Inoculation of fumigated nursery soil increases nodulation and yield of bare-root red alder (Alnus rubra Bong.)

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  • NewForests 5: 35--42, 1991. 1991 Kluwer Academic Publishers. Printed in the Netherlands.

    Inoculation of fumigated nursery soil increases nodulation and yield of bare-root red alder (Alnus rubra Bong.)

    A. B. HILGER 1, Y. TANAKA 2 and D. D. MYROLD l* Department of Crop and Soil Science, Oregon State University, Strand Agriculture Hall

    202, Corvallis, OR 97331-2213, USA; 2 Weyerhaeuser Co., Western Forestry Research Center, 505 North Pearl Street, Centralia, WA 98531, USA (*requests for offprints)

    Received 1 February 1989; accepted 11 March 1991

    Key words: actinorhizae, fertilization, Frankia, nitrogen, nodulation

    Application. Inoculation of a fumigated nursery bed with alder soil (1.9 kg m -z) resulted in earlier and greater nodulation of bare-root red alder seedlings, and increased yield of packable seedlings by 43%.

    Abstract. To determine if inoculation increases nodulation and yield of bare-root red alder (Alnus rubra Bong.), fumigated nursery plots were treated with inoculum and ammonium sulfate (28 kg N ha -a) in a factorial experiment. Inoculum was alder soil with 100 infective units of Frankia g-L Seedlings were evaluated for nodulation at age 10 wk and when lifted, at age 9 mo. Inoculation produced earlier and more extensive nodulation and increased seedling root collar diameter, height, and dry weight. Fertilization decreased seedling height, but did not decrease nodulation. No interaction of fertilization with inoculation was found. Inoculated unfertilized plots had the highest yield of packable seedlings (257 m-2), and uninoculated fertilized seedlings had the lowest yield (126 m-2).


    Red alder (Alnus rubra Bong.) is a potentially valuable species in inten- sively managed forests (Tarrant et al. 1983). Until recently, efforts to control this species, as a competitor of softwoods, have exceeded efforts to promote its culture. Quality red alder planting stock is needed but no guidelines exist for raising bare-root red alder.

    The purpose of this study was to determine if inoculation increased nodulation and growth of red alder seedlings in fumigated nursery beds. Alder seedlings are normally supplied with nitrogen fixed symbiotically, by Frankia in root nodules; alder is always nodulated in natural habitats (Bond 1976). Frankia spores are produced within sporangia in root nodules (Torrey 1987), with no mechanism for spore release into air; thus, long-range dispersal probably requires movement of soil. This process can

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    be very slow; Arveby and Huss-Danell (1988) found no evidence of Frankia dispersal into peat-bog plots from nearby alder stands over several years. Because fumigation of nursery beds might reduce the Frankia population below that required to nodulate seedlings, inoculation may be necessary.

    Nursery beds were amended with inoculum and ammonium sulfate, in factorial combinations. Inoculation increased seedling nodulation and growth. Fertilization reduced growth but did not inhibit nodulation. We concluded that inoculation was necessary for adequate nodulation and growth of seedlings in fumigated beds.

    Methods and materials

    Experimental design

    A randomized complete block design was used in a factorial experiment, with four treatments: inoculated, fertilized; not inoculated, fertilized; inoculated, not fertilized; and not inoculated, not fertilized. There were 4 replicates. The bed was divided into four blocks and the treatments randomly assigned within each block to plots 0.61-m long and 1.2-m wide, with a 15-cm buffer zone between each plot.

    Plot preparation

    Bed space at a nursery near Mima, Washington, was supplied by Weyerhaeuser Co. Beds were fumigated with methyl bromide-chlorpicrin at 389 kg ha -1, injected at 15 to 20-cm depth in September 1986, then covered with a tarpaulin for 1 week after fumigation.

    Inoculum soil was collected from the surface horizon in a young red alder stand near Corvallis, Oregon, and stored at 4 C (field-moist) for about 1 mo until used. Uninoculated bed soil and inoculum soil were bioassayed for Frankia infective units, as described below. Inoculum soil (1.9 kg m -2) was spread over each plot, and raked in to 5-cm depth, just before seeding.

    Red alder seeds, collected locally, were germinated to determine the number of pure live seed (PLS) per gram. Plots were broadcast-seeded with about 990 PLS m -2 on 29 April 1987. To protect seed from desiccation, Reemay polyester sheet (Ken-Bar Inc., 24 Gould St., Reading, Mass. 01867) was applied after sowing, and removed 19 June 1987. This cover is permeable to light, water, and air. Plots were irrigated when

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    necessary. Ammonium sulfate (13.5 g m -z, 28 kg N ha-l), dissolved in water (1 1 plot-~), was applied 10 wk after seeding.

