprays of 1.5–2.2% urea, with P supplied through the spoil. Increasing the number of 2.2% urea sprays from 1 to 3 increased the growth response from 40 to 243%. In another study, increasing the concentration of foliar-applied urea from 0 through 1% resulted in further increases in the dry weights of barley at all the levels of spoil-applied (0, 25, 75, 225 g/g) N.Foliar sprays of 0.5–1.0% NaH₂PO₄ increased the dry weights of alfalfa and barley by an average of 366% and 86%, respectively. However, the yield response of alfalfa to spoil-applied P (100 g/g) was as high as 782% compared to only 117% for barley. Alfalfa responded significantly to increasing concentrations of H₃PO₄ (0–0.3%) in foliar sprays only in the absence of spoil-applied P. With increasing rates of spoil-applied P, alfalfa yields increased steadily, but additional supply of P sprays caused leaf burning which intensified as the P concentration in sprays increased.The results of chemical analyses indicated that foliar applications were more effective than soil applications in increasing the concentration of N or P in the plants. Moreover, urea sprays increased the uptake of K, Zn, and Fe in barley, whereas spraying alfalfa with P compounds caused increases in its K and Fe content and decreases in those of Zn and Na. The results of these experiments indicated that the nutritional requirements of plants grown on coal mine spoils can be met through foliar fertilization as effectively as, or better than, through conventional soil fertilization methods.Presented at the Annual Meeting, American Society of Agronomy, Chicago, Illinois, Dec. 3–8, 1978.     

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine,USA

We followed the movements of ¹⁵N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha^–1 year^–1 using ¹⁵N-labelled nitric acid solutions (¹⁵N = 344 ) during four successive growing seasons (April–October). The ¹⁵N contents of foliage, bolewood, forests floor and mineral soil (0–5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.     

Inorganic nitrogen source utilization byFagus crenata on different soil types

The nitrogen source utilization by Fagus crenata distributed on soils with different forms of inorganic nitrogen in a cool-temperate deciduous forest in central Japan was determined by measuring foliar ¹⁵N. Two soil habitat types along a slope were delineated based on nitrogen transformation patterns, i.e., soils with high net nitrification rates and with no or low net nitrification, respectively. Despite differences in soil types, the study species, F. crenata, was distributed along the entire slope. The foliar ¹⁵N value of F. crenata from the lower slope area was significantly lower than that from the upper slope. Given the finding of a previous study that the ¹⁵N of NO₃^– was lower than that of NH₄⁺, our results indicate that reliance on NO₃^– as a nitrogen source was greater in the lower slope area than in the upper slope area. Differences in the values of foliar ¹⁵N were about 1, which is far less than the 10 ¹⁵N value of soil inorganic N reported in the previous study. This discrepancy might suggest that the study species utilized NO₃^– even in the upper site where net nitrification had not been detected. Measurements of nitrate reductase activity, an index of NO₃^– uptake, also supported this interpretation. Nitrate reduction occurred in leaves and roots at both the lower and the upper sites. Thus, the study species may be able to use NO₃^– even in soils with no net nitrification, a factor that could allow the distribution of F. crenata along the entire length of the slope.     

Effect of nitrogen on growth and water relations of radiata pine families

The effect of nitrogen fertilisation on growth, foliar nutrients and water relations of four families of radiata pine (Pinus radiata, D. Don) currently in the Australian breeding program was examined from age six to 11 years. At this stage, the stand was ready for commercial thinning. The annual rainfall at the site varied from 563 to 733 mm.Application of nitrogen fertiliser resulted in stem wood volume at age nine years of 178 m³ ha^-1 in the controls, compared with 228 m³ ha^-1 in plots treated with 600 kg N ha^-1. Pre-dawn needle water potential () measured in three consecutive summers (when rainfall ranged from 53 to 106 mm) were consistently higher (less water stress) in nitrogen fertilised than in control trees. Similarly, the water stress integral (S) decreased consistently with increasing levels of nitrogen, although total water use in fertilised trees would have been substantially higher because fertiliser application increased the leaf area index. The relationship between S and basal area was strong and paralleled that of foliar nitrogen concentration and basal area growth. Therefore, nitrogen application increased growth rates of trees by improving the nutrient status of trees and lowering the water stress on trees in summer.Families showed markedly different responses of basal area growth to nitrogen, ranging from an increase of 9.4% over three years for the least responsive family to 99.0% for the most responsive. There was no nitrogen × family interaction on or S suggesting that the large genetic variation in the growth response to nitrogen is mediated by factors other than water relations. These results have implications for managing highly productive plantations grown in an environment where rainfall is low compared to potential evapotranspiration.     

