Wednesday, August 7, 2019

Nitrogen isotope “signals” in mangrove tissues

Mat Wooller and Marilyn, Blonde Pond, Twin Cayes, Belize, 2000
         When we analyzed our first set of samples from the grids stations, we were surprised at the variations we found in the types and concentrations of nitrogen isotopes (we’ll call these “signals”) in the mangrove leaves. Fringing mangroves, primarily the red mangrove Rhizophora mangle, had nitrogen signals nearly similar to the nitrogen in air (Fogel et al., 2008). Transition red mangrove leaves had slightly different signals. Dwarf trees, which can be decades old, had very unusual signals, in fact never measured before in any plants that we knew of. If only I had a portable isotope ratio mass spectrometer (the instrument we use to make stable isotope measurements in our labs) in Belize, we would have analyzed every mangrove tree on Twin Cayes.  About one-third of our  samples were from tall, fringing trees, another third from medium height mangroves in transition zones, and the remaining third were dwarfed trees, no higher than about 1-1.5 meters tall growing in the island’s interior.
         As this was a biocomplexity project, we thought about this data in a slightly different way than we normally do when analyzing the isotope signals in plants. In biocomplexity theory, an “emergent property” is an observation that is nonlinear, that may explain organizational properties of a system. The nitrogen isotope signal of mangrove leaves was our emergent property. As far as we could tell, these were some of the most unusual nitrogen isotope signals measured in a naturally-growing plant.  At this point, there was no easy explanation for why we discovered the variation from a single species on two very small islands. The hunt for an explanation ensued and consumed Wooller, Jacobson, John Cheeseman (University of Illinois), and me with respect to this study for the next four years of the project. Fringing mangroves grow at the edges of the ecosystem, whereas the dwarf mangroves with their sculpted morphology were excellent examples of self organization.
               Feller, Lovelock, and McKee had been collecting mangrove leaf samples from their prior fertilization areas. When we saw their results, the nitrogen isotope signals of dwarf trees were strongly influenced by phosphorus fertilization and the control and nitrogen fertilized trees looked like the dwarf trees we’d analyzed from other areas. We started collecting leaves from all of the experimental trees so that we could compare recent data with samples collected several years prior. Our results for the nitrogen fertilized trees were slightly different than McKee et al. (2002), but the trends were identical. Fertilizing a mangrove tree with phosphorus changed the way the tree metabolized nitrogen.  We did not know why.
20.6 Hints from Phosphorus:
To figure out the relationship between phosphorus and nitrogen, we started a phosphorus fertilization experiment with dwarf trees that we guessed had unusual nitrogen signals we’d measured in similar trees, then collected newly grown leaves periodically over the next two years. Within 2-3 months, we recorded changes in leaf nitrogen signals documenting the timing in which the plant used phosphorus to change nitrogen metabolism. We found interior mangrove trees distributed around the islands that had been fertilized with phosphorus almost a decade ago, without subsequent phosphorus additions. These plants was similar in terms of their isotope signals to recently fertilized trees in the experimental plots. Our conclusion was that once a tree was provided a slug of phosphorus, a limiting nutrient on these islands, it held onto it for a very long period of time. Feller et al. (2005) reached the same conclusion.
         Phosphorus is important for many things in a plant, in particular for making ATP, an organism’s energy storage compounds. The enzyme (i.e., a protein that catalyzes biochemical reactions) that transports nitrogen into a plant’s roots requires several molecules of ATP, and therefore phosphorus, for each nitrate molecule transported. We concluded that the phosphorus effect related to more efficient uptake of nitrogen. We expanded our analytical tools and collected and measured the ammonium (NH4) in surface sediments, water, and air. In sediments and water samples, the nitrogen isotope “signals” were not remarkable. In microbial mats, thick strata of photosynthetic bacteria in shallow ponds, the nitrogen signals  showed us that the microbes were actively assimilating atmospheric nitrogen.

No comments:

Post a Comment

Rounding Third Base and Heading Home

Cards from Franny and Flowers the Rumbles   My daughter Dana is marrying George Goryan on June 25 at our home in Mariposa...