Weathering and Soils

Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains
of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides
new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on
major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs.

Review of Use of Isotopes in Studying the Natural History of Puerto Rico

Evaristo J. Review of Use of Isotopes in Studying the Natural History of Puerto Rico. University of Pennsylvania. 2012.

Abstract: 
This review summarizes the earth and environmental science research papers in Puerto Rico that used isotopic techniques between 1965 and 2011. The range of applications in isotope-related research in Puerto Rico has grown steadily, led by the ubiquitous utility of stable isotope ratios in biogeochemical (δ13C, δ15N) and ecological (δ13C, δ15N, δD) research. Moreover, research in climatology has grown in recent years, spanning from the evaluation of the fidelity of isotope records (δ18O, δ13C) as an environmental proxy to the elucidation of multidecadal variability for paleoclimate reconstructions (δ18O and Sr/Ca). On the other hand, in addition to using isotope ratios, hydrological studies in Puerto Rico have also used trace element data to answer flow source (δD, δ18O, 87Sr/86Sr) and solute source (Ge/Si) questions, as well as in examining groundwater/surface flow relationships (222Rn). Finally, various isotope data have been used in trying to understand geomorphological (10Be, δ30Si) and geophysical (Pb, Nd, and Sr) phenomena. It is hoped that this review will be able to contribute to stimulating future interests in isotope-related research as applicable in the LCZO or Puerto Rico, in particular, and/or in comparable humid tropical settings, in general.

Atypical soil carbon distribution across a tropical steepland forest catena.

Johnson K.D., Scatena F.N., Silver W.L. Atypical soil carbon distribution across a tropical steepland forest catena. CATENA, In Press, Corrected Proof, Available online 4 August 2011, ISSN 0341-8162, DOI: 10.1016/j.catena.2011.07.008. (http://www.sciencedirect.com/science/article/pii/S0341816211001457)

Abstract: 
Soil organic carbon (SOC) in a humid subtropical forest in Puerto Rico is higher at ridge locations compared to valleys, and therefore opposite to what is commonly observed in other forested hillslope catenas. To better understand the spatial distribution of SOC in this system, plots previously characterized by topographic position, vegetation type and stand age were related to soil depth and SOC. Additional factors were also investigated, including topographically-related differences in litter dynamics and soil chemistry. To investigate the influence of litter dynamics, the Century soil organic model was parameterized to simulate the effect of substituting valley species for ridge species. Soil chemical controls on C concentrations were investigated with multiple linear regression models using iron, aluminum and clay variables. Deeper soils were associated with indicators of higher landscape stability (older tabonuco stands established on ridges and slopes), while shallower soils persisted in more disturbed areas (younger non-tabonuco stands in valleys and on slopes). Soil depth alone accounted for 77% of the observed difference in the mean 0 to 60 cmSOC between ridge soils (deeper) and valley soils (shallower). The remaining differences in SOC were due to additional factors that lowered C concentrations at valley locations in the 0 to 10 cm pool. Model simulations showed a slight decrease in SOC when lower litter C:N was substituted for higher litter C:N, but the effects of different woody inputs on SOC were unclear. Multiple linear regression models with ammonium oxalate extractable iron and aluminum, dithionite–citrate-extractable iron and aluminum, and clay contents explained as much as 74% of the variation in C concentrations, and indicated that organo-mineral complexation may be more limited in poorly developed valley soils. Thus, topography both directly and indirectly affects SOC pools through a variety of inter-related processes that are often not quantified or captured in terrestrial carbon models.

Streams of the Montane Humid Tropics. Treatise on Geomorphology

Scatena F.N., Gupta A., 2011. Streams of the Montane Humid Tropics. Treatise on Geomorphology. Editors E. Wohl. Academic Press, San Diego Ca. Vol 9. in press April 2011

Abstract: 
Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in other fluvial systems.

Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed

Shanley, J. B., W. H. McDowell, and R. F. Stallard (2011), Long‐term patterns and short‐term dynamics of stream
solutes and suspended sediment in a rapidly weathering tropical watershed, Water Resour. Res., 47, W07515,
doi:10.1029/2010WR009788

Abstract: 
The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains, northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates among the highest in the world. We pooled stream chemistry and total suspended sediment (TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008. During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in 1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4), and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall, stream composition shifts toward that of precipitation, diluting 90% or more in the largest storms, but maintains a biogeochemical watershed signal marked by elevated K and dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang” pattern, initially increasing with flow but then decreasing at the highest flows as it becomes depleted and/or vigorous overland flow minimizes contact with watershed surfaces. TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive power of large storms in a landslide‐prone landscape. The relations of TSS and most solute concentrations with stream discharge were stable through time, suggesting minimal long‐term effects from repeated hurricane disturbance. Nitrate concentration, however, increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first‐order decay pattern. The combined data sets provide insight about important hydrologic pathways, a long‐term perspective to assess response to hurricanes, and a framework to evaluate future climate change in tropical ecosystems.

