The crucial role of organic carbon availability in driving geochemical cycles in wetland and floodplain soils

A/Prof. Luke Mosley1, Prof. Rob Fitzpatrick1, Dr Angelika Kolbl2,3, Ms Emily Leyden1, Prof. Petra Marchner4

1Acid Sulfate Soils Centre, School of Biological Sciences, University Of Adelaide, Adelaide, Australia, 2Chair of Soil Science, Technical University of Munich, Freising, Germany, 3Soil Science, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany, 4School of Agriculture, Food and Wine, University of Adelaide, Adelaide, Australia

Many of the key geochemical cycles in wetland and floodplain soils (e.g. sulfate reduction, denitrification, methanogenesis) are driven or influenced by organic matter. Microbes play a central role in many of the geochemical reactions using organic carbon as an energy source. The bioavailability of the organic carbon to microbes, rather than its total concentration, is hence of central importance. We review and synthesise findings from several studies to show that the availability of organic carbon in many hydric soils controls geochemical reactions. Particular focus is placed on organic carbon availability in acid sulfate soils, as these can create severe and sustained acidification and metal release impacts following drainage/drought. In our long term field site in the Lower River Murray (South Australia), soils have not recovered from severe acidification (pH<4) over 10 years after the drought ended. Soil organic carbon concentrations are moderate (1-2% C) in the acidified soil layers but microbial reduction reactions are very limited. In contrast, when we add available organic carbon in the laboratory the soils can recover within weeks, but only if the pH is first adjusted to above 5. This suggests toxicity by low pH or high metal (e.g. Al3+) concentrations limiting microbial activity. Solid state 13C NMR studies have revealed that, compared to the native soil organic matter, soils with fresh organic matter addition are characterized by high proportions of O/N-alkyl C, which is readily decomposed compared to other components. The concentration of terminal electron acceptors (e.g. nitrate) may also influence microbial reduction reactions. While organic carbon availability limitations can be overcome by addition of organic materials, practical difficulties may be present in amending deep soil layers in wetland soils. The potential for enhanced greenhouse gas release following soil amendment also needs consideration.

Biography: A/Prof. Luke Mosley leads a biogeochemical research group at the Waite Campus, University of Adelaide. Assessment of the role of organic carbon availability and dynamics in influencing geochemical cycles in inland and coastal wetlands has been a key research focus over the last decade. Luke is also Deputy Director of the Acid Sulfate Soil Centre at the University of Adelaide and President of Soil Science Australia.

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