Spatiotemporal Variations Affect DTPA-Extractable Heavy Metals in Coastal Salt-Affected Soils of Arid Regions

The concept of metal bioavailability in soils is increasingly becoming the key to addressing potential risks. Yet, space–time variations of heavy metal concentrations in salt-affected soils is still vague. The current work, therefore, is the first attempt to address spatial and seasonal analyses of heavy metals in a Mediterranean arid agroecosystem. This study was conducted in a coastal area in northeastern Egypt as an example. The DTPA-extractable concentrations of Cr, Co, Cu, Fe, Pb, Mn, Ni, and Zn in addition to the main properties of 70 georeferenced soil samples (0–30 cm) were determined during the wet (March) and dry (September) seasons. The results revealed that except for Cu, the concentrations of all the determined metals stood below the safe limits. On average, the concentrations of Cu were 4.1- and 5-fold the acceptable limit of 0.20 mg kg−1, respectively. The statistical analysis indicated that seasonal variations greatly affect the concentrations of Mn, Ni, and Zn. Compared with the wet season, significant increases of 1.25, 1.50, and 1.28-fold in the concentrations of these metals occurred during the dry season, respectively. The principal component analysis affirmed that the presence of Cr, Co, Fe, and Ni was closely related to geogenic factors; meanwhile, agronomic practices were likely the main inputs of Cu, Pb, and Zn. The geostatistical analysis illustrated that the geographic variability of Cr, Fe, Mn, and Zn was due to interactions of natural and stochastic processes. Farming practices controlled the spatial variability of Ni, Pb (in the wet period), and Co (in the dry period). The effect of natural processes during the wet period was evident for Cu, which showed strong spatial variability. The kriged maps showed that the concentrations of Co, Fe, and Ni tended to increase seaward and were found to be affected by pH, salt ions, and exchangeable Na+. Moreover, both silt and organic matter content had profound impacts on the spatial distribution of Cr, while the distributions of Cu, Pb, and Zn were linked to that of CaCO3 content. The suggested mechanisms governing metal bioavailability were sorption and complexation with ligands (for Co, Fe, and Ni), redox potential (for Cr), dissolution–precipitation (for Mn), and ion exchange (for Cu, Pb, and Zn). The results of this study affirm that drying–wetting cycles and spatial distribution affect the bioavailability of heavy metals in coastal salt-affected soils of arid regions. These findings imply that seasonality (wet and dry) and spatiality should be considered for monitoring and rehabilitation of degraded soils under similar ecological conditions.