Vermicompost improves maize, millet and sorghum growth in iron mine tailings

Highlights

• Chromium, iron and manganese are at high concentrations in tailings.

• Plant growth was reduced by cultivation in iron mining tailings.

• The addition of vermicompost increased plant growth.

• The vermicompost promoted greater investment in thick and very thick roots.

• Mining tailings limitations are more related to root growth than to metal toxicity.

Abstract

The Fundão dam was designed to store iron mine tailings in the region of Mariana, MG, Brazil. When it ruptured, the tailings overflowed. These tailings affected the soil due to the formation of a thick crust as a result of drying (compaction) and hindered the natural revegetation process. In this context, the use of organic fertilizers, including vermicompost, is method of reducing the physical limitations on root growth caused by soil properties and changing soil-metal interactions. For this reason, vermicompost was added to iron mine tailings, and its morphological and physiological effects on maize, millet and sorghum plants were studied. The experiment was conducted in a greenhouse using 6 dm³ pots. The plants were subjected to three treatments: mine tailings, mine tailings + vermicompost, and a reference soil. From the V3 stage onwards, biweekly growth, leaf gas exchange and chlorophyll fluorescence evaluations were performed. At the end of the experiment, dry biomass and metal, macro- and micronutrient contents were quantified, and the root morphology was evaluated. The tailings created physical limitations on root growth and had low nutrient content as well as high concentrations of chromium, iron and manganese. The addition of vermicompost favored increases in shoot and root dry biomass, increases in root length, volume, surface area and diameter, and the absorption of macro- and micronutrients, which was reflected in the growth of the studied species. In addition, vermicompost led to greater investment in thick and very thick roots, and in general, the plants showed no symptoms of metal toxicity. Considering the characteristics of the studied tailings, it can be concluded that vermicompost favors the growth of plant species and may be a viable method for beginning the recovery process in areas containing iron mine tailings.

Introduction

Mining activity has expanded throughout the world (Oladipo et al., 2016), and consequently, its negative effects on the environment have increased. In countries such as Brazil, accidents with dams have caused severe damage to the environment. Therefore, authorities are worried and are looking for new techniques to mitigate the pollution generated. In addition, many remediation programs for soils contaminated with mine tailings have been costly and inefficient for depollution. At the Aznalcóllar mine (Spain), for example, after 20 years of a remediation program in the mine overflow areas, waste pollution is still present (García-Carmona et al., 2019).

In 2015, the largest accident recorded in the mining history of Brazil, the collapse of the Fundão dam, occurred. The dam was designed to store iron mine tailings in the region of Mariana, state of Minas Gerais, Brazil, for the company Samarco S/A. With the rupture of this dam, there was an overflow of 50 million m³ of tailings, mainly consisting of iron oxide and silica, that left a trail of destruction. The tailings covered an area of 663.2 km and mixed with the soil, causing changes in soil pH, reductions in organic matter content, the leaching of chemical elements, and the addition of metals to the soil, as well as creating a thick crust due to the tailings drying (compaction), thus hindering natural revegetation and plant succession in these areas (Silva et al., 2016).

Soil recovery in environments with mine tailings is complex, and particularly in the case of the Mariana tailings, problems with compaction and heavy metals stand out. Crop species have been considered for use in decontamination processes due to their fast growth and large biomass accumulation (Vamerali et al., 2010). Additionally, many plant species of the Poaceae, Fabaceae and Brassicaceae families have high biomass yields that can compensate for the low concentration absorbed and translocated to tissues, making their performance similar or even better than that of hyperaccumulators (Vamerali et al., 2010). In addition, many of these cultivated plants show a strong increase in metal accumulation when fertilization and organic changes are applied (Sierra Aragón et al., 2019; Vamerali et al., 2010). Therefore, the use of vermicompost is justified in this case.

Regarding the physical changes in the soil generated by the tailings, this process in the soil decreases the number of macropores as a result of the high density, which results in physical resistance to the roots and reduction in aeration and the infiltration of water and nutrients; consequently, there are increases in runoff and erosive processes (Andrade et al., 2018). Fertilizer and organic matter application, as well as the planting of species with a robust root system, are among the alternatives for minimizing the effects of soil compaction.

Vermicompost is an organic amendment with high water retention capacity, porosity and nutrient availability. This compost comes from the interactions between earthworms and beneficial microorganisms for the decomposition of organic matter under vermicomposting techniques (Lazcano and Domínguez, 2011). Furthermore, the constant excretion of vermicompost by earthworm calciferous glands confers greater maturity and stability than other compost materials.

The beneficial effects of vermicompost to plant growth improvement are related to physical, chemical and biological mechanisms. The physical pathway refers to the increase in soil porosity and water retention, promoting rooting and reducing the physical limitations on plant growth caused by soil properties (Góes et al., 2011). The chemical is related to the increased nutrients availability and may affect the mobility and bioavailability of heavy metals (Huang et al., 2016; Sharma and Nagpal, 2018). Finally, the biological pathway refers to the active microbiome, which might produce humic substances and/or increase nutrients availability for plants absorption (Lazcano and Domínguez, 2011).

Species such as maize, millet and sorghum assist in soil decompaction (Calonego et al., 2011; Rivero Herrada et al., 2017) and are part of metal phytoextraction programs with cultivated plants (De Boer et al., 2018; Oladipo et al., 2016; Tavares et al., 2013; Tolentino et al., 2016; Vamerali et al., 2010). There are studies showing the use of maize, millet and sorghum plants for phytoremediation in contaminated areas. The potential of maize for extracting Cu, Zn, Pb and Cd (Xu et al., 2015), millet for extracting Cd and Ni (Asopa et al., 2017; Gupta et al., 2017), and sorghum for extracting Ni, Mn and Cr (Naeini and Rad, 2018; Padmapriya et al., 2016; Serme et al., 2015) have been described. These findings show the potential use of these plant species in the recovery of contaminated areas, and the plants can have alternative uses such as bioenergy generation.

Thus, considering the extent and severity of the disaster, the destruction of agricultural areas and the need for studies on species that can be used for the recovery of this native and agricultural environment, it was hypothesized that the addition of vermicompost to mine tailings improves tailing conditions and, consequently, the development of cultivated plants. For this reason, the objective of this study was to verify the influence of vermicompost on the morpho-physiological characteristics of maize, millet and sorghum cultivated in iron mine tailings.