Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH
4
+), nitrate (NO
3
−) or a combination thereof are expected to differ in soil N ...transformation rates and fates of NH
4
+ and NO
3
−. Using
15N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (
Hordeum vulgare L. cv.
Morex) were grown on two agricultural soils in microcosms which received either NH
4
+, NO
3
− or NH
4NO
3. Within each fertilizer treatment traces of
15NH
4
+ and
15NO
3
− were added separately. During 8 days of fertilization the fate of fertilizer
15N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45–80% of initially applied
15N was recovered in crop plants compared to only 1–10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO
3
− than NH
4
+ independent of initially applied N form. Surprisingly, no inhibitory effect of NH
4
+ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH
4
+ uptake rates, gross nitrification rates were 3–75-fold higher, indicating that nitrifiers were the strongest competitors for NH
4
+ in both soils. The rapid conversion of NH
4
+ to NO
3
− and preferential use of NO
3
− by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO
3
− and shift towards enhanced reliance on NO
3
− for their N supply.
River systems have undergone a massive transformation since the Anthropocene. The natural properties of river systems have been drastically altered and reshaped, limiting the use of management ...frameworks, their scientific knowledge base and their ability to provide adequate solutions for current problems and those of the future, such as climate change, biodiversity crisis and increased demands for water resources. To address these challenges, a socioecologically driven research agenda for river systems that complements current approaches is needed and proposed. The implementation of the concepts of social metabolism and the colonisation of natural systems into existing concepts can provide a new basis to analyse the coevolutionary coupling of social systems with ecological and hydrological (i.e., ‘socio-ecohydrological’) systems within rivers. To operationalize this research agenda, we highlight four initial core topics defined as research clusters (RCs) to address specific system properties in an integrative manner. The colonisation of natural systems by social systems is seen as a significant driver of the transformation processes in river systems. These transformation processes are influenced by connectivity (RC 1), which primarily addresses biophysical aspects and governance (RC 2), which focuses on the changes in social systems. The metabolism (RC 3) and vulnerability (RC 4) of the social and natural systems are significant aspects of the coupling of social systems and ecohydrological systems with investments, energy, resources, services and associated risks and impacts. This socio-ecohydrological research agenda complements other recent approaches, such as ‘socio-ecological’, ‘socio-hydrological’ or ‘socio-geomorphological’ systems, by focusing on the coupling of social systems with natural systems in rivers and thus, by viewing the socioeconomic features of river systems as being just as important as their natural characteristics. The proposed research agenda builds on interdisciplinarity and transdisciplinarity and requires the implementation of such programmes into the education of a new generation of river system scientists, managers and engineers who are aware of the transformation processes and the coupling between systems.
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•River systems have been massively transformed and are socio-ecohydrological systems.•A socio-ecohydrologically driven approach provides insights into coevolutionary processes.•Social metabolism and the colonisation of natural systems are underlying concepts.•Four research clusters analyse the transformation and coupling of society and nature.•Interdisciplinary and transdisciplinary approaches support the operationalization of the research agenda.
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