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DIVISION S-7—FOREST & RANGE SOILS

Nitrogen Accumulation by Conifer Seedlings and Competitor Species From ¹⁵Nitrogen-labeled Controlled-Release Fertilizer

^a Western Ag Innovations, 3-411 Downey Road, Saskatoon, SK, Canada S7N 4L8
^b Dep. of Soil Science, 51 Campus Drive, Univ. of Saskatchewan, Saskatoon, SK, Canada S7N 5A8

* Corresponding author (vanrees{at}sask.usask.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
A major impediment to the establishment of outplanted conifer seedlings is competition for available soil N by early successional species. The objective of this field study was to determine the fate of controlled-release fertilizer (CRF) N in soils with outplanted white spruce (Picea glauca [Moench] Voss.) and jack pine (Pinus banksiana Lamb.) seedlings, and the effect of weed control or vegetation management (VM) on fertilizer N accumulation. Nitrogen-15 labeled CRF was placed next to the seedling root plug during planting at four boreal mixed wood sites. After one growing season in the control plots, fertilizer N recovery as a percentage of ¹⁵N added was 4% in seedlings, 3% in competing vegetation, <1% leached, and 85% residual CRF. After two growing seasons, fertilizer N recovery was 15% in seedlings, 20% in competing vegetation, <1% leached, and 58% residual CRF. Overall, VM increased seedling fertilizer N uptake by almost 300% compared with conifer seedlings in control plots. In VM plots, fertilizer bags contained more N than in control plots after two growing seasons. In both treatments, >50% of the fertilizer N remained in the fertilizer bag, presumably remaining available in subsequent seasons. Calamagrostis (Calamagrostis canadensis) was the primary competitor for fertilizer N in both growing seasons, with minor competition from fireweed (Epilobium angustifolium L.), and aspen (Populus tremuloides Michx.). The use of a point source CRF delivery method resulted in high fertilizer use efficiency (FUE), and minimized losses to competing vegetation and leaching.

Abbreviations: CRF, controlled-release fertilizer • FUE, fertilizer-use efficiency • LSD, least significant difference • PVC, polyvinyl chloride • RCD, root collar diameter • VM, vegetation management

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN THE BOREAL FOREST, outplanted white spruce and jack pine seedlings are vulnerable to lethargic growth or mortality during the early establishment phase because of poor seedling uptake of nutrients, particularly N (Brockley, 1988; Robinson et al., 1998). Fertilization often is used to improve seedling survival and growth during this early establishment phase, alleviating the competition between outplanted conifer seedlings and early successional species for available soil N. In the past, the most commonly used method of applying nitrogenous fertilizer at time of planting was a broadcast application across the soil surface. However, this is an inefficient method for supplying N to outplanted seedlings in the boreal forest. Even when interspecific competition for fertilizer N is reduced through weed control or VM, very little of the broadcast fertilizer is taken up by the target seedling (Staples et al., 1999). Studies using ¹⁵N to examine the fate of broadcast nitrogenous fertilizers have reported low FUE, with uptake by the outplanted seedlings during the first year or two ranging from 0.4 to 10.1% of the applied fertilizer ¹⁵N (Preston and Mead, 1994; Staples et al., 1999), compared with 22.0% taken up by native early successional species (Preston et al., 1990). The excess fertilizer N not taken up by crop or noncrop vegetation is rapidly immobilized by soil microbes (Preston and Mead, 1994) or lost from the ecosystem by leaching or gaseous losses (Hulm and Killham, 1990; Preston et al., 1990; Houle and Babeux, 1994). Furthermore, depending on soil type and the rate of fertilizer application, losses of excess fertilizer N to runoff or leaching could lead to environmental problems with respect to groundwater contamination or eutrophication of surface waters (Fisher and Binkley, 2000).

