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1. Dryland irrigation increases accumulation rates of pedogenic carbonate and releases soil abiotic CO2

   https://doi.org/10.1038/s41598-021-04226-3  

   Ortiz, Anna C. ; Jin, Lixin ; Ogrinc, Nives ; Kaye, Jason ; Krajnc, Bor ; Ma, Lin ( January 2022 , Scientific Reports)

   Agricultural fields in drylands are challenged globally by limited freshwater resources for irrigation and also by elevated soil salinity and sodicity. It is well known that pedogenic carbonate is less soluble than evaporate salts and commonly forms in natural drylands. However, few studies have evaluated how irrigation loads dissolved calcium and bicarbonate to agricultural fields, accelerating formation rates of secondary calcite and simultaneously releasing abiotic CO₂to the atmosphere. This study reports one of the first geochemical and isotopic studies of such “anthropogenic” pedogenic carbonates and CO₂from irrigated drylands of southwestern United States. A pecan orchard and an alfalfa field, where flood-irrigation using the Rio Grande river is a common practice, were compared to a nearby natural dryland site. Strontium and carbon isotope ratios show that bulk pedogenic carbonates in irrigated soils at the pecan orchard primarily formed due to flood-irrigation, and that approximately 20–50% of soil CO₂in these irrigated soils is calcite-derived abiotic CO₂instead of soil-respired or atmospheric origins. Multiple variables that control the salt buildup in this region are identified and impact the crop production and soil sustainability regionally and globally. Irrigation intensity and water chemistry (irrigation water quantity and quality) dictate salt loading, and soil texture governs water infiltration and salt leaching. In the study area, agricultural soils have accumulated up to 10 wt% of calcite after just about 100 years of cultivation. These rates will likely increase in the future due to the combined effects of climate variability (reduced rainfall and more intense evaporation), use of more brackish groundwater for irrigation, and reduced porosity in soils. The enhanced accumulation rates of pedogenic carbonate are accompanied by release of large amounts of abiotic CO₂from irrigated drylands to atmosphere. Extensive field studies and modelling approaches are needed to further quantify these effluxes at local, regional and global scales.

    

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2. Investigating Ammonium By-product Removal for Ureolytic Bio-cementation Using Meter-scale Experiments

   https://doi.org/10.1038/s41598-019-54666-1  

   Lee, Minyong ; Gomez, Michael G. ; San Pablo, Alexandra C. M. ; Kolbus, Colin M. ; Graddy, Charles M. R. ; DeJong, Jason T. ; Nelson, Douglas C. ( December 2019 , Scientific Reports)

   Microbially Induced Calcite Precipitation (MICP), or bio-cementation, is a promising bio-mediated technology that can improve the engineering properties of soils through the precipitation of calcium carbonate. Despite significant advances in the technology, concerns regarding the fate of produced NH₄⁺by-products have remained largely unaddressed. In this study, five 3.7-meter long soil columns each containing one of three different soils were improved using ureolytic bio-cementation, and post-treatment NH₄⁺by-product removal was investigated during the application of 525 L of a high pH and high ionic strength rinse solution. During rinsing, reductions in aqueous NH₄⁺were observed in all columns from initial concentrations between ≈100 mM to 500 mM to final values between ≈0.3 mM and 20 mM with higher NH₄⁺concentrations observed at distances furthest from the injection well. In addition, soil V_smeasurements completed during rinse injections suggested that no significant changes in cementation integrity occurred during NH₄⁺removal. After rinsing and a 12 hour stop flow period, all column solutions achieved cumulative NH₄⁺removals exceeding 97.9%. Soil samples collected following rinsing, however, contained significant sorbed NH₄⁺masses that appeared to have a near linear relationship with surrounding aqueous NH₄⁺concentrations. While these results suggest that NH₄⁺can be successfully removed from bio-cemented soils, acceptable limits for NH₄⁺aqueous concentrations and sorbed NH₄⁺masses will likely be governed by site-specific requirements and may require further investigation and refinement of the developed techniques.

