Abstract
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.
Highlights
Future detailed analysis of soil gas monitoring data will allow studying apparent respiratory quotient (ARQ) to evaluate the relevance of subsurface processes at the different sites
We introduced an experimental set-up and design for long term soil gas monitoring
This is a prerequisite for the assessment of the homogeneity of the soil gas profile and the quantification and qualification of the estimates of the soil gas fluxes
Summary
Soils provide many functions as they represent the base of agricultural production and terrestrial ecosystem productivity. Methanogenesis is a strictly anaerobic process [5], but can occur as well in aerated soils, where oxygen-deficient zones can exist within water-saturated soil aggregates [15] Waterlogged sites such as peat and wetlands are often net sources of CH4 and have very low O2 concentrations and high CH4 concentrations in the soil profile. Soil CO2 fluxes, for example, show strong seasonal cycles in temperate climates This reflects the dependence on photosynthesis and decomposition of soil organic matter which both peak in summer [8,28]. Soil-atmosphere gas fluxes and soil gas concentrations change over time, but they differ substantially across the scales from ecosystems to soil gas profiles. Soil gas profiles usually reflect the ongoing processes of production and consumption of gases in a soil [44] and can be very different between sites. A certain minimum number of spatial replications of measurements is required to estimate representative mean fluxes of a plot or site with a defined level of uncertainty [47], which depends on the gas species
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