Spatial patterns of soil salinity in the central Argentinean Dry Chaco

Abstract

The Dry Chaco is a semi-arid ecoregion in South America that hosts one of the largest dry forests in the world, but expansion of dryland agriculture and cattle ranching led to gradual conversion of native vegetation to anthropogenic land cover. The potential impact of these newly established agricultural lands on the surrounding environment is of great concern. Local studies have shown that deforestation leads to changes in the soil-water balance, and can expedite groundwater rise and mobilization of water-soluble salts to the surface affecting plant growth and crop productivity. This study is a regional assessment of soil salinity and salinization processes in the central Argentinean Dry Chaco. It is based on extensive dataset of 492 surface and 142 subsurface samples taken along east-west transects across the Dry Chaco. Soil electrical conductivity (EC) was used as an indicator of salinity, and supplementary information on salt crusts was derived from Google Earth imagery. Subsurface salinity (i.e., measured at 100 cm depth) showed clear regional patterns in natural soil salinity that are related to the annual water budget and topography. Besides primary salinization due to arrested drainage and landscape stagnation, anthropogenic activities increased secondary salinization, especially in the agricultural areas with shallow groundwater tables and irrigated croplands. In addition, the study demonstrated that remotely-sensed vegetation indices such as the seasonal variation in the density of green vegetation are particularly suitable to monitor regional variation in soil salinity. Our results show that the extent of future dryland salinization in the Dry Chaco will mainly depend on whether areas prone to natural soil salinity are further protected from deforestation, and the magnitude and rate of groundwater rise after deforestation as conditioned by local climate and geomorphology. This better understanding of soil salinity patterns and how they are affected as a result of anthropogenic activities is important for the implementation of appropriate and effective measures to prevent severe salinization.