Leonel da Silveira Lobo Sternberg

The use of stable isotopes to study ecosystem gas exchange

Abstract Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of ”Keeling plots.” This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO2 exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limited arsenal of ecosystem-scale research tools.

Seasonal changes in the diets of migrant and non-migrant nectarivorous bats as revealed by carbon stable isotope analysis

Three species of nectar-feeding bats migrate from tropical and subtropical Mexico into the Sonoran and Chihuahuan deserts during the spring and summer months. We examined geographic and seasonal changes in the diet of one migrant species, Leptonycteris curasoae, using carbon stable isotope techniques to determine the relative importance of C3 and CAM (Cactaceae, Agavaceae) plants in its diet. We also examined the diet of a non-migratory nectar-feeding bat, Glossophaga soricina, from southern Mexico using the same techniques. We found that L. curasoae feeds extensively or exclusively on CAM plants during migration and in the northern part of its range and feeds mostly on C3 plants in southern Mexico. This bat is a year-round resident on Baja California where it is a CAM specialist. The non-migrant G. soricina feeds mostly on C3 plants year-round. Phenological data suggest that certain species of columnar cacti and at least one group of paniculate Agaves on the Mexican mainland provide a spatio-temporally predictable nectar corridor along which nectarivorous bats may migrate in the spring and fall, respectively. Different flowering schedules of Agaves in Baja California appear to promote year-round dietary specialization and perhaps non-migratory behavior in nectar-feeding bats living there.

UTILIZATION OF FRESHWATER AND OCEAN WATER BY COASTAL PLANTS OF SOUTHERN FLORIDA

The coastal vegetation of southern Florida is undergoing dramatic changes due to the instability of the ocean water-freshwater boundary. These vegetation changes will be determined by the response of each particular species to saline ocean water, particularly whether it can use ocean water or not. In this study, isotopic data were used to determine the relative usage of freshwater or ocean water by plants in the Florida keys. The results indicate that, with some exceptions, plants toward the interior of the keys were using freshwater while those toward the edge were using ocean water. A plot of the hydrogen and oxygen isotopic composition of the plant water yielded a mixing line between typical freshwater values and those of ocean water. In general, the isotopic ratios of stem water for species found in hardwood hammocks were confined to the freshwater end of the line, followed by values of stem water from mangrove margin species. found in mangroves, however, had water with extremely variable isotopic ratios, ranging from values typical of ocean water to values typical of freshwater. This variability is consistent with the hypothesis that mangroves are fully capable of growing in freshwater, but are limited to saline habitats because of competitive exclusion by fast-growing glycophilic plants.

Biogeochemical implications of the isotopic equilibrium fractionation factor between the oxygen atoms of acetone and water

Carbonyl oxygens of organic molecules undergo isotopic exchange with water during reversible hydration reactions. The equilibrium isotopic fractionation factors between the carbonyl oxygen of acetone and water at 15°, 25°, and 35°C are 1.028, 1.028, and 1.026 respectively. The differences between the δ18O values of the carbonyl oxygen of acetone and of the water with which it is in equilibrium are similar to the differences that have been observed between the δ18O values of cellulose and the water used in its synthesis by a variety of aquatic plants and animals. Additionally, the identity of the acetone-water fractionation factors at 15° and 25°C parallels the observation that the difference between the δ18O values of cellulose and water shows no temperature dependence for individual species of plants grown over the same temperature range. These results are discussed in relation to the proposal that the oxygen isotopic relationship between cellulose and water is established by isotopic exchange occurring during the hydration of carbonyl groups of the intermediates of cellulose synthesis.

Oxygen stable isotope ratios of tree‐ring cellulose: the next phase of understanding

Analysis of the oxygen isotope ratio of tree-ring cellulose is a valuable tool that can be used as a paleoclimate proxy. Our ability to use this tool has gone through different phases. The first began in the 1970s with the demonstration of empirical relationships between the oxygen isotope ratio of tree-ring cellulose and climate. These empirical relationships, however, did not provide us with the confidence that they are robust through time, across taxa and across geographical locations. The second phase began with a rudimentary understanding of the physiological and biochemical mechanisms responsible for the oxygen isotope ratios of cellulose, which is necessary to increase the power of this tool. This phase culminated in a mechanistic tree-ring model integrating concepts of physiology and biochemistry in a whole-plant system. This model made several assumptions about leaf water isotopic enrichment and biochemistry which, in the nascent third phase, are now being challenged, with surprising results. These third-phase results suggest that, contrary to the model assumption, leaf temperature across a large latitudinal gradient is remarkably constant and does not follow ambient temperature. Recent findings also indicate that the biochemistry responsible for the incorporation of the cellulose oxygen isotopic signature is not as simple as has been assumed. Interestingly, the results of these challenges have strengthened the tree-ring model. There are several other assumptions that can be investigated which will improve the utility of the tree-ring model.

