Mitigation of the Adverse Impact of Copper, Nickel, and Zinc on Soil Microorganisms and Enzymes by Mineral Sorbents

Abstract

Despite numerous studies on the influence of heavy metals on soil health, the search for effective, eco-friendly, and economically viable remediation substances is far from over. This encouraged us to carry out a study under strictly controlled conditions to test the effects of Cu²⁺, Ni²⁺, and Zn²⁺ added to soil in amounts of 150 mg·kg^-1 d.m. of soil on the soil microbiome, on the activity of two oxidoreductases and five hydrolases, and on the growth and development of the sunflower Helianthus annunus L. The remediation substances were a molecular sieve, halloysite, sepiolite, expanded clay, zeolite, and biochar. It has been demonstrated that the most severe turbulences in the soil microbiome, its activity, and the growth of Helianthus annunus L. were caused by Ni²⁺, followed by Cu²⁺, and the mildest negative effect was produced by Zn²⁺. The adverse impact of heavy metals on the soil microbiome and its activity was alleviated by the applied sorbents. Their application also contributed to the increased biomass of plants, which is significant for the successful phytoextraction of these metals from soil. Irrespective of which property was analysed, sepiolite can be recommended for the remediation of soil polluted with Ni²⁺ and zeolite-for soil polluted with Cu²⁺ and Zn²⁺. Both sorbents mitigated to the highest degree disturbances caused by the tested metals in the soil environment.

Keywords: Helianthus annuus L.; heavy metals; soil enzymes; soil microbiome; sorbents.

Figure 1

Index of the influence of heavy metals and sorbents on the number of microorganisms. Org—organotrophic bacteria; Act—actinomycetes; Fun—fungi; M—molecular sieve; H—halloysite; S—sepiolite; E—expanded clay; B—biochar; Z—zeolite; Cu—ion Cu²⁺; Ni—ion Ni²⁺; Zn—ion Zn²⁺.

Figure 2

The colony development index (CD) (a) organotrophic bacteria, (b) actinomycetes, and (c) fungi. C—control; M—molecular sieve; H—halloysite; S—sepiolite; E—expanded clay; B—biochar; Z—zeolite; C0—uncontaminated soil; Cu—ion Cu²⁺; Ni—ion Ni²⁺; Zn—ion Zn²⁺. Homogeneous groups were calculated for CD values determined for all sorbents (denoted with letters a–i).

Figure 3

Ecophysiological diversity index (EP) (a) organotrophic bacteria, (b) actinomycetes, and (c) fungi. Homogeneous groups were calculated for EP values determined for all sorbents (denoted with letters a-i). The abbreviations are explained under Figure 2.

Figure 4

Index of the influence of heavy metals and sorbents on the activity of soil enzymes. The abbreviations are explained under Figure 1.

Figure 5

Index of the influence of sorbents on the d.m. yield aerial parts of Helianthus annuus L. The abbreviations are explained under Figure 1.

Figure 6

The contribution of independent variables (η²) in influencing the plant yield, SPAD index, the number of microorganisms, and the activity of soil enzymes. Ad—adsorbent; HM—heavy metals; Y_shoots—yield of shoots; Y_roots—yield of roots; Org—organotrophic bacteria; Act—actinomycetes; Fun—fungi; Deh—dehydrogenases; Cat—catalase; Pac—acid phosphatase; Pal—alkaline phosphatase; Glu—β-glucosidase; Aryl—arylsulphatase.

Figure 7

Test results presented by PCA: (a) figures representing primary variables; (b) cases.

Figure 8

Pearson’s simple correlation coefficients, n = 112; Org—organotrophic bacteria; Act—actinomycetes; Fun—fungi; Deh—dehydrogenases; Cat—catalase; Pac—acid phosphatase; Pal—alkaline phosphatase; Glu—β-glucosidase; Aryl—arylsulphatase; Org—organotrophic bacteria; Act—actinomycetes; Fun—fungi; Y—yield of Helianthus annuus L.; CD—colony development index; EP—ecophysiological diversity index; * significant for p = 0.05.