Interactive effects of native and exotic earthworms on resource use and nutrient mineralization in a tropical wet forest soil of Puerto Rico

Lachnicht, SL, PF Hendrix, and X. Zou. 2002. Interactive effects of native and exotic earthworms on resource use and nutrient mineralization in a tropical wet forest soil of puerto rico. Biology and Fertility of Soils 36 (1) (AUG): 43-52.

Investigation of single or mixed assemblages of native Estherella sp. and exotic Pontoscolex corethrurus from a rain forest in Puerto Rico was undertaken to understand resource use patterns, and linkages with C and N mineralization in a 19-day incubation. Resource use was explored with addition of 15N-enriched leaf litter and 13Cenriched glucose to reconstructed organic and mineral soil horizons. Juvenile Estherella sp. became at least 6.06‰ more enriched in 13C than sub-adult Estherella sp. or adult P. corethrurus. Sub-adult Estherella sp. became >3.6‰ enriched in 13C over P. corethrurus. δ15N acquired by P. corethrurus was greater by 0.83–1.56‰ in the mixed-species than the single-species assemblages. δ15N of subadult Estherella sp. was enriched by 0.73–0.81‰ over juvenile Estherella sp. in the single-species assemblage. Net N immobilization occurred in the organic layer of all 15Nenriched treatments. Net N mineralization in mineral soil layers was significantly greater in microcosms with P. corethrurus than in those containing only Estherella sp.. Cumulative respiration was greatest in P. corethrurus assemblages, however, assemblages with only Estherella sp. released more 13C in respiration. P. corethrurus assimilated different N resources when incubated with, as compared to without, native Estherella sp.. δ13C and δ15N signatures acquired by assimilation of 13C and 15N differed by species, developmental stage, and competitive interactions. The results showed that alone, exotic P. corethrurus induced higher mineralization rates than native Estherella sp., but that the interaction of exotic and native species impinged on resource use by P. corethrurus, reducing the effect of the exotic species on C and N mineralization. Invasion of exotic P. corethrurus may change the mineralization potentials of C and N and their biogeochemical cycling in soils.

Greater Soil Carbon Sequestration under Nitrogen-Fixing Trees Compared with Eucalyptus Species

Resh, SC, D. Binkley, and JA Parrotta. 2002. Greater soil carbon sequestration under nitrogen-fixing trees compared with eucalyptus species RID A-2703-2010. Ecosystems 5 (3) (APR): 217-31.

Forests with nitrogen-fixing trees (N-fixers) typically accumulate more carbon (C) in soils than similar forests without N-fixing trees. This difference may develop from fundamentally different processes, with either greater accumulation of recently fixed C or reduced decomposition of older soil C. We compared the soil C pools under N-fixers with Eucalyptus (non-N-fixers) at four tropical sites: two sites on Andisol soils in Hawaii and two sites on Vertisol and Entisol soils in Puerto Rico. Using stable carbon isotope techniques, we tracked the loss of the old soil organic C from the previous C4 land use (SOC4) and the gain of new soil organic C from the C3, N-fixer, and non-N-fixer plantations (SOC3). Soils beneath N-fixing trees sequestered 0.11 + 0.07 kg m-2 y-' (mean ± one standard error) of total soil organic carbon (SOCT) compared with no change under Eucalyptus( 0.00 ± 0.07 kg m-2 y-1; P = 0.02). About 55% of the greater SOCT sequestration under the N-fixers resulted from greater retention of old SOC4, and 45% resulted from greater accretion of new SOC3. Soil N accretion under the N-fixers explained 62% of the variability of the greater retention of old SOC4 under the N-fixers. The greater retention of older soil C under N-fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

Germanium–silicon fractionation in a tropical, granitic weathering environment

Lugolobi, Festo, Andrew C. Kurtz, and Louis A. Derry. 2010. Germanium-silicon fractionation in a tropical, granitic weathering environment. Geochimica Et Cosmochimica Acta 74 (4) (FEB 15): 1294-308.

