Biogeosciences, 13, 5567–5585, 2016
www.biogeosciences.ne /13/5567/2016/
doi:10.5194/bg-13-5567-2016
© Au ho (s) 2016. CC A ibu ion 3.0 License.
T ends in soil solu ion dissol ed o ganic ca bon (DOC)
concen a ions ac oss Eu opean o es s
Ma a Camino-Se ano1, Elisabe h G a Panna ie 2, Sa a Vicca1, Sebas iaan Luyssae 3,a, Ma hieu Jona d4,
Philippe Ciais3, Be and Guene 3, Be Gielen1, Josep Peñuelas5,6, Jo di Sa dans5,6, Pe e Waldne 2, Sophia E zold2,
Guia Cecchini7, Nicholas Cla ke8, Zo an Gali´
c9, Lau e Gandois10, Ka in Hansen11, Jim Johnson12, Uwe Klinck13,
Zo a Lachmano á14, An i-Jussi Lind oos15, Henning Meesenbu g13, Tiina M. Nieminen15, Tanja G. M. Sande s16,
Kasia Sawicka17, Wal e Seidling16, Anne Thimonie 2, Elena Vanguelo a18, A ne Ve s ae en19, La s Ves e dal20, and
I an A. Janssens1
1Resea ch G oup o Plan and Vege a ion Ecology, Depa men o Biology, Uni e si y o An we p, Uni e si ei splein 1,
B-2610 Wil ijk, Belgium
2WSL, Swiss Fede al Ins i u e o Fo es , Snow and Landscape Resea ch, Zü che s asse 111, 8903, Bi mensdo , Swi ze land
3Labo a oi e des Sciences du Clima e de l’En i onnemen , LSCE/IPSL, CEA-CNRS-UVSQ, Uni e si é Pa is-Saclay,
91191 Gi -su -Y e e, F ance
4UCL-ELI, Uni e si é ca holique de Lou ain, Ea h and Li e Ins i u e, C oix du Sud 2, 1348 Lou ain-la-Neu e, Belgium
5CREAF, Ce danyola del Vallès, 08193, Ca alonia, Spain
6CSIC, Global Ecology Uni CREAF-CSIC-UAB, Ce danyola del Vallès, 08193, Ca alonia, Spain
7Depa men o Ea h Sciences, Uni e si y o Flo ence, Via La Pi a 4, 50121 Flo ence, I aly
8Di ision o En i onmen and Na u al Resou ces, No wegian Ins i u e o Bioeconomy Resea ch, 1431, Ås, No way
9Uni e si y o No i Sad-Ins i u e o Lowland Fo es y and En i onmen , 21000 No i Sad, Se bia
10EcoLab, Uni e si é de Toulouse, CNRS, INPT, UPS, A enue de l’Ag obiopole – BP 32607, 31326 Cas ane Tolosan, F ance
11IVL Swedish En i onmen al Resea ch Ins i u e, Na u al Resou ces & En i onmen al E ec s, 100 31, S ockholm, Sweden
12UCD School o Ag icul u e and Food Science, Uni e si y College Dublin, Bel ield, Dublin 4, D04 V1W8, I eland
13No hwes Ge man Fo es Resea ch Ins i u e, G ä zels . 2, 37079 Gö ingen, Ge many
14FGMRI, Fo es y and Game Managemen Resea ch Ins i u e, S nady 136, 252 02 Jílo iš ˇ
e, Czech Republic
15Na u al Resou ces Ins i u e Finland (Luke), P.O. Box 18, 01301 Van aa, Finland
16Thünen Ins i u e o Fo es Ecosys ems, Al ed-Mölle -S aße 1, 16225 Ebe swalde, Ge many
17Soil Geog aphy and Landscape G oup, Wageningen Uni e si y, P.O. Box 47, 6700 AA Wageningen, he Ne he lands
18Cen e o Ecosys em, Socie y and Biosecu i y, Fo es Resea ch, Alice Hol Lodge, W ecclesham, Fa nham,
Su ey GU10 4LH, UK
19Resea ch Ins i u e o Na u e and Fo es (INBO), Klinieks aa 25, 1070 B ussels, Belgium
20Uni e si y o Copenhagen, Depa men o Geosciences and Na u al Resou ce Managemen , Roligheds ej 23,
1958 F ede iksbe g C, Denma k
anow a : F ee Uni e si y o Ams e dam, Depa men o Ecological Science, Boelelaan 1085, 1081HV, he Ne he lands
Co espondence o: Ma a Camino-Se ano ([email p o ec ed])
Recei ed: 9 Decembe 2015 – Published in Biogeosciences Discuss.: 26 Janua y 2016
Re ised: 13 Sep embe 2016 – Accep ed: 15 Sep embe 2016 – Published: 7 Oc obe 2016
Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union.
