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Nitrogen acquisition strategies of mature Douglas%u2010fir: a case study in the northern Rocky Mountains

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

Abstract

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