Molecular Composition of Microbial Derived Necromass in Soil

Abstract:

Microbial derivatives, or necromass, are a dominant source of persistent soil organic matter (SOM), yet the chemical constituents of necromass and geochemical reactions leading to the persistence of these compounds in SOM remains to be discovered. By identifying the relationship between microbial necromass and soil geochemistry, we tested the relative importance of microbial (biotic) and mineral (abiotic) filters on necromass accumulation and persistence. Using a ¹³C-labeling incubation experiment, we tested the hypotheses that (H1) necromass accumulation varies between microbiomes associated with different crops, (H2) microbial necromass persists because of chemically complex residues and metabolites not favorable for mineralization, and (H3) necromass persistence is associated with poorly crystalline iron minerals. Soils were collected from sandy loams at the Kellogg Biological Station in MI, and silty loams at the Arlington Agricultural Research Station in WI. 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 soil. Of this, approximately 30% was recovered in the lipid, protein and metabolite pools. Lipids contained the most ¹³C (16%) and the contribution was similar in both soils. Protein from the sandy loam was significantly more enriched than from the silty loam despite similar pool sizes. Although metabolites are generally considered ephemeral, this pool was small, but highly enriched (>11000 δ¹³C), suggesting substantial recycling over the 2-month incubation. After repeated extractions, 40% of the label remained in the soil pellet. An additional ~30% of the 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. Preliminary ssNMR results suggest that this material is rich in aromatic structures. In addition, we found a substantial fraction of remaining ¹³C was associated with the mineral-associated organic matter (MAOM) pool determined by density fractionation. Our results provide novel evidence of the carbon pools that contribute to microbial necromass in soil and suggest that soil characteristics (sandy vs. silty) impact the accrual of microbial biomass residues in soil.