Porosity and organic matter distribution in jarositic phyto tubules of sulfuric soils assessed by combined µCT and NanoSIMS analysis

Abstract

Acid sulfate soils contain hypersulfidic material, e.g. pyrite (FeS₂). Under oxidizing conditions, it transforms to sulfuric material (pH < 4), which is accompanied with the formation of jarosite [KFe₃(SO₄)₂(OH)₆] along root channels (designated as jarositic phyto tubules). The encapsulation of root residues with jarosite can lead to reduced spatial availability of organic carbon which is necessary as substrate for microbes. This can limit microbial activity which might be crucial for prospective remediation success. We investigated jarositic phyto tubules by combining X-ray computed microtomography (µCT) and nanoscale secondary ion mass spectrometry (NanoSIMS), to elucidate the porosity and organic matter distribution at the spatial scale most relevant for microbial processes. We demonstrated that the jarosite can be differentiated into zones with either high or low jarosite concentrations at distances of < 0.5 mm and 0.5–1.9 mm from the relict root channel, respectively. The results showed a closer association between jarosite and organic matter in the zone with high jarosite concentration. However, the pore space in immediate vicinity of the root is almost completely filled by jarosite and the organic matter is completely encapsulated. We conclude that the overall poor accessibility of organic matter will strongly retard remediation processes of sulfuric soils after re-submergence.