Inoculating Acacia Koa with Bradyrhizobium and Applying

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HORTSCIENCE 44(2):443–446. 2009. trees can enhance species richness (Parrotta, 1992). For these reasons, interest in planting koa has increased (Pejchar and Press, 2006), Inoculating Acacia koa with especially to renew native forest bird habitat (Horiuchi and Jeffrey, 2002). Bradyrhizobium and Applying On disturbed tropical sites, planting tree seedlings may accelerate forest recovery, but Fertilizer in the Nursery: Effects success can be hampered by grass competition, poor soil nutrient availability, changes in soil conditions, harsh microclimates, and a on Nodule Formation and lack of symbiotic microorganisms (Denslow et al., 2006; Holl et al., 2000; Horiuchi and Seedling Growth Jeffrey, 2002; Scowcroft and Jeffrey, 1999) 1 with grass competition paramount (Holl R. Kasten Dumroese et al., 2000). Many sites on the island of USDA Forest Service, Rocky Mountain Research Station, 1221 South Main Hawaii are infested with the introduced Street, Moscow, ID 83843-4211 invasive Pennesitum clandestinum (kikuyu grass) (Motooka et al., 2003); this rhizoma- Douglass F. Jacobs tous grass forms dense, thick mats that Hardwood Tree Improvement and Regeneration Center, Department of Forestry impede artificial regeneration of koa forests and Natural Resources, Purdue University, West Lafayette, IN 47907 (Jeffrey and Horiuchi, 2003). In temperate forest regeneration, large nursery-grown Anthony S. Davis seedlings are preferred for outplanting on Center for Forest Nursery and Seedling Research, College of Natural sites where competition is severe because of Resources, University of Idaho, Moscow, ID 83844 improved performance compared with smaller cohorts (Jobidon et al., 1998; South Additional index words. container nursery, controlled-release fertilizer, nitrogen use effi- and Mitchell, 1999). For restoration of koa ciency, nitrogen-fixing bacteria, forest restoration forests and their important understory plants needed by endangered species, more rapid Abstract. Restoration of Acacia koa A. Gray (koa) forests on degraded sites in Hawaii is initial seedling growth gained by outplanting important for conservation of rare, endemic plants and animals and is often accomplished larger stock types may lead to quicker canopy by planting nursery-grown seedlings. To be successful after outplanting, koa seedlings closure. Canopy closure initiates the desired must access sufficient nutrients from the soil and outcompete other vegetation. Forming process of reducing competitor biomass, symbiotic associations with nitrogen-fixing microorganisms may help alleviate inherent making soil chemical properties more like nutrient deficiencies found on volcanic sites, but interactions between inoculation and those found in original forests and enhancing fertilization during production of koa seedlings are not well studied. Under operational establishment of desired understory species nursery conditions, we fertilized koa seedlings at six controlled-release fertilizer (CRF) (Scowcroft et al., 2004; Scowcroft and Jef- –3 rates (ranging from 0 to 11.9 kgÁm Osmocote Plus 15N–9P–12K), which were also either frey, 1999). inoculated with a locally collected source of nitrogen-fixing Bradyrhizobium spp. or not. Volcanic soils are inherently deficient in Fertilized seedlings were significantly larger than the control; on average, fertilized nitrogen (N) (Vitousek and Farrington, 1997) seedlings were 300% taller and had 180%, 700%, and 350% more root-collar diameter, and N availability is a primary factor limiting shoot biomass, and root biomass, respectively. Nitrogen (N) concentration was 23% to seedling growth (Nambiar and Sands, 1984). 