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.

Citation

Buss H.L., Mathur R., White A.F., and Brantley S.L. (2010): phosphorus cycling in deep saprolite, Luquillo Mountains, Puerto Rico. Chemical Geology. DOI: 10.1016/j.chemgeo.2009.08.001