    

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3. Shallow Calcium Carbonate Cycling in the North Pacific Ocean

   https://doi.org/10.1029/2022GB007388  

   Subhas, Adam V. ; Dong, Sijia ; Naviaux, John D. ; Rollins, Nick E. ; Ziveri, Patrizia ; Gray, William ; Rae, James W. B. ; Liu, Xuewu ; Byrne, Robert H. ; Chen, Sang ; et al ( May 2022 , Global Biogeochemical Cycles)

   The cycling of biologically produced calcium carbonate (CaCO₃) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situ coccolith and foraminiferal calcite dissolution rates. We combine these rates with solid phase fluxes, dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean. The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution rates of all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods) are too slow to explain the patterns of both CaCO₃sinking flux and alkalinity regeneration in the North Pacific. Using a combination of dissolved and solid‐phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO₃dissolution with a combination of ambient saturation state and oxygen consumption simultaneously explains solid‐phase CaCO₃flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. We do not need to invoke the presence of carbonate phases with higher solubilities. Instead, biomineralization and metabolic processes intimately associate the acid (CO₂) and the base (CaCO₃) in the same particles, driving the coupled shallow remineralization of organic carbon and CaCO₃. The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration within degrading particle aggregates. The coupling of these cycles acts as a major filter on the export of both organic and inorganic carbon to the deep ocean.

    

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4. Juvenile Eastern Oysters More Resilient to Extreme Ocean Acidification than Their Mud Crab Predators

   https://doi.org/10.1029/2020GC009180  

   Dodd, L. F. ; Grabowski, J. H. ; Piehler, M. F. ; Westfield, I. ; Ries, Justin B. ( February 2021 , Geochemistry, Geophysics, Geosystems)

   Ocean acidification is predicted to impair marine calcifiers' abilities to produce shells and skeletons. We conducted laboratory experiments investigating the impacts of CO₂‐induced ocean acidification (pCO₂ = 478–519, 734–835, 8,980–9,567; Ω_calcite = 7.3–5.7, 5.6–4.3, 0.6–0.7) on calcification rates of two estuarine calcifiers involved in a classic predator‐prey model system: adultPanopeus herbstii(Atlantic mud crab) and juvenileCrassostrea virginica(eastern oyster). Both oyster and crab calcification rates significantly decreased at the highestpCO₂level. Notably, however, oysters maintained positive net calcification rates in the highest pCO₂treatment that was undersaturated with respect to calcite, while mud crabs exhibited net dissolution (i.e., net loss of shell mass) in calcite‐undersaturated conditions. Secondary electron imaging of oyster shells revealed minor microstructural alterations in the moderate‐pCO₂treatment, and major microstructural and macrostructural changes (including shell dissolution, delamination of periostracum) in the high‐pCO₂treatment. These results underscore the threat that ocean acidification poses for marine organisms that produce calcium carbonate shells, illustrate the strong biological control that some marine calcifiers exert over their shell‐building process, and shows that ocean acidification differentially impacts the crab and oyster species involved in this classical predator‐prey model system.

    

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5. Constraining CaCO 3 Export and Dissolution With an Ocean Alkalinity Inverse Model

   https://doi.org/10.1029/2022GB007535  

   Liang, Hengdi ; Lunstrum, Abby M. ; Dong, Sijia ; Berelson, William M. ; John, Seth G. ( February 2023 , Global Biogeochemical Cycles)

   Ocean alkalinity plays a fundamental role in the apportionment of CO₂between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO₃) by organisms at the ocean surface and its dissolution at depth. This so‐called “CaCO₃counterpump” is poorly constrained, however, both in terms of how much CaCO₃is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO₃dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y⁻¹(from 73 m), but all converge to 1.0 Gt PIC y⁻¹export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO₃saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column.

    

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