Abstract
A shift has been predicted in future nitrogen emission scenarios from nitrous oxide to higher proportions of ammonium compounds. To investigate the interaction between increasing nitrogen load and varying nitrate:ammonium ratios (NO3−:NH4+), we performed a mesocosm experiment in an oligotrophic lake in southern Germany. We fertilized mesocosms with both roughly natural and four times the natural nitrogen wet deposition amounts in molar NO3−:NH4+ ratios of 4:1 and 1:4. We observed greater phytoplankton biomass in treatments with a relatively higher ammonium supply, but not in those with nitrate and total nitrogen load. Ammonium significantly increased the total chlorophyll a concentrations, and especially the growth of small nanophytoplankton species. The effects observed indicate that NH4+ was taken up preferentially and that spring phytoplankton in oligotrophic lakes appear to be able to respond to variations in nitrogen forms (available NO3−:NH4+ ratios) by adjusting their community composition. Such communal changes at the base of the food web may affect higher trophic levels. Therefore, the effects of varying available forms of nitrogen should also be considered in primarily phosphorus-limited aquatic systems.
Highlights
Anthropogenic intervention in the global nitrogen (N) cycle is an issue of major interest, as increasing N loads often cause measurable changes in ecosystem functioning (Vitousek et al 2010; Paerl et al 2014; Steffen et al 2015)
While the N H4+ concentration increased following the initial fertilization, it remained below 2.3 μM in treatment 1, and within a slightly unimodal range from 1.2 to 6.6 μM in treatments 2 and 3 (Fig. 1a); only treatment 4 obtained an increase of up to 28.3 μM (Fig. 1a)
The total phosphorus (TP) concentrations decreased from 0.27 to ~ 0.05 μM and averaged around 0.16 μM (Fig. 1c), with no statistical differences noted between the treatments with regard to TP (p > 0.05)
Summary
Anthropogenic intervention in the global nitrogen (N) cycle is an issue of major interest, as increasing N loads often cause measurable changes in ecosystem functioning (Vitousek et al 2010; Paerl et al 2014; Steffen et al 2015). Reactive N compounds include nitrogen oxides (NOx−) and ammonium (NH4+), which have been increasing globally due to human activities over the last century (Galloway et al 2003). It is predicted that N fertilizer surpluses will continue to increase by a global average of 23% until 2050, and by up to 75% in Latin America (Bouwman et al 2013). This may eventually double the recent average N deposition in industrialized and developing countries from 10 to 20 kg N ha−1 year−1 until 2050, and to over 50 kg N ha−1 year−1 in some Asian regions (Galloway et al 2004). Future ecosystems face a continual accumulation of total N load, and a shift in reactive N sources from NOx− towards reduced N forms
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