Unlike broadcast applications of fertilizer N that often lead to growth stimulation of noncrop plant species (Chang et al., 1996; Thevathasan et al., 2000), the use of individual seedling fertilization methods that involve a point source of CRF, such as ‘tea bags’, are less likely to promote the growth of noncrop vegetation (Anonymous, 1995). These fertilizer bags are effective in alleviating a variety of site-limiting factors such as interspecific competition and poor soil fertility (Munson et al., 1993). Many studies have discussed the associated benefits and risks of using individual seedling fertilization methods on white spruce and jack pine growth (Burdett et al., 1984; Houle and Babeux, 1994; Anonymous, 1995). However, no attempts have been made using tracers, such as ¹⁵N, to determine the fate of added CRF-N during the early establishment phase after outplanting. The objective of this study was to determine the fate of CRF-N when applied with outplanted white spruce and jack pine seedlings in the boreal forest, and the effect of VM on uptake of fertilizer N by conifer seedlings.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Study Sites
Two white spruce and two jack pine field sites were established in the boreal mixed wood forests of Saskatchewan. One white spruce, and one jack pine site were located near Alcott Creek, approximately 40 km southeast of Meadow Lake, Saskatchewan (108°34' W long., 53°88' N lat., and 108°32' W long., 53°86' N lat., respectively). The second pair of white spruce and jack pine sites was located near Wabeno Lake, approximately 80 km northwest of Prince Albert, Saskatchewan (106°44' W long., 54°33' N lat. and 106°43' W long., 55°33' N lat., respectively). The Alcott sites were mechanically harvested and delimbed at the stump and the Wabeno sites were mechanically whole-tree harvested. All of the sites were harvested between the summer and fall of 1998. The soils at each site were classified as Typic Haplocryalfs (Orthic Gray Luvisols at the Alcott and Wabeno white spruce sites, and Brunisolic Gray Luvisols at the Alcott and Wabeno jack pine sites) that have developed on glacial till with clay loam surface textures (Alcott and Wabeno white spruce sites) or glacial till overlain by a layer of well-sorted sandy material and sandy loam (Alcott jack pine) and silty loam (Wabeno jack pine) surface textures (Rostad and Ellis, 1972; Head et al., 1981). Mean annual temperatures ranged from -1.6 to -1.0°C, with average January and July temperatures of -21 and 17°C, respectively, and approximately 80 frost-free days. The mean annual precipitation was 450 mm with 70% occurring from May to September (Rostad and Ellis, 1972; Head et al., 1981).

Experimental Design
The experimental design was completely randomized, with two VM treatments (with and without weed control) replicated three times at each site. Six treatment plots (12 by 12 m) were established at each site, and in the spring of 1999, 49 (seven rows of seven seedlings) 1-yr old container-grown seedlings of white spruce or jack pine were planted by a single planter within each treatment plot (2 m spacing or 2400 stems ha^-1). Plots were separated by a 5-m buffer zone containing seedlings planted at the same density as the treatment plots. At time of planting, four seedlings within each treatment plot were planted with a Reforestation Technologies International (RTI, Salinas, CA) fertilizer bag containing 5 g of polyurethane-coated, controlled-release (12–14 mo), ¹⁵N-labeled urea (7.5 atom% ¹⁵N enrichment). The fertilizer blend was 20-6-12-6 (N-P-K-S) with micronutrients (Zn, Fe, Mg, Cu, B, Mn, Mo). Using a 2 by 2 m seedling spacing, the equivalent fertilizer rate would be approximately 6 kg N ha^-1. The remainder of the treatment plot seedlings was planted with a fertilizer bag containing nonlabeled urea fertilizer of equivalent blend. The fertilizer bag was placed 5 to 7 cm from the root plug in the same planting hole. Seedlings in the buffer zone were planted without fertilizer bags.

Treatment plots either had no weed control (Control), or weed control (VM) during the growing season. Three times during each growing season, weed control within the VM plots was maintained by aboveground scalping at ground level of noncrop vegetation using brush saws. Vegetation within a 30 cm radius of each seedling was removed by hand. All vegetation growing around seedlings planted with ¹⁵N-labeled fertilizer was kept for ¹⁵N analysis.

Seedling Survival and Growth
Initial shoot height and root collar diameter (RCD) of the 25 measurement seedlings in the center of each treatment plot were recorded immediately following outplanting and at the end of the first and second growing seasons (September 1999 and September 2000, respectively). Seedling growth responses for each treatment plot were based on the mean of the 25 measurement seedlings.

