Memory effects on greenhouse gas emissions (CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub>) following grassland restoration?

Abstract

Abstract. A five-year greenhouse gas (GHG) exchange study of the three major gas species (CO2, CH4 and N2O) from an intensively managed permanent grassland in Switzerland is presented. Measurements comprise two years (2010/2011) of manual static chamber measurements of CH4 and N2O, five years of continuous eddy covariance (EC) measurements (CO2/H2O – 2010–2014) and three years (2012–2014) of EC measurement of CH4 and N2O. Intensive grassland management included both regular and sporadic management activities. Regular management practices encompassed mowing (3–5 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-ploughing to post-ploughing GHG exchange and to identify potential memory effects of such a substantial disturbance on GHG exchange and carbon (C) and nitrogen (N) budgets. In order to include measurements carried with different observation techniques, we tested two different measurement techniques jointly in 2013, namely the manual static chamber approach and the eddy covariance technique, to quantify the GHG exchange from the observed grassland site. Our results showed that there were no memory effects on N2O and CH4 emissions after ploughing, whereas the CO2 uptake of the site considerably increased when compared to post-restoration years. In detail, we observed large losses of CO2 and N2O 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/emission peaks of N2O (defined as exceeding background emissions < 0.21 ± 0.55 nmol m−2s−1 (SE = 0.02) for at least two sequential days and the seven-day moving average exceeding background emissions) were observed for almost seven continuous months after restoration as well following organic fertilizer applications during all. Net ecosystem exchange of CO2 (NEECO2) showed a common pattern of increased uptake of CO2 in spring and reduced uptake in late fall. NEECO2 dropped to zero and became positive after each harvest event. Methane (CH4) exchange in contrast to N2O showed minor net uptake of methane seen by the static chambers and small net release of methane seen by the eddy covariance method. Overall, CH4 exchange was of negligible importance for both, the GHG budget as well as for the carbon budget of the site. Our results stress the inclusion of grassland restoration events when providing cumulative sums of C sequestration and/or global warming potentials (GWPs). Consequently, this study further highlights the need for continuous long-term GHG exchange observations as well as 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.

Referee Comments

“…Several studies (e.g. (Rutledge et al, 2017;Ammann et al, 2020, etc) show CO2 uptake in restoration and later years. Lines 89-90: If you expect CO2 losses (as per the above point), why would you expect a C gain?…”