The Chemical Ecology of Soil Organic Matter Molecular Constituents

Abstract

Soil organic matter (OM) contains vast stores of carbon, and directly supports microbial, plant, and animal life by retaining essential nutrients and water in the soil. Soil OM plays important roles in biological, chemical, and physical processes within the soil, and arguably plays a major role in maintaining long-term ecological stability in a changing world. Despite its importance, there is a great deal still unknown about soil OM chemical ecology. The development of sophisticated analytical methods have reshaped our understanding of soil OM composition, which is now believed to be comprised of plant and microbial products at various stages of decomposition. The methods also have recently been applied to study environmental change in various settings and have provided unique insight with respect to soil OM chemical ecology. The goal of this review is to highlight the methods used to characterize soil OM structure, source, and degradation that have enabled precise observations of OM and associated ecological shifts. Although the chemistry of soil OM is important in its overall fate in ecosystems, the studies conducted to date suggest that ecological function is not defined by soil OM chemistry alone. The long-standing questions regarding soil OM stability and recalcitrance will likely be answered when several molecular methods are used in tandem to closely examine structure, source, age, degradation stage, and interactions of specific OM components in soil.

Permafrost soils and carbon cycling

“…Traditionally, the "quality" of SOM (and by inference its intrinsic potential to be further decomposed, transformed and mineralized) has been evaluated by assessing its molecular composition and by applying some type of physical, chemical, or biological fractionation approach to partition the bulk SOM pool into "labile" vs. "recalcitrant" or "stable" forms (Kleber and Johnson, 2010;von Lützow et al, 2007;Simpson and Simpson, 2012). Although the chemical composition of SOM can affect the rate of decomposition, it is now recognized that (1) readily degradable "labile" carbon forms can be stabilized in soil by a variety of mechanisms or conditions that physically or chemically limit microbial access or that impact microbial activity, and (2) even the most "recalcitrant" carbon forms can be mineralized given the "right" conditions (von Lützow et al, 2006;Marschner et al, 2008;Kleber, 2010).…”

The C:N:P:S stoichiometry of soil organic matter

“…It is recognised to possess a range of turnover times, from less than one year to thousands of years (Amundson 2001), and to comprise complex chemical entities (Stevenson 1986;Kögel-Knabner 2002;Simpson and Simpson 2012). Whereas the chemical complexity was once thought to account for SOM stability, due to molecular recalcitrance, more recent thinking emphasises ecosystem properties notably sorptive protection and hindered microbial access (Schmidt et al 2011;Dungait et al 2012;Lehmann and Kleber 2015).…”

“…Whereas the chemical complexity was once thought to account for SOM stability, due to molecular recalcitrance, more recent thinking emphasises ecosystem properties notably sorptive protection and hindered microbial access (Schmidt et al 2011;Dungait et al 2012;Lehmann and Kleber 2015). The chemical structures of SOM have been elucidated principally through NMR spectroscopy (Baldock et al 1992;Hatcher et al 2001;Kögel-Knabner 2002;Simpson and Simpson 2012), while physical techniques have been used to study molecular size and aggregation (Wershaw 1999;Piccolo 2001). Radiocarbon provides information about turnover and age (Torn et al 2009;Trumbore 2009;Mills et al 2014).…”

Organic matter composition and stabilization in a polygonal tundra soil of the Lena Delta

“…However, the precise structure of soil OM or specific organic compounds cannot be determined by this method (Kögel-Knabner, 2000). More detailed information is obtained from macromolecular compounds and diagnostic lipids in soil OM (Simpson and Simpson, 2012). The dynamics of organic carbon in physically defined OM fractions are revealed by 14 C analysis.…”