HR: 0800h
AN: B31B-06    [Abstracts]
TI: Apatite Weathering and Phosphorus Availability in Deep Regolith, Luquillo Mountains, Puerto Rico
AU: * Buss, H L
EM: hlbuss@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 420, Menlo Park, CA 94025, United States
AU: Williams, J Z
EM: jzwilliam@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802, United States
AU: White, A F
EM: afwhite@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 420, Menlo Park, CA 94025, United States
AU: Brantley, S L
EM: brantley@eesi.psu.edu
AF: Penn State University, Earth and Environmental Systems Institute, University Park, PA 16802, United States
AB: 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. Here we investigate the weathering of primary minerals containing iron and phosphorous and the role of resident microorganisms in the cycling of these elements in the deep regolith of the Rio Icacos watershed in the Luquillo Experimental Forest, a tropical montane rainforest in northeastern Puerto Rico. In the Rio Icacos watershed, which has one of the fastest documented chemical weathering rates of granitic rocks in the world, the quartz diorite bedrock weathers spheroidally, producing a complex interface comprised of partially weathered rock layers called rindlets. This rindlet zone (about 0.2-3 m thick) is overlain by saprolite (2-8 m) topped by soil (0.5-1 m). Samples were taken from cores augered to 7.5 m on a ridgetop. The profile included about 5 m of regolith (soil \p saprolite), and more than 2.5 m of rindlets. A 0.5 m thick rindlet zone was also sampled in a nearby roadcut. Weathering reactions of primary minerals were examined in thin sections made from rindlets. Total chemistry was measured on all solid samples and Fe, Fe(II), and P were measured in 0.1 M NaOH and 0.5 M HCl extractions performed on augered samples. NaOH-extractable P was assumed to include inorganic and organic P that is bioavailable on both short and long timescales including P associated with secondary Fe(III)- (hydro)oxides. Concentrations of NaOH-extractable P are very low throughout the regolith but increase significantly in the rindlet zone (below 5 m depth). Residual P, believed to include primary apatite and occluded, resistant organic and inorganic forms of P, generally increases with depth. Below 5 m depth, this fraction of P is near zero. Solid-state reaction rates can be calculated for minerals in a weathering profile from the elemental distribution in the profile. Here we quantify a weathering rate for apatite using the P distribution across the rindlet zone in the Rio Icacos watershed. The resulting rate of apatite weathering is 1.1 x 10^-14 mol m^-2 s^-1. In the road cut rindlet sequence, apatite dissolves over the entire 0.5 m rindlet zone, beginning just above the un-weathered bedrock. The high concentrations of P in the NaOH extracts from the rindlet zone samples indicate that the P released during apatite weathering remains relatively accessible within this zone and in the deepest overlying saprolite. HCl-extractable total Fe and Fe(II) follow a similar trend to NaOH-extractable P except in the soil zone (0- 0.5 m), where Fe concentrations are relatively high. In the soil zone, low P concentrations in all measured fractions probably indicate intense biological scavenging of P. High densities of microorganisms in the soil decrease with depth in the regolith, but increase again near the bedrock interface. Nutrient and biological cycles in the deep saprolite profiles are decoupled between the surface layers and the saprolite-bedrock interface zone. While surface communities depend on re-cycled nutrients and atmospheric inputs, deep communities survive primarily off of nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. Extremely low available P may limit microbial growth within the saprolite, leading to the decoupling of surface and deep communities.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0448 Geomicrobiology
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 0486 Soils/pedology (1865)
DE: 1039 Alteration and weathering processes (3617)
SC: Biogeosciences [B]
MN: 2007 Joint Assembly