Characterization of recently 14C pulse‐labelled carbon from roots by fractionation of soil organic matter

Abstract

SummaryThe inability of physical and chemical techniques to separate soil organic matter into fractions that have distinct turnover rates has hampered our understanding of carbon (C) and nutrient dynamics in soil. A series of soil organic matter fractionation techniques (chemical and physical) were evaluated for their ability to distinguish a potentially labile C pool, that is ‘recent’ root and root‐derived soil C. ‘Recent’ root and root‐derived C was operationally defined as root and soil C labelled by 14CO2 pulse labelling of rye grass–clover pasture growing on undisturbed cores of soil. Most (50–94%) of total soil + root 14C activity was recovered in roots.Sequential extraction of the soil + roots with resin, 0.1 m NaOH and 1 m NaOH allocated ‘recent’ soil + root 14C to all fractions including the alkali‐insoluble residual fraction. Approximately 50% was measured in the alkali‐insoluble residue but specific activity was greater in the resin and 1 m NaOH fractions. Hot 0.5 m H2SO4 hydrolysed 80% of the 14C in the alkali‐insoluble residue of soil + roots but this diminished specific activity by recovering much non‐14C organic matter. Pre‐alkali extraction treatment with 30% H2O2 and post‐alkali treatment extractions with hot 1 m HNO3 removed organic matter with a large 14C specific activity from the alkali‐insoluble residue.Density separation failed to isolate a significant pool of ‘recent’ root‐derived 14C. The density separation of 14C‐labelled roots, and roots remixed with non‐radioactive soil, showed that the adhesion of soil particles to young 14C‐labelled roots was the likely cause of the greater proportion of 14C in the heavy fraction.Simple chemical or density fractionations of C appear unsuitable for characterizing ‘recent’ root‐derived C into fractions that can be designated labile C (short turnover time).