Caroline Buchen

Caroline Buchen successfully defended her doctoral thesis on "The fate of nitrogen after grassland renewal and grassland conversion to maize cropping – An investigation of N₂O processes and mineral N dynamics at the field scale" on 27.01.2017 at the Faculty of Architecture, Civil Engineering and Environmental Sciences of the Technical University Braunschweig. The dissertation was funded by the German Research Foundation (DFG-Research Training Group 1397 "Regulation of soil organic matter and nutrient turnover in organic agriculture").

Grassland renewal is a common agricultural management technique on intensively used grasslands, in particular in north-western Europe. The break-up of permanent grassland is associated with the release of large amounts of carbon (C) and nitrogen (N) caused by an increased mineralisation due to decomposition of soil organic matter (SOM) and the old grass sward. This additional supply of mineral N can cause enhanced N losses either in gaseous form as nitrous oxide (N₂O), an important greenhouse gas and/or as nitrate (NO₃^-) leaching. Within the research project, studies have been carried out about the persistence of this effect by using different renewal techniques (with and without mechanical sward destruction) and the processes of N₂O turnover following grassland break-up.  

For this purpose, a field trial (2013-2015) with different renewal techniques in comparison to permanent grassland and grassland conversion to maize cropping, was set up on two grassland sites (soil types: Histic Gleysol and Plaggic Anthrosol) with different SOM content and drainage regime in the north-western part of Germany.

In the first part of the study, N₂O fluxes and mineral N dynamics (0-30 cm) following grassland break-up were studied for a period of two years. In addition mineral N profiles (0-90 cm) were used to estimate the risk of NO₃^- leaching over winter. Although no effect of the different renewal treatments on the annual N₂O emissions was obtained, grassland renewal caused elevated N₂O fluxes for a short period of two months. The risk of NO₃^- leaching, however, was particularly enhanced during the first winter following grassland break-up, especially for the sandy Plaggic Anthrosol.

To investigate N₂O production processes and in particular N₂O reduction to N₂ (i.e. the end product of denitrification), the ¹⁵N gas-flux method was applied in situ during summer period in 2014 in the second part of the study. For the Histic Gleysol, large N losses via denitrification processes were obtained, in particular in form of N₂. For the Plaggic Anthrosol, lower gaseous losses were obtained.

In the third part of the study, natural abundance stable isotope signatures of soil-emitted N₂O (δ¹⁵N^bulk_N2O, δ¹⁸O_N2O and δ¹⁵N^SP_N2O = intramolecular distribution of ¹⁵N in the N₂O molecule) were used to identify sources of N₂O emission during the first year after grassland renewal (2013-2014). For the first time, the magnitude of N₂O reduction and the fraction of bacterial denitrification could be estimated simultaneously from field data using a new evaluation routine. Through the simultaneous execution of measurements, the first comparison on field scale was possible.

For the two investigated sites, a large mineralisation potential was present. This means, that a rapid development of the new grass sward or crop is necessary in order to optimise the N fertiliser utilisation and avoid N losses. Furthermore, the mineral N dynamics has to be taken into account in the N balance. By using N₂O isotopocule data, it was shown that N₂ emissions are an important pathway of losses in the N balance, which should be taken into account in the evaluation of N use efficiency in grasslands.