Growth of endomycorrhizal yellow-poplar seedlings in fumigated nursery soil

Yellow-poplar (Liriodendron tulipifera L.) has a well-earned reputation as a site sensitive species; many outplantings on sites of marginal quality have failed. Endomycorrhizal symbionts are not maintained in nursery beds that are fumigated often, therefore, most yellow-poplar seedlings grown according to standard nursery management practices are without endomycorrhizal benefits. In this study, yellow- poplar seedlings were grown in inoculated beds at the Pinson Nursery, Tennessee, to determine nursery growth benefits and seedling potential for outplanting. Watering, weeding, and similar practices were according to normal nursery schedules.

Fertilizer treatments used were (1) no fertilizer, (2) base fertilizer (500 lbs/acre 15-15-15), (3) base plus two urea topdressings (100 lbs/acre), and (4) base plus four urea topdressings. Endomycorrhizal inocula superimposed on these fertilizer treatments were (1) Glomus mosseae, (2) Glomus fasciculatus, (3) soil obtained from a yellow-poplar stand (natural incoulum), and (4) no inoculum.

Height, diameter at the root collar, and taproot diameter were greatest if at least two topdressings of urea were applied during the growing season, regardless of the endomycorrhizal incoulum source. Soil analyses showed phosphorus content to exceed 100 ppm in all cases; sometimes it was as high as 135 ppm. Potassium was also quite high. In soils with such unnaturally high levels of phosphorus, many beneficial effects of mycorrhizae could never be realized.

Top dry weight and taproot fresh weight were greatest for Glomus mosseae-infected seedlings. Other seedling growth characteristics were not significantly affected by mycorrhizal infection. Glomus mosseae-infected roots had the greatest arbuscule frequency. The number of hyphal coils in both species of Glomus-infected roots was nearly equal, but it was significantly greater than for other inocula. Natural inoculum-infected seedlings had more vesicles than other seedlings.

Base plus two urea topdressings combined with Glomus mosseae produced the most desirable seedlings for outplanting. These seedlings had well-developed, wide-spreading mycorrhizal root systems, large root collar diameters, and well balanced root/shoot ratios.