5568 M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon
Abs ac . Dissol ed o ganic ca bon (DOC) in su ace wa e s
is connec ed o DOC in soil solu ion h ough hyd ological
pa hways. The e o e, i is expec ed ha long- e m dynamics
o DOC in su ace wa e s e lec DOC ends in soil solu-
ion. Howe e , a mul i ude o si e s udies ha e ailed so a
o es ablish consis en ends in soil solu ion DOC, whe eas
inc easing concen a ions in Eu opean su ace wa e s o e
he pas decades appea o be he no m, possibly as a esul
o eco e y om acidi ica ion. The objec i es o his s udy
we e he e o e o unde s and he long- e m ends o soil so-
lu ion DOC om a la ge numbe o Eu opean o es s (ICP
Fo es s Le el II plo s) and de e mine hei main physico-
chemical and biological con ols. We applied end analysis
a wo le els: (1) o he en i e Eu opean da ase and (2) o
he indi idual ime se ies and ela ed ends wi h plo cha -
ac e is ics, i.e., soil and ege a ion p ope ies, soil solu ion
chemis y and a mosphe ic deposi ion loads. Analyses o he
en i e da ase showed an o e all inc easing end in DOC
concen a ions in he o ganic laye s, bu , a indi idual plo s
and dep hs, he e was no clea o e all end in soil solu ion
DOC. The a e change in soil solu ion DOC anged be ween
−16.8 and +23%y −1(median= +0.4%y −1)ac oss Eu-
ope. The non-signi ican ends (40%) ou numbe ed he in-
c easing (35%) and dec easing ends (25%) ac oss he 97
ICP Fo es s Le el II si es. By means o mul i a ia e s a is-
ics, we ound inc easing ends in DOC concen a ions wi h
inc easing mean ni a e (NO−
3)deposi ion and inc easing
ends in DOC concen a ions wi h dec easing mean sul a e
(SO2−
4)deposi ion, wi h he magni ude o hese ela ionships
depending on plo deposi ion his o y. While he a ibu ion o
inc easing ends in DOC o he educ ion o SO2−
4deposi-
ion could be con i med in low o medium N deposi ion a eas,
in ag eemen wi h obse a ions in su ace wa e s, his was
no he case in high N deposi ion a eas. In conclusion, long-
e m ends o soil solu ion DOC e lec ed he in e ac ions
be ween con ols ac ing a local (soil and ege a ion p ope -
ies) and egional (a mosphe ic deposi ion o SO2−
4and ino -
ganic N) scales.
1 In oduc ion
Dissol ed o ganic ca bon (DOC) in soil solu ion is he sou ce
o much o he e es ially de i ed DOC in su ace wa e s
(Ba in e al., 2009; Bianchi, 2011; Regnie e al., 2013). Soil
solu ion DOC in o es s is connec ed o s eams h ough di -
e en hyd ological pa hways: DOC mobilized in he o es
loo may be anspo ed la e ally a he in e ace o o es
loo and mine al soil o su ace wa e s o pe cola es in o
he mine al soil, whe e addi ional DOC can be mobilized
and/o DOC is pa ly adso bed on pa icle su aces and min-
e alized he ea e (Fig. 1). F om he mine al soil DOC may
be leached ei he la e ally o e ically ia g oundwa e in o
su ace wa e s (McDowell and Likens, 1988). The e o e, i
Figu e 1. Schema ic diag am illus a ing he main sou ces (in
boxes) o dissol ed o ganic ca bon (DOC) and he main p ocesses
(in bold) and ac o s (in i alics) con olling DOC concen a ions in
soils.
could be expec ed ha long- e m dynamics o DOC in su -
ace wa e s mi o hose obse ed in ecosys em soil solu-
ions.
D i e s ela ed o clima e change ( empe a u e inc ease,
p ecipi a ion change, a mosphe ic CO2inc ease), he de-
c ease in acidi ying deposi ion, o land use change and man-
agemen may indi idually o join ly explain ends in su ace
wa e DOC concen a ions (E ans e al., 2012; F eeman e
al., 2004; Oulehle e al., 2011; Sa kkola e al., 2009; Wo all
and Bu , 2004). Inc easing ai empe a u es wa m he soil,
hus s imula ing soil o ganic ma e (SOM) decomposi ion
h ough g ea e mic obial ac i i y (Da idson and Janssens,
2006; Ha ley and Ineson, 2008; Kalbi z e al., 2000). O he
d i e s, such as inc eased a mosphe ic CO2and he accumu-
la ion o a mosphe ically deposi ed ino ganic ni ogen, a e
hough o inc ease he sou ces o DOC by enhancing p i-
ma y plan p oduc i i y (i.e., h ough s imula ing oo exu-
da es o inc eased li e all) (de V ies e al., 2014; Fe e i e
al., 2014; Sucke and K ause, 2010). Changes in p ecipi a-
ion, land use and managemen (e.g. d ainage o pea lands,
changes in o es managemen o g azing sys ems) may al-
e he lux o DOC lea ing he ecosys em, bu no consis en
ends in he hyd ologic egime o land use changes ha e
been de ec ed in a eas whe e inc easing DOC ends ha e
been obse ed (Mon ei h e al., 2007).
