Evaluating heavy metal pollution risks and enzyme activity in areas with intensive hazelnut cultivation under humid ecological conditions

Unlike organic pollutants, heavy metals are often unchanging, non-degradable, and persistent in soil and water. So, they negatively impact the surrounding ecosystems and human health. In this context, the main objectives of this study were to i-) determine some physical, chemical, and biological properties and heavy metal concentrations of different soils in intensive hazelnut cultivation areas under humid ecological conditions; ii-) reveal the heavy metal pollution risks of these areas by their enrichment factor (EF), contamination factor (CF), geo-accumulation index (Igeo), degree of contamination (DC), pollution load index (PLI), and potential ecological risk index (PERI); iii-) analyze the quality of soils contaminated with heavy metals by their total enzyme activity index (TEI) and the geometric mean of enzymatic activities (GMea); iv) explore the correlation between heavy metals and soil enzyme activity indices. According to our results, the average concentrations of heavy metals in the study area ranked as Fe > Mn > Zn > Cr > Ni > Cu > Co > Pb > Cd. Based on EF, the area was evaluated as between deficiency to low enrichment and moderate enrichment for all elements except for Cd. 92.5% of Cd was in the significant enrichment class. Based on CF, the area was in the low to moderate contamination class for all elements except for Cd, with a significant, high contamination risk for Cd. According to Igeo, the area was uncontaminated to moderately contaminated for all elements except Cd, but it was at risk of moderate to strong contamination for Cd. According to DC, 12.5% of the area had a low degree of contamination, 77.5% a moderate degree, and 10% a considerable degree. Besides, for PLI, 12.5% of the area was in the perfection class, 77.5% in the baseline of pollution class, and 10% in the deterioration of site quality class. Nearly 70% of the soils had a low risk for the analyzed ecosystem and the remaining soils (30%) had moderate risk. Moreover, the area had a GMea of 69.97–208 and a TEI of 2.111–5.859. Finally, analyzing the heavy metals and soil enzyme activity indices, show that there is a negative correlation between Ni and GMea (p < 0.01) and TEI (p < 0.05).