**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 [4–8], 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 [17–19], to evaluate DOM behavior in sorption [20] or degradation experiments [21], to identify the origin or assess the fate of DOM in lake water [22–24] 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.

## **2. Material and methods**

#### **2.1 Site description**

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.

*Dissolved Organic Matter and Its Ultraviolet Absorbance at 254 Nm in Different Compartments… DOI: http://dx.doi.org/10.5772/intechopen.98861*

**Figure 1.** *The study area in North-Rhine Westphalia, Germany.*
