Drivers of long-term and short-term Nitrogen concentrations and runoff dynamics in a forested karst catchment

Excess Nitrogen (N) deposition from industrial, domestic and agricultural sources has led to increased nitrate leaching, increased gaseous N emissions, the loss of biological diversity, and has affected C sequestration in forest ecosystems. Nitrate leaching affects the purity of karst water resources, which contribute around 50 % to Austria’s drinking water supply. Here we present the first comprehensive evaluation of a 26 years record of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment in the Austrian Alps (LTER Zöbelboden), which was not affected by local N sources but solely by long-range N deposition (20-25 kg N ha^-1 y^-1 total N deposition). We inferred from soil chemical and microbial data as well as nitrate leaching, that the forest ecosystems in the catchment are likely saturated with respect to nitrogen. Consequently, 60-70% of the atmospheric N input was lost via leaching of NO₃^- to the karst aquifer or emission of N₂O to the atmosphere. However, due to high dilution DIN concentrations in the runoff rarely exceed 2 mg N l^-1. An exception were periods of forest disturbances. A number of strong storms (2007-2008) caused some major windthrows as well as single tree damages (5-10% of the catchment). Runoff concentrations of DIN showed clear responses to the disturbances with an increase (~ 1 mg N l^-1) until 2008/09 and a decreased again in 2010/11 to pre-disturbance levels. Apart from disturbances, drought years led to an increase in NO₃^- in the soil water in the following years. We observed the subsequent changes of the dynamics of DIN in runoff with a high-resolution water probe during 2018 and 2019. This data shows that the severity of the drought and the magnitude of the first rewetting event after a period of drought drives the size of the flush of DIN. It is likely that N deposition will lower with legislated emission reductions and that the currently N leaky ecosystems may immobilize more N when climate is becoming warmer in the future. However, we hypothesize that the karst aquifer will still receive DIN rich runoff water due to long-term lags in the recovery of a closed N cycle and because of expected climate events such as storms and droughts.