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Takinginto account(i) the rapid dissociation of ammonium hydrogen sulfite and decomposition of sulfites 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 hydrogen sulfite in soil can be expected to be low.

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Abiotic and biotic processes determining the fate of ammonium hydrogen sulfite in soils

Ammonium hydrogen sulfite dissociates into sulfite anions and the respective ammonium cations upon contact with soil moisture.Whereas ammonium is a natural and common component of the environment and living organisms, is rapidly degraded and not likely to bioaccumulate, sulfite anions are unstable under environmentally relevant conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of ammonium hydrogen sulfite 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) Sulfites are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. 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 sulfur (S), sulfide (HS-), sulfite (SO32-) and thiosulfate (S2O32-) 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. Other sulfur-containing microbial substrates such as dithionite (S2O42-), thiosulfates (S2O32-) or sulfite (SO32-) may also be anaerobically utilised, ultimately resulting in the reduction to sulfide (H2S).

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

Therefore, sulfites 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.