Innovative amendments derived from industrial and municipal wastes enhance plant growth and soil functions in PTE-polluted environments

Potentially toxic elements (PTE), e.g. As, Sb, Cd, Cu, Pb, Zn, can severely impact soil element cycling, organic matter turnover and soil inhabiting microbiota. Very often this has dramatic consequences for plant growth and yield which are greatly restricted in PTE-contaminated soils. The use of innovative amendments to reduce the labile pool of such soil contaminants, can result as a feasible and sustainable strategy to improve the fertility and functionality of PTE-contaminated soils as well as to exploit these latter from an agronomic point of view. Water treatment residuals (WTR), red muds (RM), organic-based materials originating from the waste cycle, e.g. municipal solid waste compost (MSWC) and biochar (BCH), have emerged in the last decades as promising amendments. In this paper, we report a synthesis of the lessons learned from research carried out in the last 20 years on the use of the above-mentioned innovative amendments for the manipulation of soil fertility and functionality in PTE-contaminated soils. The amendments considered possess physico-chemical properties useful to reduce labile PTE in soil (e.g. alkaline pH, porosity, Fe/Al phases, specific functional groups and ionic composition among the others). In addition, they contain organic and inorganic nutrients which can contribute to improve the soil chemical, microbial and biochemical status. This is often reflected by a higher organic matter content in Ac ce pt ed p ap er amended soils and/or an increase of the cation exchange capacity, available P and total N and/or dissolved organic C. As a result, soil microbial abundance, in particular heterotrophic fungi and bacteria, and enzyme activities (e.g. dehydrogenase, urease and β-glucosidase) are commonly enhanced in amended soils, while plant growth can be significantly stimulated. Overall, the obtained results suggest that the studied amendments can be used to reduce PTE bioavailability in polluted soils, improve soil microbial status and functionality, and enhance the productivity of different crops. This can offer a precious opportunity for the productive recovery of PTE-polluted soils. Introduction Soil pollution by potentially toxic elements (PTE, e.g. As, Sb, Cd, Cu, Pb and Zn) represents an increasingly urgent problem at global scale. PTE, unlike organic contaminants, are generally immutable, not degradable and persistent in soil (Adriano et al., 2004). In addition, they can be toxic to plants, animals and soil microorganisms when certain threshold levels are exceeded (Abou Jaoude et al., 2019). Unfortunately, this is not uncommon and is often due to industrial and mining activities, waste incinerators, coal and petroleum combustion, spent ammunition, battery factories, and misuse of pharmaceuticals and pesticides among the others (Silvetti et al., 2014). For instance, mining and smelting activities usually produce large amounts of tailings and waste rocks, from which PTE present in primary sulphide ores could spread in soil and other environmentally relevant compartments, e.g. surface and groundwater (Wong, 2003; Castaldi et al., 2005; Manzano et al., 2016), thus posing significant environmental and health risks. The fertility status of PTE-contaminated soils, intended as the soil capacity to support element cycling and promote plant growth, is commonly affected by the presence of PTE above certain thresholds, which interfere with many metabolic pathways impacting plant and microbial physiology (Castaldi et al., 2018; Garau et al., 2014, 2017; Visconti et al., 2018; Garau et al., 2019b). Although such soils cannot be devoted for food or feed production due to their health Ac ce pt ed p ap er hazard for humans and animals , they could (and should) be recovered with the aim of limiting the contaminants impact on soil functionality, reduce PTE spread into the environment and promote plant growth. This latter aspect is particularly relevant since the growth of selected plant species in PTE-contaminated soils can be useful for the contaminant stabilization or extraction and such strategies, i.e. phytostabilization and phytoextraction, are currently widely investigated worldwide (e.g. Kumpiene et al., 2014; Garau et al., 2014; Castaldi et al., 2018). Plant growth in PTEcontaminated environments can reduce soil erosion and spread of contaminants, limit PTE mobility and bioavailability through their immobilization in roots, reduce PTE leaching to groundwater and stimulate microbial activity through the release of root exudates (Castaldi et al., 2009, 2018; Garau et al., 2020). Moreover, the cultivation of plant species with phytoremediation capacities, but also able to produce some income, e.g. bioenergy crops or other no-food crops, can represent an innovative and sustainable approach for the recovery of PTE-contaminated soils which, however, it requires a significant improvement of soil fertility, and above all a reduction