Leaching of nitrogen forms from controlled‐release nitrogen fertilizers

Abstract

Application of soluble forms of nitrogen (N) fertilizers to sandy soils may cause leaching of nitrate N (NO₃‐N) resulting in contamination of groundwater. The leaching loss of N may be reduced to a certain extent by the use of controlled‐release N formulations. A leaching column study was conducted to evaluate the leaching of urea, ammonium N (NH₄‐N), and NO₃‐N forms from selected urea‐based controlled‐release formulations (Meister, Osmocote, and Poly‐S) and uncoated urea under eight cycles of intermittent leaching and dry conditions. Following leaching of 1,760 mL of water (equivalent to 40 cm rainfall) through the soil columns, the recovery of total N (sum of all forms) in the leachate accounted for 28, 12, 6, or 5% of the total N applied as urea, Poly‐S, Meister, and Osmocote, respectively. Loss of urea‐N from all fertilizer sources was pronounced during the initial leaching events (with the exception of Meister). Cumulative leaching of urea‐N was 10% for uncoated urea while <1.7% for the controlled‐release formulations. Cumulative leaching of NH₄‐N was 6.2% for uncoated urea while <0.5% for the controlled‐release formulations. Cumulative leaching loss of NO₃‐N was 3.78% for Osmocote, 4.6% for Meister, 10.4% for urea, and 10.5% for Poly‐S. This study demonstrates a significant reduction in leaching of N forms from controlled‐release formulations as compared to that from the soluble form.     

Effects of supplemental nitrogen on nitrogen‐assimilation enzymes,free amino nitrogen,soluble sugars,and crude protein of rice

Abstract

An upland rice variety IAC‐47 was grown in a greenhouse to determine the effect of foliar nitrogen (N) supplementation during grain development on the activity of the N assimilation enzymes, nitrate reductase (NR) and glutamine synthetase (GS), on free amino‐N content and leaf soluble sugars, and on grain crude protein content. At 10 and 20 days after anthesis (DAA), the leaves were fertilized with a liquid fertilizer containing 32% N as 12.8% urea, 9.6% ammonium (NH₄), and 9.6% nitrate (NO₃) in increasing rates corresponding to 0,20+20, 40+40, and 60+60 kg N ha^‐1. Leaves were collected twice (at 12 DAA and 14 DAA for GS activity, sugar and amino‐N content, and at 11 and 13 DAA for NRA) after each application of leaf N. The late foliar application of N increased significantly grain crude protein without a corresponding decrease in grain weight. The NR activity (NRA) increased after the foliar application of N. In the flag leaf, 60+60 kg N ha^‐1 (21 DAA) resulted in higher NRA (20x over the control), while GS activity was smaller than the control. At 22 DAA there was an increase in GS activity in the flag leaf at 20+20 N level. However, the GS activity decreased as applied N levels increased. Also at the 20+20 level, there were increases in free amino‐N in the flag leaf and second leaf at the final harvest. Throughout the experiment, plants at the 60+60 N level had the lowest levels of soluble sugars. Increases in crude protein were highest at 40+40 N level (27.9%), followed by 60+60 (18.7%).     

Uptake of ammonium‐nitrogen by blueberry plants

Blueberry plants (Vaccinium ashei Reade cv. Tifblue) and Citrus natsudaidai Hayata were compared in terms of their ability to regulate the uptake of ammonium‐nitrogen (NH₄‐N). Plants of both species were grown in N‐free nutrient solutions for three days and then transferred to nutrient solutions that contained various concentrations of NH4‐N. Blueberry plants showed increases in rates of uptake of NH₄‐N 8 to 24 h after application of NH₄‐N. At concentrations of NH₄‐N above 200 (μM, uptake rates decreased to the initial value 24 h after application of NH₄‐N and then increased. By contrast, seedlings of Citrus natsudaidai showed constant rates of uptake of NH₄‐N during the experiment. These results indicate that blueberry plants are able to repress the uptake of NH₄‐N periodically when they are exposed to high concentrations of external NH₄‐N, but not seedlings of Citrus natsudaidai.     

