Browsing by Author "Cliff, JB"

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    X-ray methods to investigate nutrient cycling in the subsurface
    (American Geophysical Union (AGU), 2021-12-18) Bone, S; Aristilde, L; Bargar, J; Klein, AR; Cliff, JB
    X-ray methods provide unique insight into the biogeochemical cycling of nutrients, such as C, P and Ca, in the subsurface, particularly when nutrient availability is mediated by interactions with the solid phase. For instance, mineral phases in the sediments may sorb nutrients, mediating their availability to microbes. In other instances, minerals can catalyze nutrient transformations. In this talk, I will give two examples of how soft and tender X-ray spectroscopies can be utilized to examine critical steps in nutrient cycling. In the first example, Ca speciation in the subsurface is investigated using a combination of scanning transmission X-ray microscopy (STXM) and tender X-ray microprobe analysis. Soils and sediments host large stores of organic carbon which can be released to the atmosphere upon mineralization. Ca may play a key role in preventing organic matter (OM) mineralization by promoting mineral sorption. Although it has long been recognized that Ca forms cation bridges that link negatively charged organic and mineral functional groups, there is little direct experimental observation of this, which is necessary for a molecular-scale, predictive understanding. We examined whether OM-Ca was co-associated with specific minerals, and with what OM types Ca was associated using nano secondary ion mass spectrometry (NanoSIMS) and STXM. The Ca speciation was probed further using (micro-) X-ray absorption near edge structure (XANES) spectroscopy. In the second example, I will discuss how XANES spectroscopy can be used to elucidate abiotic transformation of organic P to inorganic P, rendering it bioavailable. Organic P species, in particular phosphate esters such as those found in ribonucleotides, can comprise a significant fraction of total P in soils and sediments. P K-edge XANES spectroscopy was utilized to monitor dephosphorylation of ribonucleotides by ferrihydrite, and helped to elucidate how the extent of dephosphorylation was related to organic P type.