119% greater in fertilized seedlings than nonfertilized seedlings. Inoculation with Koa, however, forms a symbiotic relationship Bradyrhizobium had no significant affect on seedling characteristics except for root N with Bradyrhizobium spp. (Allen and Allen, concentration, which was 8% higher for inoculated seedlings. Fertilizer rate and 1936), widespread N-fixing bacteria found in inoculation interacted to significantly affect nodule number and nodule biomass/seedling. tropical soils (Graham and Hubbell, 1975; With inoculation, we observed an increase in nodule number (16.5 to 25.5) and biomass Nakao and Kitayama, 1996; Woomer et al., –3 (5.8 mg to 11.7 mg) with fertilizer rate up to 4.7 kgÁm but a decline to nearly zero at the 1988), and this partnership results in abun- highest rates. Without inoculation, nodule formation was scarce (3.2 nodules/seedling or dant soil N availability (Idol et al., 2007; –3 less) and only observed when CRF was 2.3 kgÁm or less. Our results suggest that high Scowcroft et al., 2004). Inoculating seeds or rates of fertilization and successful nodulation are not mutually exclusive nursery nursery stock of other Acacia species with practices. Concurrent use of fertilization and inoculation in the nursery may provide rhizobial inoculants has been shown to opportunity to produce larger seedlings that have high potential to reduce the time increase survival (Thrall et al., 2005) and needed to achieve canopy closure and thereby help achieve restoration objectives. biomass production on field sites (Herrera et al., 1993; Lal and Khanna, 1996). One reason for such improvement is that bacteria Acacia koa A. Gray (koa) is a nitrogen- fix atmospheric N that is available to seed- fixing, endemic Hawaiian hardwood tree that lings during the critical establishment phase, can grow rapidly (2.2 m/year) under optimum mitigating poor soil nutrient availability that Received for publication 17 Dec. 2008. Accepted conditions (Cole et al., 1996) and has high reduces tropical forest regrowth (Aide and for publication 13 Jan. 2009. economic value for specialty wood products Cavelier, 1994). It is well known, however, This project was supported by the USDA Forest (Yanagida et al., 2004). Koa has, however, an that increasing levels of mineral nutrition Service Center for Reforestation, Nurseries, and even higher ecological value because it is the provided by fertilization decrease nodule Genetics Resources; Purdue University; the Uni- principal, montane-forest tree that is critical formation and function (Goicoechea et al., versity of Idaho Center for Forest Nursery and for survival of several endemic, endangered 2004; Patreze and Cordeiro, 2004). Deter- Seedling Research; and the Hawaii Division of Hawaiian bird species (Scott et al., 1986). mining fertilization regimes that promote Forestry and Wildlife. We thank Ian Shigematsu, Mee-Sook Kim, Joanne Past agronomic practices destroyed or production of larger seedlings that are better Tirocke, and Amy Ross-Davis for their valuable degraded much of the koa forest (Scowcroft able to outcompete associated vegetation, contributions to the experiment. and Jeffrey, 1999), but plantations are an while concurrently providing seedlings with 1To whom reprint requests should be addressed; effective means to rapidly increase koa symbionts needed after outplanting, could e-mail [email protected]. cover; increasing cover of dominant tropical reduce nursery production time. We hypoth-

