Graduation thesis analyzes changes in nitrate mass balance in a forested watersheds. The nitrate mass balance changes occur manly because of hydrological and biogeochemical mechanisms. The studied area, the Padež stream watershed, is mainly covered with forest. Between years 2006 and 2007 the hydrometeorological conditions and streamwater chemistry of Padež stream were continuously monitored. The differences in streamwater nitrate concentrations and nitrate concentrations in precipitations and measurements of precipitations and Padež stream discharge helped us to calculated nitrate mass balance. Mass balance is analysed as a difference between inputs and outputs of nitrate per hectare of catchment area. The seasonal differences in mass balance were observed. Watershed behaves as a sink or as a source of nitrate depending on hydrological conditions and amount of mobile nitrate in soil and its concentrations in precipitations. We also try to describe the impact of global environmental changes on biogeochemical cycling of nitrate and variations in its mass balance in forested catchments.