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.