Mengqiang Zhu

Declining atmospheric sulfur (S) deposition makes S an emerging limiting macronutrient for plants, yet the stability and dynamics of soil organic S—the largest terrestrial S pool supplying plant‐available sulfate via mineralization—remain unclear. Across North American soils, total organic S is dominated by mineral‐associated organic S (MAOS; 61% ± 26%); this pool is stabilized by mineral protection but saturates at ~600 μg S/g soil. Compared with particulate organic S (POS), which is relatively labile and does not saturate, MAOS is more decomposed, chemically diverse, and climate‐sensitive. Higher temperature accelerates S mineralization in both pools, but their ability to supply available S varies with moisture regime because MAOS stabilizes in wetter soils while POS remains labile. This continental‐scale reassessment of the soil S cycle reveals that secondary minerals play a dual role in soil S cycling by stabilizing organic S and retaining sulfate, providing a mechanistic basis for nutrient management and carbon sequestration in response to climate change in an S‐scarce future.

The decomposition of organic sulfur (S) influences S availability to plants and trace metal dynamics in soils. While temperature effects on decomposition rate are known, the influence of precipitation remains less understood. We examined organic S decomposition and oxidation states in volcanic soils across two rainfall gradients on the Hawaiian Islands (mean annual precipitation [MAP]: 285-5066 mm). Higher MAP increased atmospheric sulfate deposition, which was largely converted to organic S by biological processes. However, the impact of precipitation on decomposition of organic S varied by rainfall regime. In wetter regions (MAP > 1500 mm), higher soil moisture promoted reducing conditions, resulting in markedly less oxidized and less decomposed organic S, with average oxidation state (AOS) of organic S decreasing from 4.5 to 2. In drier regions (MAP < 1500 mm), the decomposition of organic S (AOS = 4.2 ± 0.1) was high but did not correlate with MAP, likely because the soils remained sufficiently oxic for decomposition regardless of MAP and soil moisture. These trends reflect thermodynamic constraints on organic matter decomposition and suggest that soil organic matter enriched in reduced carbon tends to accumulate reduced sulfur, underscoring a strong coupling between carbon and sulfur cycles. Our findings suggest that climate change-induced shifts in precipitation and temperature could alter soil S cycling, with important consequences for nutrient availability and trace metal dynamics in terrestrial ecosystems.

The incorporation of iron (Fe) into calcium carbonate coprecipitates (CCCs) to form Fe-incorporated CCC (CCCFe) widely occurred in CCC-enriched soils and sediments. However, the molecular mechanism of phosphate (P) immobilization on CCCFe remains largely unknown. In this study, batch experiments were conducted to investigate the immobilization mechanisms of P on CCCFe with low and high Fe loadings (CCCFe_l vs CCCFe_h) using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy with surface-sensitive total electron yield (P-XANES_TEY) and bulk-sensitive fluorescence yield (P-XANES_FLY) modes. Results indicated that the high Fe loading inhibited calcite formation but favored vaterite production in the CCC and enhanced its P retention capacity. Hydroxyapatite (HAP) dominated over ferrihydrite-associated P (Fe-P) in the bulk of P sorbed on the CCCFe_h, but amorphous calcium phosphate (ACP) dominated over HAP for the CCC, suggesting Fe-induced transformation of the ACP to the HAP. Furthermore, the P-XANES_TEY analysis indicated brushite primarily formed on the surface of the P sorbed on the CCCFe_h at high P loading. Consistently, the spherical aberration-corrected scanning transmission electron microscopy analysis directly revealed ferrihydrite coating on the CCC and the presence of Ca-P and Fe-P associations. This study provides new molecular-level insights into Fe-tuned depth-dependent transformation of P on the CCC, thus benefiting P management in the CCC-enriched environmental settings.