Studies of nitrogen immobilization and mineralization in calcareous soils—IV. Changes in the organic nitrogen of light and heavy subfractions of silt- and fine clay-size particles during nitrogen turnover

A calcareous clay nd a calcareous sand, were fractionated densimetrically by dispersion in organic liquids of sp. gr. 1.59–2.06. The N contents of the light fractions decreased with increasing densities of the suspending liquids and were up to 18–23 times higher than those of the whole soils. Light fraction organic-N of both the sandy and clay soils was obtained mainly from silt-size components. However, the efficiency, with which light fraction material was obtained from the two whole soils, varied. With the clay soil, the total yield of light fraction organic-N was increased markedly by applying the densimetric technique to particle size components, rather than to the whole soil.Silt-size and fine clay-size particles from soils, sampled during rapid metabolism of microbial organic-[¹⁵N], were further fractionated densimetrically in “Nemagon”, sp. gr. 2.06. The organic-[¹⁵N] of the light and heavy subtractions changed markedly (P < 0.05) during periods of net ¹⁵N immobilization and mineralization, including a period after soil fumigation when extensive decomposition of [¹⁵N]-labelled microbial biomass occurred. Changes in the ¹⁵N of complementary light and heavy subfractions followed similar trends. Light subtraction organic-[¹⁵N] usually showed the greater relative change but the differences between the subtractions were not statistically significant. It is concluded that when small proportions only of soil organic-N are associated with macroorganic debris, as in these two soils amended wth glucose and ¹⁵NO^?₃, densimetric fractionation at a sp. gr. as high as 2.06 will yield light and heavy fractions, whose nitrogenous components are similarly available to biological attack. Enhanced metabolism of light fraction material is more likely to be demonstrated when such material consists mainly of obvious plant residues, and this may be more easily achieved by fractionation in liquids of sp. gr. <2.     

Effects of supplemental‐nitrogen on the quality of rice proteins

A study was conducted on the effect of supplemental nitrogen (N) (20 hg/ha) applied as a foliar spray or to the soil on seed production, protein percentage, and protein fractions of rice. Plants were grown in a greenhouse over two different periods of time, i.e., August 1988 to January 1989 (Period I), and December 1988 to April 1989 (Period II). Nitrogen was applied to the leaves 10 and 20 days after anthesis (DAA), and to the soil at anthesis and at 15 DAA. Average temperature was 28.7°C during Period I and 32°C during Period II, corresponding to 18.7 and 22.0 growing degree‐day/day (GDD/day), respectively. The difference in GDD/day reduced the plant cycle from 130 days during Period I to 109 days during Period II. Plants grown during Period II had larger numbers of spikelets, a higher percentage of “full grown grains”;, and higher grain weight. Although percentage crude protein was about the same for the two periods, prolamin content was increased and the albumin+globulin fraction was decreased during Period II, but with no difference in glutelin content. The increase in number of spikelets, percent full grains, and grain weight appeared to result in a greater energy demand for plants grown during Period II. This may explain the increase in prolamins, since prolamin synthesis requires less energy than globulin or albumin synthesis. There was a simultaneous decrease in albumin and globulin synthesis during Period II. The content of glutelins, which represent the major reserve proteins in rice grains, was constant during both periods.     

Evaluation of methods for nitrogen‐15 analysis of inorganic nitrogen in soil extracts: I. Steam‐distillation methods

Abstract

A study was conducted to evaluate conventional steam‐distillation techniques for N‐isotope analysis of inorganic forms of N in soil extracts. Extracts obtained with 2 M KCl from 10 diverse soils were treated with: (i) (¹⁵NH₄)₂SO₄ and KNO₃, (ii) (NH₄)₂SO₄ and K¹⁵NO₃, or (iii) KNO₃and Na¹⁵NO₂. Steam distillations were performed sequentially to determine NH₄ ⁺‐N and NO₃ ^‐‐N, and were also carried out to determine (NO₃ ^‐ + NO₂ ^‐)‐N or (NH₄ ⁺ + NO₃ ^‐ + NO₂ ^‐)‐N; a pretreatment with sulfamic acid was used to determine NO₃ ^‐‐N in the presence of NO₂ ^‐‐N. Recovery of added N ranged from 95 to 102%. Significant isotopic contamination was observed in sequential distillation of unlabeled NO₃ ^‐‐N following labeled NH₄ ⁺‐N; otherwise, analyses for ¹⁵N were usually within 1% of the values calculated by isotope‐dilution equations.     

