Increasing organic matter stocks in soils reduce atmospheric carbon dioxide (CO 2 ), but they may also promote emissions of nitrous oxide (N 2 O) by providing substrates for nitrification and denitrification and by increasing microbial O 2 consumption. The objectives of this study were to determine the effects of fertilization history, which had resulted in different soil organic matter stocks on (1) the emission rates of N 2 O and CO 2 at a constant soil moisture content of 60% water-holding capacity, (2) the short-term fluxes of N 2 O and CO 2 following the application of different fertilizers (KNO 3 vs. farmyard manure from cattle) and (3) the response to a simulated heavy rainfall event, which increased soil moisture to field capacity. Soil samples from different treatments of three long-term fertilization experiments in Germany (Methau, Spröda and Bad Lauchstädt) were incubated in a laboratory experiment with continuous determination of N 2 O and CO 2 emissions and a monitoring of soil mineral N. The long-term fertilization treatments included application of mineral N (Methau and Spröda), farmyard manure + mineral N (Methau and Spröda), farmyard manure deposition in excess (Bad Lauchstädt) and nil fertilization (Bad Lauchstädt). Long-term addition of farmyard manure increased the soil organic C (SOC) content by 55% at Methau (silt loam), by 17% at Spröda (sandy loam) and by 88% at Bad Lauchstädt (silt loam; extreme treatment which does not represent common agricultural management). Increased soil organic matter stocks induced by longterm application of farmyard manure at Methau and Spröda resulted in slightly increased N 2 O emissions at a soil moisture content of 60% water-holding capacity. However, the effect of fertilization history and SOC content on N 2 O emissions was small compared to the short-term effects induced by the current fertilizer application. At Bad Lauchstädt, high N 2 O emissions from the treatment without fertilization for 25 years indicate the importance of a sustainable soil organic matter management to maintain soil structure and soil aeration. Emissions of N 2 O following the application of nitrate and farmyard manure differed because of their specific effects on soil nitrate availability and microbial oxygen consumption. At a soil moisture content of 60% waterholding capacity, fertilizer-induced emissions were higher for farmyard manure than for nitrate. At field capacity, nitrate application induced the highest emissions. Our results indicate that feedback mechanisms of soil C sequestration on N 2 O emissions have to be considered when discussing options to increase soil C stocks.

Modeling crop growth and soil N dynamics is difficult due to the complex nature of soil–plant systems. In several studies, the DNDC model has been claimed to be well‐suited for this purpose whereas in other studies applications of the model were less successful. Objectives of this study were to test a calibration and validation scheme for DNDC‐model applications to describe a field experiment with spring wheat on a sandy soil near Darmstadt (SW Germany) using different fertilizer types (either application of mineral fertilizer and straw—MSI; or application of farmyard manure—FYM) and rates (low—MSIL, FYML; and medium—MSIM, FYMM). The model test is based on a model parameterization to best describe the case MSIL and applies this parameterization for a retrospective simulation of the other cases (MSIM, FYML, FYMM) including crop growth and N2O emissions. Soil water contents were not accurately simulated using either the DNDC default values for a loamy sand or for the next finer texture class or using results from the pedotransfer function provided by ROSETTA. After successful calibration of the soil water flow model using a soil texture class that led to the best fit of the measured water content data, grain yield of spring wheat and cumulative N2O emission were slightly underestimated by DNDC and were between 91% and 86% of the measured data. A subsequent calibration of the yields and cumulative N2O emissions from soils of the MSIL treatment gave a good prediction of crop growth and N2O emissions in the MSIM treatment, but a marked underestimation of yields of the FYM treatments. Cumulative N2O emissions were predicted well for all MSI and FYM treatments, but seasonal dynamics were not. Overall, our results indicated that for the sandy soil in Germany, site‐specific calibration was essentially required for the soil hydrology and that a calibration was useful for a subsequent prediction where greater amounts of the same fertilizer were used, but not useful for a prediction with a different fertilizer type.

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The cumulative dissertation is based on three manuscripts to which the PhD candidate predominantly contributed as the first author and one article to which she contributed as co-author. The studies are or will be published in international refereed journals: Jäger, N., Duffner, A., Ludwig, B., Flessa, H.: Long-term and short-term effects of the application of mineral and organic fertilizer on N 2 O and CO 2 emissions from a sandy soila laboratory incubation Jäger, N., Stange, C.F., Ludwig, B., Flessa, H.: Emission rates of N 2 O and CO 2 from soils with different organic matter content from three long-term fertilization experiments -a laboratory study Jäger, N., Dechow, R., Oltmanns, M., Raupp, J., Flessa, H.: Effects of different long-term fertilization treatments on soil organic matter stocks, N 2 O emissions and CH 4 uptake of a sandy soil Ludwig, B., Jäger, N., Priesack E., Flessa, H.: Application of the DNDC model to predict N 2 O emissions from sandy arable soils with differing fertilization in a long-term experiment, Journal of Plant Nutrition and Soil Science, accepted This thesis starts with a general introduction that imparts knowledge about the climaterelevant trace gases CO 2 and N 2 O. The processes of production and consumption in soils are explained and the reasons, how carbon sequestration influences the climate change and agriculture, are given. Then, the supposed mechanism of carbon sequestration leading to