Identification of plant, bacterial and fungal necro mass markers in soil organic matter via ultrahigh resolution mass spectrometry

Soil organic matter (SOM) plays a central role in the global carbon cycle, influencing for example soil fertility, biodiversity, and erosion. Recent theories predict that SOM is a blend of plant metabolites and their breakdown products, perpetually undergoing recycling and transformation driven by soil organisms such as fungi and bacteria. However, our understanding of SOM remains incomplete due to its complex chemical composition. Particularly, we lack distinct metabolite information for the majority of these compounds or sources, hampering the analysis of SOM structure, it’s genesis, as well as mechanistic understanding of soil processes. Non-targeted analysis by ultrahigh resolution mass spectrometry, foremost Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), has substantially advanced our understanding of organic matter complexity in soils and allows to gain a representative picture through the use of liquid chromatography (LC). To address the above knowledge gaps, we employed LC-FT-ICR-MS for the investigation of three primary types of necromass (maize litter, bacterial and fungal necromass) extracts, as well as aqueous SOM extracts obtained from arable topsoils (2 – 20 cm depth). Water-soluble SOM fractions can be seen as a transition state between higher molecular weight structures in soils like bio- or necromass and its decomposition end products like carbon dioxide or methane.

We employed an LC methodology capable of separating dissolved organic matter (DOM) across a broad polarity spectrum, including highly polar compounds that are typically lost during commonly employed solid-phase extraction. Our results show significant differences between farmyard manure-amended (FYM) and unamended (UF) soil DOM according to its nominal carbon oxidation state (NOSC), saturation and molecular mass, that are most prominent for the highly polar fraction of SOM. We assigned intricate markers derived from bacterial, fungal or plant necromass that indicated higher potential necromass contribution to FYM than UF soil DOM, in line with higher microbial activity in these soils. We found that necromass markers contribute most to the CHNO formula class in soil DOM, thereby explaining structural differences between FYM and UF samples.

The outcomes of our research represent an initial stride towards the identification of novel molecular markers intrinsic to soil DOM, its thermodynamic properties and N content. In the long term, these techniques will enable not only the detection of shifts in the molecular composition of soil DOM during substrate decomposition but also the recognition of alterations in structural motifs that are associated with specific necromass types. This advancement holds promise for enhancing our understanding of soil DOM dynamics and therefore may hold important implications for soil and ecosystem management.