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Taking into account (i) the rapid dissociation of ammonium thiosulfate and decomposition of thiosulfates and transformation of ammonium upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural nitrogen and sulfur cycle, (ii) ubiquitousness of ammonium and inorganic sulfur substances in soil, (iii) essentiality of sulfur, and (iv) the lack of a potential for bioaccumulation and toxicity to aquatic organisms, the hazard potential ofammonium thiosulfate in soil can be expected to be low.

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Abiotic and biotic processes determining the fate of ammonium thiosulfate in soils

Ammonium thiosulfate dissociates into ammonium cations and thiosulfate anions.Whereas ammonium is a natural and common component of the environment and living organisms, is rapidly degraded and not likely to bioaccumulate, thiosulfate anions are unstable under environmentally relevant conditions and disproportionate into sulfite anions. Sulfites are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of ammonium thiosulfate is not expected due to its inherent physico-chemical properties.

(a) Ammonium ions rapidly degrade and do not persist in soils. Available evidence points to rapid transformation of ammonium under aerobic conditions. In the terrestrial environment, ammonium is part of the nitrogen cycle. Briefly, saprophytic microorganisms such as bacteria and fungi use organic material as source of energy and food by transferring it to inorganic end products such as NH4+ (ammonium - ammonification) and NO3- (nitrate - ammonium oxidation or nitrification). Ammonium and nitrate are plant nutrients and may under anaerobic conditions be transferred to gaseous end products N2O or N2 (nitrate oxidation or denitrification) by heterotrophic bacteria such as Agrobacterium, Bacillus or Pseudomonas. Nitrification and denitrification proceed simultaneously in soil (Gisi, 1997). Any quantitatively relevant adsorption onto soils is not expected for ammonium.

(b) Thiosulfate anions are unstable under environmentally relevant conditions, including soils, and will disproportionate to sulfite.Under oxygen-rich conditions, sulfites are rapidly oxidized catalytically by (air) oxygen or by microbial action to sulfate. Microbial oxidation of reduced sulfur species including elemental thiosulfate (S2O32-), sulfur (S), sulfide (HS-) and sulfite (SO32-) is an energetically favorable reaction carried out by a wide range of organisms, i.e., sulfur oxidizing microorganisms (SOM) resulting in ultimate transformation into sulfate (SO42-, Simon and Kroneck, 2013).

In highly reduced (water-logged) soils, reduction to sulfides may take place with subsequent formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, including FeS, ZnS, PbS and CdS (Lindsay, 1979, Finster et al., 1998). Thus, under anoxic conditions, sulfate is readily reduced to sulfide by sulfate-reducing bacteria (SRM) that are common in anaerobic environments. Thiosulfates, as well as other sulfur-containing microbial substrates such as dithionite (S2O42-), or sulfite (SO32-) may also be directly anaerobically utilised, ultimately resulting in the reduction to sulfide (H2S).

A significant set of microbial populations grows by disproportionation of thiosulfate, sulfite or elemental sulfur, ultimately yielding sulfate or sulfide (Simon and Kroneck 2013 and references therein; Janssen et al. 1996, Bak and Cypionka, 1987).

In sum, thiosulfates may reasonably be considered chemically unstable under most environmental conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle.