Surface properties of stabilizing coastal dunes: combining spectral and field analyses

The coastal dunes of Israel have been undergoing a process of stabilization since 1948. One of the major features of this process is a change in the surface properties of the dunes – the development of a biological soil crust (BSC), and a change in the properties of the sand grains themselves. In Ashdod, at the southern coastal plain of Israel, sand properties that include the BSC, their fines (silt and clay) content and free iron-oxide (indicating their degree of rubification) have been analysed in detail using field and lab spectroscopy methods. In addition, sand erosion and deposition were measured using erosion pins to determine their effect on the presence of the above-mentioned factors. It was found that the BSC over these dunes is comprised of green algae that differs in its reflectance spectra from cyanobacterial crust, especially in the blue band. The crust was found to be particularly developed in the stable areas (mainly the interdunes) and on the north-facing slopes rather than on the south-facing slopes. A positive correlation was found between the crust fines and chlorophyll content, with stable areas showing more developed BSC. The stable areas showed also a lower albedo and slightly more developed reddish colour, indicating a slightly higher rate of rubification. This study demonstrates that the intensity of sand erosion/deposition rates affects soil properties, with the BSC being the fastest to react to the stabilization process (months to several years), followed by the content of fine particles (several years to a decade), whereas the rubification process is a much weaker marker and may need much longer time periods to develop (decades to centuries).     

Determination of microbial versus root‐produced CO2 in an agricultural ecosystem by means of δ13CO2 measurements in soil air

The amounts of microbial and root‐respired CO₂ in a maize/winter wheat agricultural system in south western Germany were investigated by measurements of the CO₂ mixing ratio and the ¹³C/¹²C ratio in soil air. CO₂ fluxes at the soil surface for the period of investigation (1993–1995) were also determined. Root respired CO₂ shows a strong correlation with the plant mass above ground surface of the respective vegetation (R²≥0.88); the maximum CO₂ release from roots was in August for the maize (2.0±0.5 mmol m⁻² h⁻¹) and in June for winter wheat (1.5±0.5 mmol m⁻² h⁻¹). Maximum CO₂ production by roots correlate well with the maximum amount of plant root matter. Integrating the CO₂ production over the whole growing season and normalizing to the dry root matter yields, the CO₂ production per gram dry organic root matter (DORM) of maize was found to be 0.14±0.03 gC (g DORM)⁻¹. At the sites investigated, root‐produced CO₂ contributed (16±4)% for maize, and (24±4)% for winter wheat, respectively, to the total annual CO₂ production in the soil (450±50 gC m⁻² for maize, 210±30 gC m⁻² for winter wheat).     

The H2/CO ratio of emissions from combustion sources: comparison of top-down with bottom-up measurements in southwest Germany

The hydrogen-to-carbon monoxide (H₂/CO) emission ratio of anthropogenic combustion sources was determined from more than two years of quasi-continuous atmospheric observations in Heidelberg (49°24' N, 8°42' E), located in the polluted Rhein-Neckar region. Evaluating concurrent mixing ratio changes of H₂ and CO during morning rush hours yielded mean molar H₂/CO ratios of 0.40 ± 0.06, while respective results inferred from synoptic pollution events gave a mean value of 0.31 ± 0.05 mole H₂/mole CO. After correction for the influence of the H₂ soil sink on the measured ratios, mean values of 0.46 ± 0.07 resp. 0.48 ± 0.07 mole H₂/mole CO were obtained, which are in excellent agreement with direct source studies of traffic emissions in the Heidelberg/Mannheim region (0.448 ± 0.003 mole H₂/mole CO). Including results from other European studies, our best estimate of the mean H₂/CO emission ratio from anthropogenic combustion sources (mainly traffic) ranges from 0.45 to 0.48 mole H₂/mole CO, which is about 20% smaller than the value of 0.59 mole H₂/mole CO which is frequently used as the basis to calculate global H₂ emissions from anthropogenic combustion sources.     

Seasonal variation of the molecular hydrogen uptake by soils inferred from continuous atmospheric observations in Heidelberg, southwest Germany

The dominant sink of atmospheric molecular hydrogen (H₂) is its enzymatic destruction in soils. Quantitative estimates of the global sink strength, as derived from bottom-up process studies, are, however, still associated to large uncertainties. Here we present an alternative way to estimate atmosphere-to-soil flux densities, respectively deposition velocities of H₂, based on atmospheric H₂ and ²²²Rn observations in the boundary layer. Two and a half years of continuous measurements from a polluted site in the Rhine-Neckar area have been evaluated and night-time flux densities were calculated for situations of strong nocturnal boundary layer inversions using the Radon-Tracer Method. The influences from local anthropogenic combustion sources could be detected and successfully separated by parallel measurements of carbon monoxide. Inferred daily uptake fluxes in the Heidelberg catchment area range from 0.5 to 3 × 10⁻⁸ g H₂ m⁻² s⁻¹ with a mean value of (1.28 ± 0.31) × 10⁻⁸ g H₂ m⁻² s⁻¹. Uptake rates are about 25% larger during summer than during winter, when soil moisture is high, and diffusive transport of H₂ into the soil is inhibited. The mean deposition velocity is 3.0 ± 0.7 × 10⁻² cm s⁻¹_, which is very well in line with direct measurements on similar soil types in Europe and elsewhere.