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Harvard Forest Symposium Abstract 2005

  • Title: Sarracenia can directly acquire organic nitrogen and short-circuit the inorganic nitrogen cycle
  • Primary Author: Jim Karagatzides (Harvard Forest)
  • Additional Authors: Aaron Ellison (Harvard University)
  • Abstract:

    Standard theory of plant N acquisition assumes that organic nitrogen (ON) has to be mineralized to inorganic N (IN) but research in the last decade has shown that direct uptake of ON (e.g., as intact amino acids) can be a significant N source for many species in a range of ecosystems. Species that can acquire all forms of N may have a competitive advantage over plants that acquire only some forms of N. Pitcher plants (Sarracenia spp.) are ideal systems with which to examine plant N uptake because they inhabit N-limited bogs and multiple forms of N are available to them simultaneously. These carnivorous plants augment N acquisition from rainfall, storage and roots with N derived from prey captured in their pitcher-shaped leaves. Most pitcher plants (e.g., Sarracenia flava) secrete digestive enzymes, but two species, S. purpurea and Darlingtonia californica, do not. The latter have an aquatic invertebrate food web inhabiting their pitchers that shred and mineralize prey. However, the degree to which different sources and forms of N contribute to the N budget of pitcher plants is largely unknown, in part, because the ability of pitcher plants to acquire ON directly has not been investigated previously. Our objectives were to determine if pitcher plants can acquire ON and to compare ON versus IN acquisition in species with and without digestive enzymes. Sarracenia flava and S. purpurea plants grown under greenhouse conditions received 5 atom% 15NH415NO3 or 49 atom% U-13C-15N-Glycine (a common amino acid) in a 72-hr pulse-chase experiment. The food web was flushed from S. purpurea pitchers and replaced with a 1mM N solution, the volume added was proportional to pitcher size. Sarracenia flava pitchers were fed 100 μL containing the average N load (239 μg N) fed to S. purpurea pitchers. We found substantial enrichment of 15N in pitchers from both species for both forms of N; <5% of the acquired 15N was translocated to roots or rhizomes. Sarracenia flava acquired ≈20% of the IN and ON tracer within 4 hr and this increased to a maximum of ≈47% of ON by 72 hr (Figure 1a). There was little change in the proportion of IN tracer in pitchers from 4 to 72 hr. In contrast, S. purpurea acquired ≈20% of IN within 4 hr, compared to ≈10% of ON, but ≈66% of both ON and IN were acquired after 72 hr (Figure 1b). Pitchers receiving U-glycine also were enriched in 13C and their 13C:15N ratio of 1.5 suggests direct acquisition of glycine (13C:15N=2:1). The reduced 13C:15N ratio in plant tissues is attributable to respiration and translocation of 13C from pitcher leaves. Both Sarracenia species have the versatility to acquire multiple forms of N and can short-circuit the inorganic N cycle by bypassing microbial mineralization of ON. Future work will use multiple forms of 15N and the S. purpurea microecosystem under field conditions to examine N cycling across a gradient of acidic deposition (IN) in northeastern North America.

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