HORTSCIENCE VOL. 44(2) APRIL 2009 443 esized that Bradyrhizobium nodule number inoculated half of the seedlings in each weight (mg), and nitrogen (gÁkg–1)within and biomass on container Acacia koa seed- fertilizer treatment with 10 mL of solution shoots and roots (pooled across seedlings lings would decrease with increasing fertil- per seedling to provide 107 colony-forming within treatment replicates; n = 5). Compar- izer rates and that at some level, fertilization units/seedling. The other half was not inoc- isons within predictor variables were analyzed would exclude nodule formation. Our objec- ulated. Thus, our experiment was a 6 (fertil- using least-squares means of fixed effects with tive was to ascertain the effects of increasing izer levels) · 2 (inoculation levels) · 4 a multiple comparison Tukey adjustment for fertilizer rates on Bradyrhizobium nodule (replications) randomized complete block the P values. Because nodule data violated formation and biomass under operational split-plot design with fertilizer as the parametric assumptions, nodule number and nursery conditions. whole-plot factor and inoculation as the biomass/seedling were log-transformed (add- split-plot factor. Each fertilizer–inocula- ing 0.5 to each observation given the presence Materials and Methods tion–replicate whole plot consisted of a tray of zeros). Means and SEsreflectuntransformed containing 50 D-16 containers; one half of data. In early Dec. 2006, following the basic each tray was inoculated (1200 total seed- methods of Wilkinson and Elevitch (2003), lings). Trays were randomly positioned (with Results we collected 100 Bradyrhizobium spp. 15 cm separating each tray) on benches in nodules from koa roots (roots were traced an outdoor growing area. Fertilizer rate significantly affected every back to the parent plant) at Hakalau Forest In Apr. 2007, we randomly harvested five measured response variable (Tables 1 and 2). National Wildlife Refuge on the island of plants from the centermost containers (avoid- On average, fertilized seedlings had signifi- Hawaii (long. 1949#15$ N, lat. 15519#48$ ing seedlings growing on the perimeters of the cantly larger heights (300%), root-collar W; 1941 m), rinsed them clean of soil, replications to avoid edge effects) from each diameters (180%), shoot biomass (700%), blended them in 1 L of distilled water, and fertilizer–inoculum–replicate combination (120 and root biomass (350%) than the control, refrigerated the solution in an air-tight con- sampled seedlings) and gently washed root but increasing fertilizer greater than 2.3 tainer at 4 C. Concurrently, following the systems free of medium. We measured root- kgÁm–3 did not significantly increase seedling general guidelines of Rice and Olsen (1993), collar diameter and shoot height. Nodules size (Table 2). Tissue N concentrations, we diluted the slurry, plated samples on a were counted and harvested. For each seed- however, were significantly greater with yeast extract mannitol agar media (pH 6.8 to ling, we observed three nodules in cross- increasing fertilizer rate (Table 2). The high- 7.2), incubated the inverted petri plates for 7 section using a 10· hand lens; starting at the est rate of controlled-release fertilizer (CRF) dat30C, and then counted colonies. In early midpoint of the root plug, we excised the first resulted in a 55% and 218% increase in root Jan. 2007 at the Hawaii Division of Forestry proximal, acropetal nodule, then the first and shoot N concentration, respectively, and Wildlife Kamuela (Waimea) State Tree proximal, basipetal nodule, repeating the compared with the nonfertilized control. Nursery, just south of Kamuela on the island of sequence until three nodules (if available) Lack of a significant increase in seedling Hawaii (long. 2000#24$ N, lat. 15540#38$ were collected. Shoots, roots, and nodules height, root-collar diameter, or total biomass W), we transplanted germinating (7 d after