Winfried E.H. Blum, Georg J. Lair & Jasmin Schiefer Limiting factors and time scales of soil carbon sequestration to mitigate climate change Winfried E.H. Blum, Georg J. Lair & Jasmin Schiefer Institute of Soil Research, Department for Forest and Soil Sciences University of Natural Resources and Life Sciences (BOKU), Vienna/Austria E-mail: winfried.blum@boku.ac.at

Organic carbon build-up during soil formation total available organic carbon (OC) soil organic carbon (SOC) organic carbon content (t/ha) time (year)

Critical Zone Observatory „Marchfeld“, Austria Cropland Forest Grassland Vienna Bratislava 5 km

Vienna

Soils along the age gradient in the CZO “Marchfeld” 3 3 2 increasing soil age 2 1 1 Chernozem Topsoil >4000 years Mollic Fluvisol Topsoil ~350 years Fluvisol on fine sediments above gravel; Topsoil 0-20 cm: <50 years

Texture, pH, organic C and CaCO3 content in topsoils (0-20 cm) under different land use Ancient floodplain Close to River

Soil formation with time based on build-up of A-horizons in the CZO AC C1 C2

Organic carbon accumulation in topsoils (0-20 cm) affected by different land use Zehetner et al. (GBC 2009)

Annual C- Accumulation in topsoils (0-20cm) in kg ha-1 depending on soil age (stage of development) (years) C- Accumulation (g m-2 a-1) (kg ha-1 a-1) CO2- Sequestration (kg ha-1 a-1) 0-50 200 2000 (2t) 6140 (6.14t) 100 100 1000 (1t) 3070 (3.07t) 200 50 500 (0.5t) 1535 (1.54t) 400 10 100 (0.1t) 307 (0.307t)

Calculation of C-sequestration in topsoils (0-20 cm) in t ha-1 year-1, based on the experimental results Example: Chernozem (0-20 cm depth, bulk density: 1.25 t m-3, 2% organic carbon)=50 t C ha-1 Soil age (years) C- Accumulation 0-50 2 t ha-1 a-1 = 0.04% = 0.4 ‰ 100 1 t ha-1 a-1 = 0.02% = 0.2 ‰ 200 0.5 t ha-1 a-1 = 0.01% = 0.1 ‰ 400 0.1 t ha-1 a-1 = 0.02% = 0.02 ‰

Stabilization of organic carbon in the clay fraction of cropland soils (<2µm) during soil formation TG-analysis of the clay fraction of the topsoil (0-20 cm) thermo-labile organic matter 190-390°C thermo-stabile organic matter 390-600°C Total organic C in the clay fraction (g/kg) : 62.9 61.4 60.6 68.4

Organic carbon content in clay sized fractions (<2 µm) of topsoils (0-10 cm) along the chronosequence Corg accumulation in the cropland = 1.5 mg/kg/yr

13C-NMR characterization of clay sized fractions (< 2 µm) of topsoils (0-10cm) under cropping acids, amides aromatic groups (lignin) polysaccharides (lignin, proteins) lipids, hemicellulose

Soil organic carbon accumulation over time under forest, modelled for 3 temperature scenarios (Roth-C, 0-10 cm) - 1°C + 1°C modelled for a 10 year old site using real climatic data

Summary and Outlook 1 C-sequestration is a slow process, resulting from the annual accumulation of plant and animal residues, their decomposition by metabolisation and mineralisation and their mixing with mineral soil material by bioturbation. Main factors controlling these processes are: quantity and quality of the organic residues depending on the prevailing nutritional conditions; availability of reactive mineral surfaces for C-binding resulting from weathering processes; the spheric position of these binding surfaces and their accessibility by microbes; availability of water and temperature conditions for biological processes; human interference trough soil management, changing the conditions for accumulation or decomposition of organic C.

Summary and Outlook 2 In A- horizons of soils older than 500 years most of the binding/accumulation positions for organic C are occupied. Further accumulation is only possible when new mineral binding positions become available or when organic C becomes recalcitrant or resistant against microbial decomposition. The equilibrium between decomposition and accumulation of organic C is strongly influenced by climatic conditions. Increasing temperatures decrease the capacity of soils to sequester carbon.

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10 µm

10 µm