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.

Anthropogenic disturbance of natural forest vegetation on calcareous soils alters soil organic matter composition and natural abundance of 13C and 15N in density fractions

“…These results are consistent with those reported by other authors, e.g. Nadelhoffer and Fry (1988), Accoe et al (2003) and Bird et al (2003). The results of a litter bag experiment led Connin et al (2001) to suggest that mechanisms responsible for isotopic discrimination are, at least partly, characteristic of earlier decay stages.…”

Stabilization of Soil Organic Matter: Association with Minerals or Chemical Recalcitrance?

“…By contrast, in samples with smaller SSA, which presumably contain less protective mineral phases, HF treatment released predominantly 13 C-depleted organic constituents. This finding agrees with the view that potentially biodegradable compounds enriched in 13 C such as polysaccharides and proteins can be stabilized by mineral surfaces (Bird et al 2003;Kiem and Ko¨gel-Knabner 2003) and are thus effectively removed during mineral dissolution (Dai and Johnson 1999;Schmidt and Gleixner 2005).…”

Decomposition, δ13C, and the “lignin paradox”

“…This would lead to depletion of δ 13 C in soil organic matter relative to the starting litter inputs, at least in the early stages (e.g., Flanagan et al 1999;Garten et al 2000;Quideau et al 2003). However, numerous studies (e.g., Buchmann et al 1997;Flanagan et al 1999;Ehleringer et al 2000;Feng 2002;Bird et al 2003;Quideau et al 2003) have shown that decomposition of organic matter is associated with increasing δ 13 C. The δ 13 C value usually increases with soil depth or with decreasing particle-size fraction within a depth range, as soil organic matter becomes increasingly dominated by microbial necromass stabilized by mineral association (Bird et al 2003).…”