B22A-01
Characterization of Differently Managed Mountainous Ecosystems in Complex Terrains Using Stable C, O and N Isotopes.
Mountain regions are recognized for their sensitivity with respect to land use, climate change and air pollution. The Alpine regions in Europe are not only covered with a vulnerable vegetation cover (forests and agricultural grasslands) but provide a living for a minor but considerable part of the European population. Besides important transit routes between the North and South of Europe the central alpine mountainous regions are the source of more than 90% of Europe's fresh water. On-going socio-economic changes have particular effects on land use in mountain areas. The consequences of the abandonment of mountainous agriculture range from re-invasions of forest growths to near irreversible soil erosions, which can only be stopped with labor intensive and large financial investments. Besides studying the effects of land use change the ongoing climate change represent another challenge. To understand the ecological implications of the rapidly changing and heterogeneous ecosystem numerous research activities on the structure and functioning were and are still carried out. The alterations of the magnitudes of the sources, sinks and carbon fluxes in differently managed mountainous grassland and forest ecosystems are assessed. Based on results of past and ongoing studies in mountain ecosystems on different scales various approaches to assess water and carbon relations are discussed (from Eddy flux to chamber measurements). Stable isotopes are used to analyze the different fluxes of the essential ecosystem compartments. Starting with the characterization of soil efflux measurements in pulse labeling experiments the application of isotopes for the estimation of the partial fluxes of the Net Ecosystem Exchange is evaluated. The integrative features of C and O isotopes in organic material are shown in characterizing the differences in grasslands, subject to various land use changes. As land use changes alter the micro-climate of the grasslands this is reflected in the C and O isotopic signature of the plants. The potential for the characterization of the ecosystem changes from this stored information is demonstrated.
B22A-02 INVITED
From flat ground to steep mountainous subsurface: Use of stable isotope data for watershed model development and testing
Mountain watersheds are often runoff dominated ecological systems. Much of the hydrologic flux into streams is below ground and difficult to quantify beyond the plot and hillslope scale. In recent years, stable isotope tracers have revealed much in terms of where streamwater comes from geographically in the watershed, what flowpath the water takes to the stream and how long water resides in the watershed. While this work has greatly improved our understanding of below-ground water flux, few attempts have been made to use this information for watershed model development and testing. Much of the reluctance to incorporate such effects relates to the need to define the subsurface volume of the system, something that is still very difficult with current hydrometric techniques. Here we present a new watershed modeling approach and water accounting scheme that parameterizes the subsurface mixing volume with isotope based information. We present two case studies to elucidate how these subsurface properties influence the source components and residence time of streamwater. The first case study compares two small well-studied headwater watersheds (at Maimai in New Zealand and HJ Andrews in Oregon). We use isotope data within the model to show how catchment properties such as soil depth and its distribution in space, control the age and source of streamflow at the headwater scale. In the second case study, we show how these factors influence landscape level processes in two meso-scale river basins in the Coast Range mountains of California. Here, the seasonal trend of stored water volume inferred from runoff coefficients is used to predict pre-event/event water fractions and mean residence time that we then test with isotope data and model results. Overall, our findings suggest that representing stable isotope information within a distributed watershed model structure can lead to new insights in ecohydrological behavior across multiple scales.
