Are there memory effects on greenhouse gas emissions (CO$_{2}$, N$_{2}$O and CH$_{4}$) following grassland restoration?

A 5-year greenhouse gas (GHG) exchange study of the three major gas species (CO$_{2}$, CH$_{4}$ and N$_{2}$O) from an intensively managed permanent grassland in Switzerland is presented. Measurements comprise 2 years (2010 and 2011) of manual static chamber measurements of CH$_{4}$ and N$_{2}$O, 5 years of continuous eddy covariance (EC) measurements (CO$_{2}$–H$_{2}$O – 2010–2014), and 3 years (2012–2014) of EC measurement of CH$_{4}$ and N$_{2}$O. Intensive grassland management included both regular and sporadic management activities. Regular management practices encompassed mowing (three to five cuts per year) with subsequent organic fertilizer amendments and occasional grazing, whereas sporadic management activities comprised grazing or similar activities. The primary objective of our measurements was to compare pre-plowing to post-plowing GHG exchange and to identify potential memory effects of such a substantial disturbance on GHG exchange and carbon (C) and nitrogen (N) gains and losses. In order to include measurements carried out with different observation techniques, we tested two different measurement techniques jointly in 2013, namely the manual static chamber approach and the eddy covariance technique for N$_{2}$O, to quantify the GHG exchange from the observed grassland site.
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Our results showed that there were no memory effects on N$_{2}$O and CH$_{4}$ emissions after plowing, whereas the CO$_{2}$ uptake of the site considerably increased when compared to pre-restoration years. In detail, we observed large losses of CO$_{2}$ and N$_{2}$O during the year of restoration. In contrast, the grassland acted as a carbon sink under usual management, i.e., the time periods 2010–2011 and 2013–2014. Enhanced emissions and emission peaks of N$_{2}$O (defined as exceeding background emissions 0.21 ± 0.55 nmol m$^{-2}$ s$^{-1}$ (SE = 0.02) for at least 2 sequential days and the 7 d moving average exceeding background emissions) were observed for almost 7 continuous months after restoration as well as following organic fertilizer applications during all years. Net ecosystem exchange of CO$_{2}$ (NEECO$_{2}$) showed a common pattern of increased uptake of CO$_{2}$ in spring and reduced uptake in late fall. NEECO$_{2}$ dropped to zero and became positive after each harvest event. Methane (CH$_{4}$) exchange fluctuated around zero during all years. Overall, CH$_{4}$ exchange was of negligible importance for both the GHG budget and the carbon budget of the site.

Our results stress the inclusion of grassland restoration events when providing cumulative sums of C sequestration potential and/or global warming potential (GWP). Consequently, this study further highlights the need for continuous long-term GHG exchange observations as well as for the implementation of our findings into biogeochemical process models to track potential GHG mitigation objectives as well as to predict future GHG emission scenarios reliably.