Plant Sampling
In the control treatments, a 1-m² plot (seedling-centered) was placed over each ¹⁵N-labeled seedling prior to harvesting to determine percent cover and average height of the early successional species. The aboveground understory vegetation was harvested and separated into respective species prior to plant ¹⁵N analyses. At the end of the first growing season, two of the four ¹⁵N-labeled seedlings in each treatment plot were randomly harvested. The remaining two ¹⁵N-labeled seedlings were harvested after the second season. The ¹⁵N-labeled seedlings were excavated using small hand tools to recover the entire seedling root system and minimize fine root losses. Percentage of cover, average height, and ¹⁵N enrichment data for the conifer seedlings and early successional species for each treatment plot were based on the mean of the two sampled plots. Four random root cores (10-cm diam.) were taken within the 1-m² plot to a 20 cm depth to determine ¹⁵N content in the roots of early successional species.

Soil Sampling
Prior to excavating the ¹⁵N-labeled seedling, the remnants of the fertilizer bag were excavated. After the first growing season (103 d after fertilizer ¹⁵N addition), following the removal of the fertilizer bag, a soil core (30-cm depth) was removed from directly below the RTI bag and sectioned into 5-cm increments to quantify the leaching of the fertilizer N. After the second season (464 d after the fertilizer ¹⁵N addition), the soil was sampled to a depth of 60 cm and sectioned into 10-cm increments. In addition, at each site at the start of the experiment four polyvinyl chloride (PVC) tubes (10-cm diam. by 1-m length) were installed and a fertilizer bag placed in the tube at approximately the 7-cm depth in the mineral soil. The PVC tubes were used to quantify the vertical movement of fertilizer N without the confounding effects of competing root systems. At the end of each growing season, two of the tubes were excavated and the soil sectioned into 10-cm increments.

Nitrogen-15 Analysis
Shoots and roots of the excavated seedlings were separated and the roots washed free of soil. Seedling shoots and roots, aboveground biomass and roots of early successional species were oven-dried (60°C for 72 h) and weighed. All plant tissue samples were ground separately in a Wiley mill (0.425 mm mesh) and reground in a rotating ball-bearing mill. Plant tissues were analyzed for total N and ¹⁵N enrichment using a TracerMass mass spectrometer interfaced to a RoboPrep sample converter (Europa Scientific, Crewe, UK). Soil samples and excavated fertilizer bags were oven-dried (60°C for 72 h), ground in a rotating ball-bearing mill and analyzed for total N and ¹⁵N enrichment in the same manner as the plant samples.

Vector Diagnosis
Growth and N status (i.e., concentration and content) of the conifer seedlings grown in control and VM plots were evaluated by the vector diagnosis technique (Timmer, 1991; Haase and Rose, 1995). This technique has been applied previously to studies on the effect of VM and fertilization on the early growth of white spruce seedlings (Munson et al., 1993).

Statistical Analyses
Seedling growth parameters, percentage of cover, height, and ¹⁵N enrichment data for all sites were analyzed using the General Linear Models procedure in SAS (Version 8.0, SAS Institute Inc., Cary, NC). Means comparisons were performed using least significant differences (LSD) at a significance level of 0.05. Homogeneity of variances and normality of distributions of all data sets were checked before any statistical analysis was performed, and no data transformations were necessary.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Early Successional Species Distribution
The total biomass of noncrop vegetation present within control plots after the first growing season ranged from 71.88 g m^-2 (Alcott jack pine) to 213.78 g m^-2 (Alcott white spruce) (Table 1; see Appendix for Latin names). Calamagrostis, aspen, and fireweed were the major early successional species in the control plots during the first growing season on all sites. The Alcott white spruce site had the greatest total biomass production and diversity of early successional species. Considering all sites together, during the second growing season, the principal early successional species within the control plots were calamagrostis, aspen, fireweed, raspberry, and blueberry. The Alcott white spruce site continued to have the greater diversity and abundance of early successional species; however, the vegetation biomass on the other three sites increased considerably from the first growing season. At the end of the second growing season, the total biomass of noncrop vegetation present within the control plots ranged from 241.42 g m^-2 (Alcott jack pine) to 375.85 g m^-2 (Alcott white spruce) (Table 2).

