EFFECTS OF CLAY-MINERALS AND LAND-USE ON ORGANIC-MATTER POOLS

The organic matter in density and particle size fractions of an Andiso l and an Inceptisol has been characterised by its C and N contents and infrared spectra. The soils have a similar texture but different clay mineralogy and are under two contrasting land uses represented by pas ture and cropping. The Andisol samples were taken from sites which hav e been under perennial pasture for 100 years and from a pasture site w hich has been cultivated and cropped with barley and brassica for 20 y ears. The Inceptisol samples were from sites under native forest and p erennial pasture, and from a pasture site which had been cropped with maize for 20 years. The Andisol contained allophane and ferrihydrite w hereas the Inceptisol contained mica as the main clay mineral. The C/N ratios of the dispersed-light fractions and heavy particle size fract ions from the Andisol were in a narrow range of 9-12, and this may be related to the presence of allophane and ferrihydrite. By comparison, the C/N ratio of the Inceptisol was close to 9 for the < 2 mu m fracti ons, 10-12 for the 2-20 mu m fraction, 15-18 for the > 20 mu m fractio n, and generally highest for the light fractions. This is consistent w ith the presence of clay minerals with low specific surface area. Infr ared spectra indicated that the organic matter associated with the < 2 mu m fraction was enriched in aliphatic groups. Soil carbon in the 0- 20 cm layer of the Andisol was 10 T/ha lower under cropping (134 T/ha) than under pasture (144 T/ha). By comparison, the Inceptisol was 23 T /ha lower under cropping (50 T/ha) than under pasture (73 T/ha) sugges ting that the turnover of C was more rapid in the Inceptisol. Simulati ons of C and N turnover for the Andisol using CENTURY indicated that t he pool of passive organic matter was very large. For soils with varia ble charge, of which the Andisol is an example, a modifying factor suc h as specific surface area may be required in order to give a close si mulation of C and N turnover. The data indicate that the stability of organic matter is greater in the Andisol than in the Inceptisol, and i t is less likely to be affected by cropping. Allophane and ferrihydrit e appear to have a stabilising influence on a large part of the organi c matter in the Andisol.