Download The Pitcher Plant Sarracenia purpurea Can Directly Acquire Organic Nitrogen and Short-Circuit the Inorganic Nitrogen Cycle

Abstract

Background: Despite the large stocks of organic nitrogen in soil, nitrogen availability limits plant growth in many terrestrial
ecosystems because most plants take up only inorganic nitrogen, not organic nitrogen. Although some vascular plants can
assimilate organic nitrogen directly, only recently has organic nitrogen been found to contribute significantly to the nutrient
budget of any plant. Carnivorous plants grow in extremely nutrient-poor environments and carnivory has evolved in these
plants as an alternative pathway for obtaining nutrients. We tested if the carnivorous pitcher plant Sarracenia purpurea
could directly take up intact amino acids in the field and compared uptake of organic and inorganic forms of nitrogen
across a gradient of nitrogen deposition. We hypothesized that the contribution of organic nitrogen to the nitrogen budget
of the pitcher plant would decline with increasing nitrogen deposition.

Methodology and Principal Findings: At sites in Canada (low nitrogen deposition) and the United States (high nitrogen
deposition), individual pitchers were fed two amino acids, glycine and phenylalanine, and inorganic nitrogen (as ammonium
nitrate), individually and in mixture. Plants took up intact amino acids. Acquisition of each form of nitrogen provided in
isolation exceeded uptake of the same form in mixture. At the high deposition site, uptake of organic nitrogen was higher
than uptake of inorganic nitrogen. At the low deposition site, uptake of all three forms of nitrogen was similar.
Completeness of the associated detritus-based food web that inhabits pitcher-plant leaves and breaks down captured prey
had no effect on nitrogen uptake.

Conclusions and Significance: By taking up intact amino acids, Sarracenia purpurea can short-circuit the inorganic nitrogen
cycle, thus minimizing potential bottlenecks in nitrogen availability that result from the plant's reliance for nitrogen
mineralization on a seasonally reconstructed food web operating on infrequent and irregular prey capture.