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The relationships between the ultraviolet (UV) absorbance at 254 nm and the concentration of dissolved organic matter (DOM) in bulk deposition, throughfall, forest floor solution, and soil solution in 10 cm (A-horizons), 30 cm, and 70 cm (both Bg-horizons) depths of three forested sites in North-Rhine Westphalia, Germany, were investigated over a three-year period. At first effects of pH, Ca2+ and Al3+ on molar absorptivity of DOM from forest floor solution and soil solution were investigated since the compartments differed in these properties. Neither Ca2+ nor Al3+ affected molar absorptivity in the investigated range of 1 to 100 mmolc l−1, but molar absorptivity was affected by pH (pH 3 to 8). However, compared to natural fluctuations of molar absorptivity in the field samples, the effect of pH was negligible. The correlation between UV absorbance and DOM concentration decreased in the order: bulk deposition and throughfall (r2 = 0.82 to 0.92; n = 89 to 105) > forest floor solution (r2 = 0.45 to 0.83; n = 29 to 54) > soil solution (r2 = 0.01 to 0.42; n = 29 to 56). Molar absorptivity was without any relationship to DOM concentration in bulk deposition (r2 = 0.08), throughfall (r2 = 0.01 to 0.06) and most forest floor solutions (r2 = 0.02 to 0.53). However, in soil solutions, DOM concentration and molar absorptivity were negatively correlated and showed a seasonal variation. Dissolved organic matter concentration was highest in summer and, simultaneously, molar absorptivity was lowest. This behavior could be expressed by significantly inverse exponential relationships between DOM concentration and molar absorptivity in the soil solutions of all sites and depths (r2 = 0.54 to 0.91). Seasonal fluctuations in DOM composition preclude the estimation of DOM concentration by UV absorptivity measurements in soil solutions. However, when investigating DOM dynamics in soils, the UV absorbance measurement at 254 nm and the calculation of the molar absorptivity is beneficial in recognizing fluctuations in the composition of DOM.
Research Department, Clarivate Analytics Ecuador, Ecuador
Institute of Organic Chemistry with Centre of Phytochemistry (IOCCP), Bulgarian Academy of Sciences, Bulgaria
Grand Lodge of Ecuador, Ecuador
Order of Athelstan, United Kingdom
Michael Feldman
Research Department, Clarivate Analytics Ecuador, Ecuador
Grand Lodge of Ecuador, Ecuador
Order of Athelstan, United Kingdom
Boston University, USA
*Address all correspondence to: vladimir.dimitrov@investigador.ec
1. Introduction
Compounds with loosely-bound B-electrons or non-bonding n-electrons can absorb energy in the near-ultraviolet region (200 to 380 nm) of the electromagnetic spectrum. Within the molecules of dissolved organic matter (DOM), specific segments or functional groups have this feature. Examples are functional groups containing unbound electrons, and carbon–carbon multiple bonds [1]. Unsaturation and aromaticity express this.
In a quantitative sense, the ultraviolet (UV) absorbance feature of DOM was applied for estimating DOM concentrations in waters. The kinds of samples investigated were: coastal sea water [2], lake water [3, 4], river and stream water [4, 5, 6, 7, 8], treated and untreated waste water [9, 10], peat water [11, 12], precipitation [13], throughfall [6, 13, 14], stemflow [6, 14], soil solution [6, 14], and soil extracts obtained by water or salt [15, 16]. As the absorbance of light by DOM decreases with increasing wavelength, most workers used light in the range of 250 to 330 nm. At wavelengths below 235 nm, nitrate contributes significantly to the total absorbance.
In a qualitative sense, the measurement of the UV absorbing characteristics of DOM was used in environmental studies to assess the propensity of humic substances or even bulk DOM to bind non-polar organic pollutants [17, 18, 19], to evaluate DOM behavior in sorption [20] or degradation experiments [21], to identify the origin or assess the fate of DOM in lake water [22, 23, 24] or sea water [25] and to characterize both total DOM and DOM fractions in wastewater effluents [26]. Furthermore, Weishaar et al. [27] showed the link between aromaticity and absorbance at 254 nm directly using 13C NMR spectroscopy.
Soil solution DOM has been only rarely investigated with respect to its UV absorbance. Therefore, in a field study dealing with the DOM dynamics of forested soils, DOM was analyzed in throughfall, forest floor solution and soil solution at three sites differing in vegetation and soil chemical properties. Differences were reflected in various solution compositions, i.e., varying pH and Ca2+ and Al3+concentrations. Since large seasonal differences in the UV absorbance of DOM were observed for soil solutions, the question arose whether soil solution chemical composition could affect UV absorbing characteristics of DOM.
