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Soil Science Society of America Journal 67:309-317 (2003)
© 2003 Soil Science Society of America

DIVISION S-7—FOREST & RANGE SOILS

Nitrogen Retranslocation Response of Young Picea mariana to Nitrogen-15 Supply

K. F. Salifu and V. R. Timmer*

Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, Canada M5S 3B3

* Corresponding author (vic.timmer{at}utoronto.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Nutrient loading stimulates N retranslocation, an important mechanism of N reuse in plants to support new growth. We quantified N retranslocation in young black spruce [Picea mariana (Mill.) BSP] using tracer and nontracer techniques to examine enhanced field performance after nutrient loading. Nursery reared seedlings were transplanted to sand-filled pots fertilized with 15NH4 15NO3 at rates equivalent to 0 and 200 kg N ha-1 simulating poor and rich soils. After one growing season (120 d), biomass increased (118%) on the poor soil without N gain demonstrating the significance of internal N reserves for retranslocation to new growth. Nutrient loading improved retranslocation (218%) and new biomass (156%) after planting confirming the advantage of higher preplant N reserves (175%) for later nutrient demand. Enhanced N availability in the rich soil accelerated growth (236%), N uptake (258%), and retranslocation (23%) in seedlings. Retranslocation increased with time reflecting higher N demand as seedlings become larger and suggest the process is driven by sink strength. Nontracer estimates of N retranslocation in seedlings fell short of isotopic determinations because of inability to discriminate between soil and plant derived N in tree components. Although fertilization promoted N uptake (125–258%), 15N recovery in plants averaged 12 to 19% indicating low fertilizer efficiency in young trees. Total reliance of unfertilized plants on internal N reserves for growth on the poor soil affirms the importance of retranslocation to meet plant N demands, and also exemplifies initial short-term independence on soil N for newly planted seedlings that can be prolonged by nutrient loading.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
NITROGEN RETRANSLOCATION is a key mechanism of N reuse in plants to support new growth (Kim et al., 1987; Lim and Cousens, 1986). This process is especially critical for meeting sink demands in newly planted seedlings because initial slow root growth (Burdett et al., 1984; van den Driessche, 1985; Burdett, 1990) or low soil temperatures in early spring (Chapin et al., 1986; Atkin, 1996) may limit N uptake from the soil. The importance of nutrient loading (Timmer, 1997) to build nutrient reserves and promote retranslocation and soil nutrient exploitation in the performance of newly established seedlings has been clearly demonstrated (Malik and Timmer, 1998; Xu and Timmer, 1999; Salifu and Timmer, 2001). Nutrient loading involves fertilization in excess of the demand for current growth during nursery culture to induce luxury uptake of nutrients characterized by increased internal concentration in plants without significantly changing total dry mass (Timmer, 1997). These reserves function as crucial internal nutrient sources that are depleted later to support new growth soon after planting when N stress is usually most severe (Xu and Timmer, 1999). Transplanted nutrient-loaded seedlings for example, depleted 50 vs. 32% net N by nonloaded plants from old tissues to support new growth in black spruce (Salifu and Timmer, 2001). Reliance on internal N reserves for seasonal growth of young apple (Malus domestica Borkh.) grown in sand culture (Millard and Neilsen, 1989) illustrate the importance of nutrient reserves in promoting new growth that may confer early short-term independence from soil N. Thus, nutrient loading practices that build plant N reserves in the nursery have potential, not only to increase retranslocation, but also reduce initial reliance on soil N when transplanted in the field.

Nutrient budget studies have often quantified net N retranslocation as N content differences in plant tissues before and after planting based on net transfer within plant components (Lim and Cousens, 1986; Malik and Timmer, 1998) hereafter referred to as the net approach. This method has two potential problems. First, it is assumed that leaching losses from plant tissues are negligible (Miller et al., 1976; Lim and Cousens, 1986). Second, only net transfer of N within the plant is determined that includes uptake from the soil pool plus N remobilized internally within the plant. Such estimates may be confounded because N uptake from the soil cannot be separated from that remobilized internally within plants without the use of labeled isotopes (Mead and Preston, 1994; Proe and Millard, 1994). In contrast to the net approach, use of tracers is considered more accurate for quantifying nutrient influx and afflux in plant tissues (Weinbaum et al., 1987; Millard and Proe, 1993; Proe and Millard, 1994) because direct retranslocation can be estimated at high sensitivity and precision in addition to identifying the different N pools involved (Nômmik, 1990).

