Dissolved organic matter (DOM) participates in numerous processes in soils, e.g. mineral weathering and transport of nutrients and pollutants. Nevertheless, DOM can be regarded as an easily mineralizable nutrient source not only for plants, but also for microorganisms. The C and N content of DOM (dissolved organic carbon, DOC and dissolved organic nitrogen, DON) is also important parameter of soils from point of view carbon and nitrogen cycling. Changes in DOC and DON content, as well as DOC/DON ratio of an acidic Arenosol were investigated in a pot experiment during two growing periods using oat as indicator plant. Six soil samples were taken during the investigation: in May, June and July in 2000, and in 2001, respectively. The results can be summarized as follows: (i) The DOC concentration monotonous decreased in the first year from 274 mg/kg to 159 mg/kg, whereas in 2001 the highest DOC content was measured in June (362 mg/kg). (ii) There was a decrease in DON content of the soil: the initial value (5.9 mg/kg) declined to 4.4 mg/kg in June, and 5.0 mg/kg in July in 2000. Similar to this in 2001 in the beggenning of the growing period the DON concentration was 5.6 mg/kg which reduced to 3.8 mg/kg. (iii) The DOC/DON ratio was the highest in June in both years – 58 and 91, respectively. It may be because that the N mineralization rate the highest in June, too. In May and July the values of ratio were 45 and 35 in 2000, and 32 and 43 in 2001.