Reducing topsoil salinity and raising carbon stocks through afforestation in Khorezm, Uzbekistan

Agricultural mismanagement of irrigated drylands results in severe soil degradation. Afforestation is an option for ameliorating such degraded land. We evaluated the impact afforestation has on the topsoil (0-20 cm) of salinized degraded cropland in regards to salinity, aggregate stability, and soil organic carbon (SOC) stocks in Uzbekistan, Central Asia. The effects of tree plantations established under either furrow or drip irrigation were studied four years following afforestation and two years after irrigation ceased. For comparative study we also sampled fallow land, land with 80 years of tree growth, natural forest, desert ecosystems, and paddy rice fields. Initial furrow irrigation showed to be most effective in improving soil fertility after four years of afforestation; the respective plantations of Populus euphratica and Ulmus pumila showed significant levels of reduced soil salinity and increased aggregate stability and improved SOC stocks. The comparison of the long-term afforested land with the short-term equivalent suggested a C sequestration rate of 0.09-0.15 t C ha^-1 year^-1. The SOC stocks of the long-term afforestation site exceeded those of the native forest. Hence, a rehabilitation of salt-affected cropland is feasible following the conversion into occasionally irrigated tree plantations, although it takes decades to reach steady-state conditions.