D/H ratios of environmental water recorded by D/H ratios of plant lipids

Isotope ratios of chemical components are powerful tools in the interpretation of palaeoenvironments, particularly carbonates from foraminiferans1,2, desert caliche3, and desert pavements4. Isotope ratios of plant cellulose are also indicators of environmental variables such as temperature and relative humidity5–7. Several workers have reported climatic fluctuations based on hydrogen and oxygen isotope ratios of cellulose from tree trunks8–10 and peats11,12. Here I present measurements that demonstrate that for submerged aquatic plants δD values of lipids record D/H ratios of environmental water, whereas cellulose does not. I further demonstrate that δD values of lipids in conjunction with δ18O values of cellulose provide significant information on isotope ratios of environmental water.

Hydrogen Isotopic Fractionation by Plant Roots during Water Uptake in Coastal Wetland Plants

Publisher Summary Hydrogen and oxygen isotope analyses of plant stem water are used to quantitatively determine the use of different water sources by various plants in different environments—such as white pines, desert plants, streamside trees, and coastal plants. There are differences in isotopic composition among possible water sources. There is no isotopic fractionation by plant roots during water uptake. The study discussed in this chapter compared δD and δ 18Ο values between stem and source water from several plant species growing naturally in coastal wet-land habitats. Both hydrogen and oxygen isotope ratios of stem water matched those of source water under field and greenhouse conditions when there was no isotopic fractionation during water uptake. On the other hand, differences in isotopic compositions between stem and source water indicate isotopic fractionation during water uptake. The results indicate that there is a significant hydrogen isotopic fractionation during water uptake in coastal wetland plants. Oxygen isotope ratios of stem water from coastal wetland plants, however, match those of source water.

Humidity estimate for the middle Eocene Arctic rain forest

The exquisite preservation of fossilized Metasequoia trees that grew near 808N latitude during the middle Eocene (ca. 45 Ma) in Nunavut, Canada, allowed for dD and d 18 O analyses of cellulose, techniques previously restricted to wood ,30,000 yr old. From the isotopic results, we determined that the middle Eocene Arctic atmosphere contained ;23 the water found in the region’s atmosphere today. This water vapor contributed to a middle Eocene greenhouse effect that insulated the polar region during dark polar winters.

Sea‐Level Rise and the Reduction in Pine Forests in the Florida Keys

Forests dominated by Pinus elliottii var densa have undergone a reduction in area in the Florida Keys (USA). A previous investigation interpreted the presence of halophytic species in a former pine forest in Key Largo as evidence of sea-level rise. We therefore examined aerial photos and field evidence to learn how the 15-cm rise in local sea level over the last 70 yr had affected the distribution of pines on a second island, where intact pine forests still remained in 1991. The distribution of in situ dead pine stems showed that the area occupied by pines on Sugarloaf Key was 88 ha at some time prior to the earliest available aerial photographs, in 1935. The area of pine forest was reduced to 46 ha by 1935, and continued to decrease through 1991, when it covered 30 ha. The pattern of pine mortality was related to topographic position, with the areas where pines died earliest occupying the lowest elevations. Our analysis of current vegetation patterns showed that the areas of earliest pine mortality are now populated by a higher proportion of halophytic plant assemblages than areas of more recent pine mortality. We also compared the physiological responses of pines in two portions of the island: one where pine forest reduction had been most pronounced, and a second where the extent of the forest had changed little over the past 50 yr. Both groundwater and soil water salinity were higher in the area of rapid pine forest reduction, and the pines sampled there exhibited higher physiological stress, as indicated by pre-dawn water potential and stemwood carbon isotope ratios. These results suggest that the salinization of ground- and soil water that occurs as sea level rises is a major factor in the reduction of pine forests of Sugarloaf Key. If sea level continues to increase, the Florida Keys will experience a decline in both landscape and species diversity, as species-rich upland communities are replaced by simpler mangrove communities. This pattern may also occur in other low-lying island ecosystems with limited freshwater resources.

Lake and watershed characteristics rather than climate influence nutrient limitation in shallow lakes

Both nitrogen (N) and phosphorus (P) can limit primary production in shallow lakes, but it is still debated how the importance of N and P varies in time and space. We sampled 83 shallow lakes along a latitudinal gradient (5 degrees 55 degrees S) in South America and assessed the potential nutrient limitation using different methods including nutrient ratios in sediment, water, and seston, dissolved nutrient concentrations, and occurrence of N-fixing cyanobacteria. We found that local characteristics such as soil type and associated land use in the catchment, hydrology, and also the presence of abundant submerged macrophyte growth influenced N and P limitation. We found neither a consistent variation in nutrient limitation nor indications for a steady change in denitrification along the latitudinal gradient. Contrary to findings in other regions, we did not find a relationship between the occurrence of (N-fixing and non-N-fixing) cyanobacteria and the TN:TP ratio. We found N-fixing cyanobacteria (those with heterocysts) exclusively in lakes with dissolved inorganic nitrogen (DIN) concentrations of < 100 microg/L, but notably they were also often absent in lakes with low DIN concentrations. We argue that local factors such as land use and hydrology have a stronger influence on which nutrient is limiting than climate. Furthermore, our data show that in a wide range of climates N limitation does not necessarily lead to cyanobacterial dominance.