In a previous study (Salifu and Timmer, 2001), we quantified net N retranslocation on a simulated soil N supply gradient using the net approach and demonstrated that higher plant N reserves account for improved retranslocation and growth response in transplanted nutrient-loaded seedlings. Furthermore, we showed that retranslocation varies with soil N availability and sampling time, and that enhanced soil N exploitation by transplanted nutrient-loaded seedlings was attributed to improved root growth in these plants. Further study is warranted to examine retranslocation processes with tracers to corroborate these research findings.

We labeled current uptake with 15N to allow direct quantification of isotopic N uptake, distribution and retranslocation in planted seedlings under controlled environments to confirm enhanced field performance of nutrient-loaded black spruce seedlings (Salifu and Timmer, 2001). The objectives of this research were, therefore, to determine whether (i) higher plant N reserves account for improved retranslocation and growth response in transplanted nutrient-loaded seedlings, (ii) increasing soil N supply stimulates growth in new sinks and increases the need for internal N redistribution, and (iii) retranslocation contributes more to the N demand for growth as plants become larger. In addition, we tested the hypotheses (1) that young trees may be independent of soil N soon after planting, a capacity enhanced by nutrient loading, and (2) that nutrient loading promotes soil N exploitation by accelerating root growth and extension after planting.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Nursery Culture and Transplanting Trial
Nutrient-loaded (L) and nonloaded [also referred to as conventional (C)] seedlings received a respective seasonal total of 64 and 10 mg N tree-1 and were raised for 18 wk in the nursery. Nitrogen was applied in solution form to the seedlings as a mixed fertilizer (20% N, 9% P, 17% K, and micronutrients, Plant Products Co. Ltd., Brampton, ON). The 10 mg N tree-1 treatment is commonly applied for commercial production of containerized black spruce seedlings in northern Ontario (Timmer, 1997). The 64 mg N tree-1 treatment represented fertilizer application in excess of the demand for growth to induce luxury uptake and build up of plant nutrient reserves in trees (Timmer and Aidelbaum, 1996). Weekly fertilizer application rates for loaded and conventional seedlings were calculated based on respective exponential and constant feed models described in detail by Timmer and Aidelbaum (1996) designed to synchronize supply with exponential growth and nutrient uptake of seedlings (Ingestad and Lund, 1986; Ingestad and Agren, 1995).

The year-old containerized seedlings were bare rooted by gently shaking off the growth medium from the roots followed by rinsing them in distilled water and transplanted (one per pot) in plastic pots 17 cm wide by 18 cm deep filled with acid washed sand. Bare rooting was to eliminate possible carry-over effect of last year's fertilizer in the rooting medium to dilute current year uptake. Transplanted seedlings received either 0 mg N tree-1 [control] or 250 mg N tree-1 [simulating operational silvicultural prescription of 200 kg N ha-1 under field conditions] based on mass of the furrow slice and of soil in our pots (2.5 kg pot-1). The general fertilization rates employed in silvicultural practice vary from 100 to 200 kg N ha-1 (Miller et al., 1976; Hulm and Killham, 1990; Nômmik, 1990; Chang et al., 1996; Staples et al., 1999; Chang and Preston, 2000; Salifu and Timmer, 2001). Seedlings were destructively sampled at two time periods after planting. Thus, the experiment was a 2 x 2 x 2 factorial design, testing nursery fertilization (loaded and conventional seedlings), N supply (0 and 250 mg N tree-1) hereafter referred to as the respective nutrient poor and rich soils, and sampling time (60 and 120 d) after planting and replicated three times.

Two pots represented an experimental unit, given a total of 48 pots for the entire experiment. Pots were arranged in three replicate blocks (16 pots per block) on raised benches in a heated and ventilated greenhouse at the University of Toronto, at temperature 18 to 25°C, humidity 65 to 85%, and 18-h photoperiod supplemented with sodium vapor lamps at a light intensity of 250 µmol s-1 m-2. Pots were perforated at the bottom to enhance drainage and also rotated periodically to minimize edge effects. To keep the N budget tight, plastic containers were placed under each pot and any leached solution after irrigation was returned to the pots. Nitrogen was supplied with the irrigation as 15NH4 15NO3 enriched to 5 atom%15N (34-0-0, ISOTEC Inc., USA), at three split applications starting 1, 2 and 3 wk after planting since N recovery from split doses were not significantly different from that obtained from a single application (Nômmik and Larsson, 1989). Chelated (EDTA 42% and DTPA 13%) micronutrients were applied at the rate of 0.03 g L-1 and P supplemented by KH2 P2O5 (0-52-34, Plant Products Co. Ltd., Brampton, ON) at the rate of 60 kg ha-1.to avert deficiency of other nutrients.