were separated and oven-dried 72 h at 65 C in the presence of increasing N concentration hot water soak) koa seeds collected at Haka- for biomass determination. Shoots and roots for seedlings fertilized with greater than 2.3 lau into Deepot D-16 containers (262 cm3;5- for each fertilizer–inoculation combination kgÁm–3 indicates luxury consumption (Table cm diameter and 18-cm depth; Stuewe and were pooled by replicate, ground to pass a 2). Nitrogen use efficiency decreased with Sons, Inc., Corvallis, OR) filled with a two 0.04-mm mesh, and analyzed with a LECO- increasing rate of CRF; efficiency was 17% parts Sphagnum peatmoss (Pro-Mix; Premier 600 (LECO Corp., St. Joseph, MI). Nitrogen in the 2.3 kgÁm–3 CRF treatment compared Horticulture, Dorval, Quebec, Canada) to one use efficiency was calculated by dividing the with 9% in the 11.9 kgÁm–3 treatment. part perlite (v:v) medium containing one of nutrient content (tissue concentration · bio- Although inoculation did not affect seed- six levels of controlled-release fertilizer [0, mass) of the entire seedling by the total ling size, it did significantly affect the num- 2.3, 4.7 (the ‘‘medium’’ rate on the fertilizer amount of N applied to the seedling. ber and biomass of nodules per seedling and label), 7.1, 9.5, and 11.9 kgÁm–3 Osmocote Analysis of variance using the SAS (SAS root N concentration (Table 1). Nearly all PlusÒ 15N–9P–12K (5- to 6-month longevity Inc., Cary, NC) PROC MIXED (a =0.05) nodules showed the pinkish red coloration of at 21 C; Scotts Co., Marysville, OH)]. These procedure was used to examine the effects of leghemoglobin, suggesting active N fixation. incorporation rates reflect the volume of the our predictor variables on eight koa seedling Inoculated seedlings had more (11.5 ± 1.9 medium after it was released from the bag. response variables: height (cm), root-collar versus 0.7 ± 0.3) nodules and greater nodule Within 2 d of transplanting, we diluted the 1 diameter (mm), shoot dry weight (g), root dry biomass/seedling (4.9 ± 0.9 mg versus 0.5 ± L of inoculum into 6 L of tap water, and weight (g), number of nodules, total nodule 0.2 mg) than noninoculated control seed-

Table 1. Sources of variation and P values observed for Acacia koa seedlings inoculated with Bradyrhizobium, fertilized with six rates of controlled-release fertilizer, and grown 12 weeks in a container nursery. Source of Root-collar Shoot Root Nodule Number of variation Ht diam biomass biomass biomass nodules Shoot N Root N Fertilization <0.0001 <0.0001 0.0003 0.0013 0.0003 0.0002 <0.0001 <0.0001 Inoculation 0.7925 0.9240 0.6766 0.1391 <0.0001 <0.0001 0.1547 0.0031 Interaction 0.8338 0.4864 0.7363 0.6411 <0.0001 <0.0001 0.1791 0.1390

Table 2. Means ± SEs for morphological parameters of Acacia koa seedlings inoculated with Bradyrhizobium, fertilized with six rates of controlled-release fertilizer, and grown 12 weeks in a container nursery.z Fertilizer Root-collar Shoot Root Shoot N Root N rate (kgÁm–3) Ht (cm) diam (mm) biomass (g) biomass (g) (gÁkg–1) (gÁkg–1) 0 6.8 ± 0.2 a 1.91 ± 0.03 a 0.20 ± 0.01 a 0.14 ± 0.01 a 17.6 ± 0.9 a 17.3 ± 0.6 a 2.3 17.3 ± 0.7 b 3.07 ± 0.09 b 1.12 ± 0.08 b 0.48 ± 0.03 b 21.7 ± 0.6 ab 19.2 ± 0.5 ab 4.7 18.8 ± 0.7 b 3.24 ± 0.07 b 1.22 ± 0.07 b 0.45 ± 0.03 b 27.5 ± 2.9 b 21.7 ± 0.8 bc 7.1 21.0 ± 1.1 b 3.38 ± 0.12 b 1.55 ± 0.13 b 0.50 ± 0.04 b 29.3 ± 1.3 c 24.8 ± 0.7 cd 9.5 23.1 ± 1.1 b 3.67 ± 0.13 b 1.69 ± 0.12 b 0.50 ± 0.04 b 37.4 ± 2.3 d 27.8 ± 0.9 d 11.9 22.7 ± 1.1 b 3.76 ± 0.12 b 1.81 ± 0.13 b 0.56 ± 0.05 b 38.5 ± 1.6 d 26.8 ± 1.1 d zMeans for each parameter with the same letter are not signiﬁcantly different at a = 0.05. n = 20 for height, root-collar diameter, and biomass; n = 4 for N concentration.

