OXIDIZIBLE ORGANIC CARBON FRACTIONS AND SOIL QUALITY CHANGES IN AN OXIC PALEUSTALF UNDER DIFFERENT PASTURE LEYS

To compare the effectiveness of different pasture species in restoring soil quality, changes in concentration and quality of soil organic carbon (C) were measured in the surface 10 cm of an Oxic Paleustalf (red earth) in the semiarid area of New South Wales, Australia, at the end of 4 years under lucerne (Medicago sativa cv. Trifecta), Consol lovegrass (Eragrostis curvula), and barrel medic (Medicago truncutulata cv sephi). Before the investigation, the soil had been degraded by 50 years of cropping. Soil samples were analyzed for water stable aggregation, mineralizable N, and C by three procedures: Total carbon (C) by dry combustion, oxidizible C by potassium permanganate, and oxidizible C by potassium dichromate/sulphuric acid with varying concentrations of acid. Higher dry matter production caused lucerne to be was more effective than barrel medic in increasing soil organic carbon concentration. Compared with fallow plots, total soil organic carbon concentration increased by 16, 26, and 11%, respectively, in the Consol lovegrass, lucerne, and barrel medic treatments. Nevertheless, even in the case of lucerne, the 26% increase in organic carbon in the 0-10-cm layer at the end of 4 years (7.87 vs. 9.88 g/kg) represented only 15% of the total loss in organic carbon after 50 years of cropping. Most (78-92%) of the organic carbon increases under the various pastures were of the more labile forms, as indicated by their removal under much milder oxidizing conditions than those recommended in the standard methods for organic carbon determination. Significant improvements in structural stability and nitrogen availability were detected in the perennial pasture soils. Our results suggested that the amount of organic carbon oxidizible by a modified Walkley-Black method, which involves using only half the amount of sulphuric acid, is a more sensitive indicator of the improvement in soil quality parameters under investigation, namely increases in mineralizable nitrogen and water stable aggregation. Further research is needed to verify these findings over a range of soil types and agroecosystems.