Water isotopes in desiccating lichens

The stable isotopic composition of water is routinely used as a tracer to study water exchange processes in vascular plants and ecosystems. To date, no study has focussed on isotope processes in non-vascular, poikilohydric organisms such as lichens and bryophytes. To understand basic isotope exchange processes of non-vascular plants, thallus water isotopic composition was studied in various green-algal lichens exposed to desiccation. The study indicates that lichens equilibrate with the isotopic composition of surrounding water vapour. A model was developed as a proof of concept that accounts for the specific water relations of these poikilohydric organisms. The approach incorporates first their variable thallus water potential and second a compartmentation of the thallus water into two isotopically distinct but connected water pools. Moreover, the results represent first steps towards the development of poikilohydric organisms as a recorder of ambient vapour isotopic composition

Seasonal variability of greenhouse gases in the lower troposphere above the eastern European taiga (Syktyvkar, Russia)

A three year long record of regular vertical aircraft profiling for continuous atmospheric CO2 mixing ratio measurements as well as for flask sampling to derive the climatology of other greenhouse gases (CH4, SF6 and N2O), is presented. Measurements were undertaken in the lower troposphere between 100 and 3000 m over the eastern European taiga about 100 km south east of the city of Syktyvkar (61°24′N, 52°18′E). From the continuous profiles mean CO2 mixing ratios were calculated for the atmospheric boundary layer (ABL) and for the “free troposphere” up to 3000 m. The amplitudes of the respective seasonal cycles are 22.1 ± 3.5 and 14.0 ± 2.1 ppm. ABL mixing ratios are generally larger than free tropospheric values during the winter period, and smaller during the summer due to the change of the continental biosphere from a source to a sink. The phasing of the seasonal cycles is slightly different between the two height intervals (by about 30 days), with the ABL extremes occurring earlier. Very abrupt concentration changes up to 8 ppm are observed in the free troposphere associated with changes in air mass origin. Mean CO2 mixing ratios derived from flask samples at 3000 m compare well with the respective integrated values measured in the continuous profiles above the ABL (ΔCO2 = 0.3 ± 1.6 ppm). CH4 mixing ratios also show a pronounced seasonality, and winter time vertical gradients correlate well with those of CO2. Similarly, SF6 vertical gradients are correlated with CO2 gradients possibly pointing to some anthropogenic origin of the boundary layer CO2 signal during winter. N2O and SF6 also show a slight seasonality with almost the same phasing. The main reasons for the seasonality of both gases are probably transport processes with a possible contribution from stratosphere/troposphere exchange

Modelling of biospheric CO2 gross fluxes via oxygen isotopes in a spruce forest canopy : a Rn-222 calibrated box model approach

International audienc

Observations of atmospheric variability and soil exhalation rate of radon-222 at a Russian forest site - Technical approach and deployment for boundary layer studies

A monitor for continuous observations of the atmospheric Rn-222 daughter activity has been improved and successfully implemented in a field study in the European Taiga (Fyodorovskoye Forest Reserve). The alpha-activity of the short-lived Rn-222 and Rn-220 (Pb-212) decay products, which are attached to aerosols, is accumulated on a quartz aerosol filter and assayed on line by alpha-spectroscopy. The alpha-activity from the Pb-212 daughters is determined by spectroscopy and corrected for. This monitor is suitable to measure Rn-222 activities at hourly resolution down to 0.5 Bq m(-3) with an uncertainty well below +/-20%. The prototype of this monitor is run in Heidelberg on the roof of the Institute's building about 20 m above ground. For this site, the atmospheric radioactive disequilibrium was determined between the Rn-222 daughter Po-214 and Rn-222, which has to be known in order to derive the atmospheric Rn-222 activity with the static filter method. We derived a mean disequilibrium Po-214/Rn-222 = 0.704 +/- 0.081 for various meteorological conditions through parallel Rn-222 gas measurements with a slow pulse ionisation chamber. At the Russian field site, continuous activity observations were performed from July 1998 until July 2000 with half a year's interruption in summer/fall 1999. During intensive campaigns, a second monitor was installed at Fyodorovskoye at 15.6 m (July/August 1998), and at 1.8 m (July/August 1999 and October 1999) above ground. As expected, pronounced diurnal cycles of the Rn-222 daughter activity were observed at all sites, particularly during summer when the vertical mixing conditions in the atmospheric surface layer vary strongly between day and night. The lower envelope of the continuous measurements at Fyodorovskoye and at Heidelberg changes on synoptic timescales by a factor of 4-10 due to long-range transport changes between continental to more maritime situations. Generally, the Rn-222 activity at 26.3 m height at Fyodorovskoye is lower by a factor of 2-3 compared to Heidelberg at 20 m above ground. This unexpected result is due to considerably lower Rn-222 exhalation rates from the soils measured in the footprint of the Fyodorovskoye Forest tower compared to Heidelberg. With the inverted chamber technique Rn-222 exhalation rates in the range 3.3-7.9 Bq m(-2) h(-1) were determined at Fyodorovskoye for summer 1998 and autumn 1999 (wet conditions with water table depths between 5 and 70 cm). Only during the very dry summer of 1999 the mean Rn-222 exhalation rate increased by about a factor of five. All measured exhalation rates at the Fyodorovskoye Forest are considerably smaller by a factor of 2-10 compared to observations in the vicinity of Heidelberg (ca 50-60 Bq m(-2) h(-1)) and generally in Western Europe

