Long-term manuring of soil results in divergent responses of dissolved and particulate organic matter on the molecular level

Manure addition increases amounts of soil organic matter (SOM), water-extractable organic matter (WEOM), microbial biomass, and microbial activity. Mass balances have shown that soil organic C build-up is paralleled by a comparatively low retention of the added manure C, which also declines substantially with time. The implications for SOM’s molecular composition are not fully understood, but imply transformation of manure-derived organic matter as a main driver of C accumulation. We studied four long-term manured soils (24-118 years) to unravel potential mechanisms of manure turnover and SOC build-up on the molecular level. Soils were sampled a year after the last manure application.

Bulk SOM and manure were studied directly via solid-state laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). The LDI-FT-ICR-MS results indicated that manure increased SOM's energetic potential by +0.9 ± 0.2 kJ/mol C (1.5 ± 0.4%), and this trend was confirmed by bulk elemental analysis (+5.4 ± 2.8 kJ/mol C; 12.6 ± 6.5%).  The addition of manure changed the composition of SOM components corresponding to 3–16 % of the total ion abundance compared to the controls, with the higher proportions found in longer running field trials. However, marker compounds directly related to manure explained only 2–12% of the molecular changes, while markers unrelated to the original manure signatures explained 67–84%. 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 a higher energetic potential and, despite being chemically similar to the original manure, a higher mass, suggesting that manure-derived building blocks were used for the microbial synthesis of larger molecules. Molecules with lower energetic potential disappeared in manured soil samples, mirrored by a higher oxidation state of WEOM. Consequently, we also found higher water-extractable organic C yields (normalized to soil organic C) in manured samples.

To reveal potential sources of these oxidized compounds, WEOM was studied by liquid-state FT-ICR-MS coupled with liquid chromatography, and compared to representative necromass extracts (plant, fungal, bacterial). Our results indicated a clear shift towards a more bioavailable, complex, necromass-dominated but oxidized WEOM fraction in manured soils. This finding markedly differs from the tendency towards more strongly reduced SOM, which was determined by solid-state measurements. The overlap with necromass FT-ICR-MS signatures suggested a dominant bacterial control of the changes in WEOM properties and also resulted in a stronger imprint of oxidized plant markers. Yet, the dominant fraction (83% of ion abundance) explaining the shift in oxidation state was not associated to any necromass type. This indicates an oxidation of the existing SOM reserves (“priming”).

Together, the combination of solid- and liquid-state FT-ICR-MS techniques provided complementary insight, demonstrating how manure addition affects the long-term SOC balance mirrored by SOM and WEOM composition. The comparison with potential endmembers (necromass extracts, manure) provided valuable insight into pathways of SOM turnover and will allow to identify novel process markers for future studies.