Grassland conversion to arable land and greenhouse gas emissions

Background and Objective

The conversion of grassland to arable land has strongly increased in Germany during the last years, which is in part due to the growing number of biogas plants. The ploughing-up of grassland is a potential threat for the atmosphere because it causes losses of soil organic matter and may favour soil emissions of nitrous oxide (N₂O). Due to the lack of data, a site-specific assessment of the climate effect by grassland conversion to arable land is not possible until now. The aim of this project is to investigate how the conversion of grassland in arable land influences the exchange of greenhouse gases with the atmosphere due to changes in carbon and nitrogen transformation in soils. These studies will contribute to a better understanding of land use change effects in agriculture on the emission of greenhouse gases.

Target Group

Science, Federal Ministry of Food and Agriculture, associations of agriculture and environmental protection.

Approach

Field plot experiments were set up at two sites in Kleve and Trenthorst, where greenhouse gas emissions induced by conversion of grassland to arable was studied. Carbon and nitrogen dynamics are monitored at regular intervals during two years. N₂O emissions are quantified using gas exchange measurements. Controlling processes are investigated by monitoring biological and chemical soil parameters and isotopic signatures in soil and in the gas phase. Mineralisation of organic carbon accumulated during the grassland phase is determined by CO₂ flux measurements and stable isotopic signatures of carbon. Moreover, soil carbon and nitrogen inventories will be assessed.

Our Research Questions

• To which extent is N₂O emission and nitrate leaching enhanced by conversion of grassland to arable land?
• What controls the enhanced fluxes and which processes are responsible?
• How much mineral nitrogen is lost by denitrification?
• How is the greenhouse gas emission affected by different techniques of grassland conversion (i.e. ploughing versus chemical kill-off of the sward followed by direct seeding).
• How does grassland-derived organic matter mineralize over time following the conversion?
• What is the role of soil microbial biomass during mineralisation?
• What is the impact of the different methods of grassland conversion on forage yields and need-oriented fertilization?