Self-Functionalization of Soil-Aged Biochar Surfaces by Iron Increases Nitrate Retention

Increasing retention of nutrients and organic matter (OM) is essential for strengthening soil health, sustaining food production, and environmental protection. Therefore, management strategies improving these soil characteristics are in critical need. Addition of biochar (BC) positively affects soil OM, however, effects on nutrient retention are uncertain. This results from an incomplete understanding of biogeochemical processes at the BC-soil-water interface, including interactions between the BC surface and soil derived iron (Fe), which can increase BC reactivity towards anionic macronutrients and dissolved OM. To elucidate surface changes of high temperature pine BC from in situ soil aging, an agricultural sandy loam soil was amended with 40 t ha^-1 of BC or received no BC (control) within a greenhouse experiment. Treatments (four replicates) were fertilized with 280 kg N ha^-1 of swine slurry, seeded with corn, and irrigated weekly. Soil and BC were sampled at corn harvest (10 weeks), and after root degradation (14 weeks), with weekly leachate collection until harvest. X-ray photoelectron spectroscopy (XPS) analysis suggests that the nearly pure carbon at the pristine BC surface had developed different forms of C oxidation (C-OH, C=O, and COO) over time. Additionally, at harvest Fe was present on the BC surface in Fe(II) and Fe(III) forms. Fe was absent from the pristine BC surface, and Fe had completely oxidized to Fe(III) by experiment end, indicating successive Fe mineral/oxide formation. Leaching of nitrate (NO₃^-), phosphate (PO₄^3-) and dissolved organic C (DOC) were similar for BC and control until week four, when BC surface Fe-functionalization may have begun. After week four, mass leaching was lower (55% for NO₃^-, 19% for PO₄^3-, and 28% for DOC) for BC amended soil compared to control. Due to the pronounced reduction in NO₃^- leaching, soils were extracted for NO₃^-. At experiment end (full functionalization), BC amended soils contained 8.6 times more NO₃^- compared to harvest (partial functionalization), and 2.8 times more than the control, which we postulate is mainly driven by the Fe-functionalization of BC surfaces. Our results will contribute to a better mechanistic understanding of nutrient and OM retention controlled by BC aging in soil, and to an improvement of BC based soil management strategies.