Assessment of gentle remediation options for trace element-contaminated agricultural land under semi-controlled and field conditions

The global soil resources are significantly threatened by pollution. In addition to the existing burden of contaminants in agricultural soils, the increasing anthropogenic input of metal(loid)s, further referred to as trace elements (TE), presents a major public health concern, since it endangers the food security of a rising human population. However, the growing demand for agricultural commodities will increase the pressure on fertile soils. In this context, steering the needed agricultural extensification towards arable TE-contaminated soils (TECS) could protect highly biodiverse or carbon stock land and, thereby, help reach global sustainability targets. The sustainable crop production on TECS requires effective and non-destructive measures to control relevant pollutant linkages. These are offered by gentle remediation options (GRO), the practical adoption of which is scarce across Europe as yet. This study provides different approaches of GRO applied to an agricultural soil in a characteristic post-mining region (Freiberg, Saxony (Germany)) in practical adoption (chapter 2) and under semi-controlled conditions (chapters 3 and 4). Due to severe topsoil contamination by metals (Cd, Pb, and Zn) and As, the pollutant linkages of concern at the study site are food-chain transfer, leaching to the groundwater, and tilling-related dust emissions. The overall aim was to find best management practices for coupling soil remediation with the production of marketable biomass. This was attempted via (i) in situ stabilization, alone (chapter 4.1) or combined with phytoexclusion (chapter 2), (ii) labile TE phytoextraction (chapter 3), and (iii) (aided) phytostabilization (chapter 4.2). Soil remediation by GRO was assessed with scientifically established (DGT, soil solution; chapter 4.1) and/or legally relevant chemical soil extractions (NH4NO3-solution; chapters 2 to 4) and pH measurements. Additionally, earthworms served as ecotoxicological endpoints (chapter 4.1). Initial soils, earthworms, vegetative and generative biomass produced from each approach, as well as the investigated soil additives, including fertilizers, were microwave-assisted chemically digested (HNO3, H2O2, aqua regia) prior to analysis. All environmental samples were analyzed for TE by inductively coupled plasma mass spectrometry (ICP-MS). Nutrient concentrations in soil additives and selected plant samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES). The measured TE concentrations were evaluated against applicable European and/or national thresholds. At field (chapter 2), the repeated fertilization with superphosphate and/or lime marl basically attenuated the chemical TE availability over a three years crop rotation of Brassica napus, Triticum aestivum, and Hordeum vulgare. In turn, the simultaneous phytoexclusion by low-accumulating cultivars (LAC) effectively decreased the Cd concentrations in cereal grains (by averagely 21 % in wheat and 39 % in barley). However, straw metals´ accumulation or grain As uptake partly revealed opposing trends among LAC and high-accumulating cultivars (HAC). As investigated under semi-controlled conditions (chapter 3), a sunflower (Helianthus annuus L. mutant inbred line M7 (R3B-F-U/R13M10A; test series R13F-MP)), modified towards enhanced labile TE phytoextraction by chemical mutagenesis, proved less advantageous as pre-crop for winter wheat (Triticum aestivum L. cv. Tiger) than the regionally common winter oilseed rape (Brassica napus L. cv. Lorenz). This resulted from soil alkalinization by rape, whereas sunflower mobilized more TE than it depleted from the rhizosphere. Within in situ stabilization approaches under semi-controlled conditions, a Fe- /Al-rich drinking water treatment residue (WTR), soil-applied at a rate of 1 % (m/m) prior to cultivation of wheat (Triticum aestivum L. cv. Tiger; chapter 4.1) or Szarvasi-1 (Elymus elongatus subsp. ponticus cv. Szarvasi-1; chapter 4.2), decreased the chemical availability of As, Cd, and Pb by up to 77 % , 46 %, and 61 %, respectively. Thereby, it immobilized these hazardous TE increasingly effective over time and better than a Mn-rich WTR or lime marl. The bioassays with wheat and earthworms (Dendrobaena veneta L.) showed, however, that the habitat function for biocenoses benefited more from the Fe-/Al-rich WTR when it was applied at a lower application rate (0.5 % m/m). This resulted from dose-dependent P fixation and TE entries induced by the WTR, to which Szarvasi-1 appeared insensitive. Unlike As, the availability of Cd and Zn to biota in amended soils could not be predicted by any of the applied chemical methods due to endpoint-specific binding of competing cations to the biotic ligand (plant roots, earthworm tissue), and a preferential translocation of Zn over Cd in planta. Among all studied plants, the perennials Szarvasi-1 and cup plant (Silphium perfoliatum L.; chapter 4.2) best excluded the present mixture of TE in shoots, whereby the latter exhibited growth depression. However, only grain biomass of barley and rape, and partly of low-accumulating wheat, produced at the study site presented legally compliant animal feed based on European limit values for Cd and As. Given the investigated measures´ restricted efficacy to assure forage safety, a land-use change e.g. towards the perennial Szarvasi-1, which provides a continuous plant cover at low tillage and input requirements, possibly accompanied by the monitored reuse of an Fe- /Al-rich WTR, could most promisingly control all above-stated pollutant linkages. Revenues could be generated from energy conversion or valorization in the fibrous material sector. The waste recycling of WTR in TECS, though promising, requires proper characterization, eventual process optimization, and further studies regarding long-term stability to ensure legal compliance and environmental safety. Future research and breeding efforts regarding low Cd cultivars in particular could greatly contribute to safe food or forage production at the majority of moderately contaminated sites.:Table of contents Danksagung VI Abstract VII Zusammenfassung X List of Figures XIV List of Tables XVIII Abbreviations XXII 1 Introduction 1 1.1 The contamination of agricultural soils with trace elements - a global challenge to be tackled at local scale 1 1.2 The study site 5 1.3 Objectives 7 1.4 Structure of the thesis 8 2 Field assessment of conventional fertilizers and cultivars of annual plants 11 2.1 Management of trace element-contaminated agricultural land by in situ stabilization combined with phytoexclusion over a three years crop rotation 11 3 Exploring pre-crop effects of annual oilseeds 37 3.1 Trace elements bioavailability to winter wheat (Triticum aestivum L.) grown subsequent to high biomass plants in a greenhouse study 37 4 The potentials of drinking water treatment residues and/or perennialplants 53 4.1 Trace elements bioavailability to Triticum aestivum and Dendrobaena veneta in a multielement-contaminated agricultural soil amended with drinking water treatment residues 53 4.2 (Aided) phytostabilization of trace elements using cup plant (Silphium perfoliatum L.), or tall wheatgrass (Elymus elongatus subsp. ponticus cv. Szarvasi-1) and soilapplied drinking water treatment residues 77 5 Synthesis 91 5.1 The potential of crop management practices to mitigate food chain transfer of trace elements 91 5.2 The in situ stabilization efficacy under differential experimental conditions 96 6 Future perspectives 101 References 104 List of publications 121 Erklärung 12