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) w ere 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 lu cerne (Medicago sativa cv. Trifecta), Consol lovegrass (Eragrostis curvula) , and barrel medic (Medicago truncutulata cv sephi). Before the investigati on, the soil had been degraded by 50 years of cropping, Soil samples were a nalyzed for water stable aggregation, mineralizable N, and C by three proce dures: Total carbon (C) by dry combustion, oxidizible C by potassium perman ganate, and oxidizible C by potassium dichromate/sulphuric acid with varyin g concentrations of acid, Higher dry matter production caused lucerne to be was more effective than b arrel medic in increasing soil organic carbon concentration. Compared with fallow plots, total soil organic carbon concentration increased by 1.6, 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 we re of the more labile forms, as indicated by their removal under much milde r oxidizing conditions than those recommended in the standard methods for o rganic carbon determination. Significant improvements in structural stabili ty and nitrogen availability were detected in the perennial pasture soils. Our results suggested that the amount of organic carbon oxidizible by a mod ified Walkley-Black method, which involves using only half the amount of su lphuric acid, is a more sensitive indicator of the improvement in soil qual ity parameters under investigation, namely increases ill mineralizable nitr ogen and water stable aggregation. Further research is needed to verify the se findings over a range of soil types and agroecosystems.