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.