Calcium-Organic Matter Interaction in Alkaline Soils

Multiple studies have investigated the nature and stability of organic matter (OM) associated with Fe-minerals. For example, OM sorbed onto Fe(II)-Fe(III) containing mineral, such as magnetite, is reported to be susceptible to oxidation and reduction via electron transfer. In general, the Fe-sorbed OM is stable under oxic conditions but may be bioavailable under anoxic conditions (coupled to Fe(III) reduction). In soils, like Fe- and Al-, Ca species (not redox sensitive) also plays a significant role in OM sequestration. Hence, a holistic understanding of the nature and stability of OM's associated with various mineral surfaces as well the nature of non-mineral associated OM (e.g., "Ca²⁺-OM" bridges), is crucial to predict OM dynamics in soils. Recent results from our laboratory indicated that a calcareous and alkaline soil from Prosser, WA, is a suitable candidate for such a study. Contents of environmentally significant elements, C, Fe, and Ca in this soil are low, between 2-5% of the total, each. These elemental amounts and lower amounts of particulate OM allowed us to focus on Ca-OM bridges and OM localized on various mineral forms, particularly CaCO₃. Furthermore, sparse distribution of OM on surfaces of quartz/feldspar, the predominant minerals (90+ wt.%), minor amounts of bioavailable ferrihydrite-like mineral (by ⁵⁷Fe-Mössbauer spectroscopy), and CaCO₃ crystals displaying morphology akin to biogenic CaCO₃, suggested that an understanding of the nature of Ca-OM interactions in soil will be helpful to predict, in general, SOM stability in calcareous soils. A combination of elemental mapping by microscopic (Transmission Electron Microscopy (TEM) and Scanning Transmission X-ray Microscopy (STXM)) and NEXAFS (near Extended X-ray Absorption Fine Structure Spectroscopy) studies has unambiguously shown that CaCO₃ crystals in the Prosser soil has affinity to various OM groups and the Ca is ubiquitous in OM. These results as well FT-ICR-MS analysis of water extractable dissolved OM are being used to design experiments to address relative stability of specific mineral-OM moieties under dynamic biogeochemical conditions. These insights could yield valuable information to better predict interplay of Fe and C in Fe-, C- and Ca-poor calcareous soils.