An official website of the United States government
Phosphorus (P) is an essential nutrient for sustaining life and agricultural production. Transformation of readily available P into forms that are unavailable to plants adds costs to P replenishment, which eventually translates into lower agronomic benefits and potential loss of soil P into runoff may degrade water quality. Therefore, understanding the sources and pathways of the formation of residual P pools in soils is useful information needed for the development of any technological or management efforts to minimize or inhibit the formation of such P pool and thus maximize availability to plants. In this research, we paired phosphate oxygen isotope ratios (δ18OP) with solid-state 31P NMR and quantitative XRD techniques along with general soil chemistry methods to identify the precipitation pathways of acid-extracted inorganic P (Pi) pools in an agricultural soil. Based on the comparison of isotope values of 0.5 mol L−1 NaOH-Pi, 1 mol L−1 HCl-Pi, and 10 mol L−1 HNO3-Pi pools and correlations of associated elements (Ca, Fe, and Al) in these pools, the HNO3-Pi pool appears most likely to be transformed from the NaOH-Pi pool. A narrow range of isotope values of acid-Pi pools in shallow (tilling depth) and below (where physical mixing is absent) is intriguing but likely suggests leaching of particle-bound P in deeper soils. Overall, these findings provide an improved understanding of the sources, transport, and transformation of acid-Pi pools in agricultural soils and further insights into the buildup of legacy P in soils.
more »
« less
Evolution of Oxygen Isotopologues in Phosphate and Pyrophosphate during Enzyme-Catalyzed Isotopic Exchange Reactions
Inorganic pyrophosphatase (PPase) is an enzyme that catalyzes the hydrolysis of the phosphoanhydride bond in pyrophosphate (PPi) to release inorganic phosphate (Pi) and simultaneously exchange oxygen isotopes between Pi and water. Here, we quantified the exchange kinetics of oxygen isotopes between five Pi isotopologues (P18O4, P18O316O, P18O216O2, P18O16O3, and P16O4) and water using Raman spectroscopy and 31P nuclear magnetic resonance (NMR) during the PPase-catalyzed 18O–16O isotope exchange reaction in Pi-water and PPi-water systems. At a high PPi concentration (300 mM), hydrolysis of PPi by PPase was predominant, and only a small fraction of PPi (≪1%) took part in the reversible hydrolysis–condensation reaction (PPi ↔ Pi), leading to the oxygen isotope exchange between Pi and water. We demonstrated that Raman and NMR methods can be equally applied for monitoring the kinetics of the oxygen exchange between the Pi 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.
more »
« less
- PAR ID:
- 10331403
- Date Published:
- Journal Name:
- ACS Earth and Space Chemistry
- ISSN:
- 2472-3452
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract. The northeastern US represents a mostly urban corridorimpacted by high population and fossil fuel combustion emission density.This has led to historically degraded air quality and acid rain that hasbeen a focus of regulatory-driven emissions reductions. Detailing thechemistry of atmospheric nitrate formation is critical for improving themodel representation of atmospheric chemistry and air quality. The oxygenisotopic compositions of atmospheric nitrate are useful indicators intracking nitrate formation pathways. Here, we measured oxygen isotope deltas(Δ(17O) and δ(18O)) for nitric acid (HNO3)and particulate nitrate (pNO3) from three US EPA Clean AirStatus and Trends Network (CASTNET) sites in the northeastern US fromDecember 2016 to 2018. The Δ(17O, HNO3) and δ(18O, HNO3) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ(17O, pNO3) and δ(18O, pNO3) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality ofδ(18O) and Δ(17O), with higher values observedduring winter compared to during summer, suggesting a shift in O3 to HOxradical chemistry, as expected. Unexpectedly, there was a statisticaldifference in Δ(17O) between HNO3 and pNO3, withhigher values observed for pNO3 (27.1 ± 3.8) ‰relative to HNO3 (22.7 ± 3.6) ‰, andsignificant differences in the relationship between δ(18O) andΔ(17O). This difference suggests atmospheric nitratephase-dependent oxidation chemistry that is not predicted in models. Basedon the output from GEOS-Chem and both the δ(18O) and Δ(17O) observations, we quantify the production pathways of atmosphericnitrate. The model significantly overestimated the heterogeneousN2O5 hydrolysis production for both HNO3 and pNO3, afinding consistent with observed seasonal changes in δ(18O) andΔ(17O) of HNO3 and pNO3, though large uncertaintiesremain in the quantitative transfer of δ(18O) from majoratmospheric oxidants. This comparison provides important insight into therole of oxidation chemistry in reconciling a commonly observed positive biasfor modeled atmospheric nitrate concentrations in the northeastern US.more » « less
-
Abstract Cloud condensation and hydrometeor evaporation fractionate stable isotopes of water, enriching liquid with heavy isotopes; whereupon updrafts, downdrafts, and rain vertically redistribute water and its isotopes in the lower troposphere. These vertical water fluxes through the marine boundary layer affect low cloud climate feedback and, combined with isotope fractionation, are hypothesized to explain the depletion of tropical precipitation at higher precipitation rates known as the “amount effect.” Here, an efficient and numerically stable quasi‐analytical model simulates the evaporation of raindrops and enrichment of their isotope composition. It is applied to a drop size distribution and subcloud environment representative of Atlantic trade cumulus clouds. Idealized physics experiments artificially zero out selected processes to discern the separate effects on the isotope ratio of raindrops, of exchange with the environment, evaporation, and kinetic molecular diffusion. A parameterization of size‐dependent molecular and eddy diffusion is formulated that enriches raindrops much more strongly (+5‰ for deuterated water [HDO] and +3.5‰ for O) than equilibrium evaporation as they become smaller than 1 mm. The effect on evaporated vapor is also assessed. Rain evaporation enriches subcloud vapor by +12‰ per mm rain (for HDO), explaining observations of enriched vapor in cold pools sourced by evaporatively cooled downdrafts. Drops smaller than 0.5 mm evaporate completely before falling 700 m in typical subtropical marine boundary layer conditions. The early and complete evaporation of these smaller drops in the rain size distribution enriches the vapor produced by rain evaporation.more » « less