This study had three objectives. First, to investigate the influence of various solution parameters on UV absorbing characteristics of DOM obtained from different compartments of three forested sites. Second, to check the long-term field relationship between UV absorbance and DOM concentration. Third, to evaluate the benefit of UV absorbance monitoring when investigating DOM dynamics in soils.
Field investigations were conducted in east-central North-Rhine Westphalia, Germany (Figure 1), at three adjacent forest sites within a 600 m radius. One site is stocked with mainly beech (Fagus sylvatica) and oak (Quercus robur), the second site with elm (Ulnus minor x glabra), and the third site with Norway spruce (Picea abies). Soils have developed in thin layers of sandy loess overlying glacial till, which covers underlying Upper Cretaceous limestone. Both the compacted till and the argillaceous limestone act as a water-restrictive layer, causing perched water tables in the subsoils. Soil material has stagnic properties [28], and soils are Stagnosols (beech and spruce site) and Stagnic Cambisols (elm site). The distance of the calcareous layer from the soil surface differed greatly among the sites, about 80 cm at the elm site, 95 cm at the beech site, and 135 cm at the spruce site. As can be seen in Table 1, this is reflected in soil solution parameters, which are sensitive to the presence or absence of calcareous material.
Figure 1.
The study area in North-Rhine Westphalia, Germany.
Range of pH, electrical conductivity and concentrations of Ca2+ and Al3+ during a three-year study in different compartments of three forested sites in North-Rhine Westphalia, Germany.
Electrical conductivity.
Not determined.
Not detectable.
2.2 Sampling
Bulk precipitation was collected in 3 rain gauges in a clearing about 1 km north of the three sites. Throughfall was collected in 5 polyethylene funnels at each site, 315 mm in diameter, placed 1 m above the soil surface, and draining in 2.5 l glass bottles. The funnels were covered with 3-mm mesh polyethylene screens to eliminate large organic debris. Leachates of the forest floor were collected in zero tension lysimeters, each 15 × 23 cm in size and covered with 10 mm quartz wool, with tubes at the base leading into 5 l glass bottles. Three lysimeters were installed at the base of the forest floor horizons at each site. Soil solutions were obtained in triplicate from the A- (10 cm), and Bg-horizons (30 cm and 70 cm) horizons by porous ceramic cups. Samples were obtained at weekly intervals during three years.
2.3 Analytical methods
Prior to analysis, the samples were filtered through pre-washed cellulose nitrate 0.45 μm- membrane filters. All samples were analyzed individually. Total dissolved organic C was determined by high temperature catalytic oxidation. By this procedure, dissolved C was oxidized to CO2 and quantified by a non-dispersive, infrared analyzer. A Shimadzu TOC-5050 analyzer operating at 680°C was used. Dissolved inorganic carbon was measured by quantifying the CO2 generated following phosphoric acid addition and was subtracted from total dissolved C to give DOM. Ultraviolet absorbance was measured at 254 nm in a Perkin Elmer Lambda 2 UV/VIS double beam spectrophotometer in a 1 cm path length quartz cuvette, with de-ionized water as blank. When absorbance exceeded 1.5 (mainly forest floor samples), the sample was diluted with de-ionized water, and re-read. In addition, soil solution was analyzed for electrical conductivity, pH, major cations and anions.
At sufficient low concentration Lambert–Beer‘s law can be applied (1).
A=εbcE1
with absorbance A (dimension less), concentration c (mol l−1), path length b (in cm), and the quantity ε, called molar absorptivity (l mol−1 cm−1). Molar absorptivity of DOM was calculated by rearranging Eq. (1).
2.4 Effect of pH, Ca2+, and Al3+ on molar absorptivity of DOM
In three sampling campaigns, the effects of pH, Ca2+ and Al3+ concentrations on the molar absorptivity of DOM was checked. The effect of pH was studied by using a titration system. To each one 25 ml of filtered sample 0.025 M HCl or NaOH was added until a final pH of 3, 4, 5, 6, 7, and 8 (± 0.1) was established. Addition of acid or base was performed with the titration system 725 Dosimat and pH-meter 691 (Metrohm). Dilution of the samples by adding acid or base was taken into account when calculating final DOM concentration. The influence of Ca2+ and Al3+ concentrations was studied by adding 1 ml salt solution (blank: 1 ml de-ionized water) with known amounts of CaCl2 or AlCl3 to 25 ml sample to give final concentrations of 1, 10, and 100 mmolc l−1. Ultraviolet absorbance was measured and the molar absorptivity was calculated as described above. All experiments were performed in triplicate. By variance analysis it was tested whether differences among the treatments were statistically significant (F-test, p < 0.05).
2.5 Statistical evaluation
All statistical calculations were performed with SPSS 10.0 (Program SPSS Inc., Chicago, U.S.A.).
