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.