Abstract. 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 N 2 O 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 15 N / 18 O 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 systemsWe 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.

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Summary• Species of the ectomycorrhizal genus Leccinum are generally considered to be host specialists. We determined the phylogenetic relationships between species of Leccinum from Europe and North America based on second internal transcribed spacer (ITS2) and glyceraldehyde 3-phosphate dehydrogenase ( Gapdh ).• We plotted host associations onto the phylogenies using maximum likelihood and parsimony approaches.• Resolution of the phylogeny was greater with Gapdh vs ITS2, plus the Gapdh and ITS phylogenies were highly incongruent. In Leccinum the coding region of Gapdh evolved clocklike, allowing the application of a molecular clock for the reconstruction of host specificity. Almost all species of Leccinum are highly host tree specific, except Leccinum aurantiacum , which associates with a broad range of host trees. Maximum likelihood reconstructions of the ancestral host associations show that this taxon evolved from a specialist.• Our results indicate episodes of rapid speciation coinciding with or immediately following host switches. We propose a model where host niche contraction through geographic isolation and host niche expansion through ecologically equivalent hosts drive cycles of speciation. The role of host race formation and incipient speciation is discussed.

Ectomycorrhizal seedlings of Scots pine (Pinus sylvestris) inoculated with the nitrotolerant Laccaria bicolor and the nitrophobic Suillus bovinus were exposed to ambient (350 µl l −" ) and elevated (700 µl l −" ) [CO # ]. After 79 d the seedlings were labelled for 28 d with "%CO # , after which they were harvested. "%C was determined in shoots, roots plus mycorrhizas, soil, and below-ground respiration ; nitrogen was determined in shoots and roots. Total net "%C uptake increased under elevated [CO # ]. The extra carbon did not increase the shoot mass but was translocated to the roots and resulted in a decreased shoot-to-root ratio in the Suillus-inoculated seedlings. Laccaria-inoculated seedlings did not incorporate the additional carbon in root or fungal tissue but only increased below-ground respiration. S. bovinus acquired or transferred nitrogen better than L. bicolor and enabled the seedlings to perform better with regard to net carbon uptake under elevated [CO # ]. This resulted in nitrogen concentrations in shoots of Suillus-inoculated seedlings that were twice as high as in Laccaria-inoculated seedlings, irrespective of [CO # ]. The higher nitrogen concentration in the shoots resulted in a doubling of the "%C uptake per unit shoot mass. Our results suggest that the ability of ectomycorrhizal Scots pine seedlings to respond positively to elevated atmospheric [CO # ] is strongly fungal-species specific.