Doctoral Dissertations

Date of Award

5-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

Michael D. Mullen

Committee Members

John D. Joslin, Gary S. Sayler, Donald D. Tyler

Abstract

Short-rotation woody crops (SRWC) are being considered as an alternative to burning coal for power production. Research is being conducted in West Tennessee to examine the environmental impact of converting from a traditional com silage system to growing sycamore (Platanus occidentalis L.) as a SRWC. The objective of this study was to determine what effect this conversion would have on soil microbial community structure, total microbial biomass and activities of soil microorganisms. Since soil microorganisms control the biogeochemical cycling of C, N, O, H and S in the environment, it is essential to understand how changes in crop management systems will influence these organisms.

In recent years a number of authors have expressed concerns about the usefulness of microbiological procedures such as plate-counting to describe microbial populations in the environment. In this study a battery of tests were used to measure soil biological properties. These included such traditional procedures as measuring microbial biomass by chloroform fumigation-extraction, and measuring microbial activities using enzyme assays. Several molecular and biochemical techniques have recently been developed which promise greater information and more accurate measurements of biological properties in environmental samples. Phospholipid fatty acid (PLFA) analysis and nucleic acid hybridization are two such techniques which were used in this study.

Treatments examined to show the effects of this conversion included a com silage/winter wheat system, a 2-3 year-old plantation of sycamore (called "young sycamore"), a 14-15 year-old plantation of sycamore (called "old sycamore"), and a mature hardwood forest. Soil samples were collected at four dates from April 1997 to August 1998. All tests showed that the old sycamore plantation and the mature forest had significantly more soil microbial biomass and the com and young sycamore soil. In addition, the conversion from row-cropping to SRWC caused shifts in the soil microbial community structure. The microbial community in the old sycamore and mature forest had a larger population of Gram-negative bacteria, and showed greater microbial activity than the com or young sycamore systems. Few differences were found in fungi or actinomycetes between treatments. The older forest systems had significantly more organic C and N, as well as water-extractable C than the com or young sycamore systems. Although the young sycamore system may become more like the old sycamore once the litter layer is well established and leaves begin to decompose, for the first few years of tree growth the microbial community in the young sycamore plantation is smaller, less diverse, and less active than the community in the old sycamore plantation.

This study showed good correlation between PLFA, chloroform fumigation extraction, and dehydrogenase as measures of microbial biomass. Although nucleic acid hybridization typically correlated with other measures of microbial biomass, correlation coefficients were smaller. Genetic analysis is, however, ideally suited for identifying specific genes within a complex environmental matrix. Each method has strengths and limitations, and the choice of a methodology depends on the goals of the research. PLFA was found to be a relatively simple method of gaining a broad overview of the microbial community.

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