Decadal trajectories of nitrate input and output in three nested catchments along a land use gradient

Abstract

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 10 been implemented to reduce N-inputs especially from agricultural sources, they have not always led to decreasing riverine nitrate concentrations and loads. The limited response to the measures can either be caused by the accumulation of slowly mineralized organic N in the soils acting as a biogeochemical legacy or by long travel times (TTs) of inorganic N to the streams forming a hydrological legacy. Both types of legacy are hard to distinguish from the TTs and N budgets alone. Here we jointly analyze atmospheric and agricultural N inputs with long-term observations of nitrate concentrations and discharge 15 in a mesoscale catchment in Central Germany. For three nested sub-catchments with increasing agricultural land use, we assess the catchment scale N budget, the effective TT of N. In combination with long-term trajectories of C-Q relationships we finally evaluate the potential for and the characteristics of an N-legacy. We show that in the 42-year-long observation period, the catchment received an N-input of 42 758 t, of which 97 % derived from agricultural sources. The riverine Nexport sums up to 6 592 t indicating that the catchment retained 85 % of the N-input. Removal of N by denitrification could 20 not fully explain this imbalance. Log-normal travel time distributions (TTD) for N that link the input history to the riverine export differed seasonally, with modes spanning 8-17 years. Under low-flow conditions, TTs were found to be systematically longer than during high discharges. Systematic shifts in the C-Q relationships could be attributed to significant changes in N-inputs resulting from agricultural intensification and the break-down of the East German agriculture after 1989 and to the longer travel times of nitrate during low flows compared to high flows. A chemostatic export regime of 25 nitrate was only found after several years of stabilized N-inputs. We explain these observations by the vertical migration of the N-input and the seasonally changing contribution of subsurface flow paths with differing ages and thus differing N-loads.The changes in C-Q relationships suggest a dominance of hydrological N-legacy rather than a biogeochemical N-fixation in the soils, which should result in a stronger and even increasing dampening of riverine N-concentrations after sustained high N-inputs. Despite the strong N-legacy, a chemostatic nitrate export regime is not necessarily a persistent endpoint of intense 30 agricultural land use, but rather depends on a steady replenishment of the mass of N propagating through the catchments subsurface. The input-output imbalance, the long time-lags and the lack of significant denitrification in the catchment let us Hydrol. Earth Syst. Sci. Discuss., https://doi.

Disentangling the Impact of Catchment Heterogeneity on Nitrate Export Dynamics From Event to Long‐Term Time Scales

“…Nested catchment studies are a promising approach to shedding light on the contribution of sub-catchments to nitrate export (e.g. Dupas et al, 2017;Ehrhardt et al, 2019). They enable us to analyze changes in nitrate transport along the river, to connect these changes to the specific characteristics of upstream and downstream sub-catchments, and to interpret the integrated observations of concentration, discharge (Q) and loads at the catchment outlet.…”

“…While these long-term, low-frequency data are appropriate for the identification of long-term trends, TTs and legacy stores (e.g. Ehrhardt et al, 2019;Hirsch et al, 2010), the analysis of event dynamics can only be conducted with high-frequency data (Burns et al, 2019). The time scale of single events, however, is especially important for the analysis of nitrate dynamics, because most of the annual nitrate load to the stream is transported during events (Bernal et al, 2002;Inamdar et al, 2006).…”

“…N input into the Selke catchment was calculated following the procedure described by Ehrhardt et al (2019). Here, N input refers to N surplus as the sum of three different input classes: i) agricultural N surplus, ii) atmospheric N deposition, and iii) N input from WWTPs, where i) and ii) are diffuse sources and iii) is a point source.…”

“…Apparent transit time distributions (TTDs) for nitrate were calculated by applying a methodology described by Musolff et al (2017) and Ehrhardt et al (2019). We assumed a lognormal form for the TTDs because this allows us to account for the long tails in the TTD needed to adequately reflect legacy effects.…”

“…Determining which form of legacy the missing N can be contributed to, is challenging (Van Meter & Basu, 2015, Van Meter et al, 2016 and beyond the scope of this study. Nevertheless, long-term trends in CQ relationships together with pronounced changes in N-input can give first indications on their contribution (Ehrhardt et al, 2019). In respect to hydrological legacies, a decline in N input can cause a temporal increase in the concentration heterogeneity belowground (e.g.…”