Recen ocus has mainly been on dec easing acidi ying de-
posi ion as an explana o y ac o o DOC inc eases in su -
ace wa e s in Eu ope and No h Ame ica by means o de-
c easing ionic s eng h (de Wi e al., 2007; H uška e al.,
2009) and inc easing he pH o soil solu ion, consequen ly
inc easing DOC solubili y (E ans e al., 2005; Haaland e al.,
2010; Mon ei h e al., 2007). Al hough he hypo hesis o an
inc ease in su ace wa e DOC concen a ion due o a eco -
e y om pas acidi ica ion was con i med in s udies o soil
solu ion DOC in he UK and no he n Belgium (Sawicka e
al., 2016; Vanguelo a e al., 2010; Ve s ae en e al., 2014), i
Biogeosciences, 13, 5567–5585, 2016 www.biogeosciences.ne /13/5567/2016/
M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon 5569
is no consis en wi h ends in soil solu ion DOC concen a-
ions epo ed om Finnish, No wegian, and Swedish o es s
(Lö g en and Ze e be g, 2011; Ukonmaanaho e al., 2014;
Wu e al., 2010). This inconsis ency be ween soil solu ion
DOC and s eam DOC ends could sugges ha DOC in su -
ace wa e and soil solu ion esponds di e en ly o (changes
in) en i onmen al condi ions in di e en egions (Akselsson
e al., 2013; Cla k e al., 2010; Lö g en e al., 2010). Al e na-
i ely, o he ac o s such as ee species and soil ype, may be
co-d i e s o o ganic ma e dynamics and inpu , gene a ion
and e en ion o DOC in soils.
T ends o soil solu ion DOC a y among no only o es s
bu o en also wi hin he same si e (Bo ken e al., 2011; Lö -
g en e al., 2010). Fo es cha ac e is ics such as ee species
composi ion, soil e ili y, ex u e o so p ion capaci y may
a ec he esponse o soil solu ion DOC o en i onmen al
con ols, o ins ance, by con olling he a e o soil acidi i-
ca ion h ough soil bu e ing and nu ien plan up ake p o-
cesses (Vanguelo a e al., 2010). Wi hin a si e, DOC a i-
abili y wi h soil dep h is ypically caused by di e en in en-
si y o DOC p oduc ion, ans o ma ion, and so p ion along
he soil p o ile (Fig. 1). Posi i e empo al ends in soil so-
lu ion DOC (inc easing concen a ions o e ime) ha e e-
quen ly been epo ed o he o ganic laye s and shallow soils
whe e p oduc ion and decomposi ion p ocesses con ol he
DOC concen a ion (Lö g en and Ze e be g, 2011). How-
e e , no dominan ends a e ound o he mine al soil ho i-
zons, whe e physico-chemical p ocesses, such as so p ion,
become mo e in luen ial (Bo ken e al., 2011; Buckingham e
al., 2008). Fu he mo e, p e ious s udies ha e used di e en
empo al and spa ial scales which may ha e u he added o
he inconsis ency in he DOC ends epo ed in he li e a u e
(Cla k e al., 2010).
In his con ex , he In e na ional Co-ope a i e P og amme
on Assessmen and Moni o ing o Ai Pollu ion E ec s on
Fo es s (ICP Fo es s, 2010) compiled a unique da ase con-
aining da a om mo e han 100 in ensi ely moni o ed o es
plo s (Le el II) which allow o egional ends in soil so-
lu ion DOC o o es s a a Eu opean scale o be un a eled,
as well as o s a is ical analysis o he main con ols be-
hind hese egional ends o be pe o med. Long- e m mea-
su emen s o soil solu ion DOC a e a ailable o hese plo s,
along wi h in o ma ion on abo eg ound biomass, soil p op-
e ies, and a mosphe ic deposi ion o ino ganic N and SO2−
4,
collec ed using a ha monized sampling p o ocol ac oss Eu-
ope (Fe e i and Fische , 2013). This da ase has p e iously
been used o in es iga e he spa ial a iabili y o DOC in
o es s a Eu opean scale (Camino-Se ano e al., 2014), bu
an assessmen o he empo al ends in soil solu ion DOC
using his la ge da ase has no been a emp ed so a .
The main objec i e o his s udy is o unde s and he long-
e m empo al ends o DOC concen a ions in soil solu ion
measu ed a he ICP Fo es s Le el II plo s ac oss Eu ope.
Based on he inc easing DOC ends in su ace wa e s, we
hypo hesize ha empo al ends in soil solu ion DOC will
also be posi i e, bu wi h ends a ying locally depending
on plo cha ac e is ics. We u he in es iga ed whe he plo
cha ac e is ics, speci ically clima e, ino ganic N and SO2−
4
deposi ion loads, o es ype, soil p ope ies, and changes in
soil solu ion chemis y can explain di e ences ac oss si es in
DOC ends.
2 Ma e ials and me hods
2.1 Da a desc ip ion
Soil solu ion chemis y has been moni o ed wi hin he ICP
Fo es s P og amme since he 1990s on mos Le el II plo s.
The ICP Fo es s da a we e ex ac ed om he pan-Eu opean
Fo es Moni o ing Da abase (G anke, 2013). A lis o he
Le el II plo s used o his s udy can be ound in he Sup-
plemen , Table S1. The me hods o collec ion and analy-
sis o soil solu ion used in he a ious coun ies (Swi ze -
land: G a Panna ie e al., 2011; Flande s, Belgium: Ve -
s ae en e al., 2012; Finland: Lind oos e al., 2000; UK:
Vanguelo a e al., 2010, Denma k: Hansen e al., 2007) ol-
low he ICP Fo es s manual (Nieminen, 2011). Gene ally,
lysime e s we e ins alled a se e al ixed dep hs s a ing a
0cm, de ined as he in e ace be ween he su ace o ganic
laye and unde lying mine al soil. These dep hs a e ypically
aligned wi h soil “o ganic laye ”, “mine al opsoil”, “min-
e al subsoil”, and “deepe mine al soil”, bu sampling dep hs
a y among coun ies and e en among plo s wi hin a coun y.
