No Memory Effects of Restoration on N2O Exchange above an intensively managed Grassland in Switzerland

Abstract:

Here we present 3 consecutive years of EC flux measurements of N₂O, CH₄ and CO₂) carried out in intensively managed grassland in Switzerland. Our measurements of greenhouse gas (GHG) concentrations were based on a recently developed CW-QCL absorption spectrometer to measure the concentrations of N₂O and CH₄ and an infrared gas analyzer to measure the concentrations of CO₂ and H₂O. We investigated the magnitude of trace gas emissions during a year of major disturbance (grassland restoration - including ploughing and fertilization in 2012) and the two following years representing business as usual (up to 6 harvests per year which are followed by fertilizer application, 2013 and 2014).

We observed large peaks of N₂O (up to 50 nmol m^-2 s^-1) in 2012 during thawing of the soil after the winter period and after re-sowing as well as inorganic fertilizer application at the beginning of summer. N₂O emissions following harvest and fertilizer application ranged between 2 and 7 nmol m^-2 s^-1 and background fluxes were no larger than 1 nmol m^-2 s^-1.

Fluxes of N₂O were primarily controlled by soil water content and temperature, while management activities lead to larger variation of N₂O fluxes during several days after the management event when compared to the background flux measurements. Annual flux budgets were dominated by CO₂ emissions and N₂O emissions contributed largely to the annual budget in 2012 but to a much lesser extend in the post-disturbance years (2013/2014). CH₄ flux contribution to the annual budget was negligible.

We conclude that grassland restoration results in large N₂O emissions, while not leading to larger N₂O emissions in subsequent years. Still, such specific time periods of enhanced N₂O emissions need to be considered in decadal greenhouse gas budget estimates due to the fact that a single year can offset previous carbon and nitrogen sinks.