Effect of forest disturbances of different origin on the migration of ammonium and nitrate in a montane catchment (Tatra Mountains, Poland)

• Forest disturbances strongly influence the concentration of nitrate in the stream water of nitrogen-saturated catchments. • The main source of NO 3 − −N in bark beetle-deforested catchments is snow cover. • Secondary succession in wind-deforested catchments mitigates the leaching of NO 3 − N. • Nitrification processes occurring in soils are of minor significance. • Ammonification is the main source of NH 4+ -N in soils and stream water. The Tatra Mountains, the highest range of the Carpathians, have recently experienced extensive forest disturbances, including large-scale windthrow and subsequent bark beetle outbreaks. These ecosystems are particularly vulnerable to nitrogen (N) saturation due to elevated atmospheric N deposition, making them representative of many regions worldwide. In 2013, a major windthrow event occurred in the western part of the Tatras, followed by widespread bark beetle infestation. The present study investigates the response of mineral N concentrations in soil and stream water to a natural sequence of forest disturbances across three small catchments susceptible to N saturation: a forested catchment (F), a bark beetle–affected catchment (BB), and a windthrow-affected catchment undergoing secondary succession (WD). The highest concentrations of nitrate N (NO 3 − -N) in soils and stream waters were observed in spring, indicating that snowmelt is the primary driver of NO 3 − -N mobilization during this period. However, the studied catchments exhibited contrasting responses to this seasonal N surplus. In the F and WD catchment, respectively, forest and a dense cover of shrubs and grasses efficiently assimilates NO 3 − -N from the soil, thereby limiting its transport to surface waters. In contrast, the BB catchment showed the highest NO 3 − -N concentrations in stream water, likely due to the cessation of N uptake by dying trees. In this catchment, NO 3 − -N levels in stream water reached values that pose a potential risk of eutrophication. Nitrification does not appear to be a major process supplying NO 3 − -N to the soils. Instead, the results suggest that N immobilization may play a key role in limiting NO 3 − -N leaching in spring. Elevated ammonification rates in the A horizons of soils in both disturbed catchments (BB and WD) may be the primary source of ammonium N (NH 4 + -N) detected in stream water during autumn. However, NH 4 + -N contributes only marginally to overall N losses from the studied catchments.