Changes in soil chemical properties along afforestation chronosequence in the dried Aral Sea bed, Kazakhstan, from 1990-2019

Desiccation of the Aral Sea left a vast area of unstable, saline, and barren seafloor. In response, afforestation using indigenous trees, mainly Haloxylon species, has been performed to ameliorate the soil. This study examined 1) changes in topsoil chemical properties after vegetation establishment based on 30-year chronosequence of afforested sites and 2) dynamics in the topsoil properties in two cases of naturally vegetated versus afforested areas. In August 2019, soils were sampled from the northeastern Aral Sea bed, Kazakhstan, where the shoreline retreated during the 1970s. We selected a non-saline area without any vegetation, a visibly salinized area without any natural vegetation, and 12 paired sites with predominantly sandy soil texture that were naturally vegetated or afforested during 1990, 2000, 2005, 2008, 2013, and 2017. In the 0‒10 cm soil layer, 3 points were sampled in each site and we analyzed soil pH, electrical conductivity (EC), total dissolved solids (TDS), exchangeable sodium percentage (ESP), total nitrogen (TN) and carbon (TC), total organic carbon (TOC), available phosphorus concentration (P₂O₅), exchangeable cation concentrations (K⁺, Mg²⁺, Ca²⁺, and Na⁺), and cation exchange capacity (CEC). The soil samples in chronosequence sites showed a wide range of EC, without any significant temporal trend. The pH ranged between 8.5 and 10.0, characterized soil as alkaline. CEC ranged from 9 to 45 cmol_c kg^-1. Soil pH, EC, and TDS variations among the chronosequence sites (natural or afforested) were not statistically significant. In contrast, we observed marginal increases in K⁺ and P₂O₅ after the vegetation establishment. Also, TN and TOC concentrations increased over time, significantly faster in afforested than in naturally vegetated sites. However, TC contents showed a sudden decrease in the oldest natural vegetation. This result may be partly attributed to the spatial variability in sampling locations used for the chronosequence analysis. Lastly, there were strong positive correlations among TOC, TN, K⁺, and P₂O₅; which imply an increase in soil organic materials’ contribution to nutrient accumulation and overall soil quality. In conclusion, afforestation contributed to soil amelioration but this effect was also observed in naturally vegetated sites.