A novel analytical approach for the simultaneous measurement of nitrate and DOC in soil water

Abstract

Abstract. In this paper, we present a novel approach, enabling the measurement of nitrate concentrations in natural soil porewater containing natural soil dissolved organic carbon (DOC). The method is based on UV absorbance spectroscopy, combined with fluorescence spectroscopy, for simultaneous analysis of DOC and nitrate concentrations. The analytical procedure involves deduction of the absorption caused by the DOC from the total absorbance in the UV range that is attributed to both DOC and nitrate in the water solution. The analytical concept has been successfully tested in soil water samples obtained from five agricultural sites, as well as in water samples obtained from a commercial hummus soil mixture. We believe that the new analytical concept can provide a scientific foundation for developing a sensor for real-time nitrate concentration measurements in agricultural soils. As such, it can play a significant role in reducing nitrate pollution in water resources, optimizing input application in agriculture, and decreasing food production costs.

Answers to RC 3

“…Reply to comment 6: The presented methods require site-specific calibration, to overcome the interference on the nitrate's absorbance spectrum, caused by the local DOC chemical composition in the soil. As shown in the previous work of Yeshno et al, (2019) the optical characteristics of DOC may differ from site to site, and as such compensation of interference from DOC cannot be associated only with its concentration and should account for its local chemical and optical characteristics as well. Most of the known standard, common methods to deal with the absorption caused by DOC (as detailly described under reply to comment 2 and comment 5) do not provide a site-specific solution for the local DOC, and as such was less suitable for in-situ, porewater analyses.…”

“…Lastly, none of the presented methods mentions the necessity for site-specific calibration. As shown previously in a research made by the author of this paper, due to the variability in the optical absorption characteristics of the DOC found at different agricultural sites, a site-specific calibration is required to perform adequate calibration for nitrate (Yeshno et al, 2019). The method described is based on preliminary sample chemical analyses of the DOC found in each site porewater, and obtaining a calibration equation that is dealing with the absorption/fluorescence characteristic found at each study site.…”

“…The authors applied a combination of UV absorption spectrometry (around 300 nm) with fluorescence spectrometry (excitation/emission at 350/451 nm) to demonstrate the possible feasibility of this setup when applied with single banded LED technology. The study seems to be embedded in a research project or development program, which delivered already an interesting publication with regard to a possible application of the method (Yeshno et al 2019, HESS (within the reference list this citation misses the journal. However, I have some points that need to be discussed prior to the editors decision about a possible publication in HESS.…”

Answers to RC 2

“…This data is under preparation and we hope to publish it soon. Note that lengthy field observations of nitrate measurement by spectral analyses when reduction of DOC interference was done by a site-specific algorithm, showed that once calibration was obtained it remained valid for a prolonged time interval (2 years) (Yeshno et al, 2019). This implies that the chemical composition of the DOC and thus its spectral interference in the UV range fairly stable over time, and as such allow continuous reading of nitrate by spectral techniques.…”