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...

黑云杉（black spruce）的生物量生产力，受多年冻土（permafrost）区域浅部活动层（active layer）的土壤氮素强烈限制。学界已知，树木与菌根根系（mycorrhizal roots）可通过吸收氨基酸，绕开氮限制型北方森林（boreal forest）土壤中缓慢的氮矿化（nitrogen mineralization）过程。然而，树木根系的氨基酸摄取策略，无法充分解释其在与其他植物和微生物竞争土壤氮素时的竞争优势。本研究提供证据表明，部分云杉根系可吸收完整尿素（urea）。树木根系会根据活动层厚度产生可塑性变化，以适配不同氮源的利用需求。尿素摄取仅局限于发育浅部多年冻土的土壤——这类土壤因微生物矿化活动受限而发生尿素累积。这与深部多年冻土土壤的情况形成鲜明对比：在后者生境中，树木根系主要吸收氨基酸与无机氮。在浅部多年冻土上方的较冷亚表层土壤中，光合产物（photosynthate）向细根（fine roots）的分配，有助于树木垄断尿素态氮。尽管能量投入更低……