Are there memory effects on greenhouse gas emissions (CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub>) following grassland restoration?
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Titel:
Are there memory effects on greenhouse gas emissions (CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub>) following grassland restoration?
Beschreibung:
<jats:p>Abstract. A 5-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 2 years (2010 and 2011) of manual static chamber measurements of CH4 and
N2O, 5 years of continuous eddy covariance (EC) measurements
(CO2–H2O – 2010–2014), and 3 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 (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
N2O, 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 plowing, whereas the CO2 uptake of the site
considerably increased when compared to pre-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 and emission peaks of N2O (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 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 fluctuated around zero during all years.
Overall, CH4 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.
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