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.

Spatial dependence and the relationship of soil organic carbon and soil moisture in the luquillo experimental forest, puerto rico

Wang H, Hall CAS, Cornell JD, Hall MHP.
2002. Spatial dependence and the relationship
of soil organic carbon and soil moisture in Luquillo experimental forest. Landsc.
Ecol. 17:671–84

We used geo-spatial statistical techniques to examine the spatial variation and relationship of soil organic carbon (SOC) and soil moisture (SM) in the Luquillo Experimental Forest (LEF), Puerto Rico, in order to test the hypothesis that mountainous terrain introduces spatial autocorrelation and crosscorrelation in ecosystem and soil properties. Soil samples (n = 100) were collected from the LEF in the summer of 1998 and analyzed for SOC, SM, and bulk density (BD). A global positioning system was used to georeference the location of each sampling site. At each site, elevation, slope and aspect were recorded. We calculated the isotropic and anisotropic semivariograms of soil and topographic properties, as well as the cross-variograms between SOC and SM, and between SOC and elevation. Then we used four models (random, linear, spherical and wave/hole) to test the semivariances of SOC, SM, BD, elevation, slope and aspect for spatial dependence. Our results indicate that all the studied properties except slope angle exhibit spatial dependence within the scale of sampling (200 – 1000 m sampling interval). The spatially structured variance (the variance due to the location of sampling sites) accounted for a large proportion of the sample variance for elevation (99%), BD (90%), SOC (68%), aspect (56%) and SM (44%). The ranges of spatial dependence (the distances within which parameters are spatially dependent) for aspect, SOC, elevation, SM, and BD were 9810 m, 3070 m, 1120 m, 930 m and 430 m, respectively. Cross correlograms indicate that SOC varies closely with elevation and SM depending on the distances between samples. The correlation can shift from positive to negative as the separation distance increases. Larger ranges of spatial dependence of SOC, aspect and elevation indicate that the distribution of SOC in the LEF is determined by a combination of biotic (e.g., litterfall) and abiotic factors (e.g., microclimate and topographic features) related to elevation and aspect. This demonstrates the importance of both elevation and topographic gradients in controlling climate, vegetation distribution and soil properties as well as the associated biogeochemical processes in the LEF.

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.

Speciation of water‐soluble inorganic, organic, and total nitrogen in a background marine environment: Cloud water, rainwater, and aerosol particles

Gioda, A., G. J. Reyes‐Rodríguez, G. Santos‐Figueroa, J. L. Collett Jr., S. Decesari, M. d. C. K. V. Ramos, H. J. C.
Bezerra Netto, F. R. de Aquino Neto, and O. L. Mayol‐Bracero (2011), Speciation of water‐soluble inorganic, organic, and total
nitrogen in a background marine environment: Cloud water, rainwater, and aerosol particles, J. Geophys. Res., 116, D05203,

Cloud water, rainwater, and aerosol particles were collected in Puerto Rico from December 2004 to March 2007 in order to investigate their chemical composition, relation to sources, and removal processes. The species analyzed were inorganic ions, metals, total and dissolved organic carbon (TOC, DOC), total nitrogen (TN), and organic acids. For all samples, the dominant species were marine (Na+, Cl−), representing about 50%–65% of total content. Non‐sea‐salt fraction was dominated by SO42− (17%–25%), followed by water‐soluble organic (2%–8%) and total nitrogen (2% –6%) compounds. Organic acids represented contributions to the organic fraction in cloud water of 20% and 6% for aerosol particles. Inorganic species were predominant in total nitrogen portion. The chemical composition of cloud water, rainwater, and aerosol particles were observed to be sensitive to transport patterns. Air masses from northwest Africa showed the highest concentrations of nss‐Ca2+, Fe, and Al, suggesting a crustal origin. The pH values for cloud water and rainwater observed under this transport pattern were higher than background conditions, probably due to the alkalinity associated with nss‐Ca2+. The highest concentrations of Cl− and SO42−, with lower pH, were measured during periods of influence from Soufriere Hills volcano eruptions, most likely due to emitted SO2 and HCl. Air masses from North America had an anthropogenic influence, where levels of nss‐SO42−, TOC, and TN were higher (∼4 times) than in clean air masses. These results suggest that long‐range transport could be an extra source of metals and organic/nitrogen species to the Caribbean region ecosystems.

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.

Cloud water in windward and leeward mountain forests: The stable isotope signature of orographic cloud water

Scholl, M. A., T. W. Giambelluca, S. B. Gingerich, M. A. Nullet, and L. L. Loope (2007), Cloud water in windward
and leeward mountain forests: The stable isotope signature of orographic cloud water, Water Resour. Res., 43, W12411,

Cloud water can be a significant hydrologic input to mountain forests. Because it is a precipitation source that is vulnerable to climate change, it is important to quantify amounts of cloud water input at watershed and regional scales. During this study, cloud water and rain samples were collected monthly for 2 years at sites on windward and leeward East Maui. The difference in isotopic composition between volume-weighted average cloud water and rain samples was 1.4% d18O and 12% d2H for the windward site and 2.8% d18O and 25% d2H for the leeward site, with the cloud water samples enriched in 18O and 2H relative to the rain samples. A summary of previous literature shows that fog and/or cloud water is enriched in 18O and 2H compared to rain at many locations around the world; this study documents cloud water and rain isotopic composition resulting from weather patterns common to montane environments in the trade wind latitudes. An end-member isotopic composition for cloud water was identified for each site and was used in an isotopic mixing model to estimate the proportion of precipitation input from orographic clouds. Orographic cloud water input was 37% of the total precipitation at the windward site and 46% at the leeward site. This represents an estimate of water input to the forest that could be altered by changes in cloud base altitude resulting from global climate change or deforestation.

