Manganese (Mn) oxides have a high surface area and redox potential that facilitate sorption and/or oxidation of organic carbon (OC), but their role in regulating soil C storage is relatively unexplored. Small OC compounds with distinct structures were reacted with Mn(III/IV)-oxides to investigate the effects of OC/Mn molar ratios on Mn−OC interaction mechanisms. Dissolved and solid-phase OC and Mn were measured to quantify the OC sorption to and/or the redox reaction with Mn-oxides. Mineral transformation was evaluated using X-ray diffraction and X-ray absorption spectroscopy. Higher OC/ Mn ratios resulted in higher sorption and/or redox transformation; however, interaction mechanisms differed at low or high OC/Mn ratios for some OC. Citrate, pyruvate, ascorbate, and catechol induced Mn-oxide dissolution. The average oxidation state of Mn in the solid phase did not change during the reaction with citrate, suggesting ligand-promoted mineral dissolution, but decreased significantly during reactions with the other compounds, suggesting reductive dissolution mechanisms. Phthalate primarily sorbed on Mn-oxides with no detectable formation of redox products. Mn−OC interactions led primarily to C loss through OC oxidation into inorganic C, except phthalate, which was predominantly immobilized in the solid phase. Together, these results provided detailed fundamental insights into reactions happening at organo−mineral interfaces in soils.
Inorganic pyrophosphatase (PPase) is an enzyme that catalyzes the hydrolysis of the phosphoanhydride bond in pyrophosphate (PP i ) to release inorganic phosphate (P i ) and simultaneously exchange oxygen isotopes between P i and water. Here, we quantified the exchange kinetics of oxygen isotopes between five P i isotopologues (P18O4, P18O3 16O, P18O2 16O2, P18O16O3, and P16O4) and water using Raman spectroscopy and 31P nuclear magnetic resonance (NMR) during the PPase-catalyzed 18O–16O isotope exchange reaction in P i -water and PP i -water systems. At a high PP i concentration (300 mM), hydrolysis of PP i by PPase was predominant, and only a small fraction of PP i (≪1%) took part in the reversible hydrolysis–condensation reaction (PP i ↔ P i ), leading to the oxygen isotope exchange between P i and water. We demonstrated that Raman and NMR methods can be equally applied for monitoring the kinetics of the oxygen exchange between the P i isotopologue and water. It was found that the isotope exchange determined by the spectroscopic methods was detectable as low as 0.2% 18O abundance, but the reliability below 1% was much lower. Given that high P concentrations (≥1 mM) are required in these methods, environmental application of these methods is limited to rare high P conditions in engineered and agricultural environments.
Among ubiquitous phosphorus (P) reserves in environmental matrices are ribonucleic acid (RNA) and polyphosphate (polyP), which are, respectively, organic and inorganic P-containing biopolymers. Relevant to P recycling from these biopolymers, much remains unknown about the kinetics and mechanisms of different acid phosphatases (APs) secreted by plants and soil microorganisms. Here we investigated RNA and polyP dephosphorylation by two common APs, a plant purple AP (PAP) from sweet potato and a fungal phytase from Aspergillus niger. Trends of δ18O values in released orthophosphate during each enzyme-catalyzed reaction in 18O-water implied a different extent of reactivity. Subsequent enzyme kinetics experiments revealed that A. niger phytase had 10-fold higher maximum rate for polyP dephosphorylation than the sweet potato PAP, whereas the sweet potato PAP dephosphorylated RNA at a 6-fold faster rate than A. niger phytase. Both enzymes had up to 3 orders of magnitude lower reactivity for RNA than for polyP. We determined a combined phosphodiesterase-monoesterase mechanism for RNA and terminal phosphatase mechanism for polyP using high-resolution mass spectrometry and 31P nuclear magnetic resonance, respectively. Molecular modeling with eight plant and fungal AP structures predicted substrate binding interactions consistent with the relative reactivity kinetics. Our findings implied a hierarchy in enzymatic P recycling from P-polymers by phosphatases from different biological origins, thereby influencing the relatively longer residence time of RNA versus polyP in environmental matrices. This research further sheds light on engineering strategies to enhance enzymatic recycling of biopolymer-derived P, in addition to advancing environmental predictions of this P recycling by plants and microorganisms.
Glyphosate is the active ingredient of the common herbicide Roundup. The increasing presence of glyphosate and its byproducts has raised concerns about its potential impact on the environment and human health. In this study, we used liquid chromatography mass spectrometry (LC-MS) and electrospray ionization (ESI) source Q Extactive Orbitrap mass spectrometry (ESI-Orbitrap MS) to identify glyphosate degradation products and investigated the transformation of orthophosphate released in soil. The LC-MS and ESI-Orbitrap MS results showed that glycine formed during the early stage but was rapidly utilized by soil microorganisms. AMPA was the dominant product at the late stage and was 3-6 times more persistent than glyphosate against degradation. The 18 O labeling and phosphate oxygen isotope results allowed a clear distinction of the fraction of inorganic P (P i ) derived from glyphosate, about half of which was then rapidly taken up and recycled by soil microorganisms. Soil incubation results are different from abiotic degradation with Mn-oxide in which AMPA production was suppressed. These results point that AMPA is preferred pathway of degradation in biotic degradation. The rapid cycling of P i derived from glyphosate degradation constitute a disregarded source of P that has important implications on nutrient management in agricultural as well as loss from soil to open waters.
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