The quid pro quo of dissolved organic matter biodegradability in agricultural soils: investigations of quantitative and biochemical aspects : A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Lincoln University

Abstract

Dissolved Organic Matter (DOM) serves as a primary source of mineralizable carbon (C), nitrogen (N), and other macroelements for plants and soil microbiota, acting as a linchpin of nutrient dynamics and energy flows through ecosystems. However, its mineralization through microbial activity can potentially lead to serious environmental threats. These include inflated losses as carbon dioxide (CO₂) into the atmosphere, contributing to the global greenhouse effect, or enhanced groundwater contamination and reduced soil fertility, due to leaching. The balance between processes of formation, utilization and preservation of DOM is a function of a wide range of biological, environmental and anthropogenic factors. As a result, current understanding of the quantity, composition and biodegradability potential of this dynamic soil organic matter (SOM) component in rather complex and changing systems, such as agricultural soils, remains moot. This study attempted to shed light on the quantitative and biochemical aspects of DOM underpinning its biodegradability potential. What are the features that best describe the mineralization response? How do changes in soil moisture and land use affect these and are there preferred forms and pathways that the microbial community exploits? These are some of the spread puzzle pieces of DOM studies that the present study aimed to combine. The first part of the study investigated the effect of different soil moisture levels on the quantitative and qualitative changes of DOM released from sequentially leached arable and grassland soils. The extent of soil drying before rewetting had a strong impact on the quantity of DOM released with sequential leaching, but these changes were inconsistent between land uses and poorly related to DOM biodegradability. Advancing drought levels progressively decreased differences in DOM quality, assessed by means of specific ultraviolet (UV) absorbance at 254 nm, but there remained notable changes in DOM released over the leaching course, indicating an increasing proportion of water-soluble and relatively biodegradable aromatic and/or conjugated structures. These proportions and their responsiveness to biodegradation varied, however, between land uses, questioning the involvement of components of different origin and functionality. A soil with three contrasting management histories was further considered, with the aim of identifying and characterizing, by means of fluorescence and UV indicators, specific fractions that might explain DOM mineralization response to land use. Anion exchange resin treatment of cold (CW) and hot water (HW) extracts revealed an uncharged/protonated, aliphatic, likely microbially derived sub-fraction, with contrasting biodegradability in CW and HW extracts, as opposed to the negatively charged and/or hydrophobic, aromatic, plant and SOM-derived fraction retained by the resin, which had comparable biodegradability between CW and HW extracts and land uses. The Random Forest algorithm inferred the degree of humification to be an important aspect affecting the mineralization of the CW fraction, but not of the HW fraction, conversely characterized by a large amount of phenols and organic N content. In contrast, DOM concentration was not a factor limiting biodegradability. Open questions as to what caused these inconsistencies in the mineralization response among DOM fractions and sub-fractions were finally approached at the molecular level in the last part of the study, combining data mining of fluorescence Excitation–Emission Matrices with high resolution Liquid Chromatography–Mass Spectrometry. Small peptides and quaternary ammonium compounds were attributed a leading role in DOM mineralization, which was, in contrast, demoted by optically active, less oxygenated, lipid-like and phenolic compounds. This study confirms the importance of compositional aspects of DOM in shaping the microbial metabolism and the fate of DOM biodegradation products.