Biogeosciences, 13, 1255–1268, 2016
www.biogeosciences.ne /13/1255/2016/
doi:10.5194/bg-13-1255-2016
© Au ho (s) 2016. CC A ibu ion 3.0 License.
Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion
on a mine al soil
Saa a E. Lind1, Na asinha J. Shu pali1, Olli Pel ola2, I an Mamma ella2, Niina Hy önen1, Ma ja Maljanen1,
Ma i Rä y3, Pe u Vi kajä i3, and Pe i J. Ma ikainen1
1Depa men o En i onmen al Science, Uni e si y o Eas e n Finland, Yliopis on an a 1 E, P.O. Box 1627, Kuopio Campus,
70211, Finland
2Depa men o Physics, P.O. Box 48, 00014 Uni e si y o Helsinki, Helsinki, Finland
3Na u al Resou ces Ins i u e Finland, G een Technology, Halolan ie 31 A, 71750 Maaninka, Finland
Co espondence o: Saa a E. Lind ([email p o ec ed])
Recei ed: 4 Sep embe 2015 – Published in Biogeosciences Discuss.: 19 Oc obe 2015
Re ised: 11 Feb ua y 2016 – Accep ed: 13 Feb ua y 2016 – Published: 1 Ma ch 2016
Abs ac . One o he s a egies o educe ca bon dioxide
(CO2)emissions om he ene gy sec o is o inc ease he use
o enewable ene gy sou ces such as bioene gy c ops. Bioen-
e gy is no necessa ily ca bon neu al because o g eenhouse
gas (GHG) emissions du ing biomass p oduc ion, ield man-
agemen and anspo a ion. The p esen s udy ocuses on he
cul i a ion o eed cana y g ass (RCG, Phala is a undinacea
L.), a pe ennial bioene gy c op, on a mine al soil. To quan-
i y he CO2exchange o his RCG cul i a ion sys em, and o
unde s and he key ac o s con olling i s CO2exchange, he
ne ecosys em CO2exchange (NEE) was measu ed om July
2009 un il he end o 2011 using he eddy co a iance (EC)
me hod. The RCG cul i a ion h i ed well p oducing yields
o 6200 and 6700kgDWha−1in 2010 and 2011, espec-
i ely. G oss pho osyn hesis (GPP) was con olled mainly by
adia ion om June o Sep embe . Vapou p essu e de ici
(VPD), ai empe a u e o soil mois u e did no limi pho-
osyn hesis du ing he g owing season. To al ecosys em es-
pi a ion (TER) inc eased wi h soil empe a u e, g een a ea
index and GPP. Annual NEE was −262 and −256gCm−2
in 2010 and 2011, espec i ely. Th oughou he s udy pe-
iod om July 2009 un il he end o 2011, cumula i e NEE
was −575gCm−2. Ca bon balance and i s egula o y ac o s
we e compa ed o he published esul s o a compa ison si e
on d ained o ganic soil cul i a ed wi h RCG in he same cli-
ma e. On his mine al soil si e, he RCG had highe capaci y
o ake up CO2 om he a mosphe e han on he compa ison
si e.
1 In oduc ion
An h opogenic inc ease in he a mosphe ic concen a ion o
g eenhouse gases (GHGs) has been conside ed he majo ea-
son o he global clima e wa ming (IPCC, 2013). The ca bon
dioxide (CO2)concen a ion in he a mosphe e has inc eased
om 278 o 391ppm be ween 1750 and 2011 and is s ill in-
c easing (IPCC, 2013). Ca bon dioxide emi ed o he a mo-
sphe e o igina es mainly om espi a ion (plan s and mic o-
o ganisms) and ossil uel combus ion wi h he main sinks
being pho osyn hesis and oceans (IPCC, 2013). In Finland
he ene gy sec o and ag icul u e a e he mos impo an in
he o al na ional GHG emissions (S a is ics Finland, 2014).
One o he s a egies o educe CO2emissions om he en-
e gysec o is oinc ease heuse o enewable ene gysou ces,
e.g. using biomass. Bioene gy p oduced om biomass is no
necessa ily ca bon neu al because o GHG emissions du -
ing biomass p oduc ion, ield managemen and anspo a-
ion. Li e-cycle assessmen (LCA) esul s ha e been ecen ly
epo ed o eed cana y g ass (RCG, Phala is a undinacea
L.) cul i a ion on cu -away pea lands in Finland (Shu pali e
al., 2010) and Es onia (Jä eoja e al., 2013). In hese s ud-
ies, he RCG si es we e ne sinks o CO2and hence, RCG
is sugges ed o be a good a e -use op ion o such ma ginal
soils which a e known o elease la ge amoun o CO2as a e-
sul o decomposi ion o esidual pea , when le abandoned
(Kasimi -Klemed sson e al., 1997).
Cul i a ion o RCG has been popula in Finland since he
mid-1990s and a he peak app oxima ely 19000ha (2007
and 2008) we e cul i a ed wi h RCG. Howe e , owing o
Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union.
1256 S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion
echnical di icul ies wi h he bu ning o he RCG biomass in
combus ion plan s, he scope o RCG as a sou ce o biomass
bioene gy has declined in he las ew yea s. In 2014, he a -
e age cul i a ion a ea was a ound 6000ha. Ne e heless, he
scope o RCG as a sou ce o liquid bio uel, a diges a e in
biogas plan s, oil spill abso p ion and a bu e c op be ween
e es ial and aqua ic landscape is wide (Pasila and Kymäläi-
nen, 2000; Pa ala e al., 2001; Powlson e al., 2005; Kandel
e al., 2013b).
RCG is a pe ennial c op which is well adap ed o he no h-
e n clima ic condi ions. I has a o a ion ime o up o 15
yea s. The annually ha es ed yield up o12000kgDWha−1
has been epo ed (Lewandowski e al., 2003). As a pe en-
nial c op, i has ad an ages o e heannual c opping sys ems.
The c op g ow h ollowing he i s o e win e ing s a s ea -
lie as he e-es ablishmen o he c op in he sp ing is no
needed. This cul i a ion s yle also educes he use o machin-
e y a he si e since e.g. annual illing is no equi ed.
While con inuous and long- e m measu emen s o GHG
balance om bioene gy c ops a e needed o e alua e he
a mosphe ic impac o he whole p oduc ion chain, o ou
knowledge, he e a e no GHG lux measu emen s om RCG
cul i a ion on mine al soils. Wi h his in iew, we measu ed
he CO2balance o RCG c op cul i a ion (2009–2011) on
a mine al soil by he eddy co a iance (EC) echnique. Ou
objec i es in his pape a e o quan i y and cha ac e ize he
ne ecosys em exchange (NEE) o a pe ennial c op cul i a ed
on a mine al soil and o in es iga e he ac o s con olling i s
CO2balance. Addi ionally, we aim o compa e ou indings
om he mine al soil si e o he published da a on o a RCG
cul i a ion sys em on a d ained o ganic soil ( e e ed o he e-
a e as compa ison si e) in he same clima e egion.
