Sources of soil CO2 in calcareous grassland with woody plant encroachment
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The objective of this study is to estimate the contribution of various sources that influence soil CO2 concentrations in calcareous grassland.
Materials and methods
The research was performed at the Podgorski Kras plain (45 °33′ N, 13 °55′ E, 400–430 m.a.s.l.) in the sub-Mediterranean region of Slovenia (SW Slovenia), where many meadows and pastures have been abandoned. In parallel to the measurement of soil respiration R s, soil gas was sampled for stable isotope analysis. Samples were taken biweekly at two sites, Grassland and Invaded, from July 2008 until November 2010. In addition, daily variations in concentration and stable isotope composition of soil CO2 were determined in May 2009. The partitioning of soil CO2 concentrations was performed using stable isotope mass balance calculation.
Results and discussion
The concentration and isotope composition of soil CO2 exhibited similar seasonal variations at both sites. Lower δ13CCO2 values, ranging from −28.2 to −15.2 ‰, which occurred during warm periods and higher values, up to −12.1 ‰, were typical of cold winter periods, from December to March. Organic sources were estimated to constitute between 78 and 99 % of total soil CO2 during warmer periods from May until October. This contribution was lower during the winter, ranging from 46 to 77 %. In winter, the atmospheric component to soil CO2 dominated, constituting up to 60 %. On average, the inorganic contribution was estimated to comprise 12 % of the soil CO2 at all sampling locations. The contribution of this source to soil CO2 concentration, at up to 41 %, was highest in Grassland during the growing season. The inorganic source of soil CO2 was also an important component during daily variations. The highest contribution was observed during the day, in parallel to the highest respiration rates.
The inorganic pool is shown to be an important part of soil CO2 in calcareous areas and should be considered as equal to organic CO2 as a source in soil CO2 partitioning.
KeywordsInorganic CO2 Karst grassland Organic CO2 Soil CO2 Stable isotopes
The study was supported by projects J4-1009 and J4-4224 and by the young researcher program (contract no. 1000-09-310206) financed by the Slovenian Research Agency. We thank Marjanca Jamnik, Urška Videmšek, and Anja Marinič for their help during fieldwork, Stojan Žigon for the stable isotope analysis, and Roger Pain for the linguistic corrections.
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