Influence of organic compounds on nitrogen‐fertilizer solubilization

Abstract

In the attempt to find new products which release nutrients in gradual forms, the behavior of two commercial fertilizers was studied, Nitrophoska® (N) and urea (U), covered with two organic materials, humic acid (HA) and alginic acid (AA). The release of nitrogen from the fertilizers was determined by electroultrafiltration (EUF). These applied materials on the fertilizer surface resulted in a slowing of the release of nitrogen, although strictly speaking, these compounds do not function as coated fertilizers. Their effectiveness depends on the fertilizer, for with Nitrophoska®, the addition of alginic acid was more effective, while for urea, the addition of humic acid slowed the release of nitrogen.     

Use of reflectometry for determination of nitrate‐nitrogen in well water

Recurrent monitoring of water wells is necessary to ensure that nitrate‐nitrogen (NO3‐N) concentrations in groundwater do not exceed 10 mg/L, the maximum contaminant level set by the U.S. Environmental Protection Agency. Continuous chemical analysis is often a time consuming and expensive process. A recently developed ‘Reflectoquant Analysis System’, which employs reflectometry techniques, may offer a simple and accurate method for NO3‐N analysis. The objective of this study was to evaluate the ‘Reflectoquant Analysis System’ as an alternative method for determination of NO3‐N in well water. Water samples were collected from 42 wells in Oklahoma. The samples were analyzed using the ‘Reflectoquant Analysis System’, automated cadmium reduction (Griess‐Ilosvay), ion chromatography, and phenoldisulfonic acid procedures. The linear range of the ‘Reflectoquant Analysis System’ is 1.1 to 50.6 mg/L NO3‐N. Samples exceeding this range must be diluted before analysis is performed. Excluding two wells where NO3‐N was >50.6 mg/L, simple correlation was high (r > 0.91) among the four procedures evaluated. In addition, slopes and intercepts from linear regression of NO3‐N among procedures were not significantly different. Population means obtained using the four methods were very similar. For this sample of wells, the ‘Reflectoquant Analysis System’ was precise and provided NO3‐N analysis of water samples equivalent to standard methods. Other advantages of the ‘Reflectoquant Analysis System’ are short analytical times, reduced operator training period, and competitive costs compared to standard methods.     

Regulation of accumulation of β-subunit of β-conglycinin in soybean seeds by nitrogen

Extract

The storage protein of soybean [Glycine max (L.)] seed mainly consists of glycinin, composed of acidic (38 and 45 kDa) and basic (22 kDa) subunits (Kitamura et al. 1976), and β-conglycinin composed of α′- (75 kDa), α- (72 kDa), and β-(52 kDa) subunits (Thanh and Shibasaki 1978).     

Lichen mycobionts of mycorrihizal symbiosis with nitrogen‐fixation of showy partridge pea

Effective mycorrhizal colonization is characteristic for nodulated Cassia genera that are adaptive to subhumid areas throughout the world. Growth, regeneration, and nitrogen (N) fixation occurs within regions of extreme soil and climatic environments that preclude persistent survival of other Leguminosae. Objectives of this study were to determine effective mycobiont components and adjunctive soil fertility factors governing growth, nodulation, and symbiotic N fixation of the important forage species, Showy Partridge Pea [Cassia Chamaecrista fasciculate (L.) Michx.] The perennial foliose lichen, Parmelia incurva, ubiquitous within extreme harsh drought and temperature regions, was utilized for mycorrizal mycobionts. Largest above ground plant growth, nodulation, and nitrogen fixation resulted with mycorrhizal colonization within lichen amended soil that received no other soil fertility treatments. Responses attained with phosphorus (P) and calcium (Ca) plant nutrient soil additions, without mycorrhizal mycobiont additions, were approximately half or less of effective mycorrhizal colonized plants. In general, yield response of mycorrhizal plants was reduced with plant nutrient additions throughout this study. Nitrate reductase (NR) and nitrate‐nitrogen (NO₃‐N) levels were significantly higher within nodule cytosol of nonmycorrhizal plants. Ureidoglycolate enzyme transformers and nodule cytosol ureide components were significantly greater for mycorrhizal colonized plants. These included urease (URC), allantoinase (ALTH), allantoicase (ALTC), and total ureides. However, differences were not significant for cytosol contents of pyruvate, amine‐amide N, aspartate transaminase (AST), glutamate dehydrogenase (GDH), glutamine synthetase (GS), and glutamate oxoglutarate trasaminase (GOGAT). Representative histological microscopy of mycorrhizal colonized Showy Partridge Pea are presented. Effective mycobiont propagules associative with lichen associations are apparently opportune commensal species and only functional as site specific sycophants governed by variable environmental conditions with lichen dissipation.     

