Organic matter controls re-distribution of Fe oxides in soils subjected to submersion

Pauline Geier1, Chiara Cerli2, Sabine Fiedler3, Reinhold Jahn1, Karsten Kalbitz2, Klaus Kaiser1
1 Soil Sciences, Martin Luther University Halle, Germany
2 Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, The Netherlands
3 Soil Science, Johannes Gutenberg University Mainz, Germany

O 6.7 in Biogeochemistry of wetlands

15.07.2014, 15:50-16:10, H17

Submerged rice cultivation causes loss of Fe oxides from surface soils due to strongly reduced conditions. In subsoils, despite water saturation, their content often remains constant or even increases. We assumed that the amount of dissolved organic matter released from the topsoils controls redox conditions and therefore Fe oxides in the subsoils.

In the laboratory, we exposed topsoils and upper B horizons of two Fe oxide-rich soils, an Andosol and an Alisol, to 13 anoxic–oxic cycles over one year to simulate development of soils subjected to submersion (paddy soils). Topsoils received rice straw, while the B horizons received organic C only with solutions from the topsoils, as under natural conditions. A second set of samples without straw addition was used as control. Solutions were analysed for redox potential, pH, dissolved organic C (DOC), total Fe, and Fe2+; the headspaces of the incubation vessels were analysed for CO2 and CH4; and soils were analysed for acid oxalate- and dithionite–citrate–bicarbonate-extractable Fe. Real paddy soils that developed from the test soils were used as reference.

During anoxic cycles, topsoils receiving rice straw released large amounts CO2 and CH4, indicating strong microbial activity. Consequently, Eh values dropped and the concentrations of Fe2+ increased. No dissolved Fe3+ was detected. Concentrations of DOC in topsoils were surprisingly small, indicating either strong consumption and/or strong retention of dissolved organic compounds, mainly produced from straw. During oxic cycles, little to no Fe was in solution. Also the DOC concentrations were small. Thus, input of DOC to the subsoils was little, irrespective the redox conditions. This small input of DOC caused only small drops in Eh during anoxic cycles, and so the subsoils never reached Fe-reducing conditions. In result, little to no Fe2+ remained in solutions, which means virtually all dissolved Fe entering the subsoils with topsoil solutions was removed from solution and formed into fresh Fe oxides. Extraction analyses supported these results and revealed depletion of total Fe oxides and shifts towards short range-ordered forms in topsoils over the course of experiment. In subsoils, extractable short range-ordered and, consequently, total Fe oxides increased. The findings are in agreement with the results of extraction analyses of paddy soils developed from the test soils in the field.

In summary, our study shows that submerged rice cultivation induces reductive dissolution of Fe oxides in topsoils and subsequent re-precipitation upon leaching into the less strongly reduced subsoils. The prevalence of less strongly reducing conditions in subsoils seems to be due to the limited input of DOC from the topsoils, which is also in line with the little organic C contents of the paddy subsoils. The situation may change as soon as more DOC can reach the subsoils, e.g., when the topsoils become depleted in Fe oxides and cannot retain all DOC produced.

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last modified 2014-04-04