Soil C fractionations
We separated soil organic matter into MAOM and particulate organic
matter (POM) to evaluate mechanisms underlying the persistence of
root-derived C in ingrowth cores. Due to costs, we selected three of the
six sites and processed all samples from the six end-member plots at
each site. For each sample, we used a standard size fractionation
procedure (Cambardella & Elliott 1992). Briefly, we dispersed 10 g soil
samples in 30 mL of 5% (w/v) sodium hexametaphosphate for 20 h on a
reciprocal shaker at 180 rpm and washed slurries through a 53-um sieve
using a stream of DI water. Organic matter collected on the sieve was
considered POM and the fine fraction that passed through the sieve was
considered MAOM. Both fractions were dried (80 °C), weighed and ground
to a powder. The MAOM fraction was analyzed for C concentrations and
δ13C at the Purdue Stable Isotope Facility as
described above. Root-derived MAOM-C (MAOM-Crdc; mg
MAOM-C/g bulk soil) was then calculated as:
MAOM-Crdc =\(\frac{MAOM-C\ (mg)}{bulk\ soil\ (g)}\)* Frd