Isotopic evidence reveals persistent microbial residues in soil

Microbial derivatives and necromass are dominant sources of soil organic matter (SOM), yet the specific microbiological and geochemical reactions leading to the persistence of microbial compounds in SOM remains to be discovered. Identification of the microbial taxa and classes of microbial-derived compounds that are selectively preserved may enhance our ability to manage SOM, particularly in agroecosystems. We examined how perennial and annual biofuel cropping systems influence the production and selective preservation of microbial residues. Our experiment was replicated on a sandy and a silty loam to test the relative importance of microbial (biotic) and mineral (abiotic) filters on necromass accumulation and persistence. Using a ¹³C-labeling incubation experiment, we tested the effects of cropping system and soil texture on the production and persistence of microbial-derived residues. Soils were collected from sandy loams at the Kellogg Biological Station (MI, USA) and silty loams at the Arlington Agricultural Research Station (WI, USA). These soils were amended with ¹³C-labeled glucose, which was rapidly incorporated into microbial biomass. After 2 months, ~50% of the added ¹³C remained in the bulk soil. Approximately 30% of the ¹³C remaining in the bulk soil was recovered in the lipid, protein, and metabolite pools. Lipids contained the most ¹³C (16%) and the contribution was similar in both soils. Both soils had similar protein pools, but protein from the sandy loam was significantly more enriched than protein from the silty loam. The pool of metabolites was small, but highly enriched, suggesting substantial recycling over the 2-month incubation. The majority (40%) of the whole soil ¹³C persisted in the SOM even after repeat extractions. The remaining ~30% of the whole soil ¹³C was recovered in a complex of remaining unknown debris that separates from the soil at the solvent interphase with the protein but could not be solubilized. We provide novel evidence of the carbon pools that contribute to persistent microbial residues in soil. Our results suggest that metabolites may be more important than was previously recognized. Ongoing work is identifying the labeled metabolites and characterizing the chemistry of the highly enriched protein residue fraction.