Date of Award
Doctor of Philosophy
Dennis R. Keeney
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.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Thomas Henry DeLuca
DeLuca, Thomas Henry, "Paired prairie and cultivated soils: soluble organics and nitrogen dynamics " (1993). Retrospective Theses and Dissertations. 10417.