Contamination risk by heavy metals and enzymatic stoichiometry in agricultural soils under intense use of pesticides.

ABSTRACT

Soil contamination by heavy metals (HM) from pesticides poses a serious environmental threat, affecting sustainability and agricultural productivity. Soil enzymes are essential for biochemical reactions such as organic matter decomposition and nutrient cycling and are vital for maintaining soil health. However, the effects of HM on soil enzyme activity are not yet well understood. This study examined the impact of HM contamination on enzymatic stoichiometry in regions with intensive pesticide use. We selected flower cultivation areas with 5 years (CA1) and 10 years (CA2) of pesticide exposure and a native forest area (NFA) as a reference during the dry and rainy seasons. We measured Cd, Cu, Mn, Pb, and Zn levels and employed ecological risk indices to assess contamination levels. We also analyzed enzyme activities (arylsulfatase, ß-glucosidase, acid phosphatase, urease) and enzymatic stoichiometry. CA2 exhibited the highest concentrations of Cd, Cu, and Mn in both periods, while Zn was highest in both CA1 and CA2. CA2 had higher values for all indices, indicating significant contamination. Compared with NFA, arylsulfatase activity was lower in cultivated areas during both periods, suggesting decreased soil quality. We found negative correlations between Cu, Mn, Zn, and arylsulfatase, as well as a reduction in urease with Cd; these elements also increased microbial C limitation. Our findings show that continuous pesticide input increases HM levels and that enzyme activity and stoichiometry are effective bioindicator of soil contamination. This study underscores the urgent need for guidelines to protect soils from prolonged HM buildup.