Phosphorus and iron cycling in deep saprolite,Luquillo Mountains,Puerto Rico |
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| Authors: | Heather L Buss Ryan Mathur Arthur F White Susan L Brantley |
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| Institution: | 1. School of the Environment, Flinders University, GPO Box 5100, Adelaide, SA 5001, Australia;2. National Centre for Groundwater Research and Training, Flinders University, GPO Box 5100, Adelaide, SA 5001, Australia;1. Department of Geology and Andean Geothermal Center of Excellence (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile;2. Géosciences Environnement Toulouse (Université de Toulouse, CNRS, IRD), 14 avenue Edouard Belin, 31400 Toulouse, France;3. Institut de Recherches Géologiques et Minières/Centre de Recherches Hydrologiques, BP 4110, Yaoundé, Cameroon;1. Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. Earth and Environmental Systems Institute and the Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA;3. University of Chinese Academy of Sciences, Beijing 100049, China;4. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;5. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550002, China |
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| 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. |
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