Trace metals toxicity in relation to long-term intensive agricultural production in a calcareous environment with different soil types

Although the level of trace metals in agricultural soils is small, repeated use of both agricultural techniques and agrochemicals may lead to the accumulation of dangerously high levels of some metals in different soils in the long run. In order to monitor the variations of trace metal levels and their toxicity degrees caused by cultivation activities, fractions of total and available (DTPA method) Mn, Zn, Cu, and Cd were assessed in six major soil types [Chromic Calcixerert (CC-V), Typic Calcixerolls (TC-M), Vertic Haploxerepts (VH-I), Typic Calcixererts (TC-V), Fluventic Haploxerepts (FH-I), and Typic Haploxereps (TH-I)] exposed to intensive cultivation over five decades, as well as in adjoining uncultivated soils (as reference). Soil pollution was investigated on the basis of single-factor pollution index (PI), comprehensive pollution index (PIN), pollution load index (PLI), and potential ecological risk (RI). Except for DTPA-extractable Cd, the level of both total (Mn, 700-960 mg kg(-1); Zn, 55-78 mg kg(-1); Cu, 23-39 mg kg(-1); Cd, 1.6-3.4 mg kg(-1)) and available fractions (Mn, 8-20 mg kg(-1); Zn, 0.66-2 mg kg(-1), Cu, 1.5-3 mg kg(-1); Cd, 0.42-0.7 mg kg(-1)) of the metals were below their maximum permissible limits based on national and international standards. There was a remarkable buildup of DTPA-extractable Mn (2.2-35%) and Cd (10-43%) as well as total fraction of Zn (3-19%), Cu (12-32%), and Cd (24-105%) for the majority of the examined soils mainly due to the repeated application of agrochemical inputs (e.g., chemical fertilizers, pesticides, and herbicides) and manure over a long period of time. Considering the potential ecological risk assessment of the cultivated soils (min = 44, max = 70, mean = 70), 50% of the soil types were categorized as causing low potential ecological risk (the soil types of CC-V, TC-M, and VH-I) and 50% were also considered as causing moderate potential ecological risk (the soil types of TC-I, FH-I, and TH-I). The Cd pollution was relatively more serious than other metals according to the mean values of PI, PIN, PLI, and RI. The potential ecological risk in different soil types was in the order of CC-V > TC-M > VH-I > TC-I > TH-I > FH-I, suggesting that a great variety of different soil types are responsible for the dynamics of the analyzed trace metals.