Response of summer‐planted potatoes to level of applied nitrogen and water

The irrigation and nitrogen (N) requirements of potatoes (cv. Delaware) were determined using sprinklers in a line‐source design on a Spearwood sand. Irrigation water was applied at 73 to 244% of the daily pan evaporation (Epan) and N at 0 to 800 kg N ha^‐1 (total applied) as NH₄NO₃ in 10 applications post‐planting. There was a significant yield (total and marketable) response to irrigation, at all levels of applied N, and N at all levels of applied water (P<0.001). The interaction between irrigation and N was also significant (P<0.001). There was no significant yield response to irrigation from 149% Epan (i.e., W3 treatment) to 244% Epan (i.e., W6 treatment). Irrigation at 125 and 150% of Epan was required for 95 and 99% of maximum yield, respectively, as determined from fitted Mitscherlich relationships. Critical levels of N required for 95 (417 kg ha^‐1) and 99% (703 kg ha^‐1) of maximum yield were also determined from a Mitschlerlich relationship fitted to the average of the W3 to W6 treatments. The percent total N and nitrate‐N in petioles of youngest fully expanded leaves required for 95 and 99% of maximum yield was 1.78 and 2.11, respectively, at the 10 mm tuber stage, and 0.25 and 0.80% at the 10mm plus 14 day stage (from quadratic regressions). There was a significant (P≤0.001) increase in N uptake by tubers with level of applied N from 57 kg ha^‐1 at 0 kg applied N ha^‐1 to 190 kg ha^‐1 at 800 kg applied N ha^‐1 (from a Mitscherlich relationship fitted to the average of W3 to W6 treatments). After accounting for N uptake from soil reserves (57 kg N ha^‐1), apparent recovery efficiency (RE) of fertilizer N by tubers [RE=(Up‐Uo/Np) where Up=uptake of N by the crop, Uo=uptake in absence of applied N and Np is the level of applied N, expressed as a fraction] declined from 0.28 at 100 kg applied N ha^‐1 to 0.17 at 800 kg applied N ha^‐1. There was a linear increase in ‘after cooking darkening’ (i.e., greying) of tubers with increasing level of applied N. Conversely, ‘sloughing’ (i.e., disintegration) of tubers decreased (inverse polynomial) with increasing level of applied N. Rate of irrigation had no effect on these cooking qualities. Reducing applied irrigation and N from levels required for 99% of maximum yield to levels required for 95% of maximum yield would not lead to a significant reduction in profit. This would increase apparent recovery efficiency of applied N by plants, maintain tuber quality, and reduce the impact of potato production on the water systems of the Swan coastal plain.     

Determination of nitrogen by microdiffusion in mason jars: III. Nitrogen and nitrogen‐15 in Kjeldahl digests

Abstract

Diffusion methods for quantitative determination and isotope‐ratio analysis of inorganic N in soil extracts were modified for use with Kjeldahl digests. The digest was diluted to 25 mL with deionized water, and an aliquot (to 6 mL) was transferred in a shell vial (17 mm dia., 60 mm long) to a 473‐mL (1‐pint) wide‐mouth Mason jar containing 15 mL of 8 M NaOH. The NH₃ liberated by overturning the vial inside the sealed jar was collected for 48 h at room temperature (24 h with orbital shaking) in 3 mL of boric acid‐indicator solution in a Petri dish, or in an acidified glass‐fiber disk, suspended from the Mason‐jar lid. Determinations of N and ¹⁵N by diffusion were in close agreement with analyses using conventional steam‐distillation and concentration techniques.     

