Spruce trees absorb intact urea from soils on permafrost

Biomass productivity of black spruce trees is strongly limited by soil nitrogen in shallow active layer on permafrost. Trees and mycorrhizal roots are known to absorb amino acids to bypass slow nitrogen mineralization in nitrogen-limited boreal forest soils. However, the amino acid uptake strategy of tree roots cannot fully explain their advantages in the competition for soil nitrogen with other plants and microbes. Here, we provide evidence that some spruce tree roots absorb intact urea. Tree roots develop plasticity to utilize different nitrogen sources, depending on active layer thickness. Urea uptake is limited to soils with shallow permafrost, where urea accumulates due to limited microbial mineralization activity. This contrasts with soils with deep permafrost, where tree roots absorb amino acids and inorganic nitrogen. Allocation of photosynthate to fine roots in colder subsoil above shallow permafrost provides advantages for trees monopolizing urea-nitrogen. Despite lower energy efficiency of urea utilization compared to inorganic nitrogen and amino acids, urea uptake is one of nitrogen acquisition strategies from nitrogen-starved soil. Warming-induced permafrost degradation could reduce an extent of urea-dependent drunken forests with the high potentials of soil carbon storage.