**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.

#### **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.*


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

*a Number of samples.*

*b Squared regression coefficient.*

*c Probability of error.*

#### **Table 4.**

*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.*

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.

*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.
