Soil crust development on playa surfaces of Lake Urmia and its controlling factors: New insights to combat dust sources

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The recession of lake shores, including the ones of Lake Urmia (LU), and later exposure of the lake bed sediments to wind erosion as a consequence of climate change and drought, is a devastating phenomenon worldwide. In the absence of stable surface crusts, these areas can be the source of airborne dust causing health and environmental issues. The present study aims at investigating the crust formation and development on Lake Urmia playa (LUP) surfaces through physicochemical and morphological studies and to determine the major factors affecting crust stability. To achieve the above goals, 50 soil crust samples from playa surfaces of the western LU playa were collected. The soil crust thickness and compressive strength were determined by a digital micro penetrometer. By means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), the main morphological and chemical factors responsible for the crust development and evolution in LUP were identified. Results showed that crust samples from clay flats (CFs) and clay flats-salt crusts (CF-SC) possessed the most stable crusts with a thickness of 2.2 and 1.9 cm and maximum compressive strength of 6709 and 7399 kPa, respectively, while sand sheets (Sa-sheets) showed weak and locally restricted crust development. Evaluation of correlation coefficients (CC) between soil crust physicochemical properties and crust stability parameters revealed that clay, silt, and organic matter enhance the crust stability and decrease the erodible fraction by increasing the crust thickness, while salt content quantified as Na+ concentration and electrical conductivity increase the firmness of the crusts. Interestingly, total carbonates quantified as calcium carbonate equivalent (CCE) correlated negatively with crust stability although pedogenic calcite is known as cementing agent in soils. XRD together with FTIR spectroscopy revealed that calcite is indeed the predominant mineral form in most soils with the exception of Sa-sheets with the highest share of carbonates, in which aragonite prevailed over calcite. While carbonates of pedogenic origin act as cementing agent, geologic carbonates present as coarse aragonite particles decrease soil stability. Therefore, samples with the highest total carbonate content exhibited the weakest crust, which explains the negative correlation between soil crust and CCE. SEM coupled with Energy Dispersive X-ray (EDX) and elemental mapping revealed that NaCl is the major cementing agent of the soil particles in crusts developed on LU playa surfaces. Its coexistence with phyllosilicates has increased the crust stability and resilience against wind erosion in CF and CF-SC samples while in the Sa-sheet samples, the absence of clay-size minerals (silt and clay) along with the prevalence of geologic carbonates (dolomite and aragonite) and lower NaCl content, limited crust formation and stability. This study emphasizes the importance of the intrinsic soil physicochemical properties together with the secondary (pedogenic) carbonate formation and more importantly the presence of NaCl, as the best determining factors controlling crust formation over LU playa surfaces.