No mally, ze o- ension lysime e s we e ins alled unde he
su ace o ganic laye and ension lysime e s wi hin he min-
e al soil. Howe e , in some coun ies ze o- ension lysime e s
we e also used wi hin he mine al laye s and in some en-
sion lysime e s below he o ganic laye . Mul iple collec o s
( eplica es) we e ins alled pe plo and pe dep h o assess
plo s’ spa ial a iabili y. Howe e , in some coun ies, sam-
ples om hese eplica es we e pooled be o e analyses o
a e aged p io o da a ansmission. The quali y assu ance
and con ol p ocedu es included he use o con ol cha s o
in e nal e e ence ma e ial o check long- e m compa abil-
i y wi hin na ional labo a o ies as well as pa icipa ion in
pe iodic labo a o y ing es s (e.g., Ma che o e al., 2011)
o check he in e na ional compa abili y. Da a we e epo ed
annually o he pan-Eu opean da a cen e , checked o con-
sis ency and s o ed in he pan-Eu opean Fo es Moni o ing
Da abase (G anke, 2013).
Soil wa e was usually collec ed o nigh ly o mon hly,
al hough o some plo s sampling pe iods wi h su icien soil
wa e o collec ion we e sca ce, especially in p olonged d y
pe iods o in win e due o snow and ice. A e collec ion,
he samples we e il e ed h ough a 0.45µm memb ane il e ,
s o ed below 4◦C and hen analyzed o DOC, oge he wi h
o he soil solu ion chemical p ope ies (NO−
3, Ca, Mg, NH+
4,
SO2−
4, o al dissol ed Al, o al dissol ed Fe, pH, elec ical
conduc i i y). In o ma ion on he soil solu ion chemis y a
www.biogeosciences.ne /13/5567/2016/ Biogeosciences, 13, 5567–5585, 2016
5570 M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon
he s udied plo s can be ound in he Supplemen (Tables S4–
S11). The p ecision o DOC analysis di e ed among he lab-
o a o ies. The coe icien o a ia ion o epea edly measu ed
e e ence ma e ial was 3.7% on a e age. The ime span o
soil solu ion ime se ies used o his s udy anged om 1991
o 2011, al hough co e age o his pe iod a ied om plo o
plo (Table S1).
Soil p ope ies; open ield bulk deposi ion; and h ough-
all deposi ion o NO−
3, NH+
4, and SO2−
4a e measu ed a
he same plo s as well as s em olume inc emen . The a -
mosphe ic deposi ion o NO−
3, NH+
4and SO2−
4da a co -
e s he pe iod 1999–2010 (Waldne e al., 2014). S em ol-
ume g ow h was calcula ed by he ICP Fo es s ne wo k om
diame e a b eas heigh (DBH), li e ee s a us, and ee
heigh which we e assessed o e e y ee (DBH>5cm)
wi hin a moni o ing plo app oxima ely e e y 5 yea s since
he ea ly 1990s. T ee s em olumes we e de i ed om al-
lome ic ela ionships based on diame e and heigh mea-
su emen s acco ding o De V ies e al. (2003), accoun ing
o species and egional di e ences. S em olume g ow h (in
cubic me e s) be ween wo consecu i e in en o ies was cal-
cula ed as he di e ence be ween s em olumes a he be-
ginning and he end o one in en o y pe iod o li ing ees.
S em olume da a we e co ec ed o all ees ha we e los
du ing one in en o y pe iod, including hinning. S em ol-
ume a he ime o disappea ance (assumed a hal o he ime
o he in en o y pe iod) was es ima ed om unc ions ela -
ing s em olume o s anding li ing ees a he end o he pe-
iod s. olume a he beginning o he pe iod. The me hods
used o collec ion o hese da a can be ound in he manu-
als o he ICP Fo es s Moni o ing P og amme (ICP Fo es s,
2010). The soil p ope ies a he plo s used o his s udy we e
de i ed om he ICP Fo es s agg ega ed soil da abase (AF-
SCDB.LII.2.1) (Cools and De Vos, 2014).
Since con inuous p ecipi a ion measu emen s a e no com-
monly a ailable o he Le el II plo s, p ecipi a ion measu e-
men s o he loca ion o he plo s we e ex ac ed om he
obse a ional s a ion da a o he Eu opean Clima e Assess-
men & Da ase (ECA&D) and he ENSEMBLES Obse a-
ions (E-OBS) g idded da ase (Haylock e al., 2008). We
used p ecipi a ion measu emen s ex ac ed om he E-OBS
g idded da ase o imp o e he empo al and spa ial co e -
age and o educe me hodological di e ences o p ecipi a ion
measu emen s ac oss he plo s. The E-OBS da ase con ains
daily alues o p ecipi a ion and empe a u e om s a ions
da a g idded a 0.25◦ esolu ion. When E-OBS da a we e no
a ailable, hey we e gap- illed wi h ICP Fo es s p ecipi a ion
alues gained by deposi ion measu emen s whe e a ailable.
