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Keeping carbon in the ground: Coupled cycling of organic carbon and metal oxides (Fe, Mn, Al) in UK upland soils

Dr Christian März (SEE), Dr Sheila Palmer (SoG), Dr Caroline Peacock (SEE)

Contact email: c.maerz@leeds.ac.uk

Background

Soil organic carbon (SOC) is a key parameter for healthy soil function, including its fertility, water holding capacity, and carbon storage, and it has important repercussions on both food production, flood/drought resilience, and the global carbon cycle (Lal, 2004). It is therefore of scientific, societal and economic interest to better understand SOC dynamics, and especially its interactions with the inorganic components of the soil system. This could ultimately lead to improved strategies of engineering more fertile and resilient soil systems.

In natural soils, a lot of SOC is bound to inorganic particles, especially to Al, Fe and Mn (oxyhydr)oxides. Numerous recent studies (e.g., Wagai and Mayer, 2007; Lalonde et al., 2012; Palmer et al., 2013; Riedel et al., 2013; Johnson et al., 2015; Estes et al., 2016; Shields et al., 2016) have examined the role that Fe and Mn (oxyhydr)oxides play in stabilising significant amounts of organic matter, both in soils and marine sediments (Fig. 1).

Figure 1: Relative percentages of organic carbon associated with Fe oxides (OC-Fe) in marine sediments worldwide, reaching 15-30% in most places (Lalonde et al., 2012)

However, in different climate change scenarios, flooding or drying the soil will directly translate into changing pH and redox conditions, with immediate but poorly constrained effects on the stability of Al, Fe and Mn oxides and their capacity to retain SOC within the soil system (e.g., Schwertmann, 1966; Thompson et al., 2006; Lehmann and Kleber, 2015; Emsens et al., 2016). Organic matter can be chemically transformed and then get released from the soil, either in particulate form or as organo-metallic complexes. How stable these organo-metallic complexes are under changing environmental conditions has implications for the long-term burial of carbon in soil systems or streambeds. There are also implications for the water industry as dissolved or colloidal organo-metallic complexes can contribute to unwanted coloration of water and affect its subsequent treatment for drinking water purposes.

In this project, the spatially and temporally dynamic coupling between SOC and Al, Fe and Mn oxides will be investigated in globally important carbon-rich soil types, such as peats and organomineral upland soils (Fig. 2), that have a recognised relevance as terrestrial carbon stores and within the UK are sources for much of our drinking water. The successful candidate will join a team of geochemists with long-standing experience in carbon and metal cycling in the environment.

Figure 2: Eroded peat hags in the Pennines

Objectives:

  1. Determining the quantity and relative contribution of SOC bound to Fe, Mn and Al (oxyhydr)oxides in UK upland soil systems

  2. Trace changes in this specific SOC pool in relation to soil depth, underlying lithologies, dry versus flooded areas.

  3. Quantify changes in this specific SOC pool along hydrological pathways, e.g, small stream, river banks, river sediments.

  4. Determine stability of this SOC pool under changing environmental conditions, e.g., wet versus dry, hot versus cold.

Approach & training:

Sampling locations will be uplands in the north of the UK (Fig. 2) where Dr Palmer and her group have long-standing research experience (Palmer et al., 2013). Sampling will include soil profiles and cores, soil moisture sampling, river cuttings, river sediments and suspension load to get a full picture of different stores and transport pathways of different carbon and metal fractions. The analytical approach will include geochemical analyses of the bulk sediment and water samples (dissolved and particulate organic carbon, metals, nutrients) and sequential leaching methods to extract different metal phases and the associated carbon fractions. Selected natural samples, and analogue samples created in the laboratory, will be analysed at very high spatial and chemical resolution using the latest synchrotron nanoprobes, to further investigate the mineral-organic couplings and how these might promote carbon stabilization and burial.

The student wil be trained in the laboratory procedures and analyses by Drs März, Palmer and Peacock who have substantial experience in the extraction and analytical methods to be applied (including extracting different metal phases from soil and sediment samples and total acid diegstions, analysis of dissolved phases using AAS, ICP-OES and ICP-MS, and of solid samples using XRD, CNS combustion analysis and synchrotron-based spectroscopy). Hands-on training and support will further be provided by highly qualified technicians both in the Cohen Laboratories (School of Earth and Environment) and in the School of Geography. The successful candidate will have access to a wide range of training workshops (scientific writing and presentation skills, statistics, science communication and outreach), and will be supported by the supervisors in preparing conference presentations and peer-reviewed publications.

Student qualification:

The successful candidate should have an excellent degree in an Earth Sciences, Environmental Sciences, or Soil Science discipline, a strong background and keen interest in fieldwork and analytical skills, ideally experience in conducting a research project and presenting research results to the wider scientific community.

Further reading:

Emsens W-J, Aggenbach CJS, Schoutens K, Smolders AJP, Zak D, Van Diggelen R (2016) Soil    iron content as a predictor of carbon and nutrient mobilization in rewetted fens. PLoS ONE   11, e0153166.

Estes ER, Andeer PF, Nordlund D, Wankel SD, Hansel CM (2016) Biogenic manganese oxides as reservoirs of organic carbon and proteins in terrestrial and marine environments. Geobiology, doi:10.1111/gbi.12195

Johnson K, Purvis G, Lopez-Capel E, Peacock C, Gray N, Wagner T, März C, Bowen L, Ojeda J, Finlay N, Robertson S, Worrall F, Greenwell C (2015) Towards a mechanistic understanding of carbon stabilization in manganese oxides. Nature Comm. 6, doi: 10.1038/ncomms8628.

Lalonde K, Mucci A, Ouellet A, Gelinas Y (2012) Preservation of organic matter in sediments promoted by iron. Nature 483, 198−200.

Lehmann J, Kleber M (2015) The contentious nature of soil organic matter. Nature 528, 60-68.    

Palmer SM, Clark JM, Chapman PJ, Van der Heijden GMF, Bottrell SH (2013) Effects of acid           sulphate on DOC release in mineral soils: the influence of SO42− retention and Al release. Eur. J Soil Sci. 64, 537-544.

Riedel T, Zak D, Biester H, Dittmar T (2013) Iron traps terrestrially derived dissolved organic matter at redox interfaces. PNAS 110, 10101-10125.

Schwertmann U (1966) Inhibitory effect of soil organic matter on crystallization of amorphous ferric   hydroxide. Nature 212, 645−648.

Shields MR, Bianchi TS, Gélinas Y, Allison MA, Twilley RR (2016) Enhanced terrestrial carbon preservation promoted by reactive iron in deltaic sediments. Geophys. Res. Lett. 43, 1149–1157.

Thompson A, Chadwick OA, Rancourt DG, Chorover J (2006) Iron-oxide crystallinity increases during soil redox oscillations. Geochim. Cosmochim. Acta 70, 1710-1727.

Wagai R, Mayer LM (2007) Sorptive stabilization of organic matter in soils by hydrous iron oxides. Geochim. Cosmochim. Acta 71, 25−35.

Related undergraduate subjects:

  • Earth science
  • Environmental science
  • Geology
  • Geoscience
  • Soil science