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

Abstract

<p>Soil organic matter (SOM) is an important driver of the global carbon cycle and plays a central role for the fertility, biodiversity, erosion and other key dynamics in soils. However, our understanding of SOM is still hampered by its chemical complexity. SOM is derived from a mixture of plant metabolites and their decomposition products that are continuously recycled and transformed through the action of soil organisms such as fungi and bacteria. This ultimately leads to the build-up of complex organic mixtures of plant, fungal and bacterial biomass as well as their respective decomposition products (necromass), which lack distinct metabolite information. Ultrahigh resolution mass spectrometry, such as FT-ICR-MS, has contributed much to our understanding of organic matter complexity in soils, but novel methods are needed to identify and trace molecular markers for necromass contribution. To overcome this gap, we studied three major necromass types (maize litter, bacterial and fungal necromass extracts) and arable (top-)soil aqueous SOM extracts via liquid chromatography (LC) and tandem mass spectrometry (MS/MS) coupled with FT-ICR-MS. Our main goal was to find and characterize indicative molecules (via LC-MS) and structural motifs (from MS/MS) for each necromass type, and to detect these within SOM extracts. To this end we applied an LC method that allows the separation of salts from DOM and covers a broad polarity range, including highly polar compounds that are usually lost during solid-phase extraction. MS/MS experiments were conducted on preselected features (data-independent acquisition) in positive and negative ESI modes. We further  tested the sensitivity of our method by spike experiments. Our findings represent a first step in uncovering novel molecular markers that are tracked in ongoing decomposition experiments. In the long run, these techniques will allow to not only detect changes in molecular composition of DOM during substrate decomposition, but also to detect changes in structural motifs that are linked to defined necromass types.</p>