Abstract

Biomass productivity is 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 soils. However, amino acid uptake strategy of tree roots cannot fully explain their advantages in competition for soil nitrogen with other plants and microbes. We demonstrated that black spruce roots have potentials to absorb intact urea in shallow soil on permafrost. 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 table, where roots absorb amino acids and inorganic nitrogen. Allocation of fine roots to colder subsoil above 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 for spruce growing on nitrogen-limited subarctic soil.