Germanium–silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 lmol/ mol, with individual primary mineral phases ranging between 0.5 and 7 lmol/mol. The ratios in the bulk saprolite are higher (3 lmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 lmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1–1.6 lmol/mol) primarily due to the residual accumulation of Ge-poor quartz. Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27–0.47 lmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite–bedrock interface. Massbalance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite–bedrock interface. Higher pore water Ge/Si ratios (1.2 lmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.

Evidence for nitrogen fixation associated with macroalgae from a seagrass - mangrove - coral reef system

France, R., J. Holmquist, M. Chandler, and A. Cattaneo. 1998. Delta N-15 evidence for nitrogen fixation associated with macroalgae from a seagrass-mangrove coral reef system RID H-3188-2011. Marine Ecology-Progress Series 167 : 297-9.

Early studies using acetylene reduction have suggested that macroalgae may contribute to nitrogen enrichment of waters near coral reefs via nitrogen fixation by their epiphytic cynaphytes. Our objectives were to investigate the potential of stable nitrogen isotope analysis for detecting nitrogen fixation in near-reef macroalgae, and to compare these finding with those for different fixing and non-fixing autotrophs from other systems. We made collections of near-reef algae in Puerto Rico, seagrasses and macroalgae from streams in Quebec, Canada. The mean + SD 6 1 5 ~fo r near-reef algea was 0.3 r i.G%, vaiues whlch were significantly lower than those from our other sampling areas and also well below published values for other marine benthic nitrogen fixers (ca 2%) and non-fixers (ca 6%0). Our results provide a useful test of the value of stable nitrogen analysis in detecting nitrogen fixation of near-reef algae, thereby supporting previous non-isotope work in suggesting that this macroalgalcyanophyte complex may provide an important source of fixed nitrogen to reef systems.

Coral skeletal carbon isotopes (d13C and D14C) record the delivery of terrestrial carbon to the coastal waters of Puerto Rico

Moyer, R. P., and A. G. Grottoli. 2011. Coral skeletal carbon isotopes (delta(13)C and delta(14)C) record the delivery of terrestrial carbon to the coastal waters of puerto rico. Coral Reefs 30 (3) (SEP): 791-802.

Tropical small mountainous rivers deliver a poorly quantified, but potentially significant, amount of carbon to the world’s oceans. However, few historical records of land–ocean carbon transfer exist for any region on Earth. Corals have the potential to provide such records, because they draw on dissolved inorganic carbon (DIC) for calcification. In temperate systems, the stable- (d13C) and radiocarbon (D14C) isotopes of coastal DIC are influenced by the d13C and D14C of the DIC transported from adjacent rivers. A similar pattern should exist in tropical coastal DIC and hence coral skeletons. Here, d13C and D14C measurements were made in a 56-year-old Montastraea faveolata coral growing *1 km from the mouth of the Rio Fajardo in eastern Puerto Rico. Additionally, the d13C and D14C values of the DIC of the Rio Fajardo and its adjacent coastal waters were measured during two wet and dry seasons. Three major findings were observed: (1) synchronous depletions of both d13C and D14C in the coral skeleton are annually coherent with the timing of peak river discharge, (2) riverine DIC was always more depleted in d13C and D14C than seawater DIC, and (3) the correlation of d13C and D14C was the same in both coral skeleton and the DIC of the river and coastal waters. These results indicate that coral skeletal d13C and D14C are recording the delivery of riverine DIC to the coastal ocean. Thus, coral records could be used to develop proxies of historical land– ocean carbon flux for many tropical regions. Such information could be invaluable for understanding the role of tropical land–ocean carbon flux in the context of land-use change and global climate change.

Contributions of dust to phosphorus cycling in tropical forests of the Luquillo Mountains, Puerto Rico

Pett-Ridge, Julie C. 2009. Contributions of dust to phosphorus cycling in tropical forests of the luquillo mountains, puerto rico. Biogeochemistry 94 (1) (MAY): 63-80.

