Impact of soil treatment with Nitrilo Triacetic Acid (NTA) on Cd fractionation and microbial biomass in cultivated and uncultivated calcareous soil

Abstract

Purpose

The aim of this study was to evaluate the effectiveness of nitrilotriacetic acid (NTA) on cadmium (Cd) fractions and microbial biomass in a calcareous soil spiked with Cd under cultivated (Zea mays L.) and uncultivated regime subject to soil leaching condition. Expanding investigations related to soil–plant interactions on metal-contaminated soils with insights on microbial activity and associated soil toxicity perspective provides novel perspectives on using metal-chelating agents for soil remediation.

Methods

The experimental factors were three levels of Cd contamination (0, 25, and 50 mg kg⁻¹ soil) and three levels of NTA (0, 15, and 30 mmol L⁻¹) in loamy soil under maize-cultured and non-cultured conditions. During the experiment, the adding NTA and leaching processes were performed three times.

Results

The results showed that the amount of leached Cd decreased in cultivated soil compared to uncultivated soil due to partial uptake of soluble Cd by plant roots and changes in Cd fractions in soil, so that Cd leached in Cd₅₀NTA₃₀ was 9.2 and 6.1 mg L⁻¹, respectively, in uncultivated and cultivated soils. Also, Cd leached in Cd₂₅NTA₃₀ was 5.7 and 3.1 mg L⁻¹ respectively, in uncultivated and cultivated soils. The best treatment in terms of chemical and microbial characteristics of the soil with the high percentage of Cd removed from the soil was Cd₂₅NTA₃₀ in cultivated soil. In Cd₂₅NTA₃₀ compared to Cd₂₅NTA₀ in cultivated soil, pH (0.25 unit), microbial biomass carbon (MBC, 65.0 mg kg⁻¹), and soil respiration (27.5 mg C-CO₂ kg⁻¹ 24 h⁻¹) decreased, while metabolic quotient (qCO₂, 0.05) and dissolved organic carbon (DOC, 20.0 mg L⁻¹) increased. Moreover, the changes of Cd fractions in Cd₂₅NTA₃₀ in cultivated soil compared to uncultivated soil were as follows; the exchangeable Cd (F₁, 0.27 mg kg⁻¹) and Fe/Mn-oxide-bounded Cd (F₄, 0.15 mg kg⁻¹) fractions increased, in contrast, carbonate-Cd (F₂, 2.67 mg kg⁻¹) and, organically bounded Cd (F₃, 0.06 mg kg⁻¹) fractions decreased. NTA had no significant effect on the residual fraction (F₅).

Conclusion

The use of NTA, especially in calcareous soils, where most of the Cd is bound to calcium carbonate, was able to successfully convert insoluble fractions of Cd into soluble forms and increase the removal efficiency of Cd in the phytoremediation method. NTA is a non-toxic chelating agent to improve the accumulation of Cd in maize.