of the labile (i.e. water-soluble and exchangeable) PTE fractions in soil (Fiorentino et al., 2018). A wide array of techniques has been proposed to remediate PTE-contaminated soils, most of which consist of very expensive or highly invasive treatments that can only be practiced ex-situ and have a massive impact on the ecosystem (e.g. Mulligan et al., 2001). However, alternative low input (and low cost) and more sustainable approaches have been recently proposed for in-situ remediation of polluted soils. In particular, in the last decades, a great deal of attention has been put on the evaluation of novel and less impacting strategies for gentle remediation of PTE-contaminated soils (Mench et al., 2006; Garau et al., 2014; Quintela-Sabarís et al., 2017). Such strategies are mainly based on the in-situ immobilization of the contaminants using different amendments (or sorbent materials) often deriving from the municipal or industrial waste cycle, e.g. compost, Fe-rich byproducts, biochar etc. (e.g. Castaldi et al., 2005; Garau et al., 2007, 2017; Fellet et al., 2014; Yang et al., 2016; Zhang et al., 2016; Moreno-Barriga et al., 2017). Ideally, these amendments should be able to reduce the concentration of labile and bioavailable PTE by sorption and/or (co)precipitation Ac ce pt ed p ap er reactions (Basta and McGowen, 2004; Castaldi et al., 2005; Manzano et al., 2016; Garau et al., 2017; Rocco et al., 2018), and/or by changing the contaminant speciation (Beesley and Marmiroli, 2011), thereby reducing the chemical stress imposed on plants and soil microorganisms. Importantly, the contribution of such amendments for the recovery of PTE-contaminated soils should not be limited to reduce labile PTE, as this could not be enough to promote plant growth and achieve suitable yields. For instance, adding 3% (w/w) hematite [an iron(III) oxide; Fe2O3] significantly reduced labile As in a contaminated mining soil but did not improve Phaseolus vulgaris growth which was similar to that achieved in the contaminated untreated soil (Garau et al., 2014). This was explained by the authors with bean sensitivity to Fe2O3 but could be due also to P deficiency since Fe-oxides have a great affinity for phosphates (Antelo et al., 2005; Luengo et al., 2006). It is therefore of utmost importance that amendments used for soil remediation are able to improve soil physico-chemical and biological attributes (e.g. pH, cation exchange capacity, nutrient supply, microbial abundance, diversity and functionality) other than just reducing labile PTE. The combined presence of such characteristics in each amendment can significantly contribute to plant growth in PTE-contaminated soils and can be the key to achieve economically relevant yields in such environments. In this review paper, the suitability of several amendments, mainly deriving from the municipal or industrial waste cycle, for the recovery of PTE-contaminated soils and the promotion of plant growth in such environments, will be discussed from a chemical, biochemical, and agronomic viewpoint. In particular, the main physico-chemical features of selected strategic amendments such as municipal solid waste compost (MSWC), red muds (RM), water treatment residuals (WTR) and biochar (BCH) will be presented together with their PTE-adsorption capacities. The amendments impact on the fertility, biochemical and microbial characteristics of different PTE-contaminated soils will be also discussed. Finally, the amendments potential to influence plant growth and PTE uptake in contaminated soils will be also reported with emphasis to selected grass and legume species (e.g. Lupinus albus, Pisum sativum, Phaseolus vulgaris, Triticum vulgare) as well as to Ac ce pt ed p ap er some multipurpose crops (e.g. Helichrysum italicum, Cynara cardunculus, Phragmites australis and Arundo donax). Origin and physico-chemical features of MSWC, RM, WTR and BCH The municipal and industrial waste cycle produces large amounts of by-products which almost always constitute an environmental issue with relevant economic implications. For instance, in 2019, RM deriving from the Alumina industry in Portovesme (Sardinia, Italy) amounted to approx. 20 Mm3 distributed over 160 ha located in front of the coast line at 26 m asl (Mombelli et al., 2019). Drinking water treatment plants also produce continuously large amounts of sludges, i.e. WTR, which involves considerable transport and landfill costs. In 2016, WTR production by a typical water treatment plant was estimated of 100,000 t year-1 while more than 10,000 t were produced daily on a global scale (Ahmad et al., 2016). The same can be said for compost, or more recently for biochar resulting from the transformation of organic (e.g. food and green) wastes and whose volumes are constantly increasing due to growing world population. At present, these kinds of materials or by-products mainly represent a problem (and only marginally a resource) while they could be effectively used as amendments for the reclamation of contaminated soils. In parti