The input of phosphorus (P) through mineral aerosol dust deposition may be an important component of nutrient dynamics in tropical forest ecosystems. A new dust deposition calculation is used to construct a broad analysis of the importance of dust-derived P to the P budget of a montane wet tropical forest in the Luquillo Mountains of Puerto Rico. The dust deposition calculation used here takes advantage of an internal geochemical signal (Sr isotope mass balance) to provide a spatially integrated longer-term average dust deposition flux. Dust inputs of P (0.23 ± 0.08 kg ha-1 year-1) are compared with watershed-average inputs of P to the soil through the conversion of underlying saprolite into soil (between 0.07 and 0.19 kg ha-1 year-1), and with watershed-average losses of soil P through leaching (between 0.02 and 0.14 kg ha-1 year-1) and erosion (between 0.04 and 1.38 kg ha-1 year-1). The similar magnitude of dust-derived P inputs to that of other fluxes indicates that dust is an important component of the soil and biomass P budget in this ecosystem. Dust-derived inputs of P alone are capable of completely replacing the total soil and biomass P pool on a timescale of between 2.8 ka and 7.0 ka, less than both the average soil residence time (*15 ka) and the average landslide recurrence interval (*10 ka).

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

Pett-Ridge, Julie C., Louis A. Derry, and Jenna K. Barrows. 2009. Ca/Sr and (87)sr/(86)sr ratios as tracers of ca and sr cycling in the rio icacos watershed, luquillo mountains, puerto rico. Chemical Geology 267 (1-2) (SEP 15): 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, reflecting 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 reflect 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.

Carbon Isotope Characterization of Vegetation and Soil Organic Matter in Subtropical Forests in Luquillo, Puerto Rico

Von Fischer J.C., Tieszen L.L., Carbon Isotope Characterization of Vegetation and Soil Organic Matter in Subtropical Forests in Luquillo, Puerto Rico. Biotropica Vol 27(2), 1995 pp 138-148.

We examined natural abundances of "3C 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 613C was enriched 1.6%o 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%o in the Colorado forest to -26.9%o 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%o, and the Palm forest was most positive at -26.5%o. 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 "3C depleted than the bulk vegetation. We hypothesize that the anthropogenic isotopic depletion of atmospheric CO2 (ca 1.5%o 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.

Carbon Sequestration and Plan Community Dynamics Following Reforestation of Tropical Pasture

Silver W.L., Kuppers L.M., Lugo A.E. et al. Carbon Sequestration and Plan Community Dynamics Following Reforestation of Tropical Pasture. Ecological Applications, Vol 14(4), 2004 pp 1115-1127.

Conversion of abandoned cattle pastures to secondary forests and plantations in the tropics has been proposed as a means to increase rates of carbon (C) sequestration from the atmosphere and enhance local biodiversity. We used a long-term tropical reforestation project (55–61 yr) to estimate rates of above- and belowground C sequestration and to investigate the impact of planted species on overall plant community structure. Thirteen tree species (nine native and four nonnative species) were planted as part of the reforestation effort in the mid to late 1930s. In 1992, there were 75 tree species (.9.1 cm dbh) in the forest. Overall, planted species accounted for 40% of the importance value of the forest; planted nonnative species contributed only 5% of the importance value. In the reforested ecosystem, the total soil C pool (0–60 cm depth) was larger than the aboveground C pool, and there was more soil C in the forest (102 6 10 Mg/ha [mean 6 1 SE]) than in an adjacent pasture of similar age (69 6 16 Mg/ha). Forest soil C (C3-C) increased at a rate of ;0.9 Mg·ha21·yr21, but residual pasture C (C4-C) was lost at a rate of 0.4 Mg·ha21·yr21, yielding a net gain of 33 Mg/ha as a result of 61 years of forest regrowth. Aboveground C accumulated at a rate of 1.4 6 0.05 Mg·ha21·yr21, to a total of 80 6 3 Mg/ha. A survey of 426 merchantable trees in 1959 and 1992 showed that they grew faster in the second 33 years of forest development than in the first 22 years, indicating that later stages of forest development can play an important role in C sequestration. Few indices of C cycling were correlated with plant community composition or structure. Our results indicate that significant soil C can accumulate with reforestation and that there are strong legacies of pasture use and reforestation in plant community structure and rates of plant C sequestration.

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.

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.
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