2 Ma e ials and me hods
2.1 S udy si e and ag icul u al p ac ices
The s udy si e is loca ed in Maaninka (63◦0904900 N,
27◦140300 E, 89mabo e he mean sea le el) in eas e n Fin-
land. Long- e m (30 yea s, e e ence pe iod 1981–2010; Pi i-
nen e al., 2012) annual ai empe a u e in he egion is
3.2◦C wi h Feb ua y being he coldes (−9.4◦C) and July
he wa mes (17.0◦C) mon h. The annual p ecipi a ion in he
egion is 612mm wi h a seasonal amoun o 322mm du ing
he May–Sep embe pe iod.
The expe imen al si e is a 6.3ha (280×220m) ag icul-
u al ield cul i a ed wi h RCG (c . “Pala on”). Du ing he
las 10 yea s p io o plan ing o RCG, he ield was cul-
i a ed wi h g ass (Phleum p a ense L.; Fes uca p a ensis
Huds.), ba ley (Ho deum ulga e L.) o oa (A ena sa i a
L.). Fo a de ailed soil analysis h ee 100cm deep soil pi s
we e exca a ed and eigh 6cm deep ho izons be ween 0 and
93cm we e sampled in July 2010. Th ee undis u bed soil
samples om each ho izon we e aken wi h s eel cylinde s
(heigh 6.0cm, diame e 5.7cm). Two soil samples we e used
o de e mina ion o soil physical p ope ies and one o he
chemical p ope ies. To cha ac e ize p ope ies o op soil
(0–18cm) in gene al, soil samples aken a dep hs o 0–6,
6–12, 12–18cm we e analysed sepa a ely, and mean alues
o e he ho izons we e calcula ed o each pi . The esul s
shown he e a e means (±s anda d de ia ion) o e he h ee
pi s. The soil samples we e o en d ied a 35◦C and g ound
o pass h ough a 2mm sie e. The pa icle-size dis ibu ion
was de e mined wi h he pipe e me hod (Elonen, 1971). To-
al o ganic C and o al N con en s we e de e mined by d y
combus ion using a Leco®analyse , he soil pa icle densi y
wi h a s oppe ed bo le pycnome e me hod and bulk den-
si y was calcula ed as a a io o he d y weigh (o en d ied
a 105◦C) and sampling olume o he soil. Soil pH and
elec ical conduc i i y we e measu ed in soil–wa e suspen-
sion (1:2.5 / ). The easily soluble P and exchangeable K
we e ex ac ed wi h acid ammonium ace a e a pH4.7, as de-
sc ibed by Vuo inen and Mäki ie (1955).
The soil was classi ied as a Haplic Cambisol/Regosol (Hy-
pe eu ic, Sil ic) (IUSS Wo king G oup WRB, 2007), he
opsoil being gene ally sil loam (clay mean 25±5.6%,
sil 53±9.0% and sand 22±7.8%) based on he US
Depa men o Ag icul u e (USDA) ex u al classi ica ion
sys em. The a e age soil cha ac e is ics in he opsoil
we e as ollows: pH (H2O) 5.8±0.19, elec ical conduc-
i i y 14±2.4mSm−1, soil o ganic ma e 5.2±0.90%,
o ganic ca bon 3.0±0.52%, o al ni ogen 0.2±0.03%,
C:N a io 15±0.4, he acid ammonium ace a e ex-
ac able K 104±12.9mgL−1soil, P 5.4±1.28mgL−1
soil, pa icle densi y 2.65±0.014gcm−3and bulk den-
si y 1.1±0.11gcm−3. Based on he soil mois u e e en-
ion cu e, he ield capaci y was 39.7±1.2% (soil mois-
u e ( / )) and wil ing poin was 21.6±0.8% (soil mois u e
( / )).
In he beginning o June 2009, he sowing o RCG was
done wi h a seed a e o 10.5kgha−1 oge he wi h he appli-
ca ion o a mine al e ilize (60kgNha−1, 30kgPha−1and
45kgKha−1). The ield was olled p io o and a e sow-
ing. Addi ional sowing was done o ill he seedling gaps
in June and July. He bicide (mix u e o MPCA 200gL−1,
clopy alid 20gL−1and lu oxypy 40gL−1, 2L in 200L o
wa e ha−1)was applied by he end o July 2009 o con-
ol he weeds. Mine al e ilize was applied as su ace
applica ion in sp ing 2010 (70kgNha−1, 11kgPha−1and
18kgKha−1)and sp ing 2011 (76kgNha−1, 11kgPha−1
and 19kgKha−1). The biomass p oduced du ing he i s
g owing season was no ha es ed bu le on he si e. Du ing
he ollowing yea s, he ha es ing was done in he sp ing
a e he g owing season (28 Ap il 2011 and 9 May 2012).
Thus, he sp ing 2011 was he i s ime when he c op was
ha es ed a e i s es ablishmen in he summe o 2009. As
p oduced biomass was used o bu ning, keeping he c op a
he si e o e win e is a s anda d RCG cul i a ion p ac ise in
he No dic coun ies, as sp ing ha es ing has been shown
Biogeosciences, 13, 1255–1268, 2016 www.biogeosciences.ne /13/1255/2016/
S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion 1257
o imp o e he quali y o he biomass o bu ning (Bu all,
1997). The biomass was ha es ed using a m-scale machin-
e y. The na u ally d ied ege a ion was cu wi h a con en-
ional disc mowe (wi hou condi ione ) o app ox. 5cm s ub-
ble heigh , swa hed and baled in o ound bales 1–2 days a e
cu ing.
2.2 Mic ome eo ological measu emen s
Measu emen s o CO2, la en hea (LE) and sensible hea
(H) luxes we e ca ied ou om July 2009 un il he end o
2011 using he closed-pa h EC me hod (Baldocchi, 2003). A
measu emen mas was ins alled app oxima ely in he mid-
dle o he ield and he ins umen cabin was loca ed abou
10m eas o he EC mas . The p e ailing wind di ec ion was
no he ly wi h a 24% occu ence du ing he s udy pe iod.