Nitrogen fixation and metabolism by groundwater-dependent perennial plants in a hyperarid desert

The Central Asian Taklamakan desert is characterized by a hyperarid climate with less than 50 mm annual precipitation but a permanent shallow groundwater table. The perched groundwater (2–16 m) could present a reliable and constant source of nitrogen throughout the growing season and help overcome temporal nitrogen limitations that are common in arid environments. We investigated the importance of groundwater and nitrogen fixation in the nitrogen metabolism of desert plants by assessing the possible forms and availability of soil N and atmospheric N and the seasonal variation in concentration as well as isotopic composition of plant N. Water availability was experimentally modified in the desert foreland through simulated flooding to estimate the contribution of surface water and temporally increased soil moisture for nutrient uptake and plant–water relations. The natural vegetation of the Taklamakan desert is dominated by plants with high foliar nitrogen concentrations (2–3% DM) and leaf nitrate reductase activity (NRA) (0.2–1 mol NO₂^– g^–1 FW h^–1). There is little evidence that nitrogen is a limiting resource as all perennial plants exhibited fast rates of growth. The extremely dry soil conditions preclude all but minor contributions of soil N to total plant N so that groundwater is suggested as the dominant source of N with concentrations of 100 M NO₃^–. Flood irrigation had little beneficial effect on nitrogen metabolism and growth, further confirming the dependence on groundwater. Nitrogen fixation was determined by the ¹⁵N natural abundance method and was a significant component of the N-requirement of the legume Alhagi, the average contribution of biologically fixed nitrogen in Alhagi was 54.8%. But nitrogen fixing plants had little ecological advantage owing to the more or less constant supply of N available from groundwater. From our data we conclude that the perennial species investigated have adapted to the environmental conditions through development of root systems that access groundwater to satisfy demands for both water and nutrients. This is an ecologically favourable strategy since only groundwater is a predictable and stable resource.     

Soil nitrogen transformations and nitrate utilization by Deschampsia flexuosa (L.) Trin. at two contrasting heathland sites

Two Dutch heathland sites Hoorneboeg (HB) and Ede, dominated by Deschampsia flexuosa and differing in nitrate production, were sampled for an entire growing season. A large number of soil and plant parameters were monitored in an attempt to assess the contribution of nitrate in the N supply and its assimilation by Deschampsia.Average NO₃ ^– and NH₄ ⁺ concentrations (mg kg^–1) in the top 10-cm depth were 0.03 and 2.2, respectively, for HB, and 2.1 and 6.7, respectively, for Ede. Laboratory incubations of intact cores and experiments with FH-layer suspensions showed significantly higher mineralization and nitrification rates for the Ede site during most of the season. Nitrification was largely controlled by the rate of net N-mineralization, which in turn was highly affected by soil moisture. Nitrate production was virtually zero at HB and accounted for 25% of the net N-mineralization at Ede.Shoot chemical composition showed no essential differences for the two sites, but mean in vivo (current) foliar NRA was almost 2-fold higher at Ede than at HB, indicating some utilization of nitrate at the former location. At the HB site with essentially no nitrate production, however, enzyme activities were clearly higher than basal constitutive levels in NH₄ ⁺-fed plants. Apparently, shoot NRA at the HB site became positively affected by factors other than nitrate availability and/or showed disproportional increases in response to atmospheric nitrate inputs. Root NRA displayed the same low basal level at the two sites. Nitrate fertilization (100 kg N ha^–1) yielded maximally induced foliar NRAs similar to levels found in hydroponic nitrate plants. Although no accumulation of free NO₃ ^– was observed in shoots from fertilized plots, increases in foliar concentrations of both organic N and carboxylates clearly indicated nitrate assimilation. Root NRA showed no response to nitrate addition.It is concluded that current NRA measurements in Deschampsia at heathland sites are of limited value only, especially when interpreted in isolation. A combined approach, using concurrently conducted soil and plant analyses, will allow the extent of nitrate utilization in the field to be best characterized.Publication 2013 of the Netherlands Institute of Ecology.FAX no corresponding author: +31 8306 23227     