Plant Sampling and Nutrient Analysis
One experimental unit (two pots) was randomly sampled from each block (a total of six seedlings) per treatment combination at 60 and 120 d after transplanting. After harvest, roots were washed free of the sand, and seedlings partitioned into new shoots, old shoots, and roots. The sand was sieved and any remaining root materials recovered and washed. All samples were composited by replication for nutritional analysis, but measured individually and averaged for growth analysis. Plant material was oven dried for 48 h at 70°C and ground. Quantitative lipid extraction and purification of samples was done according to Bligh and Dyer (1959). The lipid extracted and ball-milled samples were then run for total N and 15N analysis on an Isochrom Continuous Flow Stable Isotope Mass Spectrometer (Micromass, Micromass Internation Ltd., Manchester, UK) coupled to a Carla Erba Elemental analyzer (CHNS-O EA1108; Thermo Finnigan Italia, Milan, Italy) and results calculated and corrected to appropriate N standards at the stable Isotope Laboratory (University of Waterloo, Canada).

Statistical Analysis
Analysis of variance (ANOVA) was conducted on growth and nutrient variables and where appropriate means were ranked according to Waller-Duncan's multiple range tests (SAS Institute, 1989). The Anderson and McLean's (1974) linear model for the ANOVA is given as:

[1]
where Yijkl is seedling biomass, N content or concentration of the lth replicate (l = 1, 2, 3), estimated at kth time (k = 1, 2), from the jth nursery fertilization (j = 1, 2) from the ith soil N supply (i = 1, 2); µ = overall mean; Si = fixed effect of the ith soil N supply; Fj = fixed effect of the jth nursery fertilization; Tk = fixed effect of the kth time; followed by the interaction effects and {epsilon} is error associated with measured seedling biomass, N content, or concentration from bulk replicates.

Calculations
The N isotope ratio of sample was computed using the delta ({delta}) notation as:

[2]
where (atom% 15N)x and (atom% 15N)std are the respective N isotope ratios of the sample and standard (0.366, International Atomic Energy Agency, 1983) (Hauck and Bremner, 1976; Hauck et al., 1994). It was assumed that plants acquired N from two sources: (i) labeled N fertilizer from the soil pool representing direct estimates of current 15N uptake and (ii) unlabeled N previously assimilated and stored in plants as internal reserves. Recovered unlabeled N in new growth represented retranslocation or N derived from internal cycling (Millard and Neilsen, 1989; Millard and Proe, 1992). Labeled fertilizer N recovered (FNR) from the soil was computed from Eq. [3] according to Hauck and Bremner (1976), Millard and Neilsen (1989), Reddy and Reddy (1993), and Hauck et al. (1994):

[3]
where TN equals total plant N content (mg), estimated as concentration multiplied by plant dry mass; A equals atom% 15N in fertilized plant tissues, estimated from Eq. [2]; B equals atom% 15N in unfertilized plant tissues (control), estimated from Eq. [2]; C equals atom% 15N in fertilizer. Nitrogen derived from internal reserves represents retranslocation (RE) and was calculated using Eq. [4] with the assumption that the acid-washed sand supplied no N:

[4]


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Plant Growth Response
Although plants were similar in biomass before planting (Day 0, Fig. 1) , nutrient-loaded seedlings contained 22 vs. 8 mg N in nonloaded plants (Day 0, Fig. 2) . Apparently, nutrient loading increased N uptake by 175% illustrating the ability of this practice to induce luxury consumption and build internal N reserves in plants to be depleted later for improved transplanting performance. After planting, nutrient loading stimulated biomass production as shown by consistently higher (P = 0.0001, Table 1) growth in all plant tissues of loaded compared with nonloaded seedlings despite similar dry mass at planting (Fig. 1). Thus, nutrient loading accelerated biomass production by 118% on the poor soil at final harvest exemplifying superior growth performance of loaded plants noted both under greenhouse conditions (Malik and Timmer, 1995; Xu and Timmer, 1999) and in the field (Malik and Timmer, 1998). Similarly, increased N supply may have increased sink strength (P = 0.0001) and nutrient acquisition thus stimulating photosynthesis and hence plant growth as N availability is raised (Kimmins, 1997; Evans, 1989).