Vector Diagnosis
The vector diagrams (Fig. 1) compare the relative responsiveness of white spruce and jack pine seedlings to VM in terms of seedling biomass (diagonal lines), N concentration (x-axis) and N content (y-axis) using the seedlings grown in the control plots as the reference which is normalized to 100. Only the response vectors for the Alcott white spruce site are drawn in Fig. 1, to reduce clutter. After both growing seasons, vector diagnosis revealed enhanced N uptake, N concentration, and biomass production (i.e., moving toward the right across diagonal lines) in conifer seedlings grown in VM plots compared with those grown in the control plots. This vector represents a typical deficiency response because of improved N availability (Shift C; Timmer, 1991). Vegetation management increased conifer seedling N uptake and biomass production up to 73 and 23%, respectively, after the first growing season (Fig. 1a). Nitrogen uptake and biomass increased by as much as 133 and 41%, respectively, after the second growing season, compared with the seedlings grown in control plots (Fig. 1b).

View larger version (32K): [in this window] [in a new window]   Fig. 1. Vector nomograms of relative differences in N concentration, N content, and conifer seedling biomass for vegetation management treatment plot seedlings after the (a) first and (b) second growing season. Biomass and N status of seedlings grown in control plots (i.e., no vegetation management) served as the reference and was normalized to 100. Only vectors associated with the site showing the strongest positive response (i.e., Alcott white spruce) to vegetation management are shown.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Unlike broadcast fertilizer N that is taken up by early successional species (Staples et al., 1999) or rapidly immobilized by soil microorganisms (Chang et al., 1997; Chang and Preston, 2000), the majority of the added N in this study remained in the fertilizer bag after the second growing season. When averaged over all sites, >58% remained in the fertilizer bag in the control plots and 73% in the VM plots. Varying the thickness of the polyurethane coating on the fertilizer pellets contained within the fertilizer bag allows for a more controlled-release of the fertilizer N over time (Garry Hargrove, Pursell Technologies Inc., personal communication, May 2001). Furthermore, placing the fertilizer bag at depth should minimize microbial immobilization because the majority of microbial activity in boreal forest soils occurs in the humus layer of the forest floor, with comparatively low activity in the mineral soil (Walley et al., 1996). Soil microorganisms often immobilize the majority of broadcast fertilizer N within hours of application (Chang et al., 1997; Chang and Preston, 2000) and this N can remain unavailable for uptake by outplanted seedlings or early successional species even 8 yr after application (Preston and Mead, 1994). Therefore, under broadcast fertilization, early successional species are relatively small sinks for the applied N compared to the amount of fertilizer N immobilized in the soil. The use of a CRF contained in a fertilizer bag placed at depth probably minimizes the loss of fertilizer N to this effective sink and ensures that the conifer seedling takes up more of the applied fertilizer N (Table 2).

Within the boreal forest, outplanted white spruce and jack pine seedlings often cease shoot growth by late July or early August because of a limited number of growing degree-days (Brand and Janas, 1988; Hudson, 2000). This short growing season may help to explain the limited seedling fertilizer N uptake during the first year because of insufficient root growth by the outplanted seedlings (Hudson, 2000). By the second year, however, there was greater root biomass on the outplanted seedlings and a proliferation of roots near the fertilizer bag was observed when excavating the seedlings. Localized proliferation of root mass is considered a response mechanism to exploit fertile microsites (Jackson et al., 1990; Robinson, 1996) and has been reported elsewhere (Krasowski et al., 1999). This proliferation of roots probably caused enhanced N uptake from the fertilizer bag, as was indicated by the apparent greater fertilizer N uptake during the second growing season (Table 4).

