Critical Factors of Post-Harvest Nitrous Oxide Emissions from Oilseed Rape – Cereal Rotations | Evaluations Based on Field Studies and Stable Isotope Labeling

The sub-project of the project “Minderung von Treibhausgasemissionen im Rapsanbau unter besonderer Berücksichtigung der Stickstoffdüngung” of the division Plant Nutrition and Crop Physiology, Department of Crop Sciences of Georg-August University focused on steps in the nitrogen cycle that produce or interfere with N2O emissions from soils. It addressed the question of how the N cycle is modified by a winter oilseed rape – winter wheat – winter barley crop rotation. The focus of the doctoral thesis was put on the post-harvest period and the production of N2O emissions in winter oilseed rape. Several lab and field experiments were conducted: (1) Incubation experiment using oilseed rape and 15N-labelled barley straw; An incubation experiment carried out under controlled conditions aimed at comparing N addition and different straw qualities for their potential to provoke N2O emissions from soil. Treatments consisted of non-treated control soil (CK), 15N labelled barley straw (BST), oilseed rape straw (RST), 15N labelled barley straw + mineral N (BST+N), or oilseed rape straw + mineral N (RST+N). N fertilizer was applied to the soil surface as calcium ammonium-nitrate at a rate of 67.5 mg N kg-1 soil equiv. to 100 kg N ha-1 and soil moisture was adjusted to 80% water-holding capacity. The experiment covered a measurement period of 43 days. Cumulative N2O emissions in this study summed up to 3, 19, 26, 439 and 387µg N2O-N kg-1 soil 43 days-1 for CK, BST, RST, BST+N and RST+N. Application of mineral N fertilizer to the straw amended soils enhanced N2O emissions considerably in BST+N and RST+N treatments masking the effect of straw type. 15N labeling showed that only about 0.72% and 0.46% of the emitted N2O originated from straw-N in the BST and BST+N treatments after 22 days indicating a very low share of straw-borne N to the formation of N2O emissions. In agricultural practice, an N fertilization to soils amended with C-rich residues in the post-harvest period could lead to high N2O emissions. (2) Post-harvest N2O emissions as affected by N fertilizer and straw management – 2 year study at the site Reinshof; Management options to mitigate N2O emissions in oilseed rape cropping were tested in a 2-year field experiment at the field site Reinshof of the Faculty of Agricultural Sciences of Georg-August University of Goettingen. The treatments included a reduced spring N fertilization rate (1/2 of current recommendation), N fertilization of 180 kg N ha-1 and oilseed rape straw removal after harvest. N2O sampling was done from oilseed rape harvest to the beginning of the following growth season. The COUP model (Coupled heat and mass transfer model for the soil-plant-atmosphere system) was employed to uncover possible mechanisms of N2O emissions. In 2013, cumulative August-March N2O emissions ranged between 0.46±0.05 kg N2O-N ha-1 (0 kg N ha-1, with straw removal) and 1.05±0.1 kg N2O-N ha-1 (180 kg N ha-1 with straw application) whereas in 2014 N2O emissions were clearly higher accounting for 4.06±0.34 (90 kg N ha-1, with straw application) und 7.33±0.24 kg N2O-N ha-1 (unfertilized control soil with straw incorporation). There was no statistically significant effect of fertilization (p>0.05), but straw removal compared to straw incorporation slightly increased N2O emissions. In contrast to management measures, soil temperature and soil moisture showed a large influence on the rates of N2O emissions. The modeling approach indicated the importance of decomposition activity. Decomposition accelerated N cycling and in particular denitrification rates with high N2O emissions. (3) Field studies in five regions of Germany in a winter oilseed rape – winter wheat – winter barley crop rotation; For a detailed evaluation of N2O emissions in important oilseed rape cropping regions of Germany, 5 field experimental sites across Germany – Berge, Dedelow, Ihinger Hof, Hohenschulen and Merbitz – were chosen. To allow comparability, the crops were grown simultaneously from December 2012 to October 2015. Various parameters like soil temperature and water-filled pore space (WFPS) were recorded and N2O emissions were measured in the crops oilseed rape, wheat and barley and yield-related N2O emissions were calculated. To assess the impact of abiotic factors and crops, a generalized additive model was set up. The generalized additive model revealed that the abiotic factors drove N2O emissions. The impact of environmental drivers like temperature and WFPS on N2O emissions varied depending on site, but not by crop type. Fertilizer-related N2O emissions across all five sites were 0.76, 0.74 and 0.76% of the applied fertilizer N for oilseed rape, winter wheat and winter barley, respectively. N2O emissions from non-fertilized soils were not considered in this approach. Generally, the thesis demonstrated the dependence of N2O emissions on a set of factors in the post-harvest period. The factor's level of importance changed as they were varying in magnitude. Management options have to be reevaluated and adopted to fit a changing climate.