Texture effects on carbon stabilisation and storage in New Zealand soils containing predominantly 2 : 1 clays

Developing strategies to sequester carbon (C) in soils requires an understanding of the key factors that influence C stabilisation. Although fine mineral particles, especially clay, play a key role in stabilising soil organic matter (SOM), the relationship between SOM and texture is often not strong. We examined the role of the fine mineral fraction in C storage in sedimentary soils in New Zealand. Soils, representing two soil Orders (Brown and Recent) and different land use histories (total of 58 soils; 0–15 cm depth) were sampled. The concentration of C (and N) in four particle size fractions (<5, 5–20, 20–50, >50 µm) was determined (soils fractionated after dispersion by sonication). The soils had a wide range of textures and SOM; the mass proportion of clay (<5 µm) ranged from 10 to 60 g 100 g–1 and soil C from 16 to 45 g kg–1. Across both soil Orders and all land uses (dairy, sheep or beef, arable and vegetable cropping), the majority of soil C (57 to 66%) was stored in the clay fraction. However, there was no correlation (R2 = 0.02; P > 0.05) between the C concentration in whole soil and clay content. The concentration of C in the clay fraction, which varied over a wide range (35 to 135 g kg–1 clay), decreased as the mass proportion of clay increased. A similar trend in C concentration was observed for the fine (5–20 µm) silt fraction. Because of this inverse relationship between the mass of the fine fractions and their C concentration, there was little change in amount of stable C (defined as C in the <20 µm fraction) as the mass proportion of fine (<20 µm) particles increased. Differences in pyrophosphate extractable aluminium explained part of the variability in C concentration in the fine fractions; however, we were unable to identify any specific physico-chemical factor that would account for the relatively low C concentrations observed in the <5 and 5–20 µm fractions of fine-textured soils. We concluded that such soils may be under-saturated and potential may exist to store additional stable C.