The combined relationship of temperature and molybdenum concentration to nitrogen fixation byAnabaena cylindrica

The joint effects of growth temperature, incubation temperature, and molybdenum concentration on the nitrogen fixation rate ofAnabaena cylindrica were determined using the acetylene-reduction technique. The nitrogen-fixation response to increased molybdenum concentration varied among three growth temperatures (15°, 23°, and 30° C). The pattern of rate change was similar within a growth temperature but increased overall in magnitude with the three incubation temperatures (also 15°, 23°, and 30° C). The maximum rate of nitrogen fixation occurred at 30°C regardless of previous growth temperature. The minimum molybdenum concentration necessary to yield substantial acetylene reduction varied with growth temperature: at 15°C, 15g 1^–1 was effective; at 23°C, less than 5g 1^–1 was effective; and at 30°C, 50g 1^–1 was effective. At all three growth temperatures, increases in molybdenum concentration above the minimum effective concentration produced increases in acetylene reduction. However, at higher molybdenum concentrations inhibition of nitrogen fixation occurred.     

The interaction of nitrogen assimilation with photosynthesis in nitrogen deficient cells of Chlorella

Nitrogen-limited chemostat cultures of Chlorella fusca var. vacuolata, when given nitrogen in the inorganic forms of nitrate, nitrite and ammonium divert photo-generated electrons, from CO₂ fixation to nitrogen assimilation. Addition of nitrate or nitrite, but not ammonium, stimulates rate of oxygen evolution. All but the most severely nitrogen-deficient culture have increased dark respiration rates after addition of inorganic nitrogen. The nitrite reduction step of nitrogen assimilation is the most light-dependent reaction.Abbreviation DCMU 3-(3, 4-dichloro)-1-1-dimethyl urea - CCCP carbonyl cyanide m-chlorophenylhydrazone     

In vitro reconstitution of NADH: nitrate reductase in nitrate reductase-deficient mutants of barley

Summary In vitro complementation of the nitrate reductase-deficient barley mutant nar2a extracts with molybdenum cofactor from commercial xanthine oxidase resulted in reactivation of NADH: nitrate reductase activity. Maximum reactivation was achieved with 7.5 g/ml xanthine oxidase (final concentration), 10 mM glutathione (final concentration) and incubation for 30 min at room temperature (ca. 25°C). This in vitro complementation assay was used to determine the presence of functional apoprotein and molybdenum cofactor in 12 nitrate reductase-deficient barley mutants. Extracts of all nar1 alleles contained functional molybdenum cofactor (complemented with nar2a) but they lacked functional apoprotein (did not complement with molybdenum cofactor from xanthine oxidase). The nar2a, nar3a and nar3b extracts were able to donate functional apoprotein, but were poor sources of functional molybdenum cofactor. These data are in agreement with our previous assignment of nar1 to the barley NADH: nitrate reductase structural locus and nar2 and nar3 to molybdenum cofactor functions. Wild type cv. Steptoe barley seedlings grown in the absence of nitrate and lacking nitrate reductase activity contained low levels of molybdenum cofactor. Nitrate induction resulted in a several-fold increase in the measurable molybdenum cofactor levels that was correlated with the increase in nitrate reductase activity.Scientific Paper No. 6839. College of Agriculture Research Center, Washington State University, Pullman. Project Nos. 0430 and 0233. This work was supported in part by National Science Foundation Grant PCM 81-19096 and USDA Competitive Research Grant 82-CRCR-1-1112     

Phenolics,nutrition and insect herbivory in some garrigue and maquis plant species

Summary Garrigue plant species growing on a calcareous substrate in southern France had higher foliar N levels than the same species growing on a relatively lower nutrient siliceous substrate (maquis). However maquis species had significantly higher foliar levels of P, more water, higher phenolic concentrations and larger leaf areas. The cumulative amount of insect damage on garrigue and maquis plants was similar, presumably due to different nutritional advantages in each case. Soil fertilization signifincantly elevated N levels in Q. coccifera, increased total leaf areas, decreased condensed tannin levels, and these leaves showed significantly more insect damage. Some effects of burning on Q. coccifera are also described. In these shrublands, fertilization may render leaf material more nutritional for herbivores by increasing nitrogen content and decreasing condensed tannin concentration, although very heavy grazing pressure may increase levels of leaf phenolics.     

