Date of Award

Winter 2025

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Stuart Grandy

Second Advisor

Jessica Ernakovich

Third Advisor

Andrea Jilling

Abstract

Nitrogen (N) is abundant in agricultural soils during early spring due to both decomposing plant residues and initial fertilizer applications before peak crop uptake. This N is vulnerable to loss through leaching and denitrification, but a portion of this N can become stabilized in mineral-associated organic matter (MAOM). However, the short-term incorporation and long-term stability of different N sources within MAOM remain poorly understood. This study examined how fertilizer and litter derived N are incorporated and stabilized in MAOM using a three-week incubation experiment with 15N labeled fertilizer, rye root, and rye shoot additions to paired cropland and grassland soils. Sequential chemical extractions were used on the resulting MAOM fraction to assess N persistence across treatments. Fertilizer N and both types of litter-derived N rapidly formed MAOM, but their stability differed substantially. Less fertilizer-derived N moved into MAOM and it was more easily extracted, indicating weaker mineral associations. In contrast, litter-derived N, particularly the root litter with a lower C:N ratio, formed more persistent associations. The differences between N sources were also stronger in cropland soils which had lower starting soil organic matter. These findings demonstrate that MAOM can act as a short-term sink for newly added N, but its stability depends strongly on N source and soil properties. Understanding these contrasting stabilization effects can improve predictions of nutrient retention and inform soil management strategies that promote long-term N conservation.

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