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  		Fig. 1. Dry mass production in plant tissues of nutrient loaded (L) and conventional (C) black spruce seedlings sampled 0, 60, and 120 d after transplanting on nutrient poor (P), and rich (R) soils. Note scale for plant is three times that of other tissues.

 


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  		Fig. 2. Total N recovered in plant tissues of nutrient loaded (L) and conventional (C) black spruce seedlings sampled 0, 60, and 120 d after transplanting on nutrient poor (P), and rich (R) soils. Note scale for plant is twice that of other tissues.

 

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  		Table 1. Summary of analysis of variance, testing effects of single and factorial treatments: [soil N supply (S); nursery fertilization (F), and sampling time (T)] on dry mass production, and total N content in structural plant components (see Fig. 1 and 2).

 
Growth in new shoots, old shoots, and roots of loaded plants established on poor soils were similar to nonloaded seedlings planted on fertilized soils, suggesting that nutrient loading in the nursery may offset early transplanting fertilization response (Fig. 1). Nursery nutrient loading also proved more effective than field fertilization in promoting seedling performance after planting in the greenhouse (Xu and Timmer, 1999), and in the field where broadcast fertilization stimulated growth of competing vegetation (Staples et al., 1999; Imo and Timmer, 1999, 2001). Moreover, field fertilization promoted weeds that accelerated competition for light, moisture and nutrients, with a consequent reduction in tree nutrient uptake efficiency (van den Driessche, 1991; Chang et al., 1996).

Improved new growth after nutrient loading may be attributed to increased net N retranslocation characterized by rapid N depletion from old shoots (Fig. 2 and Fig. 3) to active growth sinks in expanding new shoots (Grime, 1979; Malik and Timmer, 1998; Salifu and Timmer, 2001). Presumably, the retranslocated N promoted photosynthesis and photosynthetic carbon gain in new tissues (Margolis and Brand, 1990; Nambiar and Fife, 1991). Growth sinks are sites for resource capture (Grime, 1979) placing demands on the plant to meet these requirements, thus higher growth in new shoots of loaded seedlings (36–156%, Fig. 1) may increase sink strength that further enhances resource capture. Three-way interaction effects were not significant (Table 1). However, accelerated growth in all plant tissues on rich soils, and with time (Fig. 1) signify N supply x time interaction effects (P = 0.0001, Table 1) and responsiveness of seedlings to N fertilization. Similarly, increased growth by nutrient loading at final sampling compared with their status at Day 60 demonstrates significant (P = 0.0001) loading x time interaction effects (Table 1).



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  		Fig. 3. Unlabeled N uptake and distribution in plant tissues of nutrient loaded (L) and conventional (C) black spruce seedlings sampled 0, 60, and 120 d after transplanting on nutrient poor (P), and rich (R) soils. Note scale for plant is twice that of other tissues.

 
Nitrogen Uptake and Distribution
Total Nitrogen and Unlabeled Nitrogen
Total N uptake increased (P = 0.0001, Table 1) with N supply in new shoots, old shoots, roots, and averaged 124 to 258% in plants (Fig. 2). Seedlings planted on the rich soil took up N for the entire duration of the experiment as reflected by the higher N (P = 0.0001) contents with time. By contrast, plants on poor soils failed to increase N gain suggesting new shoot and root growth was dependent entirely on retranslocated N from old tissues typified by N depletion in old shoots (Fig. 2 and 3) as similarly noted by Millard and Neilsen (1989) and Salifu and Timmer (2001). Apparently, the internal N pool met the N required for growth on the poor soil (Table 2) suggesting reduced early dependence of plants on soil N (Millard and Neilsen, 1989) and the potential of nutrient loading to enhance this capacity.


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  		Table 2. Total, labeled, and unlabeled N in nutrient-loaded (L) or conventional (C) black spruce seedlings grown on nutrient poor (P) or rich (R) soils, and sampled 60 and 120 d after planting.

 
Unlabeled N recovered in plants was unaffected by N supply (P = 0.9705), nor sampling time (P = 0.1641) but was significantly influenced by loading (P = 0.0001, Fig. 3) indicating nutrient loading in the nursery stimulated N recovery and distribution in plant tissues. Initially (to Day 60), this N was used to promote growth of new shoots to ensure a supply of carbon skeletons for further growth (Millard and Neilsen, 1989). Thereafter, shoot growth stabilized in conventional and loaded seedlings on poor soils, directing some N reserves to support root growth for enhanced exploitation of soil for growth resources (Fig. 1).

