Although shifts in community composition are widely observed with differences in soil physiochemical properties (pH, texture, minerology) little empirical evidence is available to determine what impact these differences may have on the composition of microbial biomass or the conversion of these microbial residues (necromass) to soil organic matter. We set out to test if microbial community differences, driven by soil characteristics, influences the production of residues such as lipids, proteins and amino sugars. We leveraged soils cultivated with identical switchgrass cultivars with contrasting texture and physiochemical properties: the sandy loams of the Kellogg Biological Station (KBS) in MI, and silty loams from the Arlington Agricultural Research Station (AARS) in WI. Data from the field sites indicates different bacterial and fungal communities between the two sites. We amended these soils with 13C-labeled glucose at equivalent levels to microbial biomass carbon. After two months we tracked 13C into microbial biomass residues with a Folch extraction to generate polar metabolite, protein, and lipid pools. We hypothesized lower retention of 13C in KBS soils due to their sandy texture.
Results/Conclusions:
After 2 months ~50% of the 13C remained in the soils. Of the remaining 13C 27% was extracted from silty loam (KBS) and 32% from sandy loam soil (AARS) into metabolite, protein and lipid pools. These 3 pools comprise <2% of the sample mass, but incorporated high 13C due to microbial processing of the 13C glucose. Although metabolites are considered ephemeral nature, this pool was highly enriched (>11000‰ δ13C), suggesting substantial recycling during the incubation. The small total C size of this pool (<1%) suggests retention of 13C due to recycling of these molecules or isotopic discrimination. Protein from KBS was significantly more enriched than AARS (mean 992‰ vs 291‰ δ13C respectively p-value<0.01) despite similar pool sizes. An additional 32% of retained 13C in KBS and 23% in AARS was in a complex of remaining unknown debris that separates at the interphase with protein but couldn’t be solubilized. Preliminary ssNMR results suggest this material is not proteinaceous or lipidic in nature but is rich in aromatic structures. Our results provide some of the first evidence of carbon pools that contribute to soil microbial necromass and suggest that soil characteristics impact not only microbial community composition but biomass residues in soils.