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 temperature and decreasing precipitation. 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, or organic matter accumulation, will most likely also alter these levels in soils. Although the Soil Microbial Biomass (SMB) only comprises of 1-5% of Soil Organic Matter (SOM), it can provide an early indicator of SOM dynamics as a whole due to its faster turnover time, and hence, can be used to determine soil C dynamics under changing environmental conditions. Approximately 932 million ha of land worldwide is degraded due to salinity and sodicity, usually coinciding with land available for agriculture, with salinity affecting 23% of arable land while saline-sodic soils affect a further 10% (Szabolcs 1989). 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, and slake and disperse upon wetting, form massive hardsetting structures on drying, 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 organic matter accumulation. Moreover, potential SOC losses can be higher from dispersed aggregates due to sodicity and solubilization of SOM due to salinity. Few studies are available that unambiguously demonstrate the effect of increasing salinity and sodicity on C dynamics. In this research, the effect of increasing salinity and increasing sodicity on C dynamics was determined by subjecting a non-saline non-sodic soil to one of six treatments. A low, mid or high salinity solution (EC 0.5, 10 and 30) combined with a low or high sodicity solution (SAR 1 and 30) in a factorial design was leached through a non-saline non-sodic soil in a controlled environment. The greatest increases in SMB occurred in those treatments of high-salinity high-sodicity, and high-salinity low-sodicity. This was attributed to solubilization of SOM which provided additional substrate for decomposition for the microbial population. Thus, as salinity and sodicity increase in the field, soil C is likely to be rapidly lost as a result of increased mineralization. Gypsum is the most commonly used ameliorant in sodic and saline-sodic soils to rehabilitate adverse soil environmental conditions. When soils were sampled from saline-sodic profiles in salt-scalded areas, SMB levels and soil respiration rates were found to be low in the saline-sodic soil compared to normal non-degraded soils. When the saline-sodic soils were treated with gypsum, there was no change in the SMB and respiration rates. The low levels of SMB and respiration rates were due to little or no C input into the soils of these highly degraded landscapes, as the high salinity and high sodicity levels have resulted in vegetation death. However, following the addition of organic material to the scalded soils, SMB levels increased to levels greater than that found in the non-saline non-sodic soil. The addition of gypsum (with organic material) gave no additional increases in the SMB. These experimental results indicate that in salt-affected landscapes, initial increases in salinity and sodicity results in rapid C mineralization. Biomass inputs also decrease due to declining vegetation health, followed by further losses as a result of leaching and erosion. The remaining native SOM is then mineralized, until very low levels of SOC, mostly recalcitrant C remain, which are more difficult to decompose. However, the C sequestration potential in these degraded areas is high, particularly if rehabilitation efforts are successful, as soil ecosystem function can be restored if organic material is available for decomposition when these salt-affected landscapes are revegetated. References: (1) Eswaran H, van den Berg E, Reich P (1993) Organic carbon in soils of the world. Soil Science Society of America Journal 57, 192-194. (2) Szabolcs I (1989) 'Salt affected soils.' (CRC Press: Boca Raton)