Joint analyses of nitrate transit time distributions and legacy effects in catchments with contrasting physical settings in Germany

Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have rarely led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) –or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Both legacy types determine the temporal dimension of catchment response on the one hand and the quantitative dimension on the other hand.

Here we analyze several decades of N input, water quality and discharge observations from 62 catchments in 8 federal states in Germany. The selection of catchments represents a wide range of land use, geology and soils, topography and hydroclimate. In an input-output assessment, N input from atmospheric deposition, waste water treatment and agriculture is compared with riverine N concentrations (nitrate-N) as N output. We assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the C–Q relationships, we evaluate the potential for and the characteristics of an N legacy.

Our data-driven approach shows a mean legacy of 73 % (spanning 0 – 90 %), cumulating to a total missing mass of 4270 kg N/ha a. Log-normal distributed TTs have a mean of 6 years (0.8 – 34 years) with an R² of 89 % between the convolved N input and N output. Due to the chemostatic export regime (mean CV_C/CV_Q: 0.36 < 0.5) and relatively short TTs in most of the catchments, the biogeochemical legacy seems to dominate the catchment responses nowadays. Further analyses aim to investigate the controlling parameters determining the N time lags and legacies type. A correlation analyses hint to topographic parameters, mainly slope and topographic wetness index, as main controls of the legacy i.e. that flat catchments have the tendency to higher legacies or retention.

Legacies of almost ¾ of the N input pose a challenge to the limited denitrification potential of soils and aquifers or indicate a massive N accumulation in the catchment. Latter can cause elevated N concentrations for the next decades explaining at the same time a limited response to measures. The dominant biogeochemical legacy suggests that management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of past high N loads by favoring denitrification.