Degree Type

Dissertation

Date of Award

1993

Degree Name

Doctor of Philosophy

Department

Agronomy

First Advisor

Dennis R. Keeney

Abstract

Nitrate contamination of water resources in the Midwestern U.S. is largely a result of conventional agricultural practices. In contrast, soils of prairie ecosystems do not accumulate NO[subscript]3 because of rapid NO[subscript]3 assimilation by plants and microbes. Microbial NO[subscript]3 assimilation requires a readily available C source; however, there is little known regarding available C in soils. The ability of prairie soils to recycle NO[subscript]3 makes the prairie ecosystem a useful model in N cycle studies designed to develop agricultural systems that minimize N loss;I performed field and laboratory studies on paired prairie and cultivated soils to determine how cultivation has affected levels of soluble C, sugars, and amino-N in soils and to determine whether these soluble constituents play a role in the soil N cycle;Prairie soils and greater concentrations of soluble sugars and soluble amino-N and had higher ratios of NH[subscript]4:NO[subscript]3, soluble C:inorganic N, and soluble sugar:NO[subscript]3 than their cultivated counterparts. Soluble sugars and soluble amino-N fluctuated seasonally in prairie soils. Amino-N reached a minimum in spring when microbial biomass and mineral N were increasing in both prairie and cultivated soils. Freeze-thaw events are hypothesized to be a cause of the increased N mineralization;Soluble sugars increased in soils upon addition of plant residues and remained at elevated levels until N became limiting. Cellulose additions to soil increased levels of soluble sugars only when N was not limiting. Thus demonstrating a dependence of elevated soluble sugars on soil microbial activity;Laboratory studies demonstrated that in the presence of glucose, NO[subscript]3 was more readily assimilated in cultivated soils than in prairie soils. Free amino acids released upon addition of glucose to soil may be inhibiting NO[subscript]3 assimilation in prairie soils. Field studies of the fate of applied [superscript]15NO[subscript]3 demonstrated more rapid assimilation of NO[subscript]3 in prairie soils than in cultivated soils and evidence of active dissimilatory NO[subscript]3 reduction in prairie soils;These data demonstrate the contrasting C and N dynamics of prairie and cultivated soils. Soluble sugars and amino-N may be useful indices of available C and N in soil. Cultivated Iowa soils appeared to lack sufficient energy as soluble sugars to allow microbial assimilation of ambient NO[subscript]3.

DOI

https://doi.org/10.31274/rtd-180813-13111

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Thomas Henry DeLuca

Language

en

Proquest ID

AAI9321134

File Format

application/pdf

File Size

188 pages

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