Effects of clay minerals and land use on organic matter pools

The organic matter in density and particle size fractions of an Andisol 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 pasture and cropping. The Andisol samples were taken from sites which have been under perennial pasture for 100 years and from a pasture site which has been cultivated and cropped with barley and brassica for 20 years. The Inceptisol samples were from sites under native forest and perennial pasture, and from a pasture site which had been cropped with maize for 20 years. The Andisol contained allophane and ferrihydrite whereas the Inceptisol contained mica as the main clay mineral. The C/N ratios of the dispersed-light fractions and heavy particle size fractions 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 fractions, 10-12 for the 2-20 mu m fraction, 15-18 for the > 20 mu m fraction, and generally highest for the light fractions. This is consistent with the presence of clay minerals with low specific surface area. Infrared 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) suggesting that the turnover of C was more rapid in the Inceptisol. Simulations of C and N turnover for the Andisol using CENTURY indicated that the pool of passive organic matter was very large. For soils with variable charge, of which the Andisol is an example, a modifying factor such as specific surface area may be required in order to give a close simulation of C and N turnover. The data indicate that the stability of organic matter is greater in the Andisol than in the Inceptisol, and it is less likely to be affected by cropping. Allophane and ferrihydrite appear to have a stabilising influence on a large part of the organic matter in the Andisol.