The EC ins umen a ion consis ed o an in a- ed gas anal-
yse (IRGA) o CO2and wa e apou (H2O) concen a ions
(model: Li-7000 (p ima y) o Li-6262 (backup), LiCo ) and
a sonic anemome e (model: R3-50, Gill Ins umen s L d,
UK) o wind eloci y componen s and sonic empe a u e.
The mas heigh was 2, 2.4 o 2.5m, adjus ed acco ding o
he ege a ion heigh . Excep o he wind sec o om 85 o
130◦downwind o he ins umen cabin, all wind di ec ions
we e accep able because no o he obs acles we e p esen and
he sonic anemome e in use had an omnidi ec ional geome-
y.
A hea ed gas sampling line (inne diame e 4mm, leng h
8m poly e a luo oe hylene (PTFE)+0.5m me al) wi h wo
il e s (po e size 1.0µm, PTFE, Gelman®o Millipo e®) was
used o d aw ai wi h a low a e o ini ially 6Lmin−1(un il
31 Ma ch 2011). Subsequen ly, a low a e o 9Lmin−1was
used. The IRGA was housed in a clima e-con olled cabin.
Re e ence gas low, c ea ed using soda lime and anhyd one,
also i ed wi h a Gelman® il e , was 0.3Lmin−1. The IRGA
was calib a ed app oxima ely e e y second week wi h a wo-
poin calib a ion (0 and 399µLL−1o CO2, AGA Oy, Fin-
land) and addi ionally wi h a dew poin gene a o (model:
LI-610, LiCo ) o H2O mixing a io du ing condi ions when
ai empe a u e was abo e +5◦C.
Da a collec ion was done a 10Hz using he Edisol p o-
g am (Monc ie e al., 1997). The 30min EC lux alues
we e calcula ed om he co a iance o he scala s and e -
ical wind eloci y (e.g. Aubine e al., 2000). Da a p o-
cessing was done using EddyUH pos -p ocessing so wa e
(Mamma ella e al., 2016). Despiking was done by de ining
a limi o he di e ence in subsequen da a poin s o CO2
(15µLL−1)and H2O (20mmolmol−1)concen a ions, wind
componen s (u=10ms−1, =10ms−1and w=5ms−1)
and empe a u e (5◦C). A da a poin de ined as a spike was
eplaced wi h he p e ious alue. Poin -by-poin dilu ion co -
ec ion was applied a e he despiking. Two-dimensional
coo dina e o a ion (mean la e al and e ical wind equal
o ze o) was done on he sonic anemome e wind compo-
nen s. Angle o a ack co ec ion was no applied. De end-
ing was done using block-a e aging. Lag ime due o he
gas sampling line was calcula ed by maximizing he co a i-
ance. Low- equency spec al co ec ions we e implemen ed
acco ding o Rannik and Vesala (1999). Fo high- equency
spec al co ec ions, empi ical ans e unc ion calcula ions
we e done based on he p ocedu e in oduced by Aubine e
al. (2000). Humidi y e ec s on sonic hea luxes we e co -
ec ed acco ding o Scho anus e al. (1983). F om he p o-
cessed da a, lux alues measu ed when winds we e om
behind he ins umen cabin and hose du ing ain e en s
we e emo ed. The a ailable lux da a we e u he qual-
i y con olled using il e s as ollows. We plo ed he nigh -
ime NEE wi h u∗and ound no co ela ion be ween he
wo. Ne e heless, a de aul u∗ il e o 0.1ms−1was used.
Flux was conside ed non-s a iona y ollowing Foken and
Wichu a (1996). In his pape , we used a limi o 0.4 (e.g.
40% di e ence be ween he sub-pe iods and he o al a e -
aging pe iod). Bo h skewness and ku osis o he da a we e
checked and he accep able skewness ange was se om
−3 o 3 and ku osis om 1 o 14. O e all lags (acco d-
ing o Foken e al., 2004) highe han 7 we e emo ed. Fi-
nally, he da a we e isually inspec ed. F om he a ailable
da a, app oxima ely 30% o he CO2and H lux da a and
40% o he LE lux da a we e ejec ed. The andom e -
o s o 30min a e aged and quali y-con olled CO2 luxes
we e de e mined ollowing Vicke s and Mah (1997). The
andom e o was 13, 12 and 14% du ing July–Sep embe
2009, May–Sep embe 2010 and May–Sep embe 2011, e-
spec i ely. Foo p in s we e calcula ed o each 30min a e -
aging pe iod wi h he analy ical oo p in model de eloped
by Ko mann and Meixne (2001). The model is alid wi hin
he su ace laye and i u ilizes powe law p o iles o sol ing
he oo p in sizes analy ically in a wide ange o a mosphe ic
s abili ies. Based on he analysis, 80% o he lux was ound
o o igina e om wi hin 130m adius o he mas .
The da a gap illing and lux pa i ioning was done using
he online ool (h p://www.bgc-jena.mpg.de/~MDIwo k/
eddyp oc/index.php). This gap- illing me hod conside s bo h
he co- a ia ion o he luxes wi h global adia ion, empe -
a u e and apou p essu e de ici (VPD) and empo al au o-
co ela ion o he luxes (Reichs ein e al., 2005). Flux pa i-
ioning was done excluding gap- illed da a. To al ecosys em
espi a ion (TER) was de ined as he nigh - ime measu ed
ne ecosys em CO2exchange (NEE). The eg ession be ween
nigh - ime NEE and ai empe a u e (T) was calcula ed using
an exponen ial eg ession model (Lloyd and Taylo , 1994) o
he o m
R(T)=R e eE0(1
T e −T0−1
T−T0)(1)
whe e T0= −46.021◦C, T e =10◦C and i ed pa ame e s
we e R e ( he empe a u e independen espi a ion a e) and
E0( empe a u e sensi i i y). Using he model ou pu s o
R e and E0, he hal -hou TER was es ima ed using he mea-
su ed ai empe a u e. Finally, g oss pho osyn hesis (GPP)
www.biogeosciences.ne /13/1255/2016/ Biogeosciences, 13, 1255–1268, 2016
1258 S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion
was calcula ed as a di e ence be ween NEE and TER. In his
pape , CO2 eleased o he a mosphe e is de ined as a posi i e
alue and up ake om he a mosphe e as nega i e.