Net retranslocation was computed from old shoots as the difference in total N content before and after planting based on the assumption that losses through leaching were negligible (Miller et al., 1976; Lim and Cousens 1986) and net loss translocated to growth sinks in new shoots and roots (Helmisaari, 1992; Salifu and Timmer, 2001). Retranslocation estimated by tracer according to Eq. [4] was similar on poor soils as calculated by the net approach, except for loaded seedlings at Day 120 (Table 3). The differences reflect the inability of the net approach to separate remobilized N within plants and N uptake from the soil in plant tissues as similarly noted in other studies (Mead and Preston, 1994; Proe and Millard, 1994). Moreover, rapid N uptake on rich soils (Fig. 4) may have diluted the internal N pool and led to lower net retranslocation estimates (Table 3; Mead and Preston, 1994). For example, computed net N in loaded seedlings planted on the rich soil was 2 mg and differed from tracer estimates (8 mg) at Day 60 (Table 3). Similar comparisons in nutrient loaded seedlings on the same soil at Day 120 were net estimates (3 mg N) and 11 mg N as determined by 15N.


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  		Table 3. Nitrogen retranslocation estimated from old shoots by the net approach (net N) or by tracer in new shoots (unlabeled N) of nutrient-loaded (L) or conventional (C) black spruce seedlings grown on nutrient poor (P) or rich (R) soils, and sampled 60 and 120 d after planting. Means of three replicates reported with SE in parentheses.

 


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  		Fig. 4. Labeled N uptake and distribution in plant tissues of nutrient loaded (L) and conventional (C) black spruce seedlings sampled 0, 60, and 120 d after transplanting on nutrient poor (P), and rich (R) soils. Note scale for plant is twice that of other tissues.

 
Comparison of main treatment effects Table 4 showed nutrient loading increased retranslocation by 218% (P = 0.0001), confirming improved retranslocation is stimulated by nursery nutrient loading (Imo and Timmer, 2001; Salifu and Timmer, 2001). Accelerated retranslocation with time (P = 0.0221–0.0327) agrees with the hypothesis that this process contributes more rather than less to growth demands as plants become larger (Miller, 1984), noted previously without isotope use (Millard and Neilsen, 1989; Salifu and Timmer, 2001). Similarly, increased retranslocation (23%) with N supply (P = 0.0453–0.0001) as calculated by the tracer method concurs with studies contending that retranslocation is greater in trees on fertile soils (Nambiar and Fife, 1991; Proe and Millard, 1994; Munson et al., 1995). Presumably, stronger growth sinks in expanding new shoots of plants established on the rich soil (Fig. 1) increased the need for internal N redistribution supporting a hypothesis of retranslocation driven by sink strength (Nambiar and Fife, 1991). This contrasts with diminished retranslocation response on the same soil as determined by the net approach because of problems of separating N uptake from the soil pool from N derived internally, thus obscuring interpretation of retranslocation. For example, quantified N retranslocation on the rich soil equaled 1.31 mg by the net approach and 6.10 mg by the tracer method, clearly illustrating the higher sensitivity of the isotope approach. Thus, a 366% increase in retranslocation estimated by the tracer method, illustrates sensitivity of the isotope approach.


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  		Table 4. Nitrogen retranslocation estimated from old shoots by the net approach (net N) or by tracer in new shoots (unlabeled N) and associated ANOVA for nutrient-loaded (L) or conventional (C) black spruce seedlings grown on poor or rich soils, and sampled 60 and 120 d after planting (main treatment responses).

 
To elucidate retranslocation dynamics more comprehensively, biomass (Fig. 1) and N content of old shoot (Fig. 2) along with concentration were expressed as percentage of initial status and plotted as vector nomograms (Fig. 5) . Vector direction indicated net N retranslocation (as Shift G: Imo and Timmer, 2001; Salifu and Timmer, 2001) from old shoots (since biomass increased or remained constant while N concentration and content declined) when compared with initial seedling status at planting. Greater net N retranslocation (reflected by increased vector length) was associated with higher plant N reserves in loaded seedlings (Fig. 2) agreeing with the hypothesis that the size of plant N reserves is a key variable driving retranslocation (Nambiar and Fife, 1991; Millard and Proe, 1993; Malik and Timmer, 1998; Salifu and Timmer, 2001). Accelerated net retranslocation by loaded seedlings also suggests N reserves in these plants were less structurally bound, hence more readily available for depletion (Fagerstrom and Lohm, 1977; Miller et al., 1979; Chapin, 1990). Steady-state nutrition characterized by stable internal tissue N concentration with time (as Shift B: Quoreshi and Timmer, 2000; Salifu and Timmer, 2001) was exhibited by rapid N uptake that matched growth rate of nonloaded seedlings established on rich soils (Fig. 5, bottom) up to 60 d typifying N sufficiency or adequacy, but dilution occurred (as Shift A: Imo and Timmer, 1992) by 120 d when uptake could not keep up with growth symptomatic of incipient late-season deficiency.



