the soil n cycle: new insights and key challenges

The study of soil N cycling processes has been, is, and will be at the centre of attention in soil science research. The importance of N as a nutrient for all biota; the ever-increasing rates of its anthropogenic input in terrestrial (agro)ecosystems; its resultant losses to the environment; and the complexity of the biological, physical, and chemical factors that regulate N cycling processes all contribute to the necessity of further understanding, measuring, and altering the soil N cycle. Here, we review important insights with respect to the soil N cycle that have been made over the last decade, and present a personal view on the key challenges of future research. We identify three key challenges with respect to basic N cycling processes producing gaseous emissions:
1. quantifying the importance of nitrifier denitrification and its main controlling factors;
2. characterizing the greenhouse gas mitigation potential and microbiological basis for N₂O consumption;
3. characterizing hotspots and hot moments of denitrification
Furthermore, we identified a key challenge with respect to modelling:
1. disentangling gross N transformation rates using advanced ¹⁵N / ¹⁸O tracing models
Finally, we propose four key challenges related to how ecological interactions control N cycling processes:
1. linking functional diversity of soil fauna to N cycling processes beyond mineralization;
2. determining the functional relationship between root traits and soil N cycling;
3. characterizing the control that different types of mycorrhizal symbioses exert on N cycling;
4. quantifying the contribution of non-symbiotic pathways to total N fixation fluxes in natural systems
We postulate that addressing these challenges will constitute a comprehensive research agenda with respect to the N cycle for the next decade. Such an agenda would help us to meet future challenges on food and energy security, biodiversity conservation, water and air quality, and climate stability.