As a inal s ep, he EC measu emen s we e alida ed using
he ene gy balance closu e (EBC) de e mined as he slope
o he eg ession be ween ne adia ion (Rn) and la en hea
(LE), sensible hea (H) and he g ound hea lux (G). The
EBC is exp essed in he ollowing o mula ion (A ya, 1988)
and i is a simpli ied o mula which is alid o ideal su aces,
i.e. wi h no mass and hea capaci y:
Rn=LE+H+G(2)
The EBC was de e mined using da a om only hose 30min
ime pe iods when all o he ene gy componen s we e a ail-
able. The slope o he eg ession was 0.70 in May–Sep embe
2010 and 2011. Incomple e closu e is a common p oblem
due o e.g. la ge eddies (Foken, 2008), angle o a ack issues
(Nakai e al., 2006) and also because pa o he a ailable en-
e gy is also s o ed in di e en pa s o he ecosys em (Foken,
2008). The e o e, EBC was calcula ed so ha i includes di -
e en s o age e ms, i.e. hea in he soil, c op canopy, amoun
o ene gy used in pho osyn hesis, sensible and la en hea be-
low he EC mas ( ollowing Meye s and Hollinge , 2004 and
Lind o h e al., 2010) o gi e a mo e p ecise es ima ion o he
EBC. Wi h his app oach, he slope inc eased o 0.75. The
ob ained EBC is well wi hin he ange o EBCs epo ed o
se e al FLUXNET si es by Wilson e al. (2002). Maude e
al. (2013) sugges ed ha he EBC could be used as a me ic
o sys ema ic unce ain y in EC luxes. Based on his ap-
p oach he sys ema ic unce ain ies o he EC luxes epo ed
in his s udy we e simila o hose published in o he s udies.
2.3 Suppo ing measu emen s
A wea he s a ion was se up close o he EC mas . Heigh
o he wea he s a ion mas was adjus ed acco ding o he EC
mas heigh . Suppo ing clima ic a iables, i.e. ne adia ion
(model: CNR1, Kipp&Zonen B.V.), ai empe a u e and ela-
i e humidi y (model: HMP45C, Vaisala Inc), pho osyn he -
ically ac i e adia ion (PAR, model: SKP215, Skye ins u-
men s L d.), amoun o ain all a 1m heigh (model: 52203,
R.M. Young Company), soil empe a u e a 5, 10 and 30cm
dep hs (model: 107, Campbell Scien i ic Inc.), soil mois u e
a dep hs o 5, 10 and 30cm (model: CS616, Campbell Sci-
en i ic Inc.), soil hea lux a 7.5cm dep h (model: HPF01SC,
Hukse lux) and ai p essu e (model: CS106 Vaisala PTB110
Ba ome e ) we e measu ed. Da a we e collec ed using a da -
alogge (model: CR 3000, Campbell Scien i ic Inc.). All me-
eo ological da a we e collec ed as 30min mean alues (p e-
cipi a ion as 30min sum), excep ai p essu e which was
eco ded as an hou ly mean. Suppo ing da a collec ion be-
gan on 14 Augus 2009. Sho gaps in he da a we e illed
using linea in e pola ion. I ai empe a u e, ela i e humid-
i y, p essu e o ain all da a we e missing o long pe iods,
da a om Maaninka wea he s a ion, loca ed abou 6km o
he sou heas o he si e and ope a ed by he Finnish Me eo-
ological Ins i u e (FMI), we e used.
The RCG g een a ea index (GA) was es ima ed ollow-
ing Wilson e al. (2007). Measu emen s we e done app ox-
ima ely on a weekly basis du ing he main g owing pe iod
and less equen ly in he au umn. Th ee loca ions (1×1m2)
we e selec ed and wi hin hose, h ee spo s (8×8cm2)we e
used o coun he numbe o g een s ems (Sn) and lea es (Ln)
pe uni a ea. Th ee plan s adjacen o small spo s we e se-
lec ed o measu emen s o g een a ea o lea es (La) and
s ems (Sa). Following equa ion was used o calcula e GA
(m2m−2):
GA =(Sn·Sa)+(Ln·La). (3)
Lea a ea index (LAI) was measu ed using a plan canopy
analyse (model: LAI-2000, LiCo ) wi h a 180◦ iew cap.
The LAI was measu ed close o GA plo s a he same in e -
al and a he same day as GA was es ima ed. A measu emen
was accep ed when he s anda d e o o LAI was less han
0.3 and he numbe o abo e and below ege a ion obse a-
ion pai s was mo e han 3.
Abo e-g ound biomass samples we e collec ed app oxi-
ma ely on a mon hly basis om h ee loca ions in he ield
du ing he snow- ee season in 2009, 2010 and 2011 (and
oo samples in 2009 and 2010). Abo e-g ound biomass was
collec ed om a 20×20cm2a ea. Samples we e d ied in
he o en un il (+65◦C) he weigh o he samples emained
unchanged (app oxima ely 24h) and d y weigh (DW) was
measu ed. Roo biomass (0–25cm) was sampled om he
same a eas as he abo e-g ound biomass using a soil co e
(diame e 7cm). Li ing oo s ( ine and coa se oo s) we e
picked and washed. A e d ying (+65◦C) o 24h, DW was
measu ed.
To analyse he pe o mance o he c op, wa e use e i-
ciency (WUE) was de e mined ollowing Law e al. (2002).
Fo his pu pose, e apo anspi a ion (ET) was de e mined
by di iding LE wi h he la en hea o apo iza ion
(L=2500kJkg−1). Mon hly sums o GPP and ET om he
May o Sep embe pe iod we e ob ained and WUE was de e -
mined as he slope o he linea eg ession be ween mon hly
GPP and ET. The Bowen a io was calcula ed om day ime
(PAR>20µmolm−2s−1)measu ed Hand LE luxes.
2.4 Analysis o en i onmen al ac o s go e ning CO2
exchange
The ela ionship be ween GPP and PAR was examined on
a mon hly basis om mid-May o Sep embe sepa a ely o
2010 and 2011. P io o he analysis, PAR da a we e binned a
an in e al o 10µmolm−2s−1. The bin-a e aged alues o
GPP we e plo ed agains PAR and he da a we e i ed wi h a
ec angula hype bolic model o he o m (e.g. Tho nley and
Johnson, 1990)
GPP =GPmax ·PAR·α
GPmax +PAR·α,(4)
Biogeosciences, 13, 1255–1268, 2016 www.biogeosciences.ne /13/1255/2016/
S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion 1259
whe e GPmax (µmolm−2s−1)is he heo e ical maximum
a e o pho osyn hesis a in ini e PAR and αis he appa -
en quan um yield. Addi ionally, da a wi h PAR le els g ea e
han 1000µmolm−2s−1we e used o s udy he ela ionship
be ween GPP and ai empe a u e, VPD and soil mois u e. To
analyse he ela ionship be ween GPP and GA and also LAI,
a weekly a e aged GPP was cons uc ed o hose weeks
when he plan a iables we e a ailable. These da a we e i -
ed wi h a linea eg ession.
