**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 [29–32]. 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.

#### **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


*a pH was 3, 4, 5, 6, 7, and 8.*

*b Concentrations of Ca2+ and Al3+ were 1, 10, and 100 mmolc l−1. <sup>c</sup>*

*Not significant.*

*d Not investigated since no solution could be obtained.*

#### **Table 2.**

*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.*

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.

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 (*r* 2 ranged from 0.92 to 0.93). Very different relationships between DOM concentration *Dissolved Organic Matter and Its Ultraviolet Absorbance at 254 Nm in Different Compartments… DOI: http://dx.doi.org/10.5772/intechopen.98861*

#### **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.*


*a Number of samples.*

*b Squared regression coefficient.*

*c Probability of error.*

#### **Table 3.**

*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.*

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.