3.1 Effects of pH, Ca2+, and Al3+ on molar absorptivity of DOM
The effects of changing pH, Ca2+, and Al3+ concentrations on the molar absorptivity of DOM are presented in Table 2. Changing both Ca2+ and Al3+ concentrations did not affect molar absorptivity. In contrast, the influence of pH on molar absorptivity was significant for all DOM solutions. In numerous studies the influence of pH, ionic strength, and various metal ion concentrations on the UV absorbance of humic and fulvic acids was investigated [29, 30, 31, 32]. The effects of pH can be mainly attributed to the ionization of carboxyl groups at pH < 5.5 which cause an increase in absorbance. The effects of changing ionic strength (neutral electrolyte like CaCl2) result in the suppression of the ionization of functional groups, because the particle size of humic substances decreases with increasing ionic strength. The effects observed by addition of different concentrations of metal cations like Al3+ are due to their interactions with functional groups of humic materials which alter their UV spectra when complexed. Additionally, the observed effects of changing Ca2+ concentrations are due to precipitation of strongly absorbing, high molecular weight humic constituents. However, it should be kept in mind that in the studies mentioned above, with the exception of that of Stewart and Wetzel [32], humic and fulvic acids were investigated, not natural DOM. Compared to humic and fulvic acids, natural DOM as used in this study seemed to be much less reactive to changes in Ca2+ and Al3+ concentrations, presumably due to the presence of fewer functional groups. The influence of pH was statistically significant, but compared to natural pH fluctuations which was observed during the course of the field investigation, the order of magnitude in the change of molar absorptivity was negligible. Maximum alteration was not larger than 8% of the measured range (forest floor solution at the spruce site, Table 2). Hence, the fluctuations in the UV absorbance features of DOM that were observed in the field was caused by a change in the composition of DOM and not/or only to a very low extent by varying soil solution composition.
Largest alteration ()) in the molar absorptivity (,) of dissolved organic matter at 254 nm as influenced by different pH and concentration of Ca2+ and Al3+ investigated in laboratory experiments and comparison with ranges of molar absorptivity measured during a three-year field study in different compartments of three forested sites in North-Rhine Westphalia, Germany.
pH was 3, 4, 5, 6, 7, and 8.
Concentrations of Ca2+ and Al3+ were 1, 10, and 100 mmolc l−1.
Not significant.
Not investigated since no solution could be obtained.
3.2 Ultraviolet absorption and DOM concentration
The relationship between UV absorbance and DOM concentration in all compartments of each site is shown in Figure 2. This relationship is not shown for bulk deposition. However, regression equations including those for bulk deposition are presented in Table 3. A linear positive relationship was apparent for bulk deposition as well as for throughfall for all sites (Figure 2; Table 3). The squared correlation coefficients were > 0.81. A weaker relationship was found for the forest floor solutions, especially for the spruce site (Figure 2; Table 3). Here, the squared correlation coefficient was on a low level of 0.45. In contrast to throughfall and forest floor solution, the mineral soil horizons revealed only weak relationships between UV absorbance and DOM concentration (Figure 2; Table 3). Mostly, this correlation was insignificant. In four depths, the correlations were even negative (at the spruce site in 10 and 60 cm, at the beech site in 30 and 60 cm). Generally, the significance in the relationship between UV absorbance and DOM concentration decreased in the order: bulk deposition and throughfall > forest floor solution > soil solution.
Figure 2.
Relationship between ultraviolet absorbance at 254 nm and concentration of dissolved organic matter (DOM) in throughfall (a), forest floor solution (b), and soil solution (c) of an elm, a beech, and a spruce forested site in North Rhine-Westphalia, Germany. Soil solution was obtained in 10 cm (○), 30 cm (∆), and in 70 cm (□) soil depth.
Regression analysis between ultraviolet absorbance at 254 nm (x, 1 cm cuvette) and concentration of dissolved organic matter (y, in mmol C l−1) in different compartiments of three forested sites in North Rhine-Westphalia, Germany.
Number of samples.
Squared regression coefficient.
Probability of error.
For surface waters, many studies have shown linear positive relationships between UV absorbance and DOM concentration as measured by chemical or UV oxidation methods or by high temperature combustion (see references cited in the Introduction). Throughfall and soil waters were only rarely investigated. Slightly higher or similar correlation coefficients as obtained in this study have been reported for throughfall at other forest soils in North-Rhine Westphalia [13], and eastern Austria [14]. Brandstetter et al. [14] reported a very strong relationship between DOM concentration and UV absorbance in soil solutions of forest sites (r2 ranged from 0.92 to 0.93). Very different relationships between DOM concentration and UV absorbance (at 330 nm) were given by Moore [6] for soil solutions in New Zealand. The squared correlation coefficients were 0.54, 0.63, and 0.92. Compared with Moore [6] and Brandstetter et al. [14], correlation coefficients calculated in this study were substantially lower for soil solutions. Brandstetter et al. [14] concluded that DOM content may be estimated relatively accurately by UV absorbance measurements. Their conclusion, however, can be confirmed only for bulk deposition, throughfall, and partly for forest floor solution but not for soil solution.