444 HORTSCIENCE VOL. 44(2) APRIL 2009 tively; for inoculated seedlings, mean bio- the presence of leghemoglobin. Seedlings mass/nodule was 0.4, 0.4, 0.4, 0.7, 0.4, and treated with the ‘‘low’’ label rate were 1.1 as CRF increased from 0 to 11.9 kgÁm–3. 68% taller with 52% more root-collar diameter than the traditional ‘‘dibble tube’’ Discussion (50 cm3; 2.5-cm diameter and 12-cm depth) seedlings currently used for restoration plant- Many studies stress the importance of ings on the island of Hawaii (Walters, 1981), inoculating seedlings with the correct sym- even though dibble tube seedlings were biont microorganism to achieve optimum grown at the ‘‘medium’’ label rate and all nodule formation (Galiana et al., 1990; Thrall seedlings had similar production times in the et al., 2000; Valladares et al., 2002; Wolde- nursery. meskel and Sinclair, 1998). In our study, Our 2.3 kgÁm–3 CRF rate was sufficient for inoculating nursery seedlings with a simple balancing koa seedling size and nutrient slurry of ground Bradyrhizobium nodules content with Bradyrhizobium inoculation collected from koa trees yielded subse- and nodule biomass production within the quently high levels of nodulation, akin to same production period as the current oper- work by Villar-Salvador et al. (2008). We ational standard. Future work on outplanting also observed a few sporadic nodules in the sites is needed to validate whether larger, absence of inoculation, similar to that well-fertilized koa seedlings with high levels reported by Lesueur and Duponnois (2005) of nodules have increased survival and on another Acacia species. These nodules growth compared with seedlings fertilized were most commonly found on nonfertilized, to maximize biomass and nutrient content noninoculated seedlings that, in their nutri- regardless of effects on nodulation. ent-stressed status, may have had a higher proclivity for infection. This response is Literature Cited similar to that of nonfertilized but inoculated Aide, T.M. and J. Cavelier. 1994. Barriers to seedlings, which also had the highest rates of lowland tropical forest restoration in the Sierra nodulation. Because seedlings were grown in Nevada de Santa Marta, Columbia. Restor. an outdoor compound under operational cul- Ecol. 2:219–229. tural conditions, contamination of noninocu- Allen, O.N. and E.K. Allen. 1936. Root nodule lated seedlings may have occurred by bacteria of some tropical leguminous plants: I. Fig. 1. Untransformed mean number of nodules (A) windborne deposition of soil particles or Cross inoculation studies with Vigna sinensis and total nodule biomass (B)ofcontainerAcacia through transfer by nursery workers, insect L. Soil Sci. 42:61–77. koa seedlings 12 weeks after germination by vectors, or precipitation/irrigation events. Cole, T.G., R.S. Yost, R. Kablan, and T. Olsen. controlled-release fertilizer and Bradyrhizobium Low doses of fertilizer may improve 1996. Growth potential of twelve Acacia spe- treatments. Bars indicate SE of the means, n = 4. nodule formation by N-fixing bacteria (Goi- cies on acid soils in Hawaii. For. Ecol. Mgt. coechea et al., 2004; Röhm and Werner, 80:175–196. Denslow, J.S., A.L. Uowolo, and R.F. Hughes. lings. Root N concentration of inoculated 1991). In our study using CRF, the lowest –3 2006. Limitations to seedling establishment in seedlings was 8% greater than control seed- rate of fertilizer (2.3 kgÁm ), which was a mesic Hawaiian forest. Oecologia 148: lings (2.38% ± 0.09 versus 2.21% ± 0.09). equal to the ‘‘low’’ label rate, yielded the 118–128. Fertilizer and inoculation significantly same nodule number and nodule biomass/ Galiana, A., J. Chaumont, H.G. Diem, and Y.R. interacted to affect nodule number and bio- seedling as the nonfertilized control, Dommergues. 