    Frankia bioassay

    Soil samples were bioassayed for Frankia infective units (IU) by a plant infection method, using most probable number (MPN) technique. Seeds were surface-sterilized (30% hydrogen peroxide, 20 min), sown on auto- claved acid-washed quartz sand, stratified at 4 C for 1 wk, and germi- nated at 27 C, with light. Germinants, with a radicle at least 2.5 cm long, were transferred to 50-ml centrifuge tubes containing 45 ml of 1/4- strength, N-free Hoagland's solution (Hoagland and Arnon 1950). The stem was held in a hole in the tube cap by a piece of foam stopper. Seedlings were grown for 2 wk (27 C day/22 C night, 16-h daylength) before inoculation with soil dilutions.

    Ten grams of sieved (2-mm mesh), field-moist soil, was suspended in 95 ml sterile distilled water, and ten glass beds added. After this dilution was shaken 100 times by hand, successive 10-fold dilutions were made in sterile distilled water, down to 10-Sg m1-1. Ten seedlings were inoculated, with 1 ml each, for each dilution, by running the inoculum over the root into the bioassay tube. Ten control seedlings received no inoculum. Nutrient solution was replaced every 2 wk, and at 6 wk, seedlings were examined for nodules. The numbers of nodulated seedlings at each of three successive dilutions were used to calculate the MPN (Koch 1981) of Frankia IU g-lsoil (dry weight). Uninoculated nursery soil (0--10 cm) contained fewer Frankia than bioassay can detect (1 IU g-~). The inoculum contained 100 IU g-l; thus, inoculated soil contained about 2 IU g-l, in the upper 0--5 cm.

    Seedling evaluations

    Early nodulation, at seedling age 10 wk, was estimated by randomly selecting 5 seedlings per plot, and examining roots for nodules. On 19 January 1988, all seedlings were lifted by hand, counted, and measured for height between the apical bud and the soil surface. A randomly selected 20% subsample of seedlings from each plot were also evaluated for root collar diameter (RCD, measured 2 cm above the soil surface), number of nodules, and dry weight. Roots were washed before counting the total number of nodules and the number of large (diameter > 1 cm) nodules. Roots and shoots were dried (70 C) and weighed. Seedlings were arbitrarily considered packable if at least 20 cm tall.

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    Statistical methods

    Three-way analyses of variance (ANOVA) of block x fertilizer x inoculum were done, with fertilizer x inoculum interaction included as a source of variation. Two-way ANOVAs (treatment x block) were also done to allow comparisons of treatment means. Logarithm (ln) transformation of RCD, height, dry weight, and number of nodules per plant were necessary to normalize these data; thus, plot means of log values were used in ANOVAs for these parameters. Means separation was done with Fisher- protected LSD.


    No significant (P < 0.05) fertilization x inoculation interactions were found; therefore, the effects of these two factors were evaluated independ- ently. Inoculation increased seedling nodulation, growth and yield of packable seedlings (Table 1). Inoculation increased yield by an average of 43% _ 18% (P -- 0.05). Fertilization decreased yield, but had no effect

    Table 1. Inoculation and fertilization effects on mean a number, size and nodulation of bare-root red alder seedlings, n = 8.

    Inoculation Fertilization

    Parameter (_) (+) pb (_) (+) pb

    Number at lifting Total (m -z) 443 428 0.5338 461 412 0.0898 Packable c (m -z) 159 227 0.0005 225 161 0.0007

    Size at lifting Root collar (mm) 3.2 4.1 0.0001 3.8 3.5 0.1720 Height (cm) 21.2 33.5 < 0.0001 29.5 25.2 0.0195 Shoot (g) 1.1 2.1 0.0001 1.8 1.4 0.3771 Root Dry (g) 1.0 1.4 0.0018 1.3 1.1 0.4712

    Nodulation At 10 weeks (%) 10 80 < 0.0001 43 48 0.6665 At lifting (%) 77 97 < 0.0001 89 85 0.1984 At lifting (plant -l) 1.8 4.9 < 0.0001 3.3 3.4 0.8157

    " ANOVAs for mean root collar diameter, height, dry weight, and nodules plant -~ used mean In(value) to determine main effects. Arithmetic means shown here. b Significance of factor main effect, in ANOVA. c Seedlings at least 20 cm tall.

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    on other parameters. Inoculated unfertilized seedlings were tallest; thus, this treatment produced the most packable seedlings (Table 2).

    Inoculation greatly improved early nodulation; eight times more nod- ulated seedlings were found in the inoculated than in uninoculated treat- ments, at 10 wk after seeding (Table 1). At this stage of growth, inoculated plants were already greener than the uninoculated plants. At lifting, nodulation of inoculated seedlings was still greater than of uninoculated seedlings, but 77% of the uninoculated seedlings were nodulated. Inocu- lated seedlings had more nodules per plant than did uninoculated seed- lings (Table 1). Inoculated seedlings often had nodules close to the root collar, in contrast to more distal nodules on uninoculated seedlings. We speculate that uninoculated roots only became nodulated when the