Iron and phosphorus cycling in deep saprolite, Luquillo Mountains, Puerto Rico

Buss H.L., Mathur R., White A.F., and Brantley S.L. 2010. Iron and phosphorus cycling in deep saprolite, Luquillo Mountains, Puerto Rico. Chem. Geol., 269, 52-61.

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

Twelve testable hypotheses on the Geobiology of weathering

Brantley S.L., Megonigal J.P., Scatena F.N. et al 2010. Twelve testable hypotheses on the Geobiology of weathering. Geobiology. DOI: 10.1111/j.1472-4669.2010.00264.x

Abstract: 
Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth’s surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term.(4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes.(5) Biology shapes the topography of the Critical Zone.(6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws.(7) Rising global temperatures will increase carbon losses from the Critical Zone.(8) Rising atmospheric PCO2 will increase rates and extents of mineral weathering in soils.(9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering.(10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales.(12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Chemical and mineral control of soil carbon turnover in abandoned tropical pastures

Marín-Spiotta, Erika; Swanston, Christopher W.; Torn, Margaret S.; Silver, Whendee L.; Burton, Sarah D. 2007. Chemical and mineral control of soil carbon turnover in abandoned tropical pastures.. Geoderma, doi:10.1016/j.geoderma.2007.10.001.

Abstract: 
We investigated changes in soil carbon (C) cycling with reforestation across a long-term, replicated chronosequence of tropical secondary forests regrowing on abandoned pastures. We applied CP MAS 13C NMR spectroscopy and radiocarbon modeling to soil density fractions from the top 10 cm to track changes in C chemistry and turnover during secondary forest establishment on former pastures. Our results showed that interaggregate, unattached, particulate organic C (free light fraction) and particulate C located inside soil aggregates (occluded light fraction) represent distinct soil C pools with different chemical composition and turnover rates. The signal intensity of the O-alkyl region, primarily representing carbohydrates, decreased, and alkyl C, attributed to recalcitrant waxy compounds and microbially resynthesized lipids, increased from plant litter to soil organic matter and with incorporation into soil aggregates. The alkyl/O-alkyl ratio, a common index of humification, was higher in the occluded than in the free light fraction. Greater variability in the chemical makeup of the occluded light fraction suggests that it represents material in varied stages of decomposition. Mean residence times (14C-based) of the free light fraction were significantly shorter (4±1 years) than for the heavy fraction. We report two scenarios for the occluded light fraction, one fast-cycling in which the occluded and free light fractions have similar turnover rates, and one slow-cycling, in which the occluded light fraction resembles the heavy fraction. Mean residence times of the occluded light fraction and heavy fraction in active pastures and 10-year old secondary forests in the earliest stage of succession were longer than in older secondary forests and primary forests. This is likely due to a preferential loss of physically unprotected C of more labile composition in the pastures and in the youngest successional forests, resulting in an increase in the dominance of slow-cycling C pools. Soil carbon turnover rates of the mineral-associated C in secondary forests recovering from abandoned pasture resembled those of primary forests in as little as 20 years of succession.

Soil survey of humacao area of eastern puerto rico

Boccheciamp, RA. 1977. Soil survey of the Humacao area of
Eastern Puerto Rico. United States Department of Agriculture
Soil Conservation Service.

Spatial and seasonal dynamics of surface soil carbon in the Luquillo Experimental Forest, Puerto Rico

Wang, Hongqing; Cornell, Joseph D.; Hall, Charles A.S.; Marley, David P. 2002. Spatial and seasonal dynamics of surface soil carbon in the Luquillo Experimental Forest, Puerto Rico.. Ecological Modelling 147 105-122.

Abstract: 
We developed a spatially-explicit version of the CENTURY soil model to characterize the storage and flux of soil organic carbon (SOC, 0–30 cm depth) in the Luquillo Experimental Forest (LEF), Puerto Rico as a function of climate, vegetation, and soils. The model was driven by monthly estimates of average air temperature, precipitation, and potential evapotranspiration (PET), which in turn were simulated as a function of elevation, slope, and aspect using a spatially-explicit and validated model (TOPOCLIM) of solar insolation/microclimate in mountainous areas. We simulated forest gross primary productivity (GPP) and distribution of above- and below-ground biomass production using a forest productivity model (TOPOPROD). Output from TOPOCLIM and TOPOPROD models was used to run the CENTURY soil model for 1200 months under current climate conditions and in response to potential global warming. We validated our version of CENTURY soil model using 69 soil samples taken throughout the LEF. Simulated SOC storage agrees reasonably well with the observed storage (R2=0.71). The simulated SOC storage in the top 30 cm within the LEF is highly variable, ranging from approximately 20–230 Mg/ha. The rates of decomposition were especially sensitive to changes in elevation. Carbon release rates due to decomposition were close to carbon assimilation rates and ranged from 0.6–0.96 Mg/ha per year at high elevations to 1.2–1.68 Mg/ha per year at lower elevations. Our simulations indicated that differences in elevation affect decomposition and SOC content primarily by changing microclimate. Finally, we found that a projected warming of 2.0 °C is likely to result in losses of SOC in the lower and higher elevation, but increased storage in the middle elevations in the LEF.
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