1990. Nitrogen-fixing potential mass/seedling (Table 1; Fig. 1). For both although plant biomass increased by 470%. of Acacia mangium and Acacia auriculiformis response variables, the pattern was similar. Moreover, doubling the rate of CRF appeared seedlings inoculated with Bradyrhizobium and Noninoculated seedlings without fertilizer to have no impact on nodule formation or Rhizobium spp. Biol. Fertil. Soils 9:261–267. had few nodules (3.2 ± 1.3), those receiving nodule biomass although seedling biomass Goicoechea, N., S. Merino, and M. Sańchez-Dıáz. 2.3 kgÁm–3 CRF had even fewer (0.8 ± 0.7), was not statistically increased. Although we 2004. Management of phosphorus and nitrogen and those receiving greater than 2.3 kgÁm–3 did not quantify nitrogenase activity directly, fertilization to optimize Anthyllis–Glomus– had none. Moreover, only 45% of the plants we observed that most nodules on fertilized Rhizobium symbiosis for revegetation of deser- (nine of 20) in the control and 10% (two of seedlings showed the apparent presence of tified semiarid areas. J. Plant Nutr. 27:1395– 20) of the plants in the 2.3 kg m–3 rate formed 1413. Á leghemoglobin. Graham, P. and D. Hubbell. 1975. Legume– nodules without inoculation; the remaining The benefits of improved growth with Rhizobium relationships in tropical agriculture. 109 noninoculated seedlings (91%) lacked fertilizer as well as reduction or hindrance In: Tropical forages in livestock production nodules. In contrast, nodules developed on of N fixation by excessive N application to systems. American Society of Agronomy. Spe- inoculated seedlings across all six of the tree seedlings are well described (Mrema cial Pub. 24:9–21. fertilizer treatments with a bell-shaped fre- et al., 1997; Valladares et al., 2002). Our Herrera, M.A., C.P. Salamanca, and J.M. Barea. quency distribution of increasing numbers of results indicate that fertilization rate is a more 1993. Inoculation of woody legumes with nodules (and more nodule biomass/seedling) rapid method for obtaining target plant size in selected arbuscular mycorrhizal fungi and rhi- from the nonfertilized control treatment (16.5 the nursery than the sole use of Bradyrhizoi- zobia to recover desertified Mediterranean ± 2.8; 5.8 ± 1.2 mg) up to 4.7 kgÁm–3 (25.5 ± bum inoculum but that it is possible to produce ecosystems. Appl. Environ. Microbiol. 59: 7.3; 11.7 ± 3.8 mg) followed by a sharp well-fertilized seedlings with a complement of 129–133. decline from the 4.7 kgÁm–3 fertilizer treat- nodules. Villar-Salvador et al., (2008) found Holl, K.D., M.E. Loik, E.H.V. Lin, and I.A. ment to 11.9 kgÁm–3 (0.4 ± 0.4; 0.4 ± 0.4 mg). that seedlings given high rates of fertilizer in Samuels. 2000. Tropical montane forest resto- When inoculated, the percentage of seedlings the nursery had greater size, root growth ration in Costa Rica: Overcoming barriers to dispersal and establishment. Restor. Ecol. with nodules decreased from 95% to 70% as capacity, photosynthetic rates, and N concen- –3 8:339–349. CRF rate increased from 0 to 4.7 kgÁm trations than seedlings given low rates of Horiuchi, B. and J. Jeffrey. 2002. Native plant –3 before dropping to 16% at 7.1 kgÁm and fertilizer either with or without rhizobial propagation and habitat restoration at Hakalau just 5% at the two highest CRF rates. For inoculation. For koa in our study, robust rates Forest National Wildlife Refuge, Hawaii. Natl. seedlings with nodules, mean biomass/nod- of fertilization (i.e., the ‘‘high’’ rate on the Proc. For. Conservation Nursery Assoc.—1999, ule was 0.5 and 2.1 mg for noninoculated fertilizer label; 7.1 kgÁm–3) still allowed 2000, 2001. USDA For.Serv. Proc. RMRS-P-24. seedlings receiving 0 and 2.3 kgÁm–3, respec- nodule formation and color consistent with p. 233.

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