Andruschkewitsch, R. 1* , Geisseler, D. 1 , Koch, H. J. 2 , Ludwig, B. 1

1. Effect of different tillage treatments on organic carbon stocks, contents of aggregate fractions and occluded light fraction Andruschkewitsch, R.1*, Geisseler, D.1, Koch, H. J.2, Ludwig, B.1 1Department of Environmental Chemistry, Univ. Kassel, Germany; (*andruschkewitsch@uni-kassel.de) 2Institute of Sugar Beet Research, Göttingen, Germany Background and research questions Recently incorporated organic material is rapidly integrated within macro-aggregates and therefore protected from fast decomposition (Helfrich et al. 2008). Following the physical impact of tillage, macro-aggregates disrupt and the formerly incorporated organic material is exposed to microbial decomposition (Cambardella et al. 1993; Mikha et al. 2004). Do different tillage treatments have (i) a long-term influence on the organic carbon (Corg) stocks the Corg and LF contents within the water stable aggregate fractions and (ii) a short term influence on the contents of water stable aggregate fractions? Experimental setup • Four long-term (~17 years) tillage trials on loess soils of the Institute of Sugar Beet Research with three tillage treatments: conventional (CT, 0-30 cm) mulch (MT, 0-10 cm) and no-tillage (NT) and a crop rotation consisting of: winter wheat – winter wheat – sugar beet. (Koch et al. 2009). • Comparison of the organic C (Corg) stocks and contents within water stable macro-aggregates (>250 µm) and contents of occluded light fraction (oLF) between the different tillage treatments. • Comparison of the contents of water stable aggregate fractions between the different tillage treatments over time. Results Tab. 1: Mean contents of organic C (Corg) within macro-aggregates (> 250 µm), of occluded light fraction (oLF), Corg stocks and relative crop yields for different depths and tillage treatments at t0 (wheat stand); standard deviation in brackets (n=4). Fig. 1: Average macro- and micro-aggregate contents for the different depths and tillage treatments at different sampling dates; t0: wheat stand, t1: stubble field, t2: after stubble cultivation, t3: after ploughing. Error bars show standard deviation (n=3). Different letters indicate significant differences between treatment means within one depth (p<0.05) aRelative crop yields for winter wheat and sugar beet in Germany (2004-2007): 7.5 t grain yield ha-1 and 60.5 t harvestable fresh matter ha-1, respectively • Increased Corg-stocks under MT (5.7 kg Corg m-2) in comparison with CT and NT (5.2 kg Corg m-2, Tab. 1) • Slightly increased crop yields under CT and MT than under NT (Tab. 1). • The significant increase of the Corg-contents within the macro-aggregates and of the oLF-contents under MT and NT in comparison with CT were restricted to the top 5 cm (Tab. 1). • No physical tillage effect was found (Fig. 1). Conclusions • Long term NT showed no positive effect on the Corg-stock in comparison with CT. • Higher crop yields might lead to increased aboveground and belowground biomass inputs under CT and MT. • The higher physical impact by tillage did not lead to lower aggregate contents. •  Higher organic matter inputs and more effective litter incorporation under CT and MT might compensate the physical tillage impact. • Decreasing macro-aggregate content under NT after harvest was probably due to dying off root biomass. Literature Acknowledgement This project is granted by the Deutsche Forschungsgemeinschaft (DFG-Research Training Group 1397 ‘‘Regulation of soil organic matter and nutrient turnover in organic agriculture’’). Cambardella & Elliott. (1993). Soil Sci Soc Am J. 57, 1071-1076 Helfrich et al. (2008). Soil Biol Biochem. 40, 1823-1835 Mikha et al. (2004). Soil Sci Soc Am J. 68, 809-816 Koch, H.-J et al. (2009). Eur. J. Agron. 30, 101–109