Fig. 3: Maize-derived C in density fractions of bulk soil from Halle (means and SE, n=4)

1. Carbon turnover in aggregated soils Bettina John1, Tamon Yamashita1, Bernard Ludwig2, Heiner Flessa1 1Institute of Soil Science and Forest Nutrition, University of Göttingen 2Department of Environmental Chemistry, University of Kassel www.gwdg.de/~bjohn, bjohn@gwdg.de Table 1: Soil properties of Rotthalmünster, soil acidified with HCl Introduction The turnover of soil organic carbon (SOC) depends on its position in the soil. The scope of our study is to investigate the significance of aggregate formation on the stabilization of SOC depending on fertilization and land use. We aim to determine the turnover of carbon in density and size fractions of the soil by means of 13C measurements and subsequent calculation of maize-derived percentages. • Density fractionation at Halle • Method: Density fractionation of bulk soil (< 2mm) from maize and rye sites (Fig. 1; 4). • FPOM: free particulate organic matter  between aggregates • OPOM: occluded particulate organic matter  inside aggregates. • Results: • Partioning of SOC among density fractions: • Mineral (68-75%) > OPOM1.6-2.0 g/cm³ (11-21%) > FPOM (6-10%) > OPOM< 1.6 g/cm³ (2-4.4%) • Maize-derived C in SOC (Fig. 3): FPOM > OPOM1.6-2.0 g/cm³ >OPOM< 1.6 g/cm³ • Rotthalmünster: Soil properties • Maize-derived percentages of the SOC at Rotthalmünster were about 35 % (Table 1). This higher enrichment of maize-derived SOC as compared to the maize-NPK site of the ‚Ewiger Roggen‘ at Halle, where after 39 years only 14.0 % of the C were maize-derived was due to its • higher production of biomass • input of plant residues after harvest • high percentage of silt and clay. Fig. 1. Density fractionation with sodium polytungstate solution • Combined size and density fractionation at Rotthalmünster • Method: • 1. Size fractionation of aggregates (Fig. 2) • 2. Density fractionation for each aggregate class (Fig. 1) • 3. Sieving (53 µm) of the mineral fraction. • Results: • Tillage destroyed aggregates > 1 mm which contain the major part of SOC of forest and grassland soil. • Aggregates protect carbon from decomposition as carbon content in all aggregate fractions ≥ 53 µm was higher than in the silt & clay fraction (<53 µm) (Fig. 5). Fig. 3: Maize-derived C in density fractions of bulk soil from Halle (means and SE, n=4) Fig. 2. Combined size and density fractionation FPOM (<1.6 g/ cm³ ) OPOM (<1.6 g/ cm³ ) OPOM (1.6-2.0 g/ cm³ ) Fig. 5: Distribution of C in 100 g soil among aggregate fractions of Rotthalmünster (means and SE, n=4) Fig. 4: Density fractions from Halle