Date of Award

8-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Advisor

Tharayil, Nishanth

Committee Member

Preston, Carolina M

Committee Member

Riley, Melissa

Committee Member

Rao, Apparao

Abstract

Soil organic matter decomposition is directly driven by the soil extracellular enzymes excreted by soil microorganisms. Therefore, factors which impede extracellular enzyme activity would result in retarding the rate of soil organic matter decomposition. Several factors in the soil may influence soil extracellular enzymes including the plant secondary metabolites, polyphenols- tannins and lignins. However, thus far few investigations have considered the direct and indirect impacts of polyphenols on soil enzyme activities. Hence, three investigations were conducted to understand the ecological impacts of tannins and lignins on soil extracellular enzyme activities.
Tannins, further classified as condensed and hydrolysable tannins, can hamper decomposition through the formation of the tannin-protein complex. However, investigations had not compared the reactivities of the two groups. Hence, in the first investigation the enzyme inhibition efficiency of hydrolysable and condensed tannins was compared against almond beta-glucosidase and soil enzymes collected from two soils with opposing tannin exposures. It was discerned that against almond beta-glucosidase hydrolysable tannins exhibited a higher enzyme inhibition capacity than condensed tannins. Meanwhile, in the soil enzyme inhibition was dependent upon the enzyme class and the litter chemistry history. Moreover, tannins could prevent oxidoreductase activity through their antioxidant potential, which is a novel mechanism elucidated in the decomposition setting. In the second investigation, it was discerned that the inhibiton capacity of mixed tannins is positively related to the hydrolysable content.
In the final study, the relationship of soil lignin chemistry to the degradation efficiency of soil peroxidase was investigated. Until recently, lignins were considered to be chemical resistant to enzyme mediated degradation, which suggests that the enzymatic mechanisms mediated the degradation of lignin are not fully understood. One factor that could be affecting the rate of lignin turnover in the soil is the degradation efficiency of soil peroxidase, an oxidoreductase enzyme that mediates the transformation of soil polyphenols. The degradation efficiency of soil enzymes are described by the Michaelis-Menten kinetics and Activation energy. In this study it was discerned that the degradation efficiency of peroxidase was positively associated to the vanillyl abundance of lignin. This suggests that lignin chemistry may be one of many factors governing peroxidase kinetics in the soil.
Combined, the three studies conducted discerned the direct and indirect effects of soil polyphenols on soil extracellular enzymes. By associating soil organic matter chemistry with soil extracellular activities, these basic studies have advanced our understanding of soil decomposition and bring forth new questions.

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