To be able o compa e he esul s in de ail wi h he ea -
lie indings on RCG a an o ganic soil si e in Finland (Shu -
pali e al., 2010) ano he eg ession model was used o assess
he ela ionship be ween TER and soil empe a u e, nigh -
ime measu ed NEE (PAR<5µmolm−2s−1) om May o
Sep embe sepa a ely o 2010 and 2011 was used. P io o
he analysis, he da a we e binned wi h soil empe a u e a
2.5cm dep h ( om 0 o 21.5 wi h a 0.5◦C in e al). The bin-
a e aged alues o TER we e plo ed agains soil empe a u e
and he da a we e i ed wi h an exponen ial eg ession model
o he o m (e.g. Shu pali e al., 2009)
TER =R10 ·Q(Ts/T10)
10 ,(5)
whe e Tsis he measu ed soil empe a u e (◦C) a 2.5 dep h,
T10 =10◦C and he i ed pa ame e s a e R10 (base espi a-
ion, µmolm−2s−1, a 10◦C) and Q10 ( he empe a u e sen-
si i i y coe icien ). To analyse he ela ionship be ween TER
and ege a ion, we cons uc ed weekly means om daily
TER alues o he weeks du ing which GA was es ima ed
o 2010 and 2011. To assess he ela ionship be ween GPP
and TER, daily sums o TER and GPP om May o Sep em-
be sepa a ely o 2010 and 2011 we e used in he linea e-
g ession analysis.
2.5 Compa ison si e cha ac e is ics
The compa ison si e wi h o ganic soil has been in ensi ely
s udied and se e al pape s epo esul s om i (e.g. Shu pali
e al., 2008; Hy önen e al., 2009; Shu pali e al., 2009, 2010,
2013; Gong e al., 2013). The compa ison si e is loca ed in
eas e n Finland (62◦300N, 30◦300E, 110m abo e mean sea
le el).Long- e m (30 yea s, e e ence pe iod 1981–2010) an-
nual ai empe a u e in he egion is 3.0◦C and he annual
p ecipi a ion in he egion is 613mm. The a ea was o igi-
nally an omb o ophic Sphagnum uscum pine bog ( o mo e
de ails, see Biasi e al., 2008). F om 1976 onwa ds he si e
was p epa ed o pea ex ac ion – i.e. i was d ained and he
ege a ion was emo ed. Pea ex ac ion was s a ed in 1978.
In 2001, when he pea dep hs we e be ween 20 and 85cm,
a 15ha a ea was sown wi h RCG (c . “Pala on”). Since
hen, he si e has been annually e ilized wi h 50kgNha−1,
14kgPha−1and 46kgKha−1. Lime was added as dolomi e
limes one (CaMg(CO3)2)a he a e o 7.8 ha−1in 2001 and
2006.
The a e age su ace pea cha ac e is ics we e as ollows:
pH 5.4, bulk densi y 0.42gm−3and C:N a io 40.3 (Shu -
pali e al., 2008). The clima ic condi ions du ing he yea s
2004–2007 a he si e we e such ha he annual ai empe -
a u e was 2.7, 3.7, 3.1 and 3.2◦C and annual p ecipi a ion
was 862, 544, 591, 700mm in 2004, 2005, 2006 and 2007,
espec i ely (Hy önen e al., 2009). Du ing May–Sep embe
pe iod, he p ecipi a ion was 554, 246, 249 and 423mm in
2004, 2005, 2006 and 2007, espec i ely. The di e ence o
he long- e m mean (312 mm) was app oxima ely 20% du -
ing he d y yea s (2005 and 2006) and 36 and 78% du -
ing he we yea s (2004 and 2007, espec i ely). Wa e able
le el was on a e age 0.65m, a ying om 0.4 o 0.7m du -
ing he yea s (Hy önen e al., 2009). The olume ic wa e
con en (VWC) a 30cm dep h was always high and did no
a y be ween he yea s. The VWC a su ace laye s (2.5 and
10cm dep hs) luc ua ed in esponse o p ecipi a ion e en s
and anged om 0.1 o 0.8m3m−3. The biomass a he si e
was used o bu ning pu pose and, he e o e, i was ha es ed
in he sp ing. The sp ing-ha es ed yields we e 3700, 2000,
3600 and 4700kgha−1in 2004, 2005, 2006 and 2007, e-
spec i ely (Shu pali e al., 2009). The CO2exchange was
measu ed using an open-pa h EC sys em and de ails o he
measu emen s and da a p ocessing can be ound in Shu pali
e al. (2009).
3 Resul s
3.1 Seasonal clima e and c op g ow h
The mean annual ai empe a u e a he s udy si e was 3.5,
2.2 and 4.5◦C in 2009, 2010 and 2011, espec i ely, wi h he
daily means a ying om −30.0 o +27.1◦C (Fig. 1a). An-
nual p ecipi a ion was 421, 521 and 670mm in 2009, 2010
and 2011, espec i ely. In May–Sep embe pe iod he p ecip-
i a ion was 40 and 28% lowe in 2009 (192mm) and 2010
(228mm) han he long- e m mean. P ecipi a ion was abou
he same as he long- e m mean in 2011 (327mm, Fig. 1b).
The g owing season is de ined as ha ing commenced when
he mean daily ai empe a u e exceeds 5◦C o 5 consec-
u i e days wi h no snow and ended when he mean daily
ai empe a u e is below 5◦C on 5 consecu i e days. G ow-
ing season commenced on 1 May 2009, 9 May 2010 and
23 Ap il 2011 and las ed 152, 156 and 182 days in he h ee
consecu i e yea s.
The daily a e aged VWC anged om 0.12 o
0.54m3m−3, om 0.09 o 0.37m3m−3and om 0.11
o 0.45m3m−3in 2009, 2010 and 2011, espec i ely
(Fig. 1c). The summe maxima we e eco ded a 2.5cm
dep h in July 2010 (20.9◦C) and 2011 (19.1◦C) (Fig. 1d).
Du ing he win e 2009–2010 and 2010–2011 he soil em-
pe a u es we e close o ze o. The lowes soil empe a u es
we e eco ded a 2.5cm dep h in Decembe 2009 (−7.5◦C)
and No embe 2010 (−3.4◦C).