Molybdenum (Mo) is an essential nutrient for almost all organisms. However, at high concentrations, it can be toxic to animals and plants. This study investigated the interactions of Mo(VI) with iron oxyhydroxides during ferrihydrite bioreduction in the presence of Fe(III)-reducing Geobacter sulfurreducens. Here, we showed that Mo concentration controlled ferrihydrite phase transformation, leading to Mo release. With the biotic reduction of ferrihydrite and Fe(II) production, Mo(VI) reduction and Mo(IV)O-2 formation were observed for the first time, which further immobilized Mo after surface adsorption of Mo(VI). At low Mo levels (Mo/Fe molar ratios of 1-2%), sufficient Fe(II) adsorption onto ferrihydrite resulted in its transformation into magnetite nanoparticles (>80%, similar to 25 nm), which catalyzed the reduction of Mo(VI) to form Mo(IV)O-2 and immobilized Mo. Contrastingly, at high Mo concentrations (Mo/Fe molar ratios of 5-10%), Mo(VI)O-4(2-) adsorption onto ferrihydrite limited Fe(II) adsorption; subsequently, less magnetite (15 and 100 nm, respectively) and siderite (similar to 20-30%, width and length >100 and 200 nm, respectively) with larger particle sizes formed instead, causing Mo(VI) release due to lower Mo adsorption. This study provides a comprehensive understanding of the interaction mechanisms among Geobacter sulfurreducens, Mo(VI), and iron oxyhydroxides, enabling predictions and controls of long-term Mo mobility and Fe mineral transformation under a variety of biogeochemical scenarios

The simultaneous sorption of cations and anions at the mineral-water interface can substantially alter their individual sorption characteristics; however, this phenomenon lacks a mechanistic understanding. Our study provides direct spectroscopic and modeling evidence of the molecular cosorption mechanisms of the cadmium ion (Cd²⁺) and phosphate (P) on goethite and layered manganese (Mn) oxide of birnessite, through in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR), P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and surface complexation modeling. Phosphate synergistically cosorbed with Cd on goethite predominantly through P-bridged ternary complexes (≡Fe-P-Cd) and electrostatic interactions at wide pH conditions. Likewise, P and Cd exhibited synergistic cosorption on birnessite by forming P-bridged ternary complexes (≡Mn-P-Cd) and weak competitive sorption at the layer edge sites. As pH and Cd loading increased, the surface P species transitioned from a binary complex to a ternary complex and/or Cd₃(PO₄)₂ precipitate for both goethite and birnessite. Compared to that in solution at pH 8, the formation of Cd₃(PO₄)₂ was inhibited by the presence of goethite and birnessite, ascribed to the specific adsorption of P and Cd, more pronounced in birnessite due to the stronger sorption of Cd at its vacant sites. The discovered cosorption mechanisms of P and Cd have important implications for understanding and predicting their mobility and availability in Cd-contaminated settings.

Mineral adsorption-induced molecular fractionation of dissolved organic matter (DOM) affects the composition of both DOM and OM adsorbed and thus stabilized by minerals. However, it remains unclear what mineral properties control the magnitude of DOM fractionation. Using a combined technique approach that leverages the molecular composition identified by ultrahigh resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry and adsorption isotherms, we catalogue the compositional differences that occur at the molecular level that results in fractionation due to adsorption of Suwannee River fulvic acid on aluminum (Al) and iron (Fe) oxides and a phyllosilicate (allophane) species of contrasting properties. The minerals of high solubility (i.e., amorphous Al oxide, boehmite, and allophane) exhibited much stronger DOM fractionation capabilities than the minerals of low solubility (i.e., gibbsite and Fe oxides). Specifically, the former released Al³⁺ to solution (0.05-0.35 mM) that formed complexes with OM and likely reduced the surface hydrophobicity of the mineral-OM assemblage, thus increasing the preference for adsorbing polar DOM molecules. The impacts of mineral solubility are exacerbated by the fact that interactions with DOM also enhance metal release from minerals. For sparsely soluble minerals, the mineral surface hydrophobicity, instead of solubility, appeared to be the primary control of their DOM fractionation power. Other chemical properties seemed less directly relevant than surface hydrophobicity and solubility in fractionating DOM.