2.2 Da a p epa a ion
We ex ac ed da a om plo s wi h ime se ies co e ing mo e
han 10 yea s and including mo e han 60 obse a ions o
soil solu ion DOC concen a ions o indi idual o g oups o
collec o s. Ou lie s, de ined as ±3 in e qua ile ange o he
25 and 75% quan iles o he ime se ies, we e emo ed om
each ime se ies o a oid he in luence o a ew ex eme al-
ues in he long- e m end (Schwe man e al., 2004). Values
unde 1mgL−1, which is he de ec ion limi o DOC in he
ICP Le el II plo s, we e eplaced by 1mgL−1. A e his il-
e ing, 529 ime se ies om 118 plo s, spanning om I aly
o No way, we e a ailable o analysis. Soil solu ion, p e-
cipi a ion, and empe a u e we e agg ega ed o mon hly da a
by he median o he obse a ions in each mon h and by he
sum o daily alues in he case o p ecipi a ion. Da a o in-
o ganic N (NH+
4and NO−
3)and SO2−
4 h ough all and open
ield bulk deposi ion measu ed a he plo s we e in e pola ed
o mon hly da a (Waldne e al., 2014).
The plo s we e classi ied acco ding o hei o -
es (b oadlea ed/coni e ous-domina ed) and soil ype
(Wo ld Re e ence Base (WRB), 2006), hei s em g ow h
(slow, < 6 m3ha−1y −1; in e media e, 6–12 m3ha−1y −1;
and as , >12m3ha−1y −1), and hei soil solu ion
pH (low, < 4.2; in e media e, 4.2–5; high, > 5). Plo s
we e also classi ied based on mean h ough all in-
o ganic N (NO−
3+NH+
4)deposi ion le el, de ined as
high deposi ion (HD, >15kgNha−1y −1), medium
deposi ion (MD, 5–15kgNha−1y −1), and low de-
posi ion (LD, <5kgNha−1y −1), as well as mean
h ough all SO2−
4deposi ion le el, de ined as high de-
posi ion (HD, >6kgSha−1y −1), and low deposi ion
(LD, <6kgSha−1y −1).
2.3 S a is ical me hods
Time se ies can ypically be decomposed in o andom noise,
seasonal, and end componen s (Ve bessel e al., 2010). In
his pape , we used me hods o de ec he ac ual end (change
in ime) a e emo ing he seasonal and andom noise com-
ponen s. The sequence o me hods applied is summa ized in
Fig. 2. The analysis o empo al ends in soil solu ion DOC
concen a ions was ca ied ou a wo le els: (1) he Eu opean
le el and (2) he plo le el. While he i s analysis allows
an e alua ion o he o e all end in soil solu ion DOC a a
con inen al scale, he second analysis indica es whe he he
obse ed la ge-scale ends a e occu ing a local scales as
well, and es s whe he local ends in DOC can be a ibu ed
o ce ain d i e a iables.
Linea mixed-e ec s models (LMMs) we e used o de ec
he empo al ends in soil solu ion DOC concen a ion a
Eu opean scale (Fig. 2). Fo hese models, he selec ed 529
ime se ies we e used. Fo he end analysis o indi idual
ime se ies, howe e , we ocused on he long- e m ends in
soil solu ion DOC a Eu opean o es s ha show mono onic-
i y. The e o e, DOC ime se ies we e i s analyzed using he
B eaks Fo Addi i e Seasonal and T end (BFAST) algo i hm
o de ec he p esence o b eakpoin s (Ve bessel e al., 2010;
Vicca e al., 2016), wi h he ime se ies showing b eakpoin s,
i.e., no mono onic, being disca ded (see “Desc ip ion o he
s a is ical me hods” in he Supplemen ). In o al, 258 mono-
Biogeosciences, 13, 5567–5585, 2016 www.biogeosciences.ne /13/5567/2016/
M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon 5571
Figu e 2. Flow-diag am o he sequence o me hods applied o
analysis o empo al ends o soil solu ion DOC and hei d i e s.
onic ime se ies om 97 plo s we e used o ou analysis
a e il e ing (Fig. 2). Then, mono onic end analyses we e
ca ied ou om he il e ed da ase using he seasonal Mann–
Kendall (SMK) es o mon hly DOC concen a ions (Hi sch
e al., 1982; Ma che o e al., 2013). Pa ial Mann–Kendall
(PMK) es s we e also used o es he in luence o p ecipi a-
ion as a co- a iable o de ec whe he he end migh be due
o a DOC dilu ion/concen a ion e ec (Libiselle and G im-
all, 2002). Sen (1968) slope alues we e calcula ed o SMK
and PMK. Mo eo e , LMMs we e pe o med again wi h he
il e ed da ase o compa e esul s wi h and wi hou ime se-
ies showing b eakpoin s (Fig. 2).
Fo his s udy, i e soil dep h in e als we e conside ed:
he o ganic laye (0cm), opsoil (0–20cm), in e media e
(20–40cm), subsoil (40–80cm) and deep subsoil (>80cm).