B22A-03 INVITED
Two water worlds: Isotope evidence shows that trees and streams return different pools of water to the hydrosphere
Ecohydrological coupling at the watershed scale is poorly characterized, particularly in mountainous terrain. While soil-water storage is dynamic and strongly influenced by plants, few integrated tools exist for quantifying the spatial and temporal dynamics and interactions among the major components of the terrestrial hydrologic cycle. We analyzed stable isotopes of oxygen and hydrogen in water to quantify spatial and temporal changes in precipitation, evaporation, soil water, tree water and stream discharge isotopic signatures at Watershed 10 at the HJ Andrews Experimental Forest in the Cascade Mountains of Oregon. The watershed is steep and small, ranging in elevation from 425 to 700 m within the 10 ha boundary. The region has a Mediterranean climate with very dry summers and wet winters. At the end of the wet season (June), measured soil-water storage was at its maximum, and plant water uptake occurred primarily from the soil surface. As the dry season progressed, plants relied on deeper soil water. Evaporation from the soil surface resulted in a distinct isotopic signature of tightly-bound soil water. The extent of evaporation from surface soils varied with topography and soil depth within the watershed. Our isotope data indicate that most water taken up by plants during the summer was affected by evaporation at some point, including soil water deeper than 30 cm. In contrast, mobile water reaching the stream and forming stream water did not show any evidence of evaporation even though discharge rates showed distinct diurnal cycles driven by transpiration. During the fall wet-up, soil lysimeter water and stream water were consistent with meteoric water signatures but with damping temporal variation and a several week response lag. Bulk soil water also began to look isotopically similar to lysimeter and stream water as soils "wetted-up". Nevertheless, tree water did not change with the onset of winter precipitation indicating that tree water residence time was very long during the winter dormancy period. Tree water also retained its evaporated signal. We conceptualize these ecohydrological findings as two distinct and separate pools of water held within the soil: one a mobile pool held at relatively low matric tension, making it more subject to gravitational transport to streams when more water is added to the system. The other pool is water held under higher matric tensions and has a longer residence time within the soil, and a higher propensity to be taken up by plants. Overall, our isotopic data provided insights into ecohydrological coupling that we had not expected based on our previous hydrometric- based hydrological analysis and physiologically-based tree analysis.
B22A-04
Seasonal variations in compound-specific leaf-wax lipid δD values and their relationship to environmental factors and plant physiological processes
Lipids are present in high concentrations in the leaf waxes of deciduous plants and are the final products of plant biosynthesis. Their hydrogen isotopic composition (δD) should therefore provide a time- integrated value of the leaf water used for biosynthesis and in turn the environmental and physiological factors determining leaf water enrichment. Due to the stability of lipids, their stable isotopic composition in soils and sediments could become a powerful tool to asses (paleo)hydrologic conditions. However, the relative importances of plant physiology and climatic influences as well as the timeframe over which the lipid δD value integrates these factors have not been investigated systematically. Here we explore how changes in environmental parameters and plant physiological processes over a growing season are recorded in compound-specific hydrogen isotope ratios of individual leaf-wax lipids. We sampled soil water, leaf water at dawn and midday, water vapor for hydrogen isotopic analysis, leaves for lipid analysis and recorded a number of environmental parameters (temperature, relative humidity, vapor pressure deficit among others) and plant physiological data (stomatal conductance, transpiration, photosynthetic rate) weekly over the two month growing season of wheat grass. We analyze the relative importance of plant physiological processes and environmental factors in determining leaf water enrichment and the leaf wax lipid isotopic composition. The isotopic composition of soil water and leaf water at dawn showed similar trends and increased over the growing season with short-term variations of about 40‰. Leaf wax lipid δD values varied only on the order of 20‰ over the growing season following a similar trend as soil water and leaf water at dawn. We observe a 20‰ decrease in lipid δD values only a week after a significant 40‰ decrease in the soil and leaf water isotopic composition, due to a strong rain event. These results suggest, that leaf-wax lipids are produced during the whole growing season and record the leaf water isotopic signal integrated over a timeframe on the order of a week. We observe stronger correlations of environmental conditions at dawn with the lipid δD values compared to midday conditions, suggesting more of a climatic background signal is recorded in the lipids removing the midday extremes. Systematic studies like the one presented here are essential, when interpreting variations in leaf-wax lipid δD values in soils and sediments as records of (paleo)ecosystem hydrology.
