Weathering and Soils

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.

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.

The use of chronosequences in studies of ecological succession and soil development

Walker LR, Wardle DA, Bardgett RD, Clarkson BD (2010) The
use of chronosequences in studies of ecological succession
and soil development. J Ecol 98:725–736

1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences.

carbon isotope characterization of vegetation and soil organic matter in subtropical forests in luquillo, puerto rico

Carbon Isotope Characterization of Vegetation and Soil Organic Matter in Subtropical Forests in Luquillo, Puerto Rico
Joseph C. von Fischer and Larry L. Tieszen
Vol. 27, No. 2 (Jun., 1995), pp. 138-148

We examined natural abundances of 13C in vegetation and soil organic matter (SOM) of subtropical wet and rain forests to characterize the isotopic enrichment through decomposition that has been reported for temperate forests. Soil cores and vegetative samples from the decomposition continuum (leaves, new litter, old litter, wood, and roots) were taken from each of four forest types in the Luquillo Experimental Forest, Puerto Rico. SOM δ13C was enriched 1.6% relative to aboveground litter. We found no further enrichment within the soil profile. The carbon isotope ratios of vegetation varied among forests, ranging from -28.2% in the Colorado forest to -26.9% in the Palm forest. Isotope ratios of SOM differed between forests primarily in the top 20 cm where the Colorado forest was again most negative at -28.0%, and the Palm forest was most positive at -26.5%. The isotopic differences between forests are likely attributable to differences in light regimes due to canopy density variation, soil moisture regimes, and/or recycling of CO2. Our data suggest that recalcitrant SOM is not derived directly from plant lignin since plant lignin is even more 13C depleted than the bulk vegetation. We hypothesize that the anthropogenic isotopic depletion of atmospheric CO2 (ca 1.5% in the last 150 years) accounts for some of the enrichment observed in the SOM relative to the more modern vegetation in this study and others. This study also supports other observations that under wet or anaerobic soil environments there is no isotopic enrichment during decomposition or with depth in the active profile.

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils
Whendee L. Silver, Donald J. Herman and Mary K. Firestone
Vol. 82, No. 9 (Sep., 2001), pp. 2410-2416

The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into groundwater, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using 15N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging ;0.6 mg·g21·d21. Rates of DNRA were three times greater than the combined N2O and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance of ambient C, O2, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and O2 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 mg·g21·d21 in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C or O2. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.

Soil factors predict initial plant colonization on Puerto Rican landslides

Shiels, A.B., West, C.A., Weiss, L., Klawinski, P.D. &
Walker, L.R. 2008. Soil factors predict initial plant
colonization on Puerto Rican landslides. Plant
Ecology 195: 165–178.

Tropical storms are the principal cause of landslides in montane rainforests, such as the Luquillo Experimental Forest (LEF) of Puerto Rico. A storm in 2003 caused 30 new landslides in the LEF that we used to examine prior hypotheses that slope stability and organically enriched soils are prerequisites for plant colonization. We measured slope stability and litterfall 8–13 months following landslide formation. At 13 months we also measured microtopography, soil characteristics (organic matter, particle size, total nitrogen, and water-holding capacity), elevation, distance to forest edge, and canopy cover. When all landslides were analyzed together, plant biomass and cover at 13 months were not correlated with slope stability or organic matter, but instead with soil nitrogen, clay content, waterholding capacity, and elevation. When landslides were analyzed after separating by soil type, the distance from the forest edge and slope stability combined with soil factors (excluding organic matter) predicted initial plant colonization on volcaniclastic landslides, whereas on diorite landslides none of the measured characteristics affected initial plant colonization. The life forms of the colonizing plants reflected these differences in landslide soils, as trees, shrubs, and vines colonized high clay, high nitrogen, and low elevation volcaniclastic soils, whereas herbs were the dominant colonists on high sand, low nitrogen, and high elevation diorite soils. Therefore, the predictability of the initial stage of plant succession on LEF landslides is primarily determined by soil characteristics that are related to soil type.

Litterfall and Decomposition in Relation to Soil Carbon Pools Along a Secondary Forest Chronosequence in Puerto Rico

Ostertag, R.; Marín-Spiotta, E.; Silver, W.L.; Schulten, J. 2008. Litterfall and decomposition in relation to soil carbon pools along a secondary forest chronosequence in Puerto Rico. Ecosystems. 11:701-714.

