The soil N cycle: new insights and key challenges

The study of soil N cycling processes has been, is, and will be at the center 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, measurement and mitigation of 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 for future research (Fig. 1). We identified four key questions with respect to N cycling processes: 1. How large is the contribution of non-symbiotic N fixation in natural systems? 2. How important is nitrifier denitrification and what are its main controlling factors? 3. What is the greenhouse gas mitigation potential and microbiological basis for N2O consumption? 4. How can we characterize hot-spots and hot-moments of denitrification? Furthermore, we propose three questions about proximal controls on N cycling processes: 1. How does functional diversity of soil fauna affect N cycling beyond mineralization? 2. What is the functional relationship between root traits and soil N cycling? 3. To what extent do different types of mycorrhizal symbioses (differentially) affect N cycling? Finally, we identified a key challenge with respect to modelling: 1. How can advanced 15N/18O tracing models help us to better disentangle gross N transformation rates? We postulate that addressing these questions would 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 and climate stability.