The slopes o each ime se ies we e s anda dized by di iding
hem by he median DOC concen a ion o e he sampling
pe iod ( ela i e end slope), agg ega ed o a unique plo –
soil dep h slope and classi ied by he di ec ion o he end
as signi ican ly posi i e, i.e., inc easing DOC o e ime (P,
p<0.05); signi ican ly nega i e, i.e., dec easing DOC o e
ime (N, p<0.05); and non-signi ican , i.e., no signi ican
change in DOC o e ime (NS, p≥0.05). When he e was
mo e han one collec o pe dep h in e al, he median o
he slopes was used when he di ec ion o he end (P, N,
o NS) was simila . A e agg ega ion pe plo –dep h com-
bina ion, 191 end slopes om 97 plo s we e a ailable o
analysis (Table S2). T ends o o he soil solu ion pa am-
e e s (NO−
3, Ca2+, Mg2+, NH+
4, SO2−
4, o al dissol ed Al,
o al dissol ed Fe, pH, elec ical conduc i i y), p ecipi a ion
and empe a u e we e calcula ed using he same me hodol-
ogy as o DOC. Since he esul ing s anda dized Sen slope
in %y −1( ela i e end slope) was used o all he s a is ical
analyses, om he e on we will use he gene al e m “ end
slope” in o de o simpli y.
Finally, s uc u al equa ion models (SEMs) we e pe -
o med o de e mine he capaci y o he se e al ac o s (SO2−
4
and/o NO−
3deposi ion, s em g ow h and soil solu ion chem-
is y) in explaining a iabili y in he slope o DOC ends
among he selec ed plo s (Fig. 2). We e alua ed he in lu-
ence o bo h he annual mean (kgha−1y −1)and he ends
(%y −1)in deposi ion and soil solu ion pa ame e s. All he
s a is ical analyses we e pe o med in R so wa e e sion
3.1.2 (R Co e Team, 2014) using he “ k ” (Ma che o e al.,
2013), “b as 01” (de Jong e al., 2013) and “sem” (Fox e al.,
2013) packages, excep o he LMMs ha we e pe o med
using SAS 9.3 (SAS ins i u e, Inc., Ca y, NC, USA). Mo e
de ailed in o ma ion on he s a is ical me hods used can be
ound in he Supplemen .
3 Resul s
3.1 Soil solu ion DOC ends a Eu opean scale
Fi s , empo al ends in DOC we e analyzed o all he Eu-
opean DOC da a pooled oge he by means o LMMs o es
o he p esence o o e all ends. A signi ican ly inc easing
DOC end (p<0.05) in soil solu ion collec ed wi h ze o-
ension lysime e s in he o ganic laye was obse ed mainly
unde coni e ous o es plo s (Table 1). Simila ly, a signi i-
can ly inc easing DOC end (p<0.05) in soil solu ion col-
lec ed wi h ension lysime e s was ound in deep mine al soil
(>80cm) o all si es, mainly o coni e ous o es si es (Ta-
ble 1), bu his end is based on a limi ed numbe o plo s
which a e no especially well dis ibu ed in Eu ope (75%
o Ge man plo s). By con as , non-signi ican ends we e
ound in he o he mine al soil dep h in e als (0–20, 20–
40 and 40–80cm) by means o he LMMs. When he same
analysis was applied o he il e ed Eu opean da ase , i.e.,
wi hou he ime se ies showing b eakpoin s, ewe signi -
ican ends we e obse ed: only an o e all posi i e end
(p<0.05) was ound o DOC in he o ganic laye using
ze o- ension lysime e s, again mainly unde coni e ous o -
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5572 M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon
es si es, bu no s a is ically signi ican ends we e ound in
he mine al soil (Table 1).
3.2 Soil solu ion DOC concen a ion end analysis o
indi idual ime se ies
We applied he BFAST analysis o selec he mono onic ime
se ies in o de o ensu e ha he de ec ed ends we e no in-
luenced by b eakpoin s in he ime se ies. Time se ies wi h
b eakpoin s ep esen ed mo e han 50% o he o al ime se-
ies agg ega ed by soil dep h in e al (245 ou o 436).
The indi idual end analysis using he SMK es showed
end slopes o soil solu ion DOC concen a ion anging om
−16.8 o +23%y −1(median= + 0.4%y −1, in e qua ile
ange= +4.3%y −1). Among all he ime se ies analyzed,
he non-s a is ically signi ican ends (40%, 104 ime se ies)
ou numbe ed he signi ican ly posi i e ends (35%, 91 ime
se ies) and signi ican ly nega i e ends (24%, 63 ime se-
ies) (Table 1). Thus, he e was no uni o m end in soil so-
lu ion DOC in o es s ac oss a la ge pa o Eu ope. Fu he -
mo e, he egional end di e ences we e inconsis en when
looking a di e en soil dep h in e als sepa a ely (Figs. 3
and 4), which made i di icul o d aw i m conclusions abou
he spa ial pa e n o he ends in soil solu ion DOC concen-
a ions in Eu opean o es s.
The a iabili y in ends was high, no only a con inen-
al scale bu also a plo le el (Fig. 5). We ound consis en
wi hin-plo ends only o 50 ou o he 97 si es. Mo eo e ,
some plo s e en showed di e en ends (P, N o NS) in DOC
wi hin he same dep h in e al, which was he case o 17
plo –dep h combina ions (16 in Ge many and 1 in No way),
e idencing a high small-scale plo he e ogenei y.
T end di ec ions (P, N o NS) o en di e ed among dep hs.
