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Nitrogen acquisition strategies of mature Douglas-fir: a case study in the northern Rocky Mountains

Qubain, C. A., Y. Yano and J. Hu. 2021.


Nitrogen (N) limits plant growth in temperate ecosystems, yet many evergreens exhibit low photosynthetic N use efficiency, which can be explained in part by their tendency to store more N than to use it in photosynthesis. However, it remains uncertain to what extent mature conifers translocate internal N reserves or take up N from soils to support new growth. In this study, we explored N dynamics within mature Douglas-fir (Pseudotsuga menziesii var. glauca) trees by linking N uptake in field-grown trees with seasonal soil available N. We used a branch-level mass balance approach to infer seasonal changes in total N among multiple needle and stem cohorts and bole tissue, and used foliar d15 N to evaluate N transloca- tion/uptake from soils. Soil resin-exchangeable N and net N transformation rates were measured to assess whether soils had sufficient N to support new needle growth. We estimated that after bud break, new nee- dle biomass in Douglas-fir trees accumulated an average of 0.20  0.03 mg N/branch and 0.17  0.03 mg N/branch in 2016 and 2017, respectively. While we did find some evidence of translocation of N from older stems to buds prior to bud break, we did not detect a significant drawdown of N from pre- vious years%u2019 growth during needle expansion. This suggests that the majority of N used for new growth was not reallocated from aboveground storage, but originated from the soils. This finding was further sup- ported by the d15 N data, which showed divergent d15 N patterns between older needles and buds prior to leaf flushing (indicative of translocation), but similar patterns of depletion and subsequent enrichment fol- lowing leaf expansion (indicative of N originating from soils). Overall, in order to support new growth, our study trees obtained the majority of N from the soils, suggesting tight coupling between soil available N and N uptake in the ecosystem

Key Words

conifer; evergreen; nitrogen availability; nitrogen storage; nitrogen translocation; nitrogen uptake; Pseudotsuga menziesii var. glauca/Rocky Mountain Douglas-fir