Dr Jock Churchman¹

¹University Of Adelaide, Urrbrae, 5064, Australia

Dissolved organic carbon (DOC) from wheat straw was adsorbed on clay fractions rich in both kaolinite and illite, in smectite, and in allophane that were each extracted from their soils by a physical method. Sorbed DOC was also desorbed from the resulting products. Wheat resides were incubated with the 3 types of clay fractions and 14C-labelled malic acid was added to each of the 3 soils to study carbon priming effects.

The order of both extent of adsorption and of stabilisation of organic C was allophane>smectite>kaolinite-illite, reflecting surface area (SSA). Ease of desorption showed the reverse order. Adsorption was decreased and desorption increased by removal of high SSA sesquioxides. Rate of decomposition on incubation was greatest in kaolinite-illite clay fraction, similar in the smectite and allophane fractions, and increased by removal of sesquioxides. After addition of malic acid, substantially more native C was lost from the smectitic soil than from both the allophanic soil, and the kaolinitic-illitic soil, which sustained similar losses. This latter soil shows strong microaggregation.

Regarding prospects for sequestration of C, the results from priming suggest that simple addition of labile organic matter to soils may result in a decrease, rather than an increase, in the content of organic C. In any case, added C can be easily desorbed and decomposed.

Addition of (mainly kaolinitic) clay to sandy soils, carried out to cure non-wetting, led to an increase in organic C. Addition of bentonite clay to other sandy soils led to greatly increased crop growth, but addition of compost had no effect.

Only limited areas of mineral surfaces of soils are available for the uptake of organic matter and only C adsorbed initially may be retained for a long time. World-wide, sequestration of C may be most promising on sandy soils amended by clay additions.

Biography: PhD in chemistry University of Otago, industrial ceramic research (2y), post-doctoral fellowship at the University of Wisconsin-Madison (2y), employed at the New Zealand Soil Bureau (16y), CSIRO (14y), the University of Adelaide (9y) and the University of South Australia (2y). Visiting fellowships in soil science at Reading University (UK)(1y) and the University of Western Australia (6mo.).