Nitrous oxide flux dynamics of grassland undergoing afforestation

In Ireland fertilised grasslands are major source of nitrous oxide (N2O), a powerful greenhouse gas. We present 5 years (2004–2008) of eddy-covariance (EC) observations of N2O fluxes from an ecosystem transitioning from wet managed grassland to a broadleaf forestry. One sector of the EC footprint was converted to forestry during the observation period, while the remainder of the footprint remained under intensively managed grassland. The daily N2O fluxes were responsive to the rainfall and changes in soil moisture, whereas the annual and monthly emissions depended on the amount of nitrogen fertilisers applied. During the year of forest establishment, mechanical disturbances increased N2O emissions from the forest sector, even higher than the adjacent grassland; in the following 3 years, the intensity of the nitrous oxide flux dropped to approximately one-third of the previous “grassland” level of emission. At the same time, the grassland sector recorded approximately a 20% decrease of flux intensity, coinciding with a reduction in fertiliser application. These observations led us to conclude that while reduced fertilisation and the cessation of grazing contributed to the reduction of N input, a certain period is required for the new forest to establish, settle and mature and thereby to attain a stable rate of emission of N2O emissions.