The stable isotope amount effect: New insights from NEXRAD echo tops, Luquillo Mountains, Puerto Rico

Scholl MA, Shanley JB, Zegarra JP, Coplen TB. 2009. The stable isotope
amount effect: new insights from NEXRAD echo tops, Luquillo
Mountains, Puerto Rico. Water Resources Research 45: W12407, DOI:

The stable isotope amount effect has often been invoked to explain patterns of isotopic composition of rainfall in the tropics. This paper describes a new approach, correlating the isotopic composition of precipitation with cloud height and atmospheric temperature using NEXRAD radar echo tops, which are a measure of the maximum altitude of rainfall within the clouds. The seasonal differences in echo top altitudes and their corresponding temperatures are correlated with the isotopic composition of rainfall. These results offer another factor to consider in interpretation of the seasonal variation in isotopic composition of tropical rainfall, which has previously been linked to amount or rainout effects and not to temperature effects. Rain and cloud water isotope collectors in the Luquillo Mountains in northeastern Puerto Rico were sampled monthly for three years and precipitation was analyzed for δ18O and δ2H. Precipitation enriched in 18O and 2H occurred during the winter dry season (approximately December-May) and was associated with a weather pattern of trade-wind showers and frontal systems. During the summer rainy season (approximately June-November), precipitation was depleted in 18O and 2H and originated in low pressure systems and convection associated with waves embedded in the prevailing easterly airflow. Rain substantially depleted in 18O and 2H compared to the aforementioned weather patterns occurred during large low pressure systems. Weather analysis showed that 29 % of rain input to the Luquillo Mountains was trade-wind orographic rainfall, and 30 % of rainfall could be attributed to easterly waves and low pressure systems. Isotopic signatures associated with these major climate patterns can be used to determine their influence on streamflow and groundwater recharge and to monitor possible effects of climate change on regional water resources.

Organic carbon, total nitrogen, and water-soluble ions in clouds from a tropical montane cloud forest in Puerto Rico

G.J. Reyes-Rodriguez, A. Gioda, O.L. Mayol-Bracero and J. Collett, Organic carbon, total nitrogen, and water-soluble ions in clouds from a tropical montane cloud forest in Puerto Rico, Atmospheric Environment 43 (2009), pp. 4171–4177. Abstract | Article | PDF (530 K) | View Record in Scopus | Cited By in Scopus (3)

samples collected in a mountaintop site in Puerto Rico. Cloudwater samples showed average concentrations of 1.09 mg L1 of total organic carbon (TOC), of 0.85 mg L1 for dissolved organic carbon (DOC) and of and 1.25 mg L1 for total nitrogen (TN). Concentrations of organic nitrogen (ON) changed with the origin of the air mass. Changes in their concentrationswere observed during periods under the influence of African dust (AD). The ON/TN ratios were 0.26 for the clean and 0.35 for the AD periods. Average concentrations of all these species were similar to those found in remote environments with no anthropogenic contribution. In the AD period, for cloud water the concentrations of TOC were 4 times higher and TN were 3 times higher than during periods of clean air masses associated with the trade winds. These results suggest that a significant fraction of TOC and TN in cloud and rainwater is associated to airborne particulate matter present in dust. Functional groups were identified using proton nuclear magnetic resonance (1H NMR) spectroscopy. This characterization led to the conclusion that water-soluble organic compounds in these samples are mainly aliphatic oxygenated compounds, with a small amount of aromatics. The ion chromatography results showed that the ionic specieswere predominantly of marine origin, for air masses with and without African dust influence, with cloud water concentrations of NO3  and NH4 þ much lower than from polluted areas in the US. An increase of such species as SO42, Cl, Mg2þ, Kþ and Ca2þ was seen when air masses originated from northwest Africa. The changes in the chemical composition and physical properties of clouds associated with these different types of aerosol particles could affect on cloud formation and processes.

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

Pett-Ridge J. C., Derry L. A. and Kurtz A. C. (2009) Sr isotopes as
a tracer of weathering processes and dust inputs in a tropical
granitoid watershed, Luquillo Mountains, Puerto Rico. Geochim.
Cosmochim. Acta 73, 25–43.

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. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm2 ka1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified

Ca/Sr and 87Sr/86Sr ratios as tracers of Ca and Sr cycling in the Rio Icacos watershed, Luquillo Mountains, Puerto Rico

Julie C. Pett-Ridge, Louis A. Derry, Jenna K. Barrows
Ca/Sr and 87Sr/86Sr ratios as tracers of Ca and Sr cycling in the Rio Icacos watershed, Luquillo Mountains, Puerto Rico
Chemical Geology (2009)
Volume: 267, Issue: 1-2, Publisher: Elsevier B.V., Pages: 32-45

We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bedrock-derived Sr and presumably Ca is cycled through the vegetation, re!ecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil. This is surprising because over 99% of the Ca and Sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site, and because there are large atmospheric inputs to the site of both sea salt and Saharan dust. Substantial differences in Ca and Sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site. Measured Ca/Sr ratios re!ect fractionation between these elements during biogeochemical cycling. Fractionation was particularly evident between wood and foliar tissue, but fractionation during soil exchange processes is also likely. In comparing the Ca/Sr ratios of plants, exchangeable cations, and bulk soils, we found that foliar Ca/Sr ratios were greater than exchangeable cation Ca/Sr ratios, which in turn were greater than soil Ca/Sr ratios, similar to patterns observed at other highly weathered tropical sites.
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