B22A-05
Anaerobic Methane Oxidation in Soils - revealed using 13C-labelled methane tracers
In marine sediments, anaerobic methane oxidation is a significant biogeochemical process limiting methane flux from ocean to atmosphere. To date, evidence for anaerobic methane oxidation in terrestrial environments has proved elusive, and its significance is uncertain. In this study, an isotope dilution method specifically designed to detect the process of anaerobic methane oxidation in methanogenic wetland soils is applied. Methane emissions of soils from three contrasting permanently waterlogged sites in Scotland are investigated in strictly anoxic microcosms to which 13C- labelled methane is added, and changes in the concentration and 12C/13C isotope ratios of methane and carbon dioxide are subsequently measured and used to calculate separate the separate components of the methane flux. The method used takes into account the 13C-methane associated with methanogenesis, and the amount of methane dissolved in the soil. The calculations make no prior assumptions about the kinetics of methane production or oxidation. The results indicate that methane oxidation can take place in anoxic soil environments. The clearest evidence for anaerobic methane oxidation is provided by soils from a minerotrophic fen site (pH 6.0) in Bin Forest underlain by ultra-basic and serpentine till. In the fresh soil anoxic microcosms, net consumption methane was observed, and the amount of headspace 13C-CO2 increased at a greater rate than the 12+13C-CO2, further proof of methane oxidation. A net increase in methane was measured in microcosms of soil from Murder Moss, an alkaline site, pH 6.5, with a strong calcareous influence. However, the 13C-CH4 data provided evidence of methane oxidation, both in the disappearance of C- CH4 and appearance of smaller quantities of 13C-CO2. The least alkaline (pH 5.5) microcosms, of Gateside Farm soil - a granitic till - exhibited net methanogenesis and the changes in 13C-CH4 and 13C-CO2 here followed the pattern expected if no methane is consumed. Overall, this study provides good evidence for anaerobic methane oxidation in certain wetland soils, and suggests that models predicting methane flux from wetland soils to the atmosphere could be improved by better understanding of this process.
B22A-06
The Oxygen Isotopic Signature of Nitrous Oxide is Determined by Oxygen Exchange
In order to derive accurate budget estimations and effective mitigation strategies for the greenhouse gas nitrous oxide (N2O), it is essential to identify the processes involved in its production. Analyses of the isotopic composition of N2O are increasingly used to characterize the importance of these processes. However, we argue that the reliability of results based on oxygen (O) isotopic analysis of N2O may be questioned due to insufficient consideration of O exchange between H2O and nitrogen oxides. We studied the process of O exchange in 12 widely varying soils using a novel combination of 18O and 15N tracing experiments. Incorporation of O from 18O-enriched H2O into N2O exceeded theoretical maxima based on reaction stoichiometry, revealing the presence of O exchange. Novel methodology based on the retention of the 18O:15N-enrichment ratio of NO3- in N2O allowed to quantify O exchange during denitrification: up to 97% of N2O-O originated from H2O instead of NO3-. Our results show that in soil, the main source of N2O, the conventional assumption that the O isotopic composition of N2O is determined by reaction stoichiometry and isotopic fractionation during its production does not hold. In all cases, the O isotopic signature of N2O was found to be dominated by the effect of O exchange between nitrogen oxides and water. We speculate that the implications of O exchange will extend across terrestrial and aquatic ecosystems, and possibly to other nitrogen oxides as well. Especially, a potential effect of O exchange on the O isotopic signature of NO3- needs to be studied, as this is routinely used for NO3- source determination. Our results may facilitate the development of improved methodology to study and understand the global N cycle.
B22A-07
Stable isotopic indicators of nitrous oxide and methane sources in Los Angeles, California
As urbanization increasingly encroaches upon agricultural landscapes, there are greater potential sources of greenhouse gases and other atmospheric contaminants. Measurements of the isotopic composition of trace gases have the potential to distinguish between pollutant sources and quantify the proportional contribution of agricultural activities to the total atmospheric pool. In this study, we are measuring the isotopic composition of greenhouse gases N2O and CH4 emitted from cropland, animal feeding operations, and urban activities in the South Coast Air Basin in southern California. The ultimate goal of our project is to utilize atmospheric measurements of the isotopic composition of N2O and CH4 combined with studies of source signatures to determine the proportional contributions of cropland, animal operations, and urban sources of greenhouse gases to the atmosphere. Measurements of the δ13C of methane show excellent separation between urban sources, such as vehicle emissions, power plants, oil refineries, landfills, and sewage treatment plants and agricultural sources like cows, biogas, and cattle feedlots. For nitrous oxide, soil N2O sources showed good separation from wastewater treatment facilities using δ15N and δ18O. Within soil N2O sources, the isotopic composition of N2O from cropland soils was similar to N2O emissions from urban turfgrass. These data indicate that nitrification may be as important a source of N2O as denitrification in urban soils. We are also measuring N2O fluxes from soils and from sewage treatment processes, and preliminary data indicate that urban N2O fluxes are higher than initially assumed by managers and regulatory agencies.