Soil fractionation, isotope analysis, and litter C and N fate
Ingrowth cores were destructively harvested in September and litter C and N inputs were traced into SOM pools (Ridgeway et al., 2022). A 5g subsample of dry soil from each core was separated into light POM, heavy POM, and MAOM by density and size fractionation as described in Lavallee et al. (2020). In brief, the light POM was separated through density floatation in 1.85g/mL sodium polytungstate salt solution. The remaining soil was separated into heavy POM and MAOM fractions by size separation where the MAOM fraction passes through a 53um sieve.
To trace the fate of 13C and 15N litter amendments, the soil fractions were analyzed for %C, %N, δ13C, and δ 15N using a Thermo Fisher Delta V+ isotope ratio mass spectrometer interfaced with a Carlo Erba NC2500 Elemental Analyzer. Two endmember mixing models were used to determine the proportion of C and N in each soil fraction that came from native SOM vs. the enriched litter substrate (Derrien & Amelung, 2011).