Improvement of the nitrogen uptake induced by titanium (iv) leaf supply in nitrogen‐stressed pepper seedlings

The beneficial effect of titanium (Ti) on plant metabolism can result in more profitable use of fertilizer applied to a crop. A crop chamber experiment with paprika pepper (Capsicum annuum L., cv. Bunejo) seedlings under differential nitrogen (N) concentration levels in a nutrient solution (100% N, 75% N, 50% N, and 25% N) was performed. A third of the seedlings growing under each N support level remained Ti‐untreated and were used as the reference. Another third of the seedling received one and two 0.042 mM Ti(TV) ascorbate, pH 6.0, leaf spray treatments, respectively. The biomass production of the Ti‐untreated plants was only affected by the N supply of 50% or less. The Ti(IV) leaf spray treatments produced a biomass production greater than that of the corresponding reference plants, and both the 100% N+Ti and 75% N+Ti treatments had the highest biomass production. Seedlings receiving 50% N+Ti had a level of biomass production similar to that for the 100% N without Ti reference plants. In the same way, the 25% N+Ti treatment resulted in a plant fresh weight greater than that for the Ti‐untreated reference plants, although their biomass yields were not significantly lower than that for the corresponding 100% N and 75% N Ti‐untreated reference plants. Only the 50% N and 25% N Ti‐untreated plants had definite total‐N and nitrate‐nitrogen (NO₃‐N) unbalances as compared to the other N rate‐Ti treatments.     

Salinity effects on nitrogen metabolism in plants – focusing on the activities of nitrogen metabolizing enzymes: A review

Nitrogen (N) metabolism is of great economic importance because it provides proteins and nucleic acids which in turn control many cellular activities in plants. Salinity affects different steps of N metabolism including N uptake, NO₃^? reduction, and NH₄⁺ assimilation, leading to a severe decline in crop yield. Major mechanisms of salinity effects on N metabolism are salinity-induced reductions in water availability and absorption, disruption of root membrane integrity, an inhibition of NO₃^? uptake by Cl^?, low NO₃^? loading into root xylem, alteration in the activities of N assimilating enzymes, decrease in transpiration, and reduction in relative growth rate which results in a lower N demand. However, the effects of salinity on N metabolism are multifaceted and may vary depending on many plant and soil factors. The present review deals with salinity effects on N metabolism in plants, emphasizing on the activities of N metabolizing enzymes in a saline environment.     

Nutrient charge,composition of media and nitrogen‐form affect growth of vinca

Growth of vinca [Catharanthus roseus (L.) G. Don ‘Grape Cooler'] was compared under several cultural conditions. Conditions investigated included two types of media (a peat‐lite mix and a mix containing 25% pine bark) and five types of nutrient charges in the peat‐lite media (sulfated micros, chelated micros, sulfated or chelated micros with pH adjustment to 5.5, and no charge). Nitrogen (N) source effect on growth was also investigated. Plants were grown at five different ratios of nitrate‐N to ammonium‐N. Greatest growth as measured by shoot length and shoot dry weight occurred in the peat‐lite media at either the sulfated micro or chelated micros adjusted to pH 5.5 and at the highest ratios of nitrate‐N to ammonium‐N. Root dry weight and growth were negatively affected by high levels of ammonium‐N in the fertilizer solution.     

Effect of nitrogen and sulphur fertilizers and seed inoculation with rhizobium phaseoli on the nitrogen‐sulphur relationships of bean (phaseolus vulgaris L.)

A greenhouse experiment with beans (Phaseolus vulgaris L.) was performed in order to investigate the effect of nitrogen and sulphur application and seed inoculation on the yield, leaf area, distribution of different nitrogen and sulphur fractions and N/S ratio in shoot, fruit and root.

Inoculation of plants together with nitrogen or sulphur application produces an increase in the concentration of total nitrogen and a decrease in the accumulation of nitrate‐nitrogen and sulphate‐sulphur in shoot, fruit and root. Leaf area increased more with nitrogen than with sulphur application while the highest amounts of fruit dry matter were obtained with sulphur application.

N: S ratios obtained were different according to the part of the plant tested. Sulphur fertilization decreased the N: S ratios in shoot, fruit and root. The data obtained indicate that and adequate N: S ratio can insure maximum production of yield.     

Are chlorophylls good indicators of nitrogen and phosphorus levels? 1

Aubergine plants (Solanum melongena cv. Bonica) were grown under controlled greenhouse conditions on a soil substrate supplied with organic fertilizers (15 kg/m²) mixed with calcium sulfate (CaSO₄ at 500 g/m²), with different doses of nitrogen (N as N₁ = 15, N₂ = 22.5, N₃ = 30 g/m²) in the form of ammonium nitrate (NH₄NO₃), and phosphorus (P as P₁ = 24, P₂ = 36 g/m²) as phosphorus acid H₃PO₄). Plants were sampled every 15 days, and the pigments chlorophyll a, chlorophyll b, total (a+b) and ratio (a/b), carotene, licopene, and anthocyanins were determined in the leaves. The results showed that increases in rhizosphere N led to increases in foliar concentrations of chlorophyll a and b, both individually and as total chlorophyll, independently of the dose of P applied. Total chlorophyll concentrations were directly correlated with the level of P fertilization. Carotene and licopenes reflected the influence of increasing doses of N, whereas P did not affect these pigments. Anthocyanin levels were affected by both N and P.     