Three years of aircraft-based trace gas measurements over the Fyodorovskoye southern taiga forest, 300 km north-west of Moscow

As part of the Eurosiberian Carbonflux project, regular measurements have been performed in the lower troposphere over a southern taiga forest area in Fyodorovskoye, Western Russia (56°28'N, 32°56'E). Up to 70 flights have been made between May 1998 and December 2000, plus additional intensive campaigns to study the diurnal variability of atmospheric trace gases within the boundary layer. We sampled flasks between 100 and 3000 m for analysis of CO2, δ13C and δ18O in CO2, CH4 and CO. In addition, in-situ CO2, relative humidity, pressure and temperature were performed for a better description of the vertical variability and accurate determination of the boundary layer height. The peak-to-peak amplitude of the seasonal cycle of atmospheric CO2 within the boundary layer of 26.5 ppm is about twice the one observed in the free troposphere (14.6 ppm). The spring draw down of CO2 also occurs one month earlier than in the free troposphere aloft. There is also an increase by factor of two in the vertical variability of CO2 within the free troposphere between summer and winter, which may be related to the variability of advection and mixing. Linear regression analysis applied to flask measurements of CO, CH4 and δ13C versus CO2 in the free troposphere indicates that industrial emissions over Europe are a dominant source of synoptic variability in wintertime in air masses reaching Fyodorovskoye. On the other hand, the variability of trace gases in the boundary layer observed during intensive campaigns is consistent with the patterns of proximate sources over the underlying southern taiga landscape at Fyodorovskoye.

Diurnally variable δ18O signatures of soil CO2 fluxes indicate carbonic anhydrase activity in a forest soil

Oxygen isotopes are valuable tools for studying the gas exchange between terrestrial ecosystems and the atmosphere. We determined the δ18O signatures of soil CO2 fluxes from soil chamber measurements over the diurnal cycle in September 2000, May 2001 and July 2001 in a Sitka spruce plantation in Scotland. Concurrent estimates of the δ18O composition of soil water were obtained from soil samples collected in the vicinity of the chambers. The observed δ18O signatures of net soil CO2 fluxes were diurnally variable and strongly depleted compared to those expected from a simple evasion of respired CO2 at isotopic equilibrium with soil water. We then simulated the δ18O signatures of soil CO2 fluxes using a model of soil gas exchange that includes atmospheric invasion of CO2 with concurrent isotopic equilibration with soil water and evasion of the equilibrated CO2. This brought the modeled δ18O signatures closer to the observations, but complete agreement was only achieved when acceleration of isotopic exchange between CO2 and soil water by carbonic anhydrase activity was included. We hypothesize that carbonic anhydrase is present in the litter or surface soil layers. This introduces a feedback that can result in diurnally variable δ18O signatures of net soil CO2 fluxes. Such effects can only be captured in models that have an explicit description of the canopy air space with a variable δ18O signature of CO2