The effect of soil texture and roots on the stable carbon isotope composition of soil organic carbon

Abstract

This study examines the distribution of soil organic carbon and carbon-isotopes with depth and among particle size fractions in 2 forest soil profiles of contrasting texture from Cape York Peninsula, Queensland, Australia. The profile on sand has a comparatively low inventory of carbon (557 mg/cm2 from 0–100 cm) and exhibits comparatively small variations in δ13C value. In contrast, the clay-rich profile has a much larger inventory of soil organic carbon (1725 mg/cm2 from 0–100 cm) and large variations in δ13C value occur both with depth in the profile and between different particle size fractions. The considerable differences in carbon inventories and δ13C values between the sites appear to be largely due to soil textural differences. In�the absence of fine minerals the trend in δ13C value with decreasing particle size is to similar or lower δ13C values, due to an increase in the relative abundance of low δ13C compounds in the residue left by microbial decomposition. In the presence of fine minerals, the trend is to higher δ13C values due to the stabilisation of the products of microbial decomposition by the fine minerals. Thus, the bulk δ13C value of soil organic carbon appears to be determined as much by the abundance of fine minerals in a soil profile as by isotope fractionation effects accompanying degradation. It is further postulated that an initial rapid rise in δ13C value in the upper soil layers is due to an increase in the relative importance of higher 13C, root-derived carbon immediately below the soil surface.