Influence of clay clod size and number for organic carbon distribution in sandy soil with clay addition

In agricultural soils, subsoil clay addition to sand has the potential to improve carbon sequestration by increasing soil organic carbon (OC) concentration through adsorption and occlusion. However, the factors influencing increased OC in these engineered soils are poorly understood. The addition of subsoil clay creates clods of different sizes, from a few mm up to 200 mm or more in diameter. This study assessed the i) size, number and vertical distribution of clods and OC at two clay-modified field sites and ii) effect of clod size and properties on OC in incubation experiments. The hypotheses were that smaller clods would increase and stabilise OC more than larger clods and that an even distribution of clods throughout the depth of modification will increase OC stock compared to patchy distribution. Two field sites with differing clay modification method, delved and spaded, were studied. Soil was excavated from a 30 cm quadrat in 10 cm increments down the profile, sieved into clod sizes and mass with clod number and OC concentration determined. Delving elevated clay from 40 to 60 cm depth and created few clods, which were poorly distributed in the depth of modification. Spading mixed clay from 20 to 30 cm below the soil surface and created many, smaller sized clods, which were more evenly distributed within the 0–30 cm modification depth. OC concentration was highest in the smallest clods, particularly at the soil surface. OC stock increased with clod number. Clods collected from the two field sites were further used in incubation experiments to determine the effect of clod size and properties (clay and iron concentration) on the accumulation and protection of OC. Clods (2–6 and 6–20 mm) were added to sand at 80 mg clay g⁻¹ sand and incubated 300 or 420 days at optimal moisture conditions with monthly wheat residue addition in the accumulation experiment. Smaller clods (2–6 mm) accumulated OC at a higher rate and offered greater protection from decomposition by microbes than larger clods (6–20 mm). These results support our hypotheses that smaller clods and even vertical distribution is important to increase OC. Furthermore, clod number was a critical factor in increasing OC content. We conclude that in clay-modified soils the addition of many, smaller sized clods distributed throughout the depth of modification can improve OC content.Amanda Schapel, Petra Marschner, Jock Churchma

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