The es ima ed e apo anspi a ion (ET), was 110, 330 and
370 mm in Augus –Sep embe 2009, May–Sep embe 2010
www.biogeosciences.ne /13/1255/2016/ Biogeosciences, 13, 1255–1268, 2016

1260 S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion
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Figu e 1. Clima ic condi ions a he s udy si e du ing he measu e-
men yea s. (a) Daily a e aged ai empe a u e (◦C) du ing 2009–
2011, (b) Daily p ecipi a ion (mmd−1, g ey line) and i s cumula-
i e sum (mm, black line) du ing he g owing seasons. (c) Daily a -
e aged olume ic wa e con en (VWC, m3m−3)a 2.5cm (da k
g ey line), 10cm (ligh g ey line) and 30cm (black line) du ing he
g owing seasons, om 14 Augus 2009 onwa ds. (d) Soil empe a-
u es (◦C) a he 2.5cm (da k g ey line), 10cm (ligh g ey line) and
30cm (black line) dep hs as daily means om 14 Augus 2009 un il
2 Decembe 2011.
and May–Sep embe 2011, espec i ely. Du ing hose ime
pe iods, he ecosys em used mo e wa e han was ecei ed
h ough ain all as he co esponding p ecipi a ion amoun s
we e 80, 220 and 320mm in 2009, 2010 and 2011, espec-
i ely. A clea linea ela ionship was ound be ween GPP
and ET (adjus ed R2=0.73, p<0.01, n=12) du ing he
May–Sep embe pe iod in 2010 and 2011. The wa e use e -
iciency (WUE) o he RCG cul i a ion de e mined om his
ela ionship was 12gCO2pe kgH2O. A e aged day ime
Bowen a io was 0.18 and 0.28 du ing he May–Sep embe
pe iod in 2010 and 2011, espec i ely.
Du ing he i s g owing season (2009), he ege a ion de-
elopmen was slow and he maximum plan heigh was low
when compa ed o he subsequen yea s (0.6, 1.7 and 1.8m in
2009, 2010 and 2011, espec i ely). In he ollowing yea s,
he ini ial sp ou ing in ea ly sp ing was ollowed by igo ous
plan g ow h which las ed abou 9 weeks. The apid plan
g ow h esul ed in a s eep inc ease in g een a ea (GA) and
lea a ea indices (LAI) in 2010 and 2011 (Fig. 2b, c). Bo h
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Figu e 2. Vege a ion pa ame e s de e mined on he eed ca-
na y g ass (RCG) cul i a ion. App oxima ely mon hly de e mined
abo e-g ound (g ey ba s) and oo biomass (ha ched ba s) in
gd yweigh (DW) m−2be ween week 15 and 45 in (a) 2009,
(b) 2010 and (c) 2011. Also app oxima ely weekly de e mined no -
malized g een a ea index (GA, black do s) and lea a ea index (LAI,
g ey do s) o (b) 2010 and (c) 2011 is shown.
GA and LAI le elled o in he beginning o June. The max-
imum abo e-g ound biomass was eco ded a he end o he
season (560, 1100 and 1600gDWm−2in 2009, 2010 and
2011, espec i ely) (Fig. 2a, b and c). The maximum oo
biomass was 480gDWm−2in 2010 (Fig. 2b). Depending
on he sampling occasion, 70 o 80% o he oo s we e dis-
ibu ed wi hin he 0–10cm dep h. The c op yield was 6200
and 6700kgDWha−1in 2010 and 2011, espec i ely.
3.2 CO2exchange pa e ns
3.2.1 Measu ed ne ecosys em CO2and ene gy
exchange
Measu ed 30min alues o NEE, Hand LE du ing 2009,
2010 and 2011 p io o he gap illing a e shown in Fig. 3.
In 2009, he NEE measu emen s began 45 days a e he
sowing in mid-June. The maximum ampli ude o he diu -
nal NEE cycle a ied om −26 o 20µmolm−2s−1du ing
he g owing season in 2009. The ampli ude o he diu nal
NEE cycle was no iceable a ound mid-May onwa ds un il
No embe in 2010 and 2011. The maximum ampli ude o
diu nal NEE cycle a ied om −31 o 18µmolm−2s−1and
om −37 o 20µmolm−2s−1du ing he g owing seasons in
2010 and 2011, espec i ely (Fig. 3a). Ou side he g owing
seasons, espi a o y losses domina ed he ne CO2balance.
The ecosys em CO2loss was 0.62µmolm−2s−1 om Oc o-
be 2009 o mid-May 2010, 0.76µmolm−2s−1du ing a sim-
ila pe iod in 2010–2011 and 1.1µmolm−2s−1 o a sho e
ime pe iod in 2011 (No embe and Decembe ). The diu nal
LE cycle had he maximum ampli ude du ing he summe
mon hs and anged om −30 o 400, om 0 o 400 and om
0 o 600Wm−2in 2009, 2010 and 2011, espec i ely. LE
was close o ze o du ing he non-g owing season. The am-
pli ude o diu nal Hcycle was a he maximum du ing he
summe mon hs and anged om −50 o 130, om −100 o
210 and om −100 o 190Wm−2in 2009, 2010 and 2011,
Biogeosciences, 13, 1255–1268, 2016 www.biogeosciences.ne /13/1255/2016/
S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion 1261
Figu e 3. Measu ed CO2and ene gy luxes om July 2009
o Decembe 2011. (a) Ne ecosys em CO2exchange (NEE,
µmolm−2s−1).(b) La en hea lux (LE, Wm−2).(c) Sensible hea
lux (H, Wm−2).
espec i ely. H anged om −60 o 20Wm−2du ing he
non-g owing seasons.
3.2.2 Diu nal ends
To examine he diu nal ends, he da a on ai empe a u e,
VPD, PAR and NEE in June 2010 and 2011 we e a e aged
o gene a e hal -hou diu nal means (Fig. 4). In bo h yea s,
June p esen ed condi ions o high CO2up ake du ing he day
and o CO2loss a nigh . Ai empe a u e was lowe in 2010
han in 2011 bu bo h yea s showed ypical diu nal pa e ns
wi h minimum alues du ing ea ly mo ning hou s and max-
imum alues la e in he a e noon (Fig. 4a). Simila ly, he
VPD was lowe in 2010 han 2011 (Fig. 4b). The maximum
in VPD (0.96kPa) occu ed la e a e noon in 2010 whe eas
in 2011 he maximum (0.89kPa) occu ed a ound noon. In
bo h yea s, he ampli ude o diu nal mean o empe a u e and
VPD was mode a e. The mean diu nal pa e n o NEE was
simila be ween 2010 and 2011 and he pa e ns we e ai ly
symme ical (Fig. 4d). Du ing he nigh - ime, om 22:00 o
abou 02:00 (UTC+2), CO2exchange be ween he ecosys-
em and a mosphe e was cons an and domina ed by espi-
a ion. Mean NEE du ing his ime was 4.5µmolm−2s−1in
2010 and 6.6µmolm−2s−1in 2011. In he mo ning hou s,
wi h inc easing PAR (Fig. 4c), NEE began o decline and he
ligh compensa ion poin occu ed a a PAR le el o abou
200µmolm−2s−1a a ound 05:00 (UTC+2). A e his, he
up ake domina ed he CO2balance. The peaks in mean NEE
occu ed a ound 12:00 (UTC+2) a he same ime as he
peaks in he mean PAR. The maximum mean NEE in June
Figu e 4. Mean diu nal a ia ions in June 2010 (open g ey i-
angles) and 2011 (open black ci cles). (a) Ai empe a u e (◦C).