Secondary forests are becoming increasingly widespread in the tropics, but our understanding of how secondary succession affects carbon (C) cycling and C sequestration in these ecosystems is limited. We used a well-replicated 80-year pasture to forest successional chronosequence and primary forest in Puerto Rico to explore the relationships among litterfall, litter quality, decomposition, and soil C pools. Litterfall rates recovered rapidly during early secondary succession and averaged 10.5 (± 0.1 SE) Mg/ha/y among all sites over a 2-year period. Although forest plant community composition and plant life form dominance changed during succession, litter chemistry as evaluated by sequential C fractions and by 13C-nuclear magnetic resonance spectroscopy did not change significantly with forest age, nor did leaf decomposition rates. Root decomposition was slower than leaves and was fastest in the 60-year-old sites and slowest in the 10- and 30-year-old sites. Common litter and common site experiments suggested that site conditions were more important controls than litter quality in this chronosequence. Bulk soil C content was positively correlated with hydrophobic leaf compounds, suggesting that there is greater soil C accumulation if leaf litter contains more tannins and waxy compounds relative to more labile compounds. Our results suggest that most key C fluxes associated with litter production and decomposition re-establish rapidly—within a decade or two—during tropical secondary succession. Therefore, recovery of leaf litter C cycling processes after pasture use are faster than aboveground woody biomass and species accumulation, indicating that these young secondary forests have the potential to recover litter cycling functions and provide some of the same ecosystem services of primary forests.

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher, R.C., Buss, H.L., Brantley, S.L., 2006.Aspheroidal weathering
model coupling porewater chemistry to soil thicknesses during
steady-state denudation. Earth Planet. Sci. Lett. 244, 444–457.

Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ΔV of reaction results in the build-up of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈2–3cm) and a time interval between fractures (≈200–300 a) based on an assumption of steady-state denudation at the measured rate of 0.01cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock–saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock–saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steady-state saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ∼250yr, ferric oxide is fully depleted over a four rindlet set in ∼1000yr, and saprolitization is completed in ∼5000yr in the zone containing ∼20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing. Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

Soil Survey of Caribbean National Forest and Luquillo Experimental Forest, Commonwealth of Puerto Rico

NRCS Soils Map

The CARIBBEAN NATIONAL FOREST, locally known as El Yunque or “the Forest,” is in the Luquillo Mountains and dominates the northeastern corner of Puerto Rico (fig. 1). It is one of the most popular recreation sites in Puerto Rico. Annually, almost one million tourists visit the Forest from Puerto Rico, the United States, and abroad. Puerto Rico, an island, is associated with the Greater Antilles chain of the Caribbean and is located at the southeastern end of the chain (Mitchell, 1954). The survey area includes all 27,846 acres (11,268 hectares) of the Caribbean National Forest. The “Soil Survey of the Humacao Area of Eastern Puerto Rico” (Boccheciamp and others, 1977) was published by the Soil Conservation Service in 1977. An inservice report, “The Soils of El Yunque—An Order III Soil Resource Inventory of the Caribbean National Forest,” (Ford, 1981) was completed in 1980. Other soil studies of specific areas in the Forest have been conducted (Soil Survey Staff, 1995). They can be valuable supplements to this soil survey. This report updates the previous surveys and provides additional information concerning the soils and their management.

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

Buss HL, Sak PB, Webb SM, Brantley SL. 2008. Weathering of the Rio
Blanco quartz diorite, Luquillo Mountains, Puerto Rico: coupling
oxidation, dissolution, and fracturing. Geochimica et Cosmochimica
Acta 72: 4488–4507.

In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2–2 m thick zone of partially weathered rock layers (2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive DV of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction. Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fractureinducing reaction. This reaction occurs with an expansion in d (001) from 10.0 to 10.5A ˚ , forming ‘‘altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2  1014 mol biotite m2 s1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 lm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone. Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet–saprolite interface, hornblende dissolves to completion at a rate of 6.3  1013 mol hornblende m2 s1: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O2 at the bedrock–saprolite interface.

Controls on major solutes within the drainage network of a rapidly 3 weathering tropical watershed

Bhatt, M. P., and W. H. McDowell (2007), Controls on major solutes within the drainage network of a rapidly weathering
27 tropical watershed, Water Resour. Res., 43, XXXXXX, doi:10.1029/2007WR005915.

Surface water chemistry in the main stem and source points of the Rio Icacos basin 7 (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors 8 regulating spatial variability in major solutes in a rapidly weathering landscape. We 9 sampled along the main stem as well as at small source points at high elevation where 10 fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial 11 plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base 12 cations were lower at the source points than in the main stem, and were lowest in low- 13 elevation source points. Calcium and sodium were the dominant cations at all sampling 14 points after sea-salt correction, reflecting the weathering of plagioclase feldspar 15 throughout the basin. The partial pressure of carbon dioxide (pCO2) tended to be higher, 16 and HCO3  concentrations were lower, in the low-elevation source points than at other 17 positions in the landscape. When coupled with the relatively low concentrations of Si and 18 base cations, this suggests that the availability of primary reactive minerals, rather than 19 carbonic acid concentrations, limits weathering in these low-elevation sources. 20 Mechanical denudation appears to enhance chemical weathering rates not only by 21 refreshing reactive mineral surfaces but also by contributing carbon dioxide from the 22 decomposition of organic-rich material in landslides, which occur frequently. The spatial 23 variability of major solutes appears to depend primarily on the availability of fresh primary 24 reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.
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