Deep‐freezing pretreatment and time of extraction of soil samples for inorganic nitrogen determination

Abstract

Soil samples for inorganic nitrogen (N) determination are usually deep‐frozen to prevent microbial transformations of N between sampling and analysis. For analysis, frozen soils are thawed, which may also lead to transformations of N. A specially manufactured mill for grinding frozen soil was tested to minimize these transformations. Whether the time of extraction of the samples could be extended to 20 hr to better accomondate routine work and to make the clay aggregates to disperse better during extraction was also investigated. Freezing of the samples did not produce different results to fresh soils from ammonium nitrogen (NH₄ ⁺‐N) or nitrate nitrogen (NO₃ ^‐‐N) determination. Thawing of the samples increased the concentration of NO₃ ^‐‐N in the extracts and grinding increased that of NH₄ ⁺‐N. When either thawing or grinding was applied, the total inorganic nitrogen concentration was about the same. Thawing of the ground samples increased concentrations of NO₃’‐N and NH₄ ⁺‐N in the extracts. Extending the time of extraction from 0.5 or 1 hr to 20 hr increased the concentration of NH₄ ⁺‐N in the extracts, while NO₃ ^‐‐N content was also increased slightly. It was concluded that sample pretreatment may cause serious errors in the determination of inorganic N even by methods which have proven most successful to prevent microbial transformations of nitrogen, unless the soils are extracted immediately after sampling. The period of extraction should not exceed two hours.     

Effects of N‐source,light intensity and temperature on nitrogen metabolism of bahiagrass

Under greenhouse conditions, a study was made on the effects of nitrogen (N) source (N)O₃ or NH₄), mode of application (single vs. split) and nitrification inhibition on the N‐uptake and metabolism, of bahiagrass.

Variations in light and temperature in the greenhouse affected the N‐metabolism of bahiagrass plants. Nitrate fed plants had nitrate reductase activity (NRA) pattern different from that of NH₄‐fed plants. Amino‐N accumulation patterns were similar for plants under both N‐sources, although amino‐N levels in leaves of NH₄‐fed plants were much smaller than that of NO₃ plants. Nitrate accumulation in leaves showed inverse trend to that of roots in plants fed both NO₃ or NH₄. To the sharp peaks in NO₃ levels in roots due to increases in light and temperature corresponds a sharp decrease of its levels in leaves.

For both both NO₃ or NH₄ treatments, soluble‐N accumulated most in the rhizomes of bahiagrass plants, whereas protein N accumulated most in leaves, suggesting that rhizomes had a buffering effect on the NO₃ fluxes to leaves. This presumably resulted in a lag in the NRA response of the NO₃‐fed plants to increases in light and temperature.     

Influence of nitrogen on leaf chlorophyll content and photosynthesis of ‘Golden Delicious’ apple

Abstract

The aim of the present study was to estimate the influence of different rates of soil-applied nitrogen on leaf N and chlorophyll content and photosynthesis in ‘Golden Delicious’ apple trees. Three different treatments were included: the trees were either fertilized with 80 kg N ha^?1 (N-80), 250 kg N ha^?1 (N-250) or left unfertilized (CON). Fertilization increased leaf nitrogen content, with a more prominent effect in high N application level treatment. In all treatments, a slight seasonal decrease in leaf nitrogen content was observed. N-250 treatment resulted in higher chlorophyll content; a similar effect was found late in the season for N-80 treatment. Measurements of A-C _i curves, performed on spur leaves, revealed a higher CO₂ saturated photosynthetic rate in N-250 trees compared with low application level fertilized or unfertilized trees. No effect of N fertilization on carboxylation efficiency was found, as revealed by comparisons of the initial slopes of A-C _i curves. The lack of positive effect is rather surprising, since the leaf N content was efficiently increased with application of fertilizer. Obviously, the existing pool of leaf nitrogen in non-fertilized trees does not limit Rubisco activity and efficiency.