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.
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