Inorganic Nitrogen Nutrition in Boreal Plants Is Underestimated and Insensitive to Environmental Changes

Although more and more evidences on plant uptake of soil extractable inorganic N (EIN) challenged the traditional viewpoint of soil extractable organic N (EON) as the dominant (averaging 63 ± 6%) N source to boreal plants, relative contributions between EIN and EON to boreal plants and their environmental responses remain unclear. By investigating N concentrations and natural N isotopes in soils and plants, we evaluated and compared contributions of soil EIN and EON to plants of six same genera between Alaskan tundra (AT) with lower mean annual temperature (MAT, −6.8°C) and atmospheric N deposition (AND) (AND, 0.4 kg-N/ha/yr) and northeastern China peatlands (NECP) with higher MAT (−3.3°C) and AND (5.1 kg-N/ha/yr). Soil EIN was three times less than EON, but it contributed c.a. 54 ± 7% of N among studied plants. In NECP, higher MAT and AND caused more increments (by three times) in soil EON than EIN, but soil EIN's contributions to plants did not differ between AT (52 ± 6%) and NECP (57 ± 7%). We concluded that soil EIN's contributions to boreal plants are underestimated (by 17%) and insensitive to concurring warmer climates and higher N deposition. These findings highlighted the importance of soil EIN nutrition to boreal plants and suggested potentially more EON losses under projected warming climate and increasing N pollution, which are useful for evaluating responses of N dynamics in boreal ecosystems to global changes.