Fo ins ance, in he o ganic laye , we ound mainly non-
signi ican ends, and i a end was de ec ed, i was mo e
o en posi i e han nega i e, while posi i e ends we e he
mos equen in he subsoil (below 40cm) (Table 1). Ne e -
heless, i is impo an o no e ha a s a is ical es o whe he
he e was a eal di e ence in DOC ends be ween dep hs
was no possible as he se o plo s di e ed be ween he di -
e en soil dep h in e als. Howe e , a isual compa ison o
ends o he ew plo s in which ends we e e alua ed o
mo e han h ee soil dep hs showed ha he e was no appa -
en di e ence in DOC ends be ween soil dep hs (Figs. S1
and S2).
Finally, o i ually all plo s, including p ecipi a ion as a
co- a iable in he PMK es ga e he same esul as he SMK
es , which indica es ha p ecipi a ion ( h ough dilu ion o
concen a ion e ec s) did no a ec he DOC concen a ion
ends. A dilu ion/concen a ion e ec was only de ec ed in
ou plo s (Table S1).
Figu e 3. Di ec ions o he empo al ends in soil solu ion DOC
concen a ion in he o ganic laye a plo le el. T ends we e e alu-
a ed using he seasonal Mann–Kendall es . Da a span om 1991 o
2011.
3.3 Fac o s explaining he soil solu ion DOC ends
3.3.1 E ec s o ege a ion, soil and clima e
The e was no di ec e ec o o es ype (b oadlea ed s.
coni e ous) on he di ec ion o he s a is ically signi ican
ends in soil solu ion DOC (Fig. 6a). Bo h posi i e and
nega i e ends we e equally ound unde b oadlea ed and
coni e ous o es s (χ2(1, n=97)=0.073, p=0.8). Inc eas-
ing DOC ends, howe e , occu ed mo e o en unde o es s
wi h a mean s em g ow h inc emen below 6m3ha−1y −1
o e he s udy pe iod, whe eas dec easing DOC ends we e
mo e common in o es s wi h a mean s em g ow h inc e-
men be ween 6 and 12m3ha−1y −1(χ2(2, n=53) =5.8,
p=0.05) (Fig. 6b). Only six o es s wi h a mean s em
g ow h abo e 12m3ha−1y −1we e a ailable o his s udy
( i e showing inc easing DOC ends and one showing a de-
c easing DOC end) and hus he e is no enough in o ma-
ion o d aw conclusions abou he ela ionship be ween s em
g ow h and soil solu ion DOC ends o o es s wi h e y
high s em g ow h (>12m3ha−1y −1).
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M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon 5573
Table 1. Tempo al ends o DOC concen a ions ob ained wi h he linea mixed models (LMM) buil o di e en o es ypes, soil dep h
in e als and collec o ypes wi h he en i e da ase (wi h b eakpoin s) and wi h he da ase wi hou ime se ies showing b eakpoin s (wi hou
b eakpoin s) and he seasonal Mann–Kendall (SMK) es s. The able shows he median DOC concen a ions in mg L−1([DOC]), ela i e
end slope ( slope in %y −1), he numbe o obse a ions (n) and he p alue. Fo he SMK es s, he numbe o ime se ies showing
signi ican nega i e (N), non-signi ican (NS) and signi ican posi i e (P) ends is shown and he in e qua ile ange o he slope is be ween
b acke s. LMMs o which no s a is ically signi ican end was de ec ed (p>0.1) a e ep esen ed in oman ype, he LMMs o which
a signi ican end is de ec ed a e in bold (p< 0.05) and in i alics (0.05 < p< 0.1) (O: o ganic laye ; M02: mine al soil 0–20cm; M24:
mine al soil 20–40cm; M48: mine al soil 40–80cm; M8: mine al soil > 80 cm; TL: ension lysime e ; ZTL: ze o- ension lysime e ; n.s.: no
signi ican ).
