Urea uptake by spruce tree roots in permafrost-affected soils

Biomass productivity of black spruce trees is strongly limited by soil nitrogen (N) in shallow active layer on permafrost. Trees and mycorrhizal roots are known to absorb amino acids to bypass slow N mineralization in Nlimited boreal forest soils. However, amino acid uptake strategy of tree roots cannot fully explain their advantages in competition for soil N with other plants and microbes. We investigated the potentials of tree roots to absorb urea as well as amino acids and inorganic N, using tracer experiments of 13C, 15N-labelled glutamic acid and urea and 15N-labelled ammonium nitrate in black spruce forests growing on permafrost-affected soils with different permafrost table depths in the eastern edge of the Mackenzie river delta (northwest Canada). We demonstrated that black spruce roots have potentials to absorb intact urea but only in soils with shallow permafrost, where urea accumulates due to limited microbial mineralization activity. This contrasts with soils with deep permafrost table, where roots absorb amino acids and inorganic N. Allocation of fine roots to colder subsoil above permafrost provides advantages for trees monopolizing urea-N. Despite lower energy efficiency of urea utilization compared to inorganic N and amino acids, urea uptake is one of N acquisition strategies for spruce growing on N-limited subarctic soil.