Isotopic fractionation accompanying fertilizer nitrogen transformations in soil and trees of a Scots spine ecosystem

Foliage from a mature stand of Scots pine (Pinus sylvestris L.) receiving increasing doses of ammonium nitrate and urea nitrogen was assayed during the five subsequent growing seasons for total N concentration and ¹⁵N abundance. The aim of the study was to examine the potential of the ¹⁵N technique to provide estimates on fertilizer N recovery and its fate in the ecosystem. The ¹⁵N abundance in the foliage increased in proportion to the dose of fertilizer application. This was generally owing to the fact that the ¹⁵N of the fertilizer N was significantly higher than that in the soil inorganic-N pool, as well as in the needle biomass of the Scots pine trees on the nonfertilized plots. Due to ¹⁵N isotope discrimination occurring during N transformations in soil the relationship was however not very close. Calculations based on the principle of isotope dilution yielded only rough and, in some cases, even misleading estimates of the fraction of the fertilizer-derived nitrogen (N_dff) in the needles. This was especially the case for the urea-N, which undergoes significant isotopic fractionation during the process of ammonia volatilization and possibly microbial NH₄ ⁺ assimilation in soil. Over five growing seasons, foliar total N concentration peaked at the end of the second season while the ¹⁵N abundance continued to increase. Although large methodological errors may be involved when interpreting natural ¹⁵N abundance, the measurement of ¹⁵N seems to provide semi-quantitative information about fertilizer N accumulation and transformation processes in coniferous ecosystems. A better understanding of the tree and soil processes causing isotopic fractionation is a prerequisite for correct interpretation of ¹⁵N data.     

Interactions between nitrate uptake and N2 fixation in white clover

Summary White clover (Trifolium repens L.) plants grown in pots and supplied with the same concentration x days of¹⁵N labelled nitrate, but in contrasting patterns and doses had similar N concentrations but differed in the proportions devived from N₂ fixation and nitrate. N₂-fixation and nodule dry weight responded rapidly (2–3 days) to changes in nitrate availability. Plants exposed frequently to small doses of nitrate took up more nitrate (and hence relied less on N₂-fixation) and had greater dry weights and shoot: root ratios than those exposed to larger doses less often. In mixed ryegrass (Lolium perenne L.)/clover communities clover's ability to either successfully compete for nitrate or fix N₂ gave it consistently higher N concentrations than grass whether they were given high or low nitrate nutrient. This higher N concentration was accompanied by greater dry weights than grass in the low nitrate swards but not where high levels of nitrate were applied.     

Natural<Superscript>15</Superscript> N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (¹⁵N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of ¹⁵N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of ¹⁵N between soil N pools and plants, and by using an existing model. ¹⁵N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO₃^– pool at two sites with different nitrification rates in a temperature forest in Japan. ¹⁵N of plants was similar to that of soil NO₃^– in the high-nitrification site. Because of high foliar NRA and the large foliar NO₃^– pool at this site, we concluded that plant ¹⁵N indicated a great reliance of plants on soil NO₃^– there. However, many ¹⁵N of soil N overlapped each other at the other site, and ¹⁵N could not provide definitive evidence of the N source. The existing model was verified by measured ¹⁵N of soil inorganic N and it explained the variations of plant ¹⁵N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare ¹⁵N signatures from different ecosystems and to understand N dynamics.     

NITROGEN UPTAKE AND ASSIMILATION KINETICS IN ALEXANDRIUM MINUTUM (DYNOPHYCEAE): EFFECT OF N‐LIMITED GROWTH RATE ON NITRATE AND AMMONIUM INTERACTIONS1

Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)–limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (K_s=0.26±0.03 and 0.31±0.04 μmol·L^?1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (=0.42–0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N‐sufficient cells (I_max～0.9 and K_i=0.31–0.56 μmol·L^?1). In the case of N assimilation, ammonium was also the preferred form in N‐deficient cells (=0.54–0.72), whereas in N‐sufficient cells, both N sources were equally preferred (=0.90–1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest‐growing N‐deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.     

