Long-term effects of manure addition on soil organic matter molecular composition: Carbon transformation as a major driver of energetic potential

Long-term addition of farmyard manure supports the accumulation of microbial carbon (C) and soil organic matter (SOM), but the effects on energy storage remain unknown. In particular, it remains unresolved whether manure or the stimulation of microbial transformations explains the increased microbial imprint. The latter would suggest that the accumulation of SOM transformation products controls energy storage, rather than manure directly. We hypothesized that the overlap with original manure signatures could be used as a measure of SOM transformation and its effect on SOM's nominal oxidation state of C (NOSC) and energetic potential ΔG⁰C_OX. We employed solid-state laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS) to study molecular signatures of manure samples and topsoil from four long-term field experiments receiving manure, and unfertilized controls. In line with bulk elemental analysis, LDI-FT-ICR-MS suggested that manure increased SOM's energetic potential (0.7–1.2 kJ/mol C). Manure addition changed SOM composition by 3–16% of total ion abundance as compared to controls, being larger in longer-running field experiments. Markers unrelated to original manure signatures (i.e., indirect effects) explained 67–84% of molecular changes while markers directly related to manure explained only 2–12%. Long-term manure addition resulted in increased saturation, oxidation and molecular weight, and decreased aromaticity of SOM as compared to unfertilized soils. Accumulated molecules had higher energetic potentials and were, despite being chemically similar to original manure, elevated in mass, suggesting potential use of manure-derived building blocks for microbial synthesis of larger molecules. Molecules with lower energetic potential disappeared in manured samples, mirrored by a higher oxidation state of water-extractable organic matter, pointing to an increased solubility of SOM. Our results indicate a uniform shift in SOM properties upon manure addition, but highlight the role of site-specific trajectories of SOM compositional change. We discuss the implications of manure-induced microbial transformations for energy storage and long-term stability of SOM.