Synchrotron resolved microscale mineralogy and elemental composition of plinthic and manganiferous nodules from agricultural soils

Authors: Fischel, Matthew; TAPPERO, RYAN - Brookhaven National Laboratory; VICTOR, TIFFANY - Brookhaven National Laboratory; Maul, Jude; Cavigelli, Michel; Duan, Shuiwang; YAN, JIALI - Chuzhou University; RABENHORST, MARTIN - Department Of Environmental Science & Technology, College Of Agriculture And Natural Resources, Uni

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2026
Publication Date: 6/30/2026
Citation: Fischel, M.H., Tappero, R., Victor, T., Maul, J.E., Cavigelli, M.A., Duan, S., Yan, J., Rabenhorst, M. 2026. Synchrotron resolved microscale mineralogy and elemental composition of plinthic and manganiferous nodules from agricultural soils. Geoderma. 472. Article 117767. https://doi.org/10.1016/j.geoderma.2026.117767.
DOI: https://doi.org/10.1016/j.geoderma.2026.117767

Interpretive Summary: Half of soil is made of minerals that can form hard clumps called concretions that are rarely studied because they are mistakenly viewed as inert rocks rather than an active component of the soil. Concretions can affect how nutrients and heavy metal contaminants from fertilizers and pesticides move in soil, with unknown implications for crop yield and quality. Two types of mineral concretions were analyzed in agricultural fields in the Farming Systems Project, a 30-year old system measuring the long-term impact of conventional and organic farm management on crop yield and soil health at the USDA-ARS Beltsville Agricultural Research Center in Maryland, to determine how farm management impacts concretion elemental and mineral composition and implications for contaminant and nutrient cycling. Results showed that the concretions actively accumulate copper and phosphorus from the soil. While accumulating copper is positive because of the potential to prevent crop copper toxicity, accumulating phosphorus locks this nutrient away and reduces the phosphorus available to crops, forcing farmers to apply more of this essential nutrient. This work identifies a new way nutrients and toxic heavy metals are nearly indefinitely locked away in mineral concretions in soils. However, this pathway is not accounted for in soil testing because the concretions are larger than the soil fraction (i.e. they are not easily sampled and sent to a lab). This research provides crucial information on rarely studied but important soil features, which can improve soil fertility management in agricultural systems and help soil testing companies and extension agents better understand phosphorus and heavy metal cycling to give sounder soil fertility recommendations to farmers based on the presence of mineral concretions in their soils.

Technical Abstract: Iron and manganese oxides are strong sorbents and elemental scavengers in the environment that can form macroscopic nodules in soils. Despite their importance in the geochemical cycling of nutrients and contaminants, nodules are rarely characterized. When analyzed, they are often combined into bulk samples, obscuring individual characteristics. This study investigates the composition and mineralogy of individual plinthic and manganiferous nodules in soil from cropping systems in the Farming Systems Project in Beltsville, Maryland. Elemental composition indicates the accumulation of contaminants and nutrients, including copper in all nodules and high levels of phosphorus in select nodules. Sequential extractions determine that highly crystalline minerals comprise the nodules, locking away these nutrients and contaminants. Bulk X-ray diffraction (XRD) demonstrated distinct mineralogies in the plinthic and manganiferous nodules, with the former comprised of quartz, goethite, and kaolinite, and the latter consisting of quartz, Mn2O3, and hematite. Synchrotron and laboratory-based X-ray fluorescence (XRF) mapping gives critical insight into the elemental distribution within the nodules and elucidates spatial elemental associations, including the coupling of manganese and iron in the manganiferous nodules. XRF maps paired with µXRD provide microscale spatial resolution in mineralogy, which resolved several phases not apparent in bulk XRD, manganese’s association with goethite, and minerals indicative of redox transformations, including magnetite and maghemite. These novel results provide critical insight into the microscale mineralogy and individual elemental composition of plinthic and manganiferous nodules, as well as how they alter bulk soil geochemistry in ways largely unaccounted for in research on soil systems.