The physiological basis of increased biomass partitioning to roots upon nitrogen deprivation in young clonal tea (Camellia sinensis (L.) O. Kuntz)

Deprivation of nitrogen (N) increases assimilate partitioning towards roots at the expense of that to shoots. This study was done to determine the physiological basis of increased root growth of tea (sCammellia sinensis L.) under N shortage. Nine-month-old clonal tea (clone TRI2025) was grown in quartz sand under naturally lit glasshouse conditions. Three levels of N (0, 3.75 and 7.5 mM N) were incorporated in to the nutrient solution and applied daily. Plant growth, photosynthesis, root respiration and plant N contents were measured at 10-day intervals over a 45-day period. Root dry weight showed a sharp increase during the first 15 days after the plants were transferred to 0 mM N, whereas no such increase was shown in plants transferred to 7.5 mM N. In contrast, shoot dry weight increased at 7.5 mM N and was significantly greater than at 0 mM N, where no increase was observed. Due to the above changes, root weight ratio increased and leaf weight ratio decreased during the first 15 days of N deprivation. Leaf photosynthetic rates did not vary between N levels during the initial 15-day period. Thereafter, photosynthetic rates were greater at 7.5 mM and 3.75 mM N than at 0 mM N. Root respiration rate decreased at 0 mM N, whereas it increased at 3.75 and 7.5 mM N, probably because of the greater respiratory cost for nitrate uptake. Root respiratory costs associated with maintenance (R _m) and nitrate uptake (R _u) were calculated to investigate whether the sharp increase of root growth observed upon nitrogen deprivation was solely due to the reduced respiratory costs for nitrate uptake. The estimated values for R _m and R _u were 3.241 × 10^–4 mol CO₂ g^–1 (root dry matter) s^–1 and 0.64 mol CO₂ (mol N)^–1, respectively. Calculations showed that decreased respiratory costs for nitrate uptake could not solely account for the significant increase of root biomass upon N deprivation. Therefore, it is concluded that a significant shift in assimilate partitioning towards roots occurs immediately following N deprivation in tea.     

Diagnosis of nitrogen deficiency and toxicity of Eucalyptus globulus seedlings by foliar analysis

The relationship between shoot growth and foliar nitrogen (N) in E. globulus seedlings was studied in the glasshouse to determine standard values for N deficiency and toxicity diagnosis. Seedlings were grown for 9 weeks in yellow sand, at 10 rates of N, applied as ammonium sulphate, calcium nitrate or ammonium nitrate. Shoot dry weight (DW) increased linearly with N rate for all forms of N in the deficiency range. Seedlings continued to respond to higher rates of ammonium and ammonium nitrate than to nitrate. Maximum shoot DW for nitrate fed plants and ammonium nitrate fed plants were 51% and 84% respectively of ammonium fed plants. Total N concentration in the youngest fully expanded leaf (YFEL) ranged from 1.0% to 3.3% in deficient and adequate plants. The critical N concentration for deficiency diagnosis (corresponding to 90% maximum yield) in the YFEL, determined from these growth response curves averaged over all N forms, was 2.6% N. For ammonium nitrate fed plants, total N concentration in the YFEL for the severely deficient, deficient, adequate, and toxic ranges were <1.4%, 1.4–2.5%, 2.6–3.5%, > 4.3%. High total N concentrations were associated with growth depression and toxicity symptoms, which differed with N form. For nitrate fed plants, a total N concentration above 3.3% in the YFEL was associated with severe growth depression, and leaf tip necrosis. The adequate concentration range for ammonium nitrate was similar to values found on a field trial with 7 month old E. globulus trees grown on an exforest site.     

Response of coastal phytoplankton to ammonium and nitrate pulses: seasonal variations of nitrogen uptake and regeneration

Seasonal variation in uptake and regeneration of ammonium and nitrate in a coastal lagoon was studied using ¹⁵N incorporation in particulate matter and by measuring changes in particulate nitrogen. Uptake and regeneration rates were two orders of magnitude lower in winter than in summer. Summer uptake values were 2.8 and 2.2 mol N.l^–1.d^–1 for ammonium and nitrate, respectively. Regeneration rates were 2.9 and 2.1 mol N.l^–1.d^–1 for ammonium and nitrate respectively. Regeneration/uptake ratios were often below one, indicating that water column processes were not sufficient to satisfy the phytoplankton nitrogen demand. This implies a role of other sources of nitrogen, such as macrofauna (oysters and epibionts) and sediment. Phytoplankton was well adapted to the seasonal variations in resources, with mixotrophic dinoflagellates dominant in winter, and fast growing diatoms in summer. In winter and spring, ammonium was clearly preferred to nitrate as a nitrogen source, but nitrate was an important nitrogen source in summer because of high nitrification rates. Despite low nutrient levels, the high rates of nitrogen regeneration in summer as well as the simultaneous uptake of nitrate and ammonium allow high phytoplankton growth rates which in turn enable high oyster production.     

