Abstract Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light attenuation. Because absorption by DOM depends on its composition, UV–Vis absorbance is used to constrain DOM composition, source, and amount. Ferric iron, Fe(III), also absorbs in the UV–Vis; when Fe(III) is present, DOM-attributed absorbance is overestimated. Here, we explore how differing behavior of DOM and Fe(III) at the catchment scale impacts UV–Vis absorbance and evaluate how system-specific variability impacts the effectiveness of existing Fe(III) correction factors in a temperate watershed. We sampled five sites in the Connecticut River mainstem bi-weekly for ~ 1.5 years, and seven sites in the Connecticut River watershed once during the summer 2019. We utilized size fractionation to isolate the impact of DOM and Fe(III) on absorbance and show that variable contributions of Fe(III) to absorbance at 254 nm (a 254 ) and 412 nm (a 412 ) by size fraction complicates correction for Fe(III). We demonstrate that the overestimation of DOM-attributed absorbance by Fe(III) is correlated to the Fe(III):dissolved organic carbon concentration ratio; thus, overestimation can be high even when Fe(III) is low. a 254 overestimation is highly variable even within a single system, but can be as high as 53%. Finally, we illustrate that UV-Vis overestimation might impart bias to seasonal, discharge, and land-use trends in DOM quality. Together, these findings argue that Fe(III) should be measured in tandem with UV–Vis absorbance for estimates of CDOM composition or amount.
more »
« less
Differentiation Between Humic and Non-Humic Substances Using Alkaline Extraction and Ultraviolet Spectroscopy
Although humic substances are the principal ingredients in processed humic products, there has been no practical way to determine if a material is humified, allowing fake products to be used by farmers instead of genuine humic substances.
To develop a test method using conventional laboratory techniques to determine if a material is humified.
A neutralized extract is prepared using the standardized extraction protocols specified in ISO 19822:2018(E). A portion of the extract is used to determine the concentration of dissolved organic matter on an ash-free basis. A portion of the remaining neutralized extract is diluted to a concentration of 30 mg/kg of dissolved organic matter and transferred to a quartz UV cuvette for ultraviolet-visible (UV-Vis) spectroscopy. UV-Vis absorbance is recorded over a wavelength range of 220–500 nm at 5 nm intervals. The absorbance data are normalized by conversion to scaled absorbance, which is compared to a reference scaled absorbance spectral curve for humic substances to determine if the tested material is humic or non-humic.
This method was able to differentiate legitimate humic substances from non-humic adulterants in a multiple-laboratory validation study (P ≤ 0.05).
This method can differentiate humic from non-humic substances in materials intended to be used as ingredients in commercial humic products or for research.
This method uses common laboratory procedures and equipment.
- NSF-PAR ID:
- 10396562
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of AOAC INTERNATIONAL
- ISSN:
- 1060-3271
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Large rivers are the main arteries for transportation of carbon to the ocean; yet, how hydrology and anthropogenic disturbances may change the composition and export of dissolved organic matter along large river continuums is largely unknown. The Yangtze River has a watershed area of 1.80 × 106 km2. It originates from the Qinghai‐Tibet Plateau and flows 6300 km eastward through the center of China. We collected samples (
n = 271) along the river continuum and analyzed weekly samples at the most downstream situated gauging station in 2017–2018 and gathered long‐term (2006–2018) water quality data. We found higher gross domestic product, population density, and urban and agricultural land use downstream than upstream of the Three Gorges Dam, coinciding with higher dissolved organic carbon (DOC), UV absorption (a 254), specific ultraviolet absorbance (SUVA254), parallel factor analysis‐derived C1–C5, aliphatic compounds, and lowera 250:a 365and spectral slope (S 275–295). Chemical oxygen demand, humic‐like C1–C2 and C6, and protein‐like C4 and C7 increased, while dissolved oxygen and ammonium decreased with increasing discharge at most of the sites studied, including the intensively monitored downstream site. The annual DOC fluxes were ca. 1.5–1.8 Tg yr−1, and 12–18% was biodegradable in a 28‐d bio‐incubation. Our results highlight that urbanization and stormwater periods enhanced the export of both terrestrial organic‐rich substances and household effluents from nearshore residential areas. Our study emphasizes the continued need to protect the Yangtze River watershed as increased organic carbon loading or altered composition and bio‐lability may change the ecosystem function and carbon cycling. -
more » « less