-
Greenland stadials and interstadials (GS/GI) were millennial climate oscillations during the last glacial period that were originally identified in Greenland ice cores but that have been correlated with environmental change around much of the globe, including in monsoon regimes, with enhanced monsoon rainfall coincident with North Atlantic warming. Hydroclimate variability associated with GS/GI have been investigated in detail using terrestrial (primarily oxygen isotopes in stalagmites) and marine records, particularly for the Southeast Asian monsoon. However, a considerably smaller number of terrestrial records preserve these events in the Indian summer monsoon (ISM), the primary water source for ~2 billion people across South Asia. Here we present the first glacial-age speleothem stable isotope time series from Nepal, located in the ISM regime. UK-1 is a 187 mm tall aragonite stalagmite from the Pokhara Valley of central Nepal, ~150 km west of Kathmandu. The chronology of UK-1, which was established by 8 U/Th dates, all of which fall in stratigraphic order (within the errors), reveals continuous growth from 34,350-31,500 yr BP (Marine Isotope Stage 3); age uncertainties average ±84 yr. Stable isotope samples were measured every 1 mm, corresponding to a temporal resolution of 18 yr. Oxygen isotope ratios range from -5.6‰ to -7.6‰, and share the same timing and structure as Greenland (inter)stadials GS/GI 6 and 5.2 in the NGRIP record. We interpret this as reflecting an amount effect response to a strengthened ISM driven by more (less) poleward migration of the intertropical convergence zone during periods of northern hemisphere warming (cooling). This clear millennial signal in UK-1 is a somewhat unexpected result given that amount effects in oxygen isotopes in precipitation are weak (R^2=0.1) in this area today. UK-1 carbon isotope ratios range from -3‰ to -6‰ (excluding a small number of negative spikes) and exhibit variability coarsely similar to the NGRIP record, with lower (higher) values generally corresponding to GI (GS), possibly due to prior calcite precipitation in voids above the cave concomitant with changes in precipitation. Some periods of antiphasing between carbon and oxygen are also apparent and may reflect flushing of soil carbon dioxide during particularly wet phases.more » « less
-
Outcrops of brittle faults are rare in Marie Byrd Land, West Antarctica, because fault damage zones commonly undergo enhanced erosion and form bedrock troughs occupied by glacier ice. Where exposures do exist, faults yield information about regional strain in the West Antarctic Rift System (WARS) and may host minerals that contain a record of the temperature and chemistry of fluids during regional-scale faulting. In MBL’s southern Ford Ranges, bordering Ross Sea, a distinctive fault array was sampled that hosts tourmaline and quartz, a mineral-pair that can provide temperature and composition of fault-associated fluids, using 18O. Host rocks are tourmaline-free. At three separate sites, fault surfaces display strongly aligned tourmaline, suggesting that mineralization occurred during tectonism. One site features highly polished, or mirrored, surfaces, a characteristic that may indicate tourmaline precipitation during seismic slip. The orientation and kinematics of the high angle faults are NNW-striking: normal-slip, and WNW-ESE striking: right-lateral strike-slip. The timing of mineralization is yet to be determined, but viable possibilities are that the faults formed during broad intracontinental extension during formation of Ross Embayment in the Cretaceous, or during development of deep, narrow basins beneath the RIS grounding zone, in the Neogene (newly detected, see Tankersley et al., this meeting). Once formed, tourmaline is resistant to chemical and isotopic re-equilibration, and therefore can retain a record of its conditions during formation. We used oxygen isotope compositions of tourmaline and quartz pairs to investigate temperatures, fluid-rock ratios, and fluid sources, with bearing on fault-localized flux of fluids and geothermal heat. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces, as well as quartz separated from host rock in the same hand samples. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. ∆Qtz-Trm values fall between 1.3 and 2.0, and 18O of quartz in faults closely resembles 18O of host rock quartz. We tentatively determine the water oxygen isotope ratio as greater than ~7.7 ‰. Plutonic-metamorphic associations in the immediate region, and comparisons with similar faults elsewhere (i.e. Isola d’Elba, Italy), suggest temperatures as high as 500°C for the fluids that circulated into the faults. The data are interpreted to show that brittle faults provided pathways for hot fluids derived from mid-crustal processes to make their way to shallow crustal depths. 18O values indicate magmatic and/or metamorphic fluid sources, with minor to no introduction of meteoric fluids. Tourmaline-quartz pairs did not attain equilibrium, likely due to tourmaline’s rapid crystallization. On-going investigation includes analysis of H and B isotopes in tourmaline, which will better characterize the relationship between fault-hosted and mid-crustal fluids.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsearthspacechem.2c00050