Effect of elevated atmospheric carbon dioxide on the use of foliar application of Bacillus thuringiensis

Toxins from Bacillus thuringiensis have beenused as pest management tools for more than 50 years. The effect of these toxins depends on the quantityof Bacillus thuringiensis (Bt) toxins ingestedby susceptible insects. Food ingestion is affected byCO₂ concentration; plants grown in elevatedCO₂ often have increased carbon/nitrogen ratios(C/N), resulting in greater leaf area consumption. Therefore, we hypothesized that elevated CO₂would improve the efficacy of foliar applications ofB. thuringiensis. Cotton plants were grown ateither ambient (360–380 l/l) or elevated CO₂(900 l/l). Groups of plants in both CO₂treatments were exposed to low (30 mg/kg soil/week) orhigh (130 mg/kg soil/week) nitrogen (N) fertilizationlevels in a split plot design. The resulting plantswere assessed for N and carbon (C) contents. Leafdisks from the same plants were dipped in a Btsolution and then fed to Spodoptera exigua(Hübner), an insect species of considerableeconomic importance. Elevated CO₂ significantlyreduced total N, and increased the C/N. Nitrogenfertilization significantly affected consumption byearly stadia larvae, larval weight gain, and relativegrowth rate (RGR). Interactions between CO₂concentration and N fertilization level significantlyimpacted late stadia larval food consumption, andthrough differential Bt toxin intake, affectedduration of larval stage and mortality to the adultstage. We conclude that the elevated atmosphericCO₂ concentrations expected in the next centurywill interact with commercial fertilization practicesto enhance the efficacy of B. thuringiensisformulations applied topically to crops. Theimplications for improved control are discussed.     

Is birch a suitable reference in estimating dinitrogen fixation in alders by isotope dilution?

Seedlings ofAlnus incana (nodulated and non-nodulated) andBetula papyrifera were fertilized with varying amounts (0, 10, 25, 50, 100, 250 and 500 g N g^–1 soil) of labelled ammonium-N or nitrate-N ( 5.2 A% excess¹⁵N as ammonium sulphate or potassium nitrate). After 4 months in the greenhouse,¹⁵N excess in the plants were determined and an isotope dilution equation was applied to determine the percent of biomass N fixed by theA. incana/Frankia system. When ammonium was used as the sole N source and birch as the non-fixing reference, N-fixation accounted for 95%, 87% and 60% of the plant nitrogen yields with 10, 25 and 50 g N g^–1 rates, additions respectively. At the 100 g N g^–1 fertilization and above N-fixation accounted for less than 10% of the N yield. Similar results were obtained when non-nodulatedA. incana was used as non-fixing reference. With nitrate as the sole N source, N-fixation accounted for 98%, 97%, 97%, 86%, 56% and 12% of N yield with 10, 25, 50, 100, 250 and 500 g N g^–1 additions respectively. These values were similar for both types of reference plants. The direct isotope dilution method was compared to that of the total nitrogen difference method. There was good agreement between the two methods up to 50 g N g^–1 for ammonium and up to 100 g N g^–1 for nitrate. The difference method produced negative values at high concentrations of nitrogen fertilization. Again similar results were obtained by the two reference plants. The results indicate that birch can be used as a non-fixing control in isotope dilution studies but that care must be exercised in selecting the type and quantity of labelled nitrogen fertilizer.     

Regional Assessment of N Saturation using Foliar and Root \varvec {\delta}^{\bf 15}{\bf N}

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar than the regional driver of N deposition. Foliar increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root was more tightly coupled to forest floor properties than was foliar . We observed a pattern of decreasing foliar values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar included species composition and climate. Relationships between foliar and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar , due to the importance of ammonium deposition in this region. Our results suggest that examining values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.