THE EFFECTS OF SALINITY AND SODICITY ON SOIL ORGANIC CARBON STOCKS AND FLUXES: AN OVERVIEW Vanessa N.L. Wong 1,2 , Richard S.B. Greene 1 , Brian W. Murphy 3 , Ram Dalal 2,4 , Surender Mann 2,5 and Graham Farquhar 2,6 1 School of Resources, Environment and Society, The Australian National University; CRC LEME 2 CRC for Greenhouse Accounting 3 NSW Department of Planning, Infrastructure and Natural Resources 4 Queensland Department of Natural Resources, Mines and Water 5 WA Department of Industry and Resources 6 Research School of Biological Sciences, The Australian National University INTRODUCTION Soil is the world’s largest terrestrial carbon (C) sink, and is estimated to contain approximately 1600 Pg of C to a depth of one metre (Eswaran et al., 1993). The distribution of soil organic C (SOC) largely follows gradients similar to biomass accumulation, increasing with increasing precipitation and decreasing temperature. As a result, SOC levels are a function of inputs, dominated by plant litter contributions and rhizodeposition, and losses such as leaching, erosion and heterotrophic respiration. Therefore, changes in biomass inputs, which affect organic matter accumulation, will most likely also alter these levels in soils. Although the soil microbial biomass (SMB) only comprises 1-5% of soil organic matter (SOM; Sparling, 1992), it can provide an early indicator of SOM dynamics as a whole due to its faster turnover time. Hence, it can be used to determine soil C dynamics under changing environmental conditions (Killham, 1994). Approximately 932 million ha of land worldwide are degraded due to salinity and sodicity, usually coinciding with land available for agriculture. Of this area, salinity affects 23 % of arable land while saline-sodic soils affect a further 10 % (Szabolcs, 1989). In Australia, approximately 17 million ha of land is affected by salinity, while sodicity affects approximately 340 million ha (Szabolcs, 1989), usually coinciding with agricultural areas. Soils affected by salinity, i.e. those soils high in soluble salts, are characterised by rising watertables and waterlogging of lower lying areas in the landscape. Sodic soils are high in exchangeable sodium, slake and disperse upon wetting. On drying, massive hardsetting structures are formed, which suffer from poor soil-water relations largely related to decreased permeability, infiltration and the formation of surface crusts. In these degraded areas, SOC levels are likely to be affected by declining vegetation health and hence, decreasing biomass inputs and concomitant lower levels of SOM accumulation. An increase in salinity and sodicity directly impacts upon plant vigour through changes in osmotic potential, ion toxicities and deficiencies. Indirect effects on vegetation can result from altered soil conditions such as increased dispersion and decreased permeability. Moreover, potential SOC losses can be higher from dispersed aggregates due to sodicity and solubilisation of SOM as a result of salinity. Therefore, changes in salinity and sodicity affect soil physical and chemical properties, which subsequently alter nutrient cycles and decomposition processes. The risk of erosion is increased, while soil physical and chemical properties are altered, impacting upon aggregation and nutrient cycling as well as biotic activity. Therefore, there is a clear linkage between land management practices through their effect on salinity and sodicity and their potential to alter soil C stocks and fluxes in the landscape, particularly in regards to land degradation and subsequent rehabilitation efforts (eg. Wong et al., 2005). Despite the large area affected by salinity and sodicity, data on the magnitude and mechanism of changes in soil C stocks in these degraded environments remains sparse. In addition, few studies are available that unambiguously demonstrate the effect of increasing salinity and sodicity on soil C dynamics (eg. Nelson et al., 1997, Nelson et al., 1996, Pankhurst et al., 2001, Sarig et al., 1993). The overall aim of this project is to determine how soil C stocks and turnover rates are affected by land degradation through increasing salinity and sodicity, and the extent of hysteresis these systems exhibit upon rehabilitation. The project has the following objectives: Regolith 2006 - Consolidation and Dispersion of Ideas 367