Anthropogenic changes of nitrogen loads in a small river: external nutrient sources vs. internal turnover processes

Abstract

Anthropogenic nutrient inputs increase the N-load in many aquatic systems, leading to eutrophication and potential changes of biological N-retention capacity. In this study, nitrate inputs in a small river were investigated along a gradient of anthropogenic influence. We aimed to determine changes in nitrate load and isotope signatures in the water column and to identify the anthropogenic influence on biological nitrogen assimilation and nitrification or denitrification in sediments. In seasonal sampling campaigns, we analysed dissolved inorganic nitrogen concentrations, and stable isotopes of nitrate. To differentiate rates of nitrate production and consumption in the pristine vs. agricultural river section, intact sediment cores were incubated with N-labelled nitrate. δN values of nitrate in the pristine river section were low, reflecting natural sources, but, as expected, increased with nitrate concentration in all seasons along the gradient. In general, nitrate retention and consumption were higher in the anthropogenically impacted than in the pristine river section, and nitrate consumption exceeded production. In addition to our measurements, modelled results also show that even in a small river, the anthropogenically enhanced consumption capacity is overwhelmed by surplus N-inputs, and nitrate consumption cannot increase in turn with external loads.

Spatial variations in sedimentary N-transformation rates in the North Sea (German Bight)

“…15 N at. % excess (Brase et al, 2018), V is the volume of the overlying water, and A is the surface area of the sediment. (All rates are given in mmol N m −2 d −1 .…”

“…More recently, Neumann et al (2017) used pore-water NO − 3 concentration gradient profiles to determine NO − 3 consumption rates in the German Bight. Stable isotope techniques offer several approaches to quantify N turnover processes, and 15 N tracer studies have been widely used to determine N-transformation rates (e.g., nitrification and denitrification) (Brase et al, 2018;Sanders et al, 2018). The isotope dilution method can be used to distinguish between net and gross rates and so help to unravel several N processes, such as ammonification and assimilation or nitrification and denitrification.…”

Spatial variations in sedimentary N-transformation rates in the North Sea (German Bight)

“…where Cstart is the initial NH4 + or NO3concentration, Cend is the concentration at time t, and f 15 Nstart and f 15 Nend represent 15 N atom% excess (Brase et al, 2018), V is the volume of the overlying water and A is the surface area of the sediment. All rates are given in mmol m -2 d -1…”

“…Stable isotope techniques offer several approaches to quantify N turnover processes, and 15 N tracer studies have been widely used to determine N transformation rates (e.g. nitrification and denitrification) (Brase et al, 2018;Deutsch et al, 2009;Henriksen and Kemp, 1988;Sanders et al, 2018;Wankel et al, 2011). In this study, we use an isotope dilution method that can unravel several N-processes like ammonification, assimilation, nitrification, denitrification, dissimilatory NO3reduction to NH4 + (DNRA) and sedimentary NO3consumption / N2 production within sediments.…”

Editorial to special section ‘Application of isotopes in river water research’

“…Examples for different scales of pollution reach from small to large scale river catchments affected by urban and agricultural activities. Brase et al [5] investigated nitrogen loads in the regional watershed of the Holtemme, Harz Mountains, Germany. In the article by Wang et al [6] land use effects and intensive agricultural activities were investigated by nitrogen isotopes and hydrogeochemical tracers.…”