2.4 Physical fractionation of soil organic matter
Total organic carbon (TOC) and total nitrogen (TN) were determined using
a ThermoFlash 2000 NC Soil Analyzer. For organic C analysis, carbonates
were removed from soil samples by acid fumigation with HCl prior to
analysis (Harris et al., 2001).
Soils were fractionated by size after aggregate dispersion following the
method described by Cambardella and Elliott (1992) and Sokol and
Bradford (2019). A 10.5-g portion of each sample, sieved at 2 mm, was
shaken for 18 hr in 5 g L-1 sodium hexametaphosphate,
Na6[(PO3)6]. After
dispersion, samples were sieved again at 53 μm using a vibratory sieve
shaker (AS 200, Retsch). Soil POM and MAOM represented the fraction
remaining on the sieve (>53 μm) and that which passed
through the sieve (<53 μm), respectively. All fractions were
oven-dried at 60°C, weighed, and ground with a zirconium ball mill (MM
400, Retsch) for analyses. Carbon and nitrogen contents of POM and MAOM
fractions were analysed as described for TOC and TN. Hereafter,
particulate organic and mineral-associated organic C contents are
referred to as POC and MAOC, respectively.