The impact of single and combined amendment of elemental sulphur and graphene oxide on soil microbiome and nutrient transformation activities.

Background: Sulphur (S) deficiency has emerged in recent years in European soils due to the decreased occurrence of acid rains. Elemental sulphur (S) is highly beneficial as a source of S in agriculture, but it must be oxidized to a plant-accessible form. Micro- or nano-formulated S may undergo accelerated transformation, as the oxidation rate of S indirectly depends on particle size. Graphene oxide (GO) is a 2D-carbon-based nanomaterial with benefits as soil amendment, which could modulate the processes of S oxidation. Micro-and nano-sized composites, comprised of S and GO, were tested as soil amendments in a pot experiment with unplanted soil to assess their effects on soil microbial biomass, activity, and transformation to sulphates. Fourteen different variants were tested, based on solely added GO, solely added micro- or nano-sized S (each in three different doses) and on a combination of all S doses with GO.

Results: Compared to unamended soil, nano-S and nano-S+GO increased soil pH(CaCl). Micro-S (at a dose 4 g kg) increased soil pH(CaCl), whereas micro-S+GO (at a dose 4 g kg) decreased soil pH(CaCl). The total bacterial and ammonium oxidizer microbial abundance decreased due to micro-S and nano-S amendment, with an indirect dependence on the amended dose. This trend was alleviated by the co-application of GO. Urease activity showed a distinct response to micro-S+GO (decreased value) and nano-S+GO amendment (increased value). Arylsulfatase was enhanced by micro-S+GO, while sulphur reducing bacteria (dsr) increased proliferation due to high micro-S and nano-S, and co-amendment of both with GO. In comparison to nano-S, the amendment of micro-S+GO more increased soluble sulphur content more significantly.

Conclusions: Under the conditions of this soil experiment, graphene oxide exhibited a significant effect on the process of sulphur oxidation.