The journey of a molecule related by Aldo Leopold
is an exercise in poignant prose;
but here, if I may be so bold,
I make an effort to transpose
his Odyssey, to the lexicon of biogeochemistry.
And since X is no fitting name for this history,
we will equally consider she
as carbon and as nitrogen,
and trace the path of N and C
from limestone cliff through cyclic sea.
As NaNO3, nitrogen has well-known solubility.
and natural fertilizing ability;
but for those who prefer C
the mineral source can be
easily, ubiquitous deposits of CaCO3.
First a plant took root up there,
and split the stone and let in air
bearing H2O and CO2,
from which carbonic acid grew
(with the carboxylic additions of oak root too),
which weathered the rock, from whence the bur-oak drew
a solution of nitrate
some calcium bicarbonate,
which when assimilated may
eventually have constructed a strand of DNA.
So were exhumed our fated friends, but to what end:
victim to assimilatory metabolism
and kept interred in organisms.
Turn by turn they onward flowed
until such time as their owner decomposed.
Through each phase of this biological pump,
from plant to beast to human race,
both N and C keep different pace:
depending on their placement
as reactant or cellular cement.
When an organism in which they’d happen to reside
would finally succumb to time’s tide,
came then the turn of fungi
and hungry bacteria to ammonify,
and percolate the soil with carbon dioxide.
Then a root would re-uptake, to assimilate in a leaf,
before a mouse physically bore them beneath
the surface realm of wind and rain
to there delay another refrain.
And while N and C wait to again respire
biological cycling is halted by wildfire:
the combustion of biomass
oxidizing N and C en masse,
and losing N2 and CO2 to air
but storing charred organic matter beneath the surface soot.
With the return of legumes and their bacteria-laden roots
nitrogen fixation from atmospheric
to NH4+ (always anaerobic),
which in turn was nitrified, to NO3-
which fueled respiration, and C again was free.
So began a race, hydraulically transported by slope
with impetus again
derived from rain,
delayed by cyclic residence
in all manner of heterotroph.
The seaward rush of N and C progressed until they found
themselves united in urea or aminos;
after sulphate reduction in anoxic silt
and dissimilatory metabolism, they plunge into
(presumably) the Gulf of Mexico.
This entirely new system was not for Aldo to explore.
To cycles in the ocean, more pressing than on land,
he gave the brand
of prison, with which I disagree:
for no system is a prison in terms of biogeochemistry.
While passing through a gateway estuary
the nutrient spiral roulette
could spit into the Atlantic
or as here presumed: waste organic matter.
DOC is dominant, one fifth of all the latter.
N is surface bio-limiting,
while C is bio-intermediate,
both can be dissolved or particulate:
a functional difference in size, meaning that one sinks
while the distribution of DOM is circulation driven.
When in the euphotic zone
most forms of OM are, prone
to surface photochemical reduction,
and NH4+ driven regenerative production.
Though rates vary by geography, N remineralization
occurs, first by ammonium-nitrate oxidation,
followed by denitrification
(to solute N2 and H3PO4),
then by metal ions, and sulphate reduction (of which I’ll mention more).
The generated biomass of net primary production,
feeds a chain of organisms
with less efficient metabolism,
this bio-pump trophically concentrates
once-dissolved OM into heavy fecal particulates.
But this and other sinking POM will not go unused:
at aphotic depths it is reduced
to CO2 and nutrients
by heterotrophs reliant
on settling, and the vital circulation which upwells once more.
Then the NO3- and CO2 are used by coccolithophores,
to fuel new production of their plates
from Ca2+ and bicarbonate,
and when the algae die, the CaCO3
quickly settles to benthos and becomes sedimentary.
When I consider that only a quarter of OM
(regardless the sort)
makes it to the ocean floor
by settling or transport;
and 90% is remineralized for a few reactions more,
I can say with confidence, that the chance is rather small,
that even through many iterations
they’ll see the abyssal plain at all.
So, though C and N may languish
in sediment and solution and fish
it’s never so long, speaking geologically,
despite the volume of the sea,
before carbon and nitrogen
are returned to air and land
through crustal subduction and steady ebullition.