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 few 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 not always 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). Here, we compare atmospheric and agricultural N inputs with longterm observations of riverine nitrate concentrations and loads in a central German mesoscale catchment with three nested subcatchments of increasing agricultural land use. Based on a data-driven approach, 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.We show that in the 40-year-long observation period, the catchment (270 km 2 ) with 60 % agricultural area received an N input of 53 437 t, while it exported 6592 t, indicating an overall retention of 88 %. Removal of N by denitrification could not sufficiently explain this imbalance. Log-normal travel time distributions (TTDs) that link the N input history to the riverine export differed seasonally, with modes spanning 7-22 years and the mean TTs being systematically shorter during the high-flow season as compared to low-flow conditions. Systematic shifts in the C-Q relationships were noticed over time that could be attributed to strong changes in N inputs resulting from agricultural intensification before 1989, the break-down of East German agriculture after 1989 and the seasonal differences in TTs. A chemostatic export regime of nitrate was only found after several years of stabilized N inputs. The changes in C-Q relationships suggest a dominance of the hydrological N legacy over the biogeochemical N fixation in the soils, as we expected to observe a stronger and even increasing dampening of the riverine N concentrations after sustained high N inputs. Our analyses reveal an imbalance between N input and output, long timelags and a lack of significant denitrification in the catchment. All these suggest that catchment management needs to address both a longer-term reduction of N inputs and shorterterm mitigation of today's high N loads. The latter may be covered by interventions triggering denitrification, such as hedgerows around agricultural fields, riparian buffers zones or constructed wetlands. Further joint analyses of N budgets and TTs covering a higher variety of catchments will provide a deeper insight into N trajectories and their controlling parameters.

Predicting Nutrient Incontinence in the Anthropocene at Watershed Scales

“…Based on our analysis of 49 watersheds, 27 of which had longterm nutrient and discharge estimates, hydrological properties set the initial attenuation capacity, with secondary effects from biogeochemical conditions and land-use parameters. These results corroborate findings from other regions where runoff strongly controls nutrient attenuation and flux (Covino et al, 2010;Zarnetske et al, 2018;Ehrhardt et al, 2019). However, we point out that runoff is a high-level parameter that interacts FIGURE 7 | Correlation analysis of N attenuation metrics and biogeochemical proxies.…”

“…Nutrient retention and removal (hereafter "attenuation") involve multiple hydrological and biogeochemical processes that are difficult to characterize because many of them are not directly observable due to long timescales or inaccessibility (e.g., groundwater processes) (Aquilina et al, 2018;Kolbe et al, 2019), 4-dimensional variation in subsurface characteristics (Sebilo et al, 2013;Musolff et al, 2015), and nutrient legacies (Van Meter and Basu, 2017;Ehrhardt et al, 2019). Consequently, to identify the ecological drivers of nutrient attenuation at watershed scales, we selected tracers or proxies that could be associated with hydrological flowpath, residence time, and biogeochemical reactions (Pinay et al, 2015;Abbott et al, 2016).…”

“…We used NO − 3 concentration for these calculations because it accounted for 92% of measured total N, and TP to account for all measured P ( Figure S2). One weakness of both the mass-balance and residual metrics of nutrient attenuation is that apparent attenuation can occur if significant time lags exits between nutrient inputs and export (Basu et al, 2011;Ehrhardt et al, 2019). Over the past two decades, agricultural practices have improved and many nutrient point sources have been eliminated in western France (Poisvert et al, 2017;Abbott et al, 2018b;Dupas et al, 2018), and we reasoned that the slope of the decline in nutrient concentration would be negatively correlated with hydrological nutrient legacy (i.e., watersheds with more groundwater and soil nutrient storage would have slower rates of nutrient decrease after inputs ceased or were reduced).…”

“…There are three general fates for nutrients moving through the soils, riparian zones, surface waters, and aquifers of a watershed: (1) Retention (i.e., a long or short-term delay) by biological and physical processes, including nutrient uptake, sorption, or hydrological residence time (Covino et al, 2010;Sebilo et al, 2013;Van Meter et al, 2016;Dupas et al, 2017;Ehrhardt et al, 2019), (2) Vertical removal to the atmosphere or lithosphere, including denitrification, aeolian transport, or mineral precipitation with various metals (Groffman et al, 2006;Seitzinger et al, 2006;Pinay et al, 2018;Randall et al, 2019), and (3) Longitudinal export from the watershed via surface or subsurface flow (Burt and Pinay, 2005;Seitzinger et al, 2010;Abbott et al, 2018a). The reactivity and mobility of organic and inorganic nutrients depend on and influence biogeochemical conditions (Abbott et al, 2016;Bernhardt et al, 2017), meaning that the fate of carbon, N, and P can vary substantially through time (e.g., storm events or seasons) and in space (e.g., different watersheds or biomes) (Dupas et al, 2016;Moatar et al, 2017;Musolff et al, 2017;Minaudo et al, 2019).…”

“…Despite advances in understanding nutrient dynamics, it remains difficult to quantify nutrient retention and removal on timescales matching hydrological and nutrient residence times (Vitousek, 2004;Pinay et al, 2015;Ehrhardt et al, 2019). Additionally, many nutrient attenuation studies focus on single-nutrient dynamics, despite evidence that eutrophication is caused by interactions among multiple nutrients and factors (Carpenter et al, 1998;Elser et al, 2007;Paerl et al, 2016;Hobbie et al, 2017;Le Moal et al, 2019).…”