Long Term Soil Gas Monitoring as Tool to Understand Soil Processes

Soils provide many functions as they represent a habitat for flora and fauna, supply water, nutrient, and anchorage for plant growth and more. They can also be considered as large bioreactors in which many processes occur that involve the consumption and production of different gas species. Soils can be a source and sink for greenhouse gases. During the last decades this topic attracted special attention. Most studies on soil-atmosphere gas fluxes used chamber methods or micro-meteorological methods. Soil gas fluxes can also be calculated from vertical soil gas profiles which can provide additional insights into the underlying processes. We present a design for sampling and measuring soil gas concentration profiles that was developed to facilitate long term monitoring. Long term monitoring requires minimization of the impact of repeated measurements on the plot and also minimization of the routine workload while the quality of the measurement needs to be maintained continuously high. We used permanently installed gas wells that allowed passive gas sampling at different depths. Soil gas monitoring set ups were installed on 13 plots at 6 forest sites in South West Germany between 1998 and 2010. Until now, soil gas was sampled monthly and analysed for CO2, N2O, CH4, O2, N2, Ar, and C2H4 using gas chromatography. We present typical time series and profiles of soil gas concentrations and fluxes of a selected site as an example. We discuss the effect of different calculation approaches and conclude that flux estimates of O2, CO2 and CH4 can be considered as highly reliable, whereas N2O flux estimates include a higher uncertainty. We point out the potential of the data and suggest ideas for future research questions for which soil gas monitoring would provide the ideal data basis. Combining and linking the soil gas data with additional environmental data promises new insights and understanding of soil processes

Dynamics of Soil CO2 Efflux and Vertical CO2 Production in a European Beech and a Scots Pine Forest

The conversion of coniferous forest to deciduous forest is accompanied by changes in the vertical distribution of fine roots and soil organic carbon (SOC) content. It is unclear how these changes affect soil CO2 efflux and vertical soil CO2 production, considering changing climate. Here, we present the results of a 6-year study on CO2 efflux, covering relatively warm-dry and cool-wet years. A combination of the flux-gradient method and closed chamber measurements was used to study the CO2 efflux and the vertical distribution of soil CO2 production in a beech (Fagus sylvatica L.) and a pine (Pinus sylvestris L.) forest in northeast Germany. We observed, on average, similar CO2 efflux with 517 (±126) and 559 (±78) g C m–2 a–1 for the beech site and the pine site, respectively. CO2 efflux at the beech site exceeded that at the pine site during the wet year 2017, whereas in dry years, the opposite was the case. Water availability as indicated by precipitation was the primary determining long-term factor of CO2 efflux, whereas seasonal variation was mainly affected by soil temperature, and—in the case of beech—additionally by soil water content. CO2 efflux decreased more dramatically (-43%) at the beech site than at the pine site (-22%) during the warm-dry year 2018 compared to the cool-wet year 2017. We assumed that drought reduces heterotrophic respiration (Rh) at both sites, but additionally decreases autotrophic respiration (Ra) at the beech stand. Soil CO2 production at the beech site ranged over a greater soil depth than at the pine site, attributed to different fine root distribution. The organic layer and the A horizon contributed 47 and 68% of total CO2 efflux at the beech site and the pine site, respectively. The seasonal patterns of different CO2 efflux between both sites were assumed to relate to different phases of tree physiological activity of deciduous compared to evergreen tree species

Graphene synthesis on SiO2 using pulsed laser deposition with bilayer predominance

International audienceHere we report the low-defect synthesis of bilayer graphene film on SiO 2 with a nickel catalyst using pulsed laser deposition combined with rapid thermal annealing. A parametric study was performed with various initial amorphous carbon (a-C) film thicknesses and annealing temperatures and a fixed nickel catalyst film thickness. Raman spectra and mapping over large areas of up to 100 × 100 μm² were used to investigate the structure and the defects of graphene films. Optimal conditions for graphene growth were an initial a-C film thickness of 2 nm and an annealing temperature of 900°C. Results showed that 76% of the optimized film contained graphene bilayers, and 18% of the optimized film contained graphene monolayers. A transmittance of 87% at 550 nm is observed without any transfer process from the SiO 2 substrate. This paper presents experimental guidelines for optimal synthesis conditions to control graphene growth by pulsed laser deposition