Universität zu Köln

Monitoring of reducing conditions in soils and implications for biogeochemical processes

Dorau, Kristof (2017) Monitoring of reducing conditions in soils and implications for biogeochemical processes. PhD thesis, Universität zu Köln.

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    Abstract

    Reducing conditions in soils alter a variety of biogeochemical processes that affects the mobility of nutrients and pollutants, the emission of greenhouse gases, and the formation of redoximorphic features. Hence, a precise characterization and monitoring of reducing conditions is important for land use management and risk assessment. Conventionally, platinum (Pt) electrodes are used to measure the redox potential (EH) for delineation of the soil redox status. In combination with a data-logger system the EH can be monitored at high temporal resolution but with the shortcoming of cost-intensive technical equipment. Alternatively, the Indicator of Reduction In Soils (IRIS) method can be used to delineate soil reducing conditions that consists of synthesized iron (FeIII) oxides coated onto white polyvinyl chloride (PVC) bars. The bars are installed for a period of 30 days in the soil and visually assessed for the effects of reduction by the depletion of the FeIII oxide coating. Currently, a differentiation into redox classes of oxidizing (EH > 300 mV; oxygen (O2) is present in the soil), weakly reducing (EH 300 to 100 mV; manganese (MnIII,IV) oxide reduc-tion), and moderately reducing (EH 100 to –100 mV; FeIII oxide reduction) soil conditions can only be achieved by Pt electrodes. The objectives of this thesis are to investigate the temporal and spatial distribution of the EH by different monitoring approaches. Automated Pt electrode measurements on hourly basis (2011 to 2014) were compared with measurements on a weekly basis (1990 to 1993) to query the benefit of high temporal resolution EH monitoring. Additionally, the IRIS method was adopted by coating MnIII,IV oxides onto white PVC bars (in the following referred to as ‘Mn and Fe redox bars’) and evaluated for their potential to differentiate into weakly and moderately reducing soil conditions in field and laboratory experiments. An important characteristic of MnIII,IV and FeIII oxides is their functioning as a sorbent for ele-ments in soil solution. Therefore, the oxide coatings of previously field-installed redox bars were extracted and analyzed to investigate the element sorption behavior. Finally, long-term groundwater data (1997 to 2012) was analyzed to assess trends of the water table (WT) depth development along a 17 km2 sized mesotrophic fen, which is relevant for the onset of reduction and with impact on the soil redox status. All field-experiments were conducted at different study sites in North-West Germany. Daily EH readings by Pt electrodes were sufficient to equally derive trends of the redox class distribution over time compared with hourly measurements but a loss of information occurred when weekly or monthly EH readings were performed. Since fluctuations up to 540 mV were measured within a day, hourly readings were essential to identify biogeo-chemical processes. Besides EH measurements by Pt electrodes, it was possible to facili-tate a durable MnIII,IV oxide coating onto white PVC bars enabling to monitor the soil redox status. Laboratory experiments at defined EH-pH conditions went along with MnIII,IV oxide dissolution under weakly reducing conditions. The capillary fringe above the groundwater surface was identified as a hot spot for MnIII,IV oxide reduction with minor FeIII oxide remov-al. Hence, the simultaneous application of Mn and Fe redox bars enabled to differentiate zones of weakly and moderately reducing soil conditions. It was found that the presence of ferrous Fe2+ (reductant) in soil solution mediated an abiotic redox reaction with the for-mation of durable FeIII oxide coatings along Mn redox bars. Subsequently, MnIII,IV oxides (oxidant) transformed to manganous Mn2+ being removed from the PVC surface. The Fe precipitates and the remaining MnIII,IV oxide coating differed remarkably in color enabling to quantify the percentage area of these patterns over time. Thereby, temporally and spatially diverse pathways of FeIII oxide forming processes can be studied. Selective chemical ex-traction of these oxide coatings along Mn redox bars verified a preferential sorption of cati-onic elements (e.g. copper, lead, zinc) to the surface of MnIII,IV oxides, whereas the posi-tively charged surface of in situ formed FeIII oxides were enriched in elements having an oxyanionic character (e.g. arsenic, molybdenum, phosphorus, vanadium). Moreover, in situ formed FeIII oxides (‘field’-Fe oxides) revealed higher element loadings compared with synthesized FeIII oxides along Fe redox bars (‘lab’-Fe oxides). In accordance with this find-ing, field-Fe oxides were solely composed of short-range ordered minerals exhibiting a higher sorption capacity. The WT depth was the main driver for the above-mentioned pro-cesses and controlled the (i) EH dynamics, (ii) oxide removal along Mn and Fe redox bars, and (iii) element relocation into the topsoil by capillary rise. In this regard, WT depth read-ings along a mesotrophic fen indicated a lowering by on average 20 cm at 39 out of 46 monitoring wells. A meteorological forecast of the climatic water balance until the year 2100 indicated that the development of decreasing WT depths will be intensified. Obvious-ly, this will have impact on the soil redox status and associated biogeochemical processes within the top soil. Overall, monitoring by Mn and Fe redox bars improves the understanding of the spatial and temporal distribution of soil reducing conditions but does not capture the ‘true’ dynam-ic nature that can be determined by Pt electrodes. A continuous monitoring of the soil re-dox status by Pt electrodes or redox bars over decadal time scales is important to evaluate the impact of climate change on biogeochemical processes in waterlogged soils.

    Item Type: Thesis (PhD thesis)
    Creators:
    CreatorsEmail
    Dorau, Kristofkristof.dorau@gmx.de
    URN: urn:nbn:de:hbz:38-74398
    Subjects: Natural sciences and mathematics
    Chemistry and allied sciences
    Earth sciences
    Civic and landscape art
    Geography and travel
    Uncontrolled Keywords:
    KeywordsLanguage
    redox potential (EH), environmental monitoring, redox bars, manganese and iron oxides, soil reducing conditions, element sorption, climate change, wetland restoration managementEnglish
    Redoxpotential (EH), Umweltmonitoring, Redox-Stäbe, Mangan- und Eisenoxide, Reduzierende Bedingungen in Böden, Elementsorption, Klimawandel, Renaturierung von FeuchtgebietenGerman
    Faculty: Mathematisch-Naturwissenschaftliche Fakultät
    Divisions: Mathematisch-Naturwissenschaftliche Fakultät > Geographisches Institut
    Language: English
    Date: February 2017
    Date Type: Publication
    Date of oral exam: 13 April 2016
    Full Text Status: Public
    Date Deposited: 14 Mar 2017 09:01:13
    Referee
    NameAcademic Title
    Mansfeldt, TimProf. Dr.
    Kehl, MartinPD Dr.
    URI: http://kups.ub.uni-koeln.de/id/eprint/7439

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