『Abstract
Rapid weathering and erosion rates in mountainous tropical watersheds
lead to highly variable soil and saprolite thicknesses which in
turn impact nutrient fluxes and biological populations. In the
Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains
high microorganism densities at the surface and at depth overlying
bedrock. We test the hypotheses that the organisms at depth are
limited by the availability of two nutrients, P and Fe. Many tropical
soils are P-limited, rather than N-limited, and dissolution of
apatite is the dominant source of P. We document patterns of apatite
weathering and of bioavailable Fe derived from the weathering
of primary minerals hornblende and biotite in cores augered to
7.5 m on a ridgetop as compared to spheroidally weathering bedrock
sampled in a nearby roadcut.
Iron isotopic compositions of 0.5 N HCl extracts of soil and
saprolite range from about δ56Fe = 0 to -0.1‰ throughout
the saprolite except at the surface and at 5 m depth where δ56Fe
= -0.26 to -0.64‰. The enrichment of light isotopes in HCl-extractable
Fe in the soil and at the saprolite-bedrock interface is consistent
with active Fe cycling and consistent with the locations of high
cell densities and Fe(II)-oxidizing bacteria, identified previously.
To evaluate the potential P-limitation of Fe-cycling bacteria
in the profile, solid-state concentrations of P were measured
as a function of depth in the soil, saprolite, and weathering
bedrock. Weathering apatite crystals were examined in thin sections
and an apatite dissolution rate of 6.8×10-14 mol m-2
s-1 was calculated. While surface communities depend
on recycled nutrients and atmospheric inputs, deep communities
survive primarily on nutrients released by the weathering bedrock
and thus are tightly coupled to processes related to saprolite
formation including mineral weathering. While low available P
may limit microbial activity within the middle saprolite, fluxes
of P from apatite weathering should be sufficient to support robust
growth of microorganisms in the deep saprolite.
Keywords: Phosphorus; Iron isotopes; Saprolite; Apatite weathering
rate; Fe(II)-oxidizing bacteria』
1. Introduction
2. Methods
2.1. Field site and sample collection
2.2. Phosphorus and apatite analyses
2.3. Fe isotope analysis
3. Results
4. Discussion
4.1. Phosphorus flux and apatite weathering
4.2. Fe isotopic composition of the regolith profile
5. Conclusions
Acknowledgements
References