**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


*b Not determined.*

*c Not detectable.*

#### **Table 1.**

*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.*

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).

$$A = \mathbf{e} \, bc \, \tag{1}$$

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).
