Dissolved Organic Matter and Its Ultraviolet Absorbance at 254 Nm in Different Compartments of Three Forested Sites

Abstract

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.