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