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Effects of Trifolium-Rhizobium symbiosis on Pinus contorta regeneration, forest soil, and selected native plant species Trowbridge, R. L


This study reports on the early effects of the Trifolium hybridum-Rhizobium symbiosis on Pinus contorta Doug, ex Loud (lodgepole pine), soil, and selected native plant species. Four rates of seeding (0, 10, 20, and 30 kg/ha) using inoculated Trifolium hybridum (alsike clover) seed were applied to three different site preparation treatments (broadcast burn, windrow burn, and mechanical scraping) using a split-plot design. Alsike clover and the Rhizobium inoculant were found to have excellent establishment and infectivity, and the symbiosis was assessed to be fixing nitrogen effectively. No effect of site preparation treatments was observed on establishment of the symbiosis, and clover-seeded plots averaged 76% cover by the end of the third growing season. The symbiosis had no significant (p < 0.05) effects on lodgepole pine total or incremental height or survival during the first three growing seasons, nor was there any observed effect on lodgepole pine foliar total nitrogen (N) concentration and ઠ¹⁵N values at the end of the second growing season. Small, but significant (p < 0.05) decreases were observed for lodgepole pine total and incremental diameter in the second and third growing seasons, as well as needle mass in the second growing season. The growth decreases were probably attributable to the effect of shading by the clover cover. However, lodgepole pine seedlings overtopped the clover by the end of the third growing season and shade effects are likely to decrease as tree seedlings continue to grow. After one growing season, the symbiosis significantly (p < 0.05) increased mineralizable N in the forest floor and mineral (0-15 cm) soil layers. However, no significant changes in total N were detectable. The changes in mineralizable N were likely a measure of increased microbial biomass attributable to greater amounts of rhizosphere soil in clover-seeded plots compared to controls. Available phosphorus (P) in the forest floor significantly (p < 0.05) decreased as rate of seeding increased after one growing season. The decrease of forest floor available P may be attributed to greater assimilation of P in clover-seeded plots for plant and microbial growth, as well as the additional requirements for P in the supply of biological energy needed for active N₂ fixation. All native plant species had low cover values which made interpretation of results difficult. However, percent cover of Calamagrostis canadensis, Rosa acicularis, and Spiraea betulifolia were significantly less in clover-seeded plots compared to controls at the end of the second growing season. Replacement of some herb and low-growing shrub species by legume-Rhizobium symbiosis may be desirable if the net result is an increase in site N without detrimental effects to tree crop species. It is recommended that the legume-Rhizobium symbiosis be established in the early regeneration of lodgepole pine plantations on similar sites that are inherently N deficient and have experienced further site N depletion through forestry practices such as slashburning.

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