Deciphering the Energy Use Channels in Soil Organic Matter: Impacts of Long-term Farmyard Manure Addition and Microbial Necromass Revealed by LC-FT-ICR-MS

Soil organic matter (SOM) plays a central role in the global carbon cycle and contributes to storage of C and energy in soils. Farmyard manure (FYM) addition to arable soils is a measure to increase SOM content, microbial activity and abundance of microbial metabolites (e.g., necromass (NM) markers). However, understanding the mechanistic links between soil dynamics and energy storage is hampered due to the chemical complexity of SOM. Non-targeted molecular-level methods like liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR-MS) can be used to explore the complex dynamics of SOM, revealing energetic fingerprints and long-term changes in SOM due to FYM addition. We compared water-extractable organic matter (WEOM) from soils of four long-term FYM addition experiments with representative WEOM signatures from maize, bacterial and fungal NM. Long-term FYM addition increased the complexity of WEOM, most pronounced in polar, unsaturated, oxidised and energy-poor compounds. These changes were linked to a 2-3-fold increase in bacterial, plant and fungal NM signatures. Especially bacterial NM in FYM-amended WEOM indicated a shift in dominant energy use channels. Control soils showed a much lower overlap with all NMs, but indicated a higher dominance of fungal energy-use channels, especially for N-containing compounds. A large fraction of WEOM signals (79% in FYM-amended, 94% in control soils) was unrelated to any of the three NM signatures, and was also mainly responsible for the shift in nominal oxidation state of carbon (NOSC) between the fertilisation treatments. LC-FT-ICR-MS provided access to ~600 novel microbial NM markers which are readily soluble and compositionally distinct from classical NM markers (ergosterol, aminosugars, etc.). Overall, we highlight novel insights into NM contribution to SOM by LC-FT-ICR-MS, and how it can assist to constrain compositional and energetic impacts of FYM addition on soils.