Isolation, interpretation, and implications of physical soil organic matter fractions in soil systems
Date
2024
Journal Title
Journal ISSN
Volume Title
Abstract
Soil organic matter (SOM) is crucial to sustained ecosystem function, due to its role in regulating nutrient cycling, carbon (C) storage, and soil structure relevant to both food production and climate regulation. Since the early 1990s, physical fractionation methods have been used to separate bulk SOM into discrete components. The central aim of these methodologies is to simplify the complex heterogeneity of the bulk SOM pool by isolating fractions with more homogenous chemistries, formation pathways, and mechanisms of persistence. By understanding the relative distribution of C and nitrogen (N) among these various fractions, we gain appreciable insight into the mechanisms underlying fundamental soil biogeochemical processes. Despite their historic use, however, significant questions remain regarding the means of proper isolation and interpretation. This dissertation looks to these questions directly, reviewing and then interrogating the methods by which fractions separated before applying those fractionation schemes to answer key questions relating SOM to ecosystem function. The first section reviews the history and current state of physical fractionation methodologies, before using a triangulation of experimental evidence, including chemical, isotopic, and spectral indicators, to identify the best practices for laboratory use. These chapters advance our current understanding of SOM biogeochemistry by drawing an explicit link between the conceptual definitions of SOM fractions and the various procedural definitions that have been used historically. Across a range of soils representative of agricultural land in the United States, we show that fractionation methods that separate particulate organic matter (POM) fraction by density isolate fractions more in line with the conceptual definition of POM than the more frequently used size separation. This work aims to unify understanding across the field of soil biogeochemistry and allows for more robust analyses and modeling efforts. The subsequent chapters use this approach to investigate fundamental questions around SOM stability and persistence. The mineral associated organic matter (MAOM) fraction has long been understood to be relatively stable, with slower turnover times and a more homogenous composition as compared to POM. Its accumulation has thus been discussed as a target for climate change mitigation. We leveraged a unique long-term experimental site with archived samples stretching back over 60 years to test this assumption, aiming to identify a dynamic fraction of MAOM by comparing the SOM composition of plots that had not received organic inputs over the course of the experiment against plots that had received regular inputs for six decades. Our spectral and isotopic analyses showed that a dynamic fraction of the MAOM existed and was primarily composed of plant derived compounds. As the exchangeable MAOM pool was exhausted due to a lack of fresh C inputs, we found that the composition of the MAOM pool became more strongly dominated by microbial byproducts. This work represents useful evidence towards a holistic understanding of the dynamic nature of SOM, and forces reimagining of long-held paradigmatic views. One challenge in the current SOM biogeochemistry landscape is that often questions exist downstream of methodologies, such that the fractions that can be isolated drive the research that is conducted. By first identifying robust methodologies, in the second half of this dissertation we were able to ask specific questions about the link between SOM dynamics and ecosystem function. To this end, we pursued three different lines of inquiry: a field study in which the objective was to link the fractional distribution of C and N to yield stability in agricultural systems, a field study that seeks to understand the persistence dynamics of SOM over a decadal scale in grassland systems, and a laboratory incubation that aims to discern the relative contributions of POM and MAOM in regard to plant available N. The first field study used samples from 9 working farms across the Central United States to better understand how SOM might moderate the spatiotemporal stability of crop yields at the field scale. Yield instability is a major cause of economic and environmental distress in row crop systems, and regional studies have suggested that increasing SOM may be able to mitigate variation in yield across time and space. The chapter presented here is the first study that attempts to identify a mechanistic link between SOM fractions and yield stability. In disagreement with regional and county scale studies, we found that SOM abundance was not linked to increased yield stability in cropping systems. Rather, unstable yield zones had significantly higher SOM content than stable zones, particularly in regard to the POM fraction. This work indicates that at the subfield scale, interactions between climate, topography, and management may be driving spatial patterns of both yield stability and SOM accumulation. This is a key insight, implying that some of the relationships between SOM and agronomic outcomes are scale dependent, and highlighting the need for field scale work to maintain relevance to growers. The second field study produced novel insights, tracing isotopically enriched litter and pyrogenic organic matter (PyOM) through various SOM fractions over the course of a decade, one of the longest tracer experiments that has occurred in grassland ecosystems. We found that after 10 years, the majority of the remaining litter derived C and N inputs were stored in the MAOM fraction, a result well aligned with our hypotheses. Interestingly though, the litter derived MAOM fraction formed rapidly (~ 1 year) and persisted at a relatively similar concentration for the duration of the study. This suggests the potential for divergent persistence mechanisms of POM and MAOM, implying less inter-fraction transfer than previous frameworks have proposed and prompting re-evaluation of the mechanisms of MAOM formation and persistence. In contrast, the applied PyOM remained almost completely in the POM fraction over the 10-year period, reinforcing both the heterogeneity of the bulk SOM pool, and the myriad of persistence mechanisms that stabilize various SOM fractions. Given that PyOM is ubiquitous in soil regardless of burn history and can persist for hundreds of years, this result has critical importance for our understanding of turnover time of the POM fraction, and suggests that we may be underestimating the dynamic nature of POM when PyOM is not accounted for. Finally, in a lab incubation experiment, we took advantage of recent advances in isotopic measurement to prove recent theories around MAOM N accessibility. Whereas POM is often thought of as the fraction that provides nutrients in the short term, our two-week incubation showed that under certain conditions, the majority of plant available N may be derived from the MAOM fraction. This work validates proposed frameworks and is an important step towards understanding coupled C and N management in agroecosystems that could improve N use efficiency and increase producer sustainability. Overall, the work in this dissertation aims to provide a comprehensive overview of how fractions can and should be isolated, and the information gained via this fractionation. By clarifying and advancing methodology to quantify SOM components and the understanding of their contribution to critical soil functions for the sustainability of food production and the mitigation of climate change this dissertation represents a major step forward for the study, modeling and managing of SOM in agricultural systems.
Description
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Embargo expires: 08/16/2025.
Subject
physical fractionation
soil biogeochemistry
soil spectroscopy
POM
MAOM
soil organic matter