Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Dryland Salinization. Analysis of salinity levels in the Argentinian Dry Chaco

Auteur : Galindo Díaz, Marta

Université de soutenance : Katholieke Universiteit Leuven

Grade : Master ICP Master of Science in Sustainable Development 2021

Résumé
Dry forests’ deforestation rates are among the highest for all ecosystems. One of the world’s largest deforestation hotspots is the South-American Dry Chaco. The region is characterized by a semi-arid to subhumid climate and low topographic gradients. In combination with shallow groundwater tables, the region is prone to soil salinization. Large-scale deforestation processes in the region disrupt the hydrological cycle and increase the risk for soil salinization. Under forests, deep percolation of water and groundwater recharge are low as all precipitation input is used for evapotranspiration. Consequently, salts can accumulate in the soil mantle. By replacing native vegetation with agricultural crops, the deep percolation of water towards the groundwater table increases, which can have severe impacts on soil salinity. In this thesis, we have developed an exploratory analysis to assess the salinity levels across the Argentinean Dry Chaco. Soil samples were collected in forested and neighbouring deforested areas at different depth intervals – down to 2 m. The methodology used in this thesis is pioneer compared to the other studies found in the Dry Chaco. Salinity values were quantified by determining the electroconductivity (EC) and the total concentration of salts present in each sample. Contrary to other studies which restricted the salinity quantification to EC and chloride content, underestimating the salinity levels in the Dry Chaco. For this purpose, a saturated paste – which simulates the natural environment occurring in moisture-saturated soils – was obtained. EC values were measured using a YSI ProDSS instrument. The concentration of ions was determined by Inductively Coupled Plasma (ICP) and Liquid Phase Ion Chromatography (IC). After some calculations to validate our results, we compare the salts’ concentration among land uses, depth intervals, and deforestation years. Such calculations were made based upon the mass concentration (mg/kg) of each sample, which indicates the exact concentration of each ion in the soil at the time the samples were taken. Our results indicate that the impact of deforestation on soil salinity is spatially dependent. In areas with deep groundwater tables, clear signs of salt leaching are observed, whereas signs of dryland salinity are observed in areas with a low and flat topography. Additionally, temporal variation has been demonstrated since older plots showed a lower concentration of salts in the vadose zone. When studying the ions’ composition we found high concentrations of calcium, sodium, chloride, sulphate, and nitrate. Only chloride concentration was relatively higher in forest plots, while sodium and nitrates were higher in agricultural crops. In Western Australia, different management practices to control salinity have been implemented. Such measures included the plantation of herbaceous perennial legumes which exhibit efficient water use and can be employed as pasture for livestock ; the drainage of groundwater table, which very often implies environmental damage ; and the development of modelling techniques to assess salinization levels. Being able to monitor the effectiveness of Australian management practices helps to establish a good framework to control natural and secondary salinity in the Dry Chaco.

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