3.3 Molar absorptivity and DOM concentration
A prerequisite for estimating DOM concentrations by UV absorbance measurements is that the UV absorbing features of DOM do not vary with time for the solution investigated. A measure for the UV absorbing feature of DOM is the molar absorptivity. The relationship between DOM concentration and molar absorptivity for the compartments of each site is presented in Figure 3, and the statistical computation is given in Table 4 including those for bulk deposition. As can be seen from Figure 3 and Table 4, the relationship between DOM concentration and molar absorptivity was very weak for bulk deposition, for throughfall and for forest floor solution at almost all sites. In other words, the molar absorptivity did not depend significantly on DOM concentration during the course of the investigation. Hence, DOM from these compartments showed a relatively uniform UV absorbing feature. In opposite, the mineral soil horizons revealed a unique relationship not reported previously (Figure 3; Table 4): Concentrations of DOM and molar absorptivity were significantly negatively correlated, which could be best described by inverse exponential regression equations with squared correlation coefficients ranging from 0.54 to 0.91. Thus, molar absorptivity clearly depends on DOM concentration. The higher is the DOM concentration, the lower is the molar absorptivity and vice versa. Consequently, a strong variation in the composition of DOM in relation to its UV absorbing features existed in the mineral soils horizons. Notably, this was valid for all mineral horizons, independent of vegetation and soil properties.
Figure 3.
Relationship between molar absorptivity at 254 nm and concentration of dissolved organic matter (DOM) in throughfall (a), forest floor solution (b), and soil solution (c) of an elm, a beech, and a spruce forested site in North Rhine-Westphalia, Germany. Soil solution was obtained in 10 cm (○), 30 cm (∆), and in 70 cm (□) soil depth.
Regression analysis between concentration of dissolved organic matter (x, mmol C l−1) and molar absorptivity at 254 nm (y, 1 mol−1 cm−1) in different compartiments of three forested sites in North-Rhine Westphalia, Germany.
Number of samples.
Squared regression coefficient.
Probability of error.
Tipping et al. [4] found that, in lakes waters during summer, the production of non-absorbing (340 nm) DOM by phytoplankton lowered molar absorptivity of the lake water DOM. During winter, in stream water more DOM was present in a form which absorbs at 250 nm [8]. This was attributed to lower microbial activity, among other things, since microbes produce low molecular mass aliphatics with less UV absorbing features. If microorganisms in soils also produce preferentially non-absorbing DOM, then microbial growth and resultant excretion and lysis of cells should result in lower values of molar absorptivity during the summer season. It is beyond the scope of this paper to elucidate all aspects of the DOM dynamics in these soils, but it should be mentioned that for the mineral soil horizons the DOM concentrations peaked in summer and molar absorptivity was lowest. Vice versa, during winter, a time with low microbial activity, DOM concentrations were lowest and molar absorptivity was highest in soil solution. Thus, the varying composition of DOM within the soil compartments seemed to be at least partly due to microbially controlled processes. This has an important implication in that strong fluctuations in DOM composition preclude the estimation of DOM content by UV absorbance measurement. However, as the molar absorptivity depends on molecular size and aromaticity of DOM [26, 33, 34], the monitoring of the UV absorbing features of DOM is a simple but meaningful tool when investigating DOM dynamics in soils.
In contrast to bulk deposition and throughfall, DOM concentrations in forest floor solution and especially mineral soil solution cannot be estimated by UV spectrometry at 254 nm. This is caused by strong seasonal fluctuations of the molar absorptivity of DOM. However, for DOM monitoring studies the UV absorbance measurement at 254 nm is a suitable method for recognizing fluctuations in the composition of soil DOM.
The author is grateful to Bernd Köhler, Bernd Steinweg, Lutz Huischen, Timo Kluttig, Thilo Rennert and Kai Gockel for field and laboratory assistance. Laboratory assistance was also given by Heidi Biernath, Willi Gosda and Gerlind Wilde. Technical support was provided by Heinrich Wolfsperger and Dr. Norbert Krahmer, Geologischer Dienst Nordrhein-Westfalen, Germany.
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Written By
Vladimir Dimitrov and Michael Feldman
Submitted: 03 March 2021Reviewed: 11 June 2021Published: 14 December 2022
Open access peer-reviewed chapter