At the end of each growing season, there was very little movement of the fertilizer N below the fertilizer bag (Tables 5 and 6), with or without (i.e., PVC tube) competing root systems, suggesting that the fertilizer bags minimized fertilizer N losses to leaching. Regardless of the site, after the second year, the majority of the leached fertilizer N (i.e., within 10 cm of the fertilizer bag) would still be accessible to the white spruce and jack pine root systems at this depth (Strong and La Roi, 1983; Krasowski et al., 1996). Although there were no differences in leaching depth of fertilizer N between the sites, greater leaching of the fertilizer N was expected at the Alcott jack pine site because of the coarser-textured soils. During both growing seasons, the Alcott jack pine site received from 35 to 40% lower than normal rainfall (data not shown). The limited leaching of fertilizer N at this site may in part be attributed to the relatively low amount of precipitation received.

Relative Competitiveness of Early Successional Species for Fertilizer Nitrogen
The lower fertilizer N uptake by conifer seedlings grown in control plots compared with VM plots is the result of two competitive mechanisms: direct competition for the uptake of applied fertilizer N with early successional species, and indirect competition by the early successional species for other site resources, thus inhibiting overall conifer seedling growth, resulting in reduced fertilizer N uptake. Optimizing silvicultural practices (i.e., VM and fertilization) in young conifer plantations requires an understanding of the relative competitiveness of early successional species, which affect the survival and growth of outplanted seedlings during the early establishment phase (Bell et al., 2000). Early successional species distribution within the ¹⁵N-labeled seedling measurement plots indicated that the majority of the principal competitors in boreal forest plantations were present (Bell, 1991); however, only calamagrostis, fireweed, and aspen were able to take up measurable amounts of the fertilizer N. The lack of fertilizer N uptake by other very competitive species was surprising. This is especially true for red raspberry, which is a strong competitor for site resources (Bell, 1991) and was abundant at the Wabeno sites. In the present study, calamagrostis was the only species to accumulate fertilizer N during the first growing season and the primary species in the second growing season, although there were numerous woody species present in the ¹⁵N-labeled seedling measurement plots. This supports the highly competitive nature of this perennial grass species reported by others (Eis, 1981; Lieffers et al., 1993).

The ability of early successional species to take up the applied fertilizer N is dependent on root morphological and physiological characteristics. The capacity of calamagrostis to absorb N released from the fertilizer bag is indicative of its capability to exploit soil resources through an extensive fibrous root system (Ländhausser and Lieffers, 1994; Hangs, 2001). Hangs (2001) reported calamagrostis to exhibit the greatest maximal uptake rates (i.e., I_max) and affinity (i.e., K_m) for NH⁺₄ and NO^-₃ uptake among selected early successional species in the boreal forest. Furthermore, the relative competitiveness of calamagrostis to inhibit shoot and root growth and fertilizer N uptake by planted white spruce and jack pine seedlings was greater than aspen and fireweed (Hangs, 2001). Calamagrostis was capable of taking up over 60% of the applied NH₄NO₃ after the 90-d growth period, compared with <10% uptake for fireweed and <3% uptake for aspen (Hangs, 2001).

Finally, the lower amount of fertilizer N remaining in the fertilizer bags within control plots compared with the treated plots at the end of the second growing season is because of uptake by early successional species. Furthermore, the lower fertilizer N uptake by early successional species at the Wabeno jack pine site compared with the other sites probably is a function of the lower abundance of calamagrostis at this site (data not shown).

