SOIL ACIDIFICATION AND THE CARBON-CYCLE IN A CROPPING SOIL OF NORTH-EASTERN VICTORIA

Changes in soil organic matter were determined for a long-term (1975-9 5) experiment at the Rutherglen Research Institute in north-eastern Vi ctoria. The crop rotations in this experiment were continuous lupins ( LL) and continuous wheat (WW). The soil at this site was a solodic or Yellow Dermosol with a soil pH of 6.08 (pH in 0.01 M CaCl2 1:5) in 197 5 in the surface 10 cm, which had declined by 0.8 and 1.5 pH units for WW and LL, respectively, in the 0-20 cm soil zone by 1992. Acidificat ion rates decreased with increasing soil depth. The acidification rate in the 0-60 cm soil zone was 12.5 kmol(H+)/ha.year for the LL rotatio n and 4.6 kmol(H+)/ha.year for the WW rotation. The amount of CaCO3 re quired to neutralise the acidification of wheat-lupin rotations as cal culated in this paper was up to 3.8 t/ha.10 years for a WLWL rotation or 3.3 t/ha10.years for a WWL rotation; these amounts are significantl y higher than previously reported rates. In this paper, we calculate t he impact of changes in soil carbon (C) status over time, and therefor e soil buffering, on the rates of acidification in incremental soil la yers to a depth of 60 cm. Total organic C for these rotations in 1992 was 1.12% for WW and 1.17% for LL in the 0-10 cm soil zone. An investi gation of the humic and fulvic acid fractions of these 2 rotations to a depth of 60 cm showed that the LL rotation had significantly higher (P < 0.05) C at depth than the WW rotation. Acidification due to the n et decrease in soil C over the 15-year study period plus acidification due to the alkali removed in the seed was calculated to be -4.88 kmol (H+)/ha.year for the LL rotation and -6.52 kmol(H+)/ha.year for the WW rotation.