Evidence for foliar endophytic nitrogen fixation in a widely distributed subalpine conifer

Abstract

SummaryConiferous forest nitrogen (N) budgets indicate unknown sources of N. A consistent association between limber pine (Pinus flexilis) and potential N 2 -fixing acetic acid bacteria (AAB) indicates that native foliar endophytes may supply subalpine forests with N.To assess whether the P. flexilis-AAB association is consistent across years, we re-sampled P. flexilis twigs at Niwot Ridge, CO and characterized needle endophyte communities via 16S rRNA Illumina sequencing. To investigate whether endophytes have access to foliar N 2 , we incubated twigs with 13 N 2 -enriched air and imaged radioisotope distribution in needles, the first experiment of its kind using 13 N. We used the acetylene reduction assay to test for nitrogenase activity within P. flexilis twigs four times from June to September. We found evidence for N 2 fixation in P. flexilis foliage. N 2 diffused readily into needles and nitrogenase activity was positive across sampling dates. We estimate that this association could provide 6.8-13.6 lg N m À2 d À1 to P. flexilis stands. AAB dominated the P. flexilis needle endophyte community. We propose that foliar endophytes represent a low-cost, evolutionarily stable N 2 -fixing strategy for long-lived conifers. This novel source of biological N 2 fixation has fundamental implications for understanding forest N budgets.

Associative nitrogen fixation in nodules of the conifer Lepidothamnus fonkii (Podocarpaceae) inhabiting ombrotrophic bogs in southern Patagonia

“…In this latter study, A. vinelandii accumulated up to 50 μM of ammonium and was capable of supporting algal growth in N-free medium. Although 15 N recovered in leaf biomass might partly arise from a contribution of foliar endophytic nitrogen fixation, as reported for Pinus flexilis 56, a translocation of N from nodules to leaves seems more likely due to higher 15 N enrichment in the stem compared to leaf biomass.…”

Plant host identity and soil macronutrients explain little variation in sapling endophyte community composition: Is disturbance an alternative explanation?

“…Though little is known about their interactions with plant hosts, bacteria of Mycobacteriaceae have recently been isolated from stems of common grasses and Pinus needles (e.g. Koskimaki, Hankala, Suorsa, Nylund, & Pirtttila, , Moyes et al, ). Thus, it is possible that endophytes in this taxon may commonly function as pathogens.…”

The importance of phyllosphere on plant functional ecology: a phyllo trait manifesto

“…However, bryophytes may also play a role in leaf nitrogen balance either by reciprocally transferring nitrogen to host leaves (Wanek & Portl, ) or creating favorable microclimatic conditions for cyanobacteria (Bentley, ; Turetsky, ). Microbial assemblages such as biofilms harboring cyanobacteria and other nitrogen fixing bacteria, including endophytic bacteria, might also transfer nitrogen to the leaf (Coley & Kursar, ; Moyes et al ., ; Rigonato et al ., ; Lambais et al ., ). In this line, Laforest‐Lapointe et al .…”