(b) Vapou p essu e de ici (VPD, kPa). (c) Pho osyn he ically ac-
i e adia ion (PAR, µmolm−2s−1).(d) Ne ecosys em CO2ex-
change (NEE, µmolm−2s−1). Da a a e hal -hou means wi h s an-
da d e o .
was −21 and −23µmolm−2s−12010 and 2011, espec-
i ely. Wi h declining PAR le els, he plan CO2up ake also
declined. The seconda y ligh compensa ion poin occu ed
a a ound 20:00 (UTC+2).
3.2.3 Daily pa e ns
Seasonal pa e ns o daily sums o GPP, TER and NEE a e
shown in Fig. 5. F om he s a o NEE measu emen s in
la e July o mid-Augus in 2009, he si e was a ne sou ce
o CO2 o he a mosphe e. By mid-Augus , GPP began o
o e whelm TER u ning he si e in o a CO2sink. Du ing he
g owing season, he maximum daily alues o NEE, TER
and GPP we e −5.8, 9.7 and −10.5gCm−2d−1, espec-
i ely. The up ake o CO2ended by la e Oc obe . Respi-
a ion le elled o by mid-Decembe . F om mid-Decembe
2009 un il May 2010, TER emained low a an a e age a e
o 0.46gCm−2d−1. In May 2010 and 2011, he daily GPP
and TER we e clea ly dis inguishable. Du ing he g owing
season, he maximum daily alues o NEE, TER and GPP
we e −9.4, 11.5 and −18.0gCm−2d−1, espec i ely. Res-
www.biogeosciences.ne /13/1255/2016/ Biogeosciences, 13, 1255–1268, 2016
1262 S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion
Figu e 5. The componen s o daily CO2exchange o e he mea-
su emen pe iod. Daily sum o ne ecosys em CO2exchange (NEE,
g ey ba s), g oss p ima y p oduc ion (GPP, open black ci cles)
and o al ecosys em espi a ion (TER, open g ey iangles) as
gCm−2d−1. Ho izon al solid black lines show he ze o le el and
e ical dashed black lines ma k beginning o he yea .
pi a ion le elled o a he end o No embe and TER e-
mained low du ing he win e ime un il he beginning o May
in 2011. Win e ime TER a e aged 0.51gCm−2d−1. Du -
ing he g owing season in 2011, he maximum daily alues
o NEE, TER and GPP we e simila o hose in 2010. Res-
pi a ion le elled o by he beginning o Decembe , wi h an
a e age alue o 0.76gCm−2d−1 o Decembe 2011.
3.3 Fac o s con olling CO2exchange
3.3.1 G oss pho osyn hesis
The s ong ela ionships be ween bin-a e aged GPP and PAR
om May o Sep embe in 2010 and 2011 can be seen
in Fig. 6a–e. The ec angula hype bolic model p o ided
good i s o he da a (adjus ed R2>0.90, Table 1) excep in
May 2010 and 2011 (adjus ed R2=0.52 and R2=0.76, e-
spec i ely) and all ela ionships we e s a is ically signi ican
(p<0.01). The e was no clea indica ion o GPP sa u a ion
e en a PAR le els close o 1800µmolm−2s−1du ing June
and July (Fig. 6a–e). The es ima ed mon hly GPmax alues
a e shown in Table 1. The e we e no di e ences in he GPmax
alues o May, June and July du ing 2010 and 2011, whe eas
in Augus and especially in Sep embe , he mon hly a e age
GPmax was highe in 2011 han in 2010. The seasonal a ia-
ion in mon hly GPmax alues was clea (Table 1) and in May,
Sep embe and Augus , he mon hly a e aged GPmax we e
low while he maximum alues we e obse ed in June and
July. The ange o he mon hly α- alues (quan um yield) a -
ied om −0.04 o −0.06 in 2010 and om −0.05 o −0.07
in 2011. Fu he analysis unde condi ions wi h PAR le el
g ea e han 1000µmolm−2s−1 e ealed ha e ec o o he
clima ic a iables such as ai empe a u e, he VPD and soil
mois u e on GPP was masked by he dominan ole o PAR.
We s udied he ela ionships be ween weekly a e aged
GPP, GA and LAI. GPP inc eased wi h an inc easing GA im-
plying a posi i e linea ela ionship be ween hese a iables;
Figu e 6. Rela ionship o g oss p ima y p oduc ion (GPP)
o inciden pho osyn he ically ac i e adia ion (PAR). Mea-
su ed mon hly (mid-May–Sep embe ) GPP (µmolm−2s−1)
a e aged wi h binned (s eps o 10µmolm−2s−1)PAR
(µmolm−2s−1) o 2010 (closed g ey iangles) and 2011
(closed black ci cles). Da a a e i ed wi h nonlinea eg ession
(GPP=(GPmax ×PAR×α/(GPmax+PAR×α)) be ween GPP
and PAR ( i esul s in Table 1). Only measu ed da a we e used in
he analysis.
he adjus ed R2 alue o he eg ession was 0.28 in 2010
(p=0.011) and 0.45 in 2011 (p<0.01). A ela ionship be-
ween GPP and LAI was no e iden in 2010; howe e , hey
we e be e co ela ed in 2011 wi h an adjus ed R2 alue o
0.42 (p<0.01).
3.3.2 Ecosys em espi a ion
The e was a clea ela ionship be ween bin-a e aged nigh -
ime TER and soil empe a u e om May o Sep embe in
2010 and 2011 (Fig. 7a). The exponen ial eg ession model
p o ided good i s o he da a (adjus ed R20.71 and 0.69 o
2010 and 2011, espec i ely) and he ela ionships we e s a-
is ically signi ican (p<0.01). The Q10 alues we e simila
be ween he 2 yea s (2.17 and 2.35). The R10 alues we e
1.75 and 1.66µmolm−2s−1in 2010 and 2011, espec i ely.
