ORGANIC-CARBON AND ASSOCIATED SOIL PROPERTIES OF A RED EARTH AFTER 10 YEARS OF ROTATION UNDER DIFFERENT STUBBLE AND TILLAGE PRACTICES

Differences in soil organic carbon level as a result of different tillage and stubble management practices under continuous cropping were studied in a 10 years old wheat/lupin rotation experiment on a red earth at Wagga Wagga, New South Wales. Stubble burning and tillage had a similar impact in reducing the total amounts of organic carbon in the top 0.2 m of soil. There was no significant difference between the conventional cultivation (3 cultivations) and reduced cultivation (1 cultivation) systems. A 31% difference in organic carbon in the top 0.1 m (2.42% v. 1.68%) was found between the extreme management practices, i.e. direct drill / stubble retained treatment and the conventional/stubble burnt treatment. These results highlight the important effect of management practice on soil organic carbon level under continuous cropping. Tillage had the additional effect of changing the distribution of organic carbon resulting in higher level in the 0.10-0.15 m layer. The reduction m organic carbon was accompanied by significant losses in total nitrogen, exchangeable calcium and magnesium, as well as reduction in biological activity and aggregate stability. Loss of 1% organic carbon resulted in a loss of 2.97 cmole(+) kg soil-1 of negative charge. However, C/N ratio remained constant at 12-1 under different tillage and stubble treatments. Finally, while stubble burning resulted in pH increase, tillage led to a significant reduction in soil pH (5.38 to 4.98) in the 0-0.05 m layer due to increased exchangeable Al brought to the soil surface as a result of an inversion action.