title: Transport times and retention as tools to understand temporal and spatial nitrogen dynamics in catchments
Creators: name: Ehrhardt, Sophie
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Nitrate
subject: Hydrology
subject: Travel times
subject: Catchment
subject: Water quality
subject: Agriculture
subject: 550

description: Excess nitrogen (N) deteriorates water quality and causes eutrophication of surface waters. Regulations to reduce anthropogenic N inputs to the biosphere often show no or only delayed effects in receiving surface waters, hinting at large legacy N stores built up in soils (biogeochemical legacy) and groundwater (hydrological legacy). However, due to the high complexity of interacting hydrological and biogeochemical processes in catchments, legacy stores are poorly understood. This PhD project aims to advance the knowledge about these N legacy stores across a large variety of European catchments. The main objective was a quantification and separation of N retention mechanisms in catchments: legacy stores and removal via denitrification, to contribute to a more effective and science-based water quality management. To this end, the PhD work was designed in three studies. Although facing differences in data availability and testing different methods, all studies associated long-term N inputs with observed riverine nitrate concentrations and loads. A key method applied here, was the adaption of the travel time concept (Kirchner et al. 2000) to infer catchment-scale N transport times from time series of diffuse N input and riverine N output. The first study (Study 1) focused on the Central German Holtemme catchment, which is divided into three nested subcatchments with an increasing agricultural impact. Due to this gradient and an agricultural history with strong temporal shifts in fertilizer usage, the catchment was well suited to enable a better process understanding of N transport through subcatchments. In the second study (Study 2) long-term data from 16 French catchments were investigated to determine legacy stores and their controls. Study 2 used the Generalized Likelihood Uncertainty Estimation (GLUE) approach for the quantification in the hydrologic N transport timescales. By merging findings and methods from Study 1 and 2, Study 3 advanced the knowledge about the temporal and spatial variation of N retention mechanisms. In a large-scale setting, retention mechanisms and their dominant controls were analyzed across 238 diverse western European catchments. The catchment in Study 1 showed an overall N retention of 88 %. The log-normal transfer functions, fitted to the input-output data from different subcatchments and seasons, yielded highest N exports 7 – 22 years after the N input. In line with an inter-annual shift in concentration-discharge relations observed in the stream, Study 1 concluded a dominant hydrologic legacy. The temporal riverine N dynamics were dominantly shaped by a seasonally changing activation of differently loaded subsurface flow paths. Based on the identified dominance of hydrologic N legacy, the study suggested measures that trigger denitrification to avoid future massive release of N that is still present in the groundwater. In the western French catchments (Study 2), 45 – 88 % of the diffuse N input was retained within the catchments, two-thirds of which were attributed to storage in the topsoil. Although this biogeochemical legacy in the soil was different to the hydrologic N legacy in the Holtemme catchment (Study 1), the overall high retention was comparable in both studies. While differences in N retention between the catchments could be explained by differences in average runoff, the varying peak exports (2 – 14 years after N input) were partly attributable to spatial differences in the lithology. Due to the N accumulation in the soil, the study suggested that catchment management should address the recycling of the stored soil N through agroecosystem practices. The large-scale analysis in Study 3, conducted across diverse catchment settings, showed a median of peak exports of 5 years after the input (up to 34 years) with longer transport times being evident in catchments with high potential evapotranspiration and low precipitation seasonality. On average, almost three quarters of the diffuse N input were retained in the catchments with a retention efficiency that was specifically high in catchments with low specific discharge and thick, unconsolidated aquifers. This study confirmed a wide-spread presence of strong N retention across western European catchments. Given the overall short transport times but high retention of N, Study 3 concluded a dominance of biogeochemical legacy rather than groundwater N storage or substantial N removal via denitrification in catchments across western Europe. In line with Study 2, management should aim to recycle accumulated soil N to prevent long-term leaching, which poses a risk to aquatic ecosystems. In summary, the three studies advanced the knowledge of the N dynamics and their spatial variation at catchment scale, relevant to management. By the use of differing and partly refined methods, the data driven input-output-assessment highlighted the interplay between biogeochemical processes, hydrological transport and anthropogenic activities. This improved knowledge on catchment response and its controls is useful in developing strategies for more effective water quality management. Along those lines, this PhD project proposed methods to assess the most promising locations for the implementation of measures. Based on this, catchment managers can develop science-based concepts to efficiently tackle the N legacy components and to curb current N losses to rivers.