Abstract Dissolved organic carbon (DOC) flux from rivers in the pan‐Arctic watershed represents an important connection between major terrestrial carbon stocks and the Arctic Ocean. Previous estimates of Arctic carbon flux and dissolved organic matter (DOM) seasonal dynamics have relied predominantly on measurements from the six major Arctic rivers, yet these may not be representative of northern high‐latitude constrained smaller watersheds. Here, we evaluate DOC concentration and DOM composition in the Onega River, a small Arctic watershed, using optical measurements and ultrahigh resolution mass spectrometry. Compared to the six largest Arctic rivers, DOC, absorbance at
a 350, and indicators of terrestrial DOM (e.g., specific UV absorbance at 254 nm, modified aromaticity index, relative abundance of condensed aromatics and polyphenolics) were elevated in the Onega throughout the year. Seasonality was also generally muted in comparison to the major Arctic rivers with relatively elevated DOC and terrestrial markers in both spring and fall seasons. The Onega exhibits a strong relationship betweena 350and DOC, and its organic‐rich nature is apparent in its high DOC yield (4.85 g m2yr−1), and higher chromophoric DOM per unit DOC than the six largest Arctic rivers. As DOC yield from the Onega may be more representative of smaller northern high‐latitude rivers, we derived a new pan‐Arctic DOC flux scaling estimate which is over 50% higher than previous estimates scaled solely from the six major Arctic rivers. These observations suggest that smaller northern high‐latitude rivers may be underrepresented in Arctic carbon flux models and highlights uncertainty around constraining the export of DOC to the Arctic Ocean. -
more » « less
Abstract Arctic rivers are sensitive to climate and environmental change, but the biogeochemical response remains poorly understood. Monthly size‐fractionated dissolved organic matter (DOM) samples from the lower Yukon River were characterized using UV–visible, fluorescence, and Fourier transform‐infrared (FT‐IR) spectroscopy techniques. The EEM‐PARAFAC analysis revealed three major fluorescent DOM components, including two humic‐like components (C480and C400) and one protein‐like component (C310), with their relative importance following the order of C480 ≥ C400 > C310in the high‐molecular‐weight DOM (1 kDa–0.4
μ m) and C400 > C480 > C310in the low‐molecular‐weight DOM pool (< 1 kDa). Transformation in DOM and change in sources were manifested in major fluorescent components and optical properties, including biological index (BIX), humification index (HIX), spectral slope (S 275–295) and specific UV absorbance at 254 nm (SUVA254). These changes occurred within different DOM size‐fractions and among ice‐covered, spring freshet, and open seasons. Joint analysis of EEM and FT‐IR spectra using a data fusion technique showed that humic‐like DOM is mostly associated with C─H, C═C, and C─O bonds, while protein‐like DOM is correlated more with C─N and N─H related structures. DOM aromaticity and the ratios of HIX to BIX and protein‐like to humic‐like components may be used as a compelling proxy to measure change in source waters and to infer permafrost dynamics. Our results provide insight into the seasonal variation in DOM composition for different size‐fractions in the lower Yukon River, and a baseline dataset against which future changes can be understood in the context of arctic basin biogeochemical cycling. -
more » « less
Abstract Tidal wetlands are a significant source of dissolved organic matter (DOM) to coastal ecosystems, which impacts nutrient cycling, light exposure, carbon dynamics, phytoplankton activity, microbial growth, and ecosystem productivity. There is a wide variety of research on the properties and sources of DOM; however, little is known about the characteristics and degradation of DOM specifically sourced from tidal wetland plants. By conducting microbial and combined UV exposure and microbial incubation experiments of leachates from fresh and senescent plants in Chesapeake Bay wetlands, it was demonstrated that senescent material leached more dissolved organic carbon (DOC) than fresh material (77.9 ± 54.3 vs 21.6 ± 11.8 mg DOC L−1, respectively). Degradation followed an exponential decay pattern, and the senescent material averaged 50.5 ± 9.45% biodegradable DOC (%BDOC), or the loss of DOC due to microbial degradation. In comparison, the fresh material averaged a greater %BDOC (72.6 ± 19.2%). Percent remaining of absorbance (83.3 ± 26.7% for fresh, 90.1 ± 10.8% for senescent) was greater than percent remaining DOC, indicating that colored DOM is less bioavailable than non-colored material. Concentrations of DOC leached, %BDOC, and SUVA280 varied between species, indicating that the species composition of the marsh likely impacts the quantity and quality of exported DOC. Comparing the UV + microbial to the microbial only incubations did not reveal any clear effects on %BDOC but UV exposure enhanced loss of absorbance during subsequent dark incubation. These results demonstrate the impacts of senescence on the quality and concentration of DOM leached from tidal wetland plants, and that microbes combined with UV impact the degradation of this DOM differently from microbes alone.
