Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Douglas L. Karlen

Second Advisor

Timothy B. Parkin


Conceptual models implicate the fungi as important mechanisms in soil aggregate formation but the lack of suitable assays for routine quantification of fungal biomass make validation of these models evasive. The objectives of these studies were threefold: (1) to evaluate the effectiveness of ergosterol as an indicator of fungal biomass; (2) to evaluate interrelationships between aggregate-forming processes induced by fungi and those caused by physical and chemical factors; (3) to evaluate potential soil quality indicators under different tillage and residue management systems. Objective 1 focused on determining the efficiency of recovering ergosterol from spiked soil samples and measuring ergosterol of nine fungal species. Objective 2 involved a series of laboratory experiments using two autoclaved soils, residue additions (ground alfalfa (Medicago sativa) and reed canarygrass (Phalaris arundinacea)), and freeze/thaw and wet/dry cycles in a factorial design with three replicates for each factor (i.e., a total of 108 experimental units). Objective 3 evaluated wet aggregate stability, microbial biomass, respiration, total C and N, and soil ergosterol content as a function of tillage or residue management in field studies at three research farms. Results indicate the ergosterol assay (Objective 1) has low variability and high extraction efficiency. Fungal ergosterol content ranged from 0.38 to 10.40 mg kg[superscript]-1. Addition of Chaetomium sp. to autoclaved soil resulted in increased aggregation (Objective 2). Subjecting autoclaved soil samples to either freeze/thaw cycles, wet/dry cycles, or crop residue additions decreased aggregation, suggesting that the mechanism by which crop residues increase aggregation is through stimulation of the fungal biomass. Residue, in the absence of Chaetomium sp., actually decreased aggregation. Field study results (Objective 3) indicate that crop residue management impacts fungal populations with corresponding changes in aggregate stability. This research suggests that ergosterol is an effective indicator of living fungal biomass and that fungi are important mediators of aggregate formation and stabilization. These studies suggest that aggregate stability and ergosterol content can be used as indicators for assessing soil quality. As we strive for sustainability in our agricultural systems, an improved understanding of interactions between soil management practices and fungal populations is an important consideration.



Digital Repository @ Iowa State University,

Copyright Owner

Neal Samuel Eash



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File Size

216 pages