Agronomy and Horticulture, Department of


First Advisor

Rhae Drijber

Date of this Version


Document Type



A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Agronomy and Horticulture, Under the Supervision of Professor Rhae A. Drijber. Lincoln, Nebraska: August 2022

Copyright © 2022 Morgan R. McPherson


Arbuscular mycorrhizal fungi (AMF) remain a vital obligate symbiont of nearly all plants. It is well established that the symbiosis between AMF and host plant improves plant nutrient acquisition, alleviates abiotic and biotic environmental stressors, defends against plant pathogens, and contributes to overall plant fitness and productivity through modification of the soil habitat. Modifications include increased soil aggregation and stability, carbon sequestration through provision of fungal wall precursors to soil organic matter (SOM) formation, and enhanced nutrient cycling in the mycorrhizosphere. The goal of this dissertation was to assess how AMF respond to nitrogen (N) fertilization regimes in maize cropping systems of increasing crop rotational diversity. Two, long-term field sites were used to evaluate AMF responses to N application during maize growth. The first site was a conventionally tilled and rainfed site in Elora, Ontario, Canada at the University of Guelph, hereafter referred to as Canadian Nitrogen Study (CNS). CNS evaluates contrasting mineral fertilization rates applied either continuously for 10 years or shocked with a higher/lower N rate once every five years. We demonstrate that soil AMF biomass is more responsive to current season N application rates than historical N regimes and supports our prior research showing that extramatrical AMF biomass declines with increasing N applied. The second site is a rainfed, no-till maize system managed by the United States Department of Agriculture (USDA) and referred to as Crop Rotation Study (CRS). CRS was sampled seasonally over two years for soil biological and chemical properties and was designed to evaluate soil C and N stocks in diverse rotations with continuous corn under three levels of N fertilization. We found a similar inverse relationship of extramatrical AMF to N application rate as in CNS and demonstrate how N fertilization drove AMF biomass dynamics in the soil. Due to the agronomic importance of maize, it is necessary to cultivate sustainable management practices that contribute to resilient mycorrhizal communities and SOC stabilization.

Advisor: Rhae A. Drijber