Master of Science Thesis submitted 1993

Effects of Inoculation of Degraded Soils with Undisturbed Soil on Tree Seedling Growth and Soil Microbial Properties

Michael Rodriguez


A bottomland hardwood forest creation project is being carried out in a borrow pit in Core Point, North Carolina by the Texasgulf Corporation. Topsoil had been stripped and stockpiled on the site for 8 years prior to initiation of the project. The stripping and stockpiling of topsoil results in a decline of microbial biomass, activity, and diversity, rendering the soil less capable of supporting plant life. Similarly, the poor chemical, physical, and biological properties of exposed subsoil results in very poor plant growth.

Stockpiled topsoil and exposed subsoil were collected from the project site and used as growth media for containerized loblolly pine (Pinus taeda) seedlings. Half of the soils were mixed with 1% of soil collected from a nearby young mixed pine and hardwood forest in an attempt to inoculate them with beneficial microorganisms, particularly mycorrhizal fungi. After 9 months, the mean dry weight of the inoculated seedlings grown in the stockpiled topsoil was 2.4 X greater than that of the control seedlings. The mean dry weight of those inoculated seedlings that were grown in exposed subsoil were 3.4 X greater than control seedlings. The inoculated seedlings had greater mycorrhizal infection.

After the stockpiled topsoil was respread, red maple (Acer rubrum) and sweetgum (Liquidambar styraciflua) seedlings were planted in 3 plots - one subsoil, and 2 respread topsoil. The seedlings were inoculated by transferring small quantities of soil collected from a nearby forest to the seedling planting holes. A third treatment of pasteurized transfer soil was used. After 8 months, there were no differences between treatments in growth or soil microbial biomass. However, in the subsoil plot, mean dehydrogenase activity of the rhizospheres of the inoculated seedlings was double that of the other two treatments across time, and was consistently greater. Dehydrogenase activity and microbial biomass of two undisturbed reference forests' soils were an order of magnitude greater than the project site's soils. In one of the respread topsoil plots, mean foliar N concentration of the inoculated seedlings was 1.2 X greater than that of the seedlings receiving the other two treatments.

Inoculation of the potted loblolly pines was clearly beneficial, but the results of inoculation in the field were of questionable value. There is little hope that the productivity of the subsoil plot will substantially improve with or without inoculation due to its poor chemical and physical properties. The minimal improvement that occurred in the inoculated seedlings of the respread topsoil plots will not likely result in any long-term benefits. The benefits of inoculation, however, may become more evident in subsequent growing seasons when the seedlings are not so highly stressed. Continued monitoring of soil dehydrogenase activity and microbial biomass could serve as a useful index of the recovery of the project site's soils.