Dr Amanda Schapel1, Mr David Davenport1, Dr Melissa Fraser1, Prof Petra Marschner2
1Primary Industries and Regions South Australia, , Australia, 2The University of Adelaide, School of Agriculture, Food and Wine, Glen Osmond , Australia
Sandy soils cover a large proportion of Australia’s agricultural region and are significant soil types globally. These soils often have low productivity because of limited water and nutrient capacity. The poor plant growth and therefore low organic carbon (OC) input along with rapid decomposition because of low clay concentration makes it difficult to increase OC concentration. For this reason, sandy soils are often omitted from global agricultural soil carbon sequestration models. There is large potential to increase carbon sequestration if the OC storage capacity of sandy soils can be increased through clay addition. The addition of subsoil clay to sandy soil is a practice used in South Australia, Victoria and Western Australia to overcome water repellence, improve water and nutrient retention, and increase plant productivity. It also has the potential to increase soil OC storage through improved OC input and increased capacity to stabilise the new OC by binding to the added clay. To determine if subsoil clay addition to sandy soil increases OC storage, soil cores were collected to 50 cm depth from South Australian field sites and assessed for OC and clay concentration and select physical and chemical properties of clay-amended treatments compared to unamended sands. Clay amendment treatments differed in the amount of subsoil clay added to the surface 30 cm of sand and the depth of incorporation (shallow and deep). The greatest potential for increased OC storage in clay-amended soil was in the 10-30 cm depth with a positive correlation between OC and clay stock. Even vertical distribution of many, small clods of clay within the surface 30 cm was a key factor to increase OC storage in clay-amended soils. Subsoil clay properties and the amount added to the surface 30 cm, as well as depth to undisturbed subsoil also influenced OC stock.