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  		Fig. 5. Temporal dynamics of relative change in plant dry mass, N content, and concentration in old shoots of nutrient loaded (L, top) and conventional (C, bottom) black spruce seedlings after transplanting on nutrient poor (P) and rich (R) soils. Vectors reflect progressions in time (d) from T0 to T60, and T60 to T120. Note scale difference between nomograms.

 
Labeled Nitrogen Recovered from Soil Pool
Applied fertilizer N recovery by plants calculated from Eq. [3] show 15N recovery can be ranked as new shoot > root > old shoot (Fig. 4) and significantly (P = 0.0070–0001) increased with time, except for old shoots (P = 0.1028) suggesting new shoots were major sinks for N as similarly found by Marshall and McMullan (1976) and Chang et al. (1996). Higher root growth in transplanted nutrient-loaded seedlings (Fig. 1) increased soil N uptake in respective plants and roots by 28 and 37% at Day 60 (Fig. 4) as also noted by Imo and Timmer (2001) and Salifu and Timmer (2001). Fertilizer N recoveries based on 15N studies averaged 6 to 12% in black spruce (Knowles and Lefebvre, 1972; Chang et al., 1996), and ranged from 5 to 25% in Sitka spruce [Picea sitchensis (Bong.) Car., Hulm and Killham, 1990], and in lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm., Preston et al., 1990). Recovered 15N as a percentage of applied 15N fertilizer (250 mg tree-1) in our plants (loaded seedlings planted on rich soils) from Day 60 to 120 averaged 12 to 19% (Table 2, Fig. 4) and compared well with those found for black spruce (Knowles and Lefebvre, 1972) and the other species (Preston et al., 1990; Chang et al., 1996). Low fertilizer recovery efficiency in plant tissues suggests substantial pools of N may be retained in the soil beyond root exploitation assuming negligible losses through NH3–N volatilization, denitrification and leaching. Mechanism(s) that limited capacity of trees for N uptake under abundant N supply found in this study and in others (Millard and Neilsen, 1989; Preston et al., 1990; Mead and Preston, 1994) warrants further study.

CONCLUSION
Nutrient loading in the nursery induced luxury uptake of N that increased N accumulation by 175% in nutrient-loaded seedlings prior to planting. These reserves functioned as critical internal N sources that were depleted soon after planting to enhance growth in new tissues of nutrient-loaded seedlings. The reliance on internal reserves for the growth of trees established on nutrient poor soils demonstrates not only the importance of retranslocation in meeting plant N demand, but also confirms short-term independence on soil N for newly planted seedlings that can be prolonged by nutrient loading. Higher plant N reserves accounted for improved retranslocation in nutrient-loaded trees consistent with results of previous field studies. Increased retranslocation with N supply confirmed enhanced retranslocation response as N availability is raised. Accelerated retranslocation with time supported the contention that retranslocation contributes increasingly to growth demand as seedlings become larger (Miller, 1984). Nutrient-loaded seedlings contained 28% more N vs. nonloaded plants at Day 60 and extra root growth probably accounted for greater soil N exploitation. Fertilizer 15N recovery in the seedlings was low and averaged 12 to 19% within the range reported in other studies (Millard and Neilsen, 1989; Mead and Preston, 1994). More research is needed to explain the limited capacity for fertilizer uptake by trees in plant-soil systems.


   ACKNOWLEDGMENTS
 
Financial support for the research was partly provided by the Natural Sciences and Engineering Research Council of Canada (NSERC). Assistance from University of Toronto open Fellowships, Faculty of Forestry and E.E. Johnson Postgraduate Forestry Awards are gratefully acknowledged. We also thank North-Gro Development Ltd. for help in growing seedlings and for providing storage facilities for hardening and winter storage of plants.

Received for publication October 19, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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