Conifer Seedling Growth
Interspecific competition affected conifer shoot growth by the end of the first growing season and continued through the second growing season. Outplanted conifer seedlings grown within the control plots (i.e., no VM) had smaller shoot growth increments compared with seedlings growing in the VM plots at all sites except the Alcott jack pine site (Table 3). Although white spruce and jack pine seedling height generally does not show a consistent response to competition (Brand, 1990; Morris et al., 1990), the height growth increment of both of these conifers after two growing seasons was reduced by competitive pressure from early successional species within the control plots. The sensitivity of white spruce and jack pine stem diameter growth to competitive conditions is well documented (Sutton, 1995). In our study both white spruce and jack pine seedlings within the control plots had lower RCD growth compared with the VM plots. The influence of interspecific competition on height and RCD growth directly affects the stem volume and signifies a response to root competition (Ländhausser and Lieffers, 1998).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
When fertilizer N uptake is expressed as a percentage of the fertilizer N released from the fertilizer bag, results from this study support the hypothesis that using a point source CRF delivery method promotes high FUE by outplanted white spruce and jack pine seedlings. Despite some accumulation of fertilizer N by calamagrostis, the use of a CRF contained in fertilizer bags resulted in minimal losses to competing vegetation within the ¹⁵N-labeled seedling measurement plots. Except for minor accumulation of fertilizer N by fireweed and aspen during the second growing season, there was no fertilizer N uptake by any other early successional species after 2 yr. Unlike broadcast applications, where microbial N immobilization reduces fertilizer N availability, the use of CRF resulted in more than 50% of the original fertilizer N remaining available for conifer seedling uptake after two growing seasons. Some leaching of the fertilizer N occurred, although the fertilizer N still was accessible by the conifer seedling root system.

Although the CRF-N was applied in a fertilizer bag placed close to the conifer seedling root plug, calamagrostis was capable of accumulating significant amounts of the fertilizer N. Future research is needed to assess the fate of fertilizer ¹⁵N using other individual seedling fertilization methods (i.e., nutrient-loaded root plugs containing CRF). Finally, the ability of calamagrostis to successfully compete with the outplanted conifers for the applied fertilizer N, despite the placement of the fertilizer close to the seedling root plug, clearly indicates the tenacity of this grass species in the field during the early establishment phase. Prioritizing the removal of calamagrostis from plantations should increase the FUE and benefit the early growth of outplanted white spruce and jack pine seedlings in boreal forest sites.

APPENDIX

Early successional species common names and Latin nomenclature.

Common name Latin nomenclature Aspen Populus tremuloides (Michx.) Bicknell's geranium Geranium bicknelii Britt. Birch Betula papyrifera Marsh. Bishop's-cap Mitella nuda L. Blueberry Vaccinium angustifolium Ait. Bunchberry Cornus canadensis L. Calamagrostis Calamagrostis canadensis   (Michx.) Beauv. Canada thistle Cirsium arvense (L.) Scop. Dandelion Krigia biflora (Walt.) Blake Dewberry Rubus pubescens Raf. Dry-ground cranberry Vaccinium vitis-idaea (Lodd) Fireweed Epilobium angustifolium L. Gooseberry Ribes oxyacanthoides L. Green alder Alnus crispa (Ait.) Pursh. Honeysuckle Lonicera involucrate Horsetail Equisetum sylvaticum L. Jack pine Pinus banksiana Lamb. Labrador tea Ledum groenlandicum Lindley's aster Aster ciliolatus Lindl. Low bush cranberry Viburnum edule (Michx.) Raf. Narrow-leafed hawk's-beard Crepis tectorum L. Palmate-leafed colt's-foot Petasites palmatus (Ait.) A. Gray Pea vine Lathyrus venosus Muhl. Raspberry Rubus idaeus L. Rose Rosa acicularis Lindl. Sarsaparilla Aralia nudicaulis L. Solomon's-seal Smilacina trifolia (L.) Desf. Starflower Trientalis borealis Raf. Strawberry Fragaria virginiana Twinflower Linnaea borealis L. Vetch Vicia Americana Muhl. White spruce Picea glauca (Moench) Voss Willow Salix humilis Marsh.

	ACKNOWLEDGMENTS

 
The authors thank M. Young of Weyerhaeuser Saskatchewan Ltd. and R. Nesdoly of Mistik Management Ltd. for providing the field sites and ¹⁵N for this study, D. Jackson, B. Bailey, R. Block, M. Bock, D. Greenough, and S. Lieffers for providing assistance in the field, M. Stocki for mass spectrometry analysis, and N. Anderson (Reforestation Technologies International, Salinas, CA) for preparing and supplying the controlled-release fertilizer bags used in this research. The Natural Sciences and Engineering Research Council of Canada funded this study. Publication No. R885, Saskatchewan Centre for Soil Research, Saskatoon, SK Canada.

Received for publication September 10, 2001.

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