Addi ionally, TER inc eased wi h he inc easing GA in 2010
(Fig. 7b), howe e , he linea co ela ion was no s a is ically
signi ican (adjus ed R2=0.16, p=0.053). TER and GA
we e be e co ela ed in 2011 (adjus ed R2=0.51, p<0.01).
The e was a s ong posi i e linea ela ionship be ween TER
and GPP (p<0.01) in bo h yea s (Fig. 7c). GPP explained
82 and 75% o he a ia ion in he TER in 2010 and 2011,
espec i ely.
3.4 Annual balance
The es ima ed annual balances o TER, GPP and NEE a e
shown in Table 2. The si e ac ed as a CO2sink du ing
he s udied yea s and he annual NEE was −56.7, −262
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S. E. Lind e al.: Ca bon dioxide exchange o a pe ennial bioene gy c op cul i a ion 1263
Table 1. Mon hly i esul s o a ec angula hype bolic model oge he wi h a e age clima ic condi ions – he i esul s be ween g oss p ima y
p oduc ion (GPP, µmolm−2s−1)binned wi h pho osyn he ically ac i e adia ion (PAR, µmolm−2s−1, bins om 0 o 1800µmolm−2s−1
wi h an in e al o 10µmolm−2s−1) om mid-May o Sep embe in 2010 and 2011. A ec angula hype bolic model o he o m
GPP=(GPmax ×PAR×α/(GPmax+PAR×α), whe e GPmax (±SE, µmolm−2s−1)is he heo e ical maximum a e o pho osyn hesis a
in ini e PAR and α(±SE) is he appa en quan um yield – i.e. he ini ial slope o he ligh esponse cu e was used. Adjus ed R2o eg ession
and numbe o PAR bins (n) a e shown. Also mon hly a e age (±SD) o ai empe a u e (T,◦C), olume ic wa e con en (VWC, m3m−3)
a 2.5cm dep h and apou p essu e de ici (VPD, kPa) a e shown oge he wi h numbe o ain e en days (when p ecipi a ion>0.2mm) in
mon h, p ecipi a ion sum (p ec., mmmo−1)and mon hly a e aged g een a ea (GA, m2m−2)and lea a ea (LAI, m2m−2)indices.
Mon h GPmax α R2n T VWC VPD P ec. sum GA LAI
(µmolm−2s−1)(◦C) (m3m−3)(kPa) e en s
2010
May −21.5±1.7 −0.057±0.009 0.52 133 14.3±5.3 0.26±0.05 0.65±0.6 7 23 8.7 1.8
Jun −44.5±1.7 −0.047±0.002 0.93 158 13.0±4.6 0.26±0.05 0.54±0.4 9 72 19.0 4.3
Jul −40.1±1.1 −0.053±0.002 0.95 163 21.0±4.7 0.14±0.03 0.85±0. 7 7 34 17.2 4.0
Aug −25.2±0.7 −0.057±0.003 0.91 148 15.8±6.2 0.14±0.05 0.53±0.5 14 42 14.0 3.9
Sep −18.1±2.2 −0.040±0.007 0.93 19 9.8±3.9 0.21±0.04 0.14±0.2 16 53 14.1 4.0
2011
May −21.2±1.0 −0.056±0.005 0.76 134 11.2±4.0 0.30±0.03 0.45±0.4 11 38 5.7 1.8
Jun −45.8±1.4 −0.060±0.002 0.94 163 16.1±4.9 0.21±0.05 0.73±0.6 11 41 16.2 4.6
Jul −40.4±1.5 −0.050±0.002 0.92 154 19.1±4.4 0.20±0.06 0.65±0.5 11 91 15.5 5.3
Aug −29.9±1.0 −0.069±0.004 0.90 141 15.0±3.5 0.25±0.05 0.38±0.4 10 80 12.5 3.7
Sep −24.2±0.7 −0.074±0.004 0.94 103 11.1±3.3 0.31±0.04 0.20±0.2 13 70 8.0 4.3
Figu e 7. Rela ionships be ween o al ecosys em espi a ion (TER)
and en i onmen al a iables. (a) TER (µmolm−2s−1)and soil
empe a u e (◦C) a 2.5cm dep h (binned wi h s eps o 0.5◦C)
in May–Sep embe pe iod i ed wi h an exponen ial nonlinea e-
g ession (TER=R10 ×Q(Ts/ T10)
10 , whe e R10 and Q10 a e i -
ed pa ame e s). (b) Weekly a e aged TER (gCm−2d−1)and
g een a ea index (GA, m3m−3)in May–Oc obe pe iod i ed
wi h linea eg ession. (c) Daily alues o TER (gCm−2d−1)and
g oss p ima y p oduc ion (GPP, gCm−2d−1, binned wi h s eps o
0.25gCm−2d−1)in May–Sep embe pe iod i ed wi h linea e-
g ession. Closed g ey iangles a e da a o 2010 and closed black
ci cles o 2011. Fi esul s a e gi en in he ex .
and −256gCm−2in 2009 (23 July o 31 Decembe ), 2010
and 2011, espec i ely. The pa e n in NEE accumula ion is
shown in Fig 8. Du ing he 3-week ime pe iod om la e
July o mid-Augus 2009, he si e ac ed as a sou ce o a -
mosphe ic CO2. A e he ansi ion om a sou ce o a sink
in mid-Augus 2009, he si e seques e ed a mosphe e CO2
o abou 60 days leading o a nega i e cumula i e NEE o
−160gCm−2. Du ing he win e do mancy pe iod ( om
Figu e 8. Cumula i e NEE o e he s udy pe iod. Nega i e alues
indica e up ake o CO2and posi i e alues emission o he a mo-
sphe e. Ho izon al solid black lines show he ze o le el and e ical
dashed black lines ma k beginning o he yea .
la e Oc obe 2009 o May 2010) he si e los 183gCm−2
and he cumula i e NEE was 23gCm−2. A e his, he
si e was an annual CO2sink, since he summe ime up-
ake was highe han he win e ime CO2loss. In 2010, CO2
up ake pe iod las ed app oxima ely 120 days (May o mid-
Sep embe ) and in mid-Sep embe he cumula i e NEE was
−403gCm−2. Du ing he second win e do mancy, om
mid-Sep embe 2010 o mid-May 2011, he si e los app ox-
ima ely 168gCm−2. In 2011, he CO2up ake pe iod las ed
abou 135 days ( om mid-May o ea ly Oc obe ) wi h a cu-
mula i e NEE o −679gCm−2by he end o his season.
By he end o 2011, he cumula i e NEE was −575gCm−2.
This inal cumula i e alue o CO2-C ep esen s he amoun
o ca bon he si e accumula ed om he s a o he measu e-
men s in July 2009 un il he end o 2011.
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