Date of Award

Summer 2019

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

Stuart Grandy

Second Advisor

Alexandra R Contosta

Third Advisor

Serita Frey

Abstract

As the largest terrestrial sink for carbon (C) and a critical source of nitrogen (N) for plants, soil organic matter (SOM) is a major driver of ecosystem function. It is critical to understand the mechanistic controls on SOM in order to improve models of global C cycling and to develop accurate measures of soil fertility. SOM consists of a wide spectrum of compounds, varying in chemical characteristics and function. The chemical and physical fractionation of SOM is a valuable tool for distilling this complexity into meaningful and distinct pools: detrital or particulate organic matter (POM), which contains mostly recent litter inputs at early stages of decomposition, and mineral-associated organic matter (MAOM), which is far more processed, consisting of small organic compounds bound to reactive mineral surfaces. For decades, MAOM has been studied primarily for its capacity to sequester soil C and N. In this dissertation, my research reveals the under-appreciated role of clay minerals in mediating the short-term accrual and turnover of SOM. I examine the mechanistic controls on MAOM and specifically, how agricultural management and plant-microbe interactions influence C and N within MAOM.

Agricultural practices can directly impact the capacity for soils to store MAOM. Approaches that minimize soil disturbance, such as conservation tillage, and those that increase crop residue input and diversity, such as cover cropping, can facilitate the rapid accrual of N within MAOM (Chapter 1). Through this research, I found that MAOM N may also be an important, but overlooked, source of N for crops. This work led me to develop a conceptual framework in which I synthesized literature from the fields of geochemistry and soil biology to investigate the potential mechanisms that drive MAOM turnover. Although this conceptual work stands alone (Chapter 2), the hypotheses and ideas therein form the basis for my experimental work. My overarching hypothesis addresses the biochemical strategies that plants employ to disrupt mineral-organic interactions and release both C and N from MAOM. Specifically, I examine two mechanisms by which plant root inputs may stimulate the destabilization and turnover of both C and N within MAOM: belowground root C inputs, specifically in the form of sugars and organic acids, can stimulate MAOM decomposition through indirect and direct mechanisms, respectively.

Through a series of laboratory incubations, I demonstrate that simulated root exudates can stimulate the mobilization of both C and N from MAOM through microbial and non-microbial pathways (Chapter 3). Additions of a sugar substrate, glucose, were associated with the microbial-mediated mineralization of C and N from MAOM. The organic acid substrate, oxalic acid, was associated with the direct and concomitant mobilization of DON and metals into exchangeable and soluble pools. Most notably, both substrates stimulated the respiration of MAOM-C (i.e., positive priming), with total increases ranging from 35–89%. Our results provide evidence for pathways of MAOM destabilization, and more generally reveal that a pool of soil nutrients generally considered passive or inert has the potential to function as a significant source of C and N.

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