Collec o ype Laye [DOC] LMM (wi h b eakpoin s) LMM (wi hou b eakpoin s) SMK (wi hou b eakpoin s)
n slope p alue n slope p alue slope N NS P
In b oadlea ed and coni e ous o es s
TL O 47.3 3133 6.75 0.078 1168 −0.30 n.s. −1.03 (±1.65) 1 3 1
M02 12.9 19311 0.10 n.s. 8917 −1.06 n.s. 0.16 (±4.78) 17 29 21
M24 4.93 7700 2.69 n.s. 3404 3.66 n.s. 0.6 (±9.03) 11 12 11
M48 3.66 24614 0.95 n.s. 11065 0.80 n.s. 0.67 (±4.76) 22 30 32
M8 3.27 9378 6.78 0.0036 3394 3.41 n.s. 1.007 (±8.79) 8 9 16
ZTL O 37.9 8136 3.75 <0.001 4659 1.63 0.0939 1.7 (±4.28) 3 16 8
M02 30.7 3389 −0.54 n.s. 445 0.17 n.s. −0.7 (±1.85) 0 3 1
M24 17.3 739 0.36 n.s. 0 0 0
M48 4.73 654 −3.37 n.s. 336 1.05 n.s. 1.07 (±3.08) 1 2 1
M8 3.7 118 1.39 n.s. 0 0 0
In b oadlea ed o es s
TL O 41.4 637 −5.96 n.s. 475 −0.17 n.s. −0.3 (±0.9) 0 2 0
M02 8.80 8397 3.07 0.0764 3104 0.51 n.s. 0.89 (±5.94) 4 7 10
M24 3.78 2584 −0.05 n.s. 928 6.01 n.s. 1.03 (±11.31) 3 5 4
M48 2.60 10635 −0.93 n.s. 4634 2.46 n.s. 1.51 (±5.31) 11 8 16
M8 2.60 4354 −6.85 0.0672 1797 −0.10 n.s. 0.3 (±6.28) 4 5 6
ZTL O 33.3 4057 0.37 n.s. 1956 −0.90 n.s. 0.96 (±5.47) 2 7 3
M02 4.26 608 0.26 n.s. 192 1.88 n.s. 2.72 0 0 1
M24 20.4 94 11.80 0.026 0 0 0
M48 3.42 427 −2.84 n.s. 0 0 1 0
M8 2.42 34 −36.18 <0.001 0 0 0
In coni e ous o es s
TL O 49.0 2496 8.15 0.0633 693 1.33 n.s. −1.06 (±2.25) 1 1 1
M02 15.7 10914 −0.97 n.s. 5813 −1.60 n.s. −0.04 (±3.98) 13 22 11
M24 5.72 5116 2.71 n.s. 2476 3.66 n.s. −0.3 (±7.82) 7 7 8
M48 4.44 13979 1.24 n.s. 6431 0.05 n.s. 0.3 (±4.32) 16 22 11
M8 3.70 5024 9.93 <0.001 1597 7.58 n.s. 2.89 (±10.28) 4 4 10
ZTL O 42.9 4079 3.59 0.0018 2703 3.09 0.0045 1.85 (±2.88) 1 9 5
M02 36.9 2781 −0.60 n.s. 253 −1.44 n.s. −0.83 (±0.4) 0 3 0
M24 16.3 645 0.23 n.s. 0 0 0
M48 44.0 227 −0.39 n.s. 251 −0.55 n.s. 2.14 (±3.66) 1 1 1
M8 4.14 84 13.87 0.0995 0 0 0
The DOC ends also a ied among soil ypes; mo e han
hal o he plo s showing a consis en inc easing DOC end
a all e alua ed soil dep h in e als we e loca ed in Cambisols
(6 ou o 11 plo s), which a e a he e ile soils, whe eas
plo s showing consis en nega i e ends co e ed six di e -
en soil ypes. O he soil p ope ies, like clay con en , ca ion
exchange capaci y o pH, did no clea ly di e be ween si es
wi h posi i e and nega i e DOC ends (Table 2). I is e-
ma kable ha ends in soil solu ion pH, Mg and Ca con-
cen a ions we e simila ac oss plo s wi h bo h posi i e and
nega i e DOC ends. Soil solu ion pH inc eased dis inc ly
www.biogeosciences.ne /13/5567/2016/ Biogeosciences, 13, 5567–5585, 2016
5574 M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon
Figu e 4. Di ec ions o empo al ends in soil solu ion DOC concen a ion a plo le el in he mine al soil o soil laye s: (a) opsoil
(0–20cm), (b) in e media e (20–40cm), (c) subsoil (40–80cm) and (d) deep subsoil (> 80 cm). T ends we e e alua ed using he seasonal
Mann–Kendall es . Da a span om 1991 o 2011.
in almos all he si es, while Ca and Mg dec eased ma kedly
(Table 2).
Finally, no signi ican co ela ions we e ound be ween
ends in empe a u e o p ecipi a ion and ends in soil so-
lu ion DOC, wi h he excep ion o a posi i e co ela ion be-
ween ends in soil solu ion DOC in he soil dep h in e al
20–40cm and he end in empe a u e ( =0.47, p=0.03).
3.3.2 E ec s o mean and ends in a mosphe ic
deposi ion and soil solu ion pa ame e s
Analysis o di e en models ha could explain he DOC
ends using he o e all da ase indica ed bo h di ec and in-
di ec e ec s o he annual mean SO2−
4and NO−
3 h ough-
all a mosphe ic deposi ion on he end slopes o DOC.
Biogeosciences, 13, 5567–5585, 2016 www.biogeosciences.ne /13/5567/2016/
M. Camino-Se ano e al.: T ends in soil solu ion dissol ed o ganic ca bon 5575
Table 2. Si e p ope ies o he 13 plo s showing consis en nega i e ends (N) o DOC concen a ions and o he 12 plo s showing consis en
posi i e ends (P) o DOC concen a ions. Soil p ope ies (clay pe cen age, C/N a io, pH(CaCl2), ca ion exchange capaci y (CEC)) a e o
he soil dep h in e al 0–20cm. Mean a mosphe ic deposi ion (ino ganic N and SO2−
4)is h ough all deposi ion om 1999 o 2010. When
h ough all deposi ion was no a ailable, bulk deposi ion is p esen ed wi h an as e isk. Rela i e end slopes ( slope) in soil solu ion pH,
Ca2+and Mg2+concen a ions we e calcula ed using he seasonal Mann–Kendall es .
Code end Soil ype Clay C/N pH CEC MAP MAT N depos. SO2−
4depos. slope pH slope Ca2+ slope Mg2+
Plo (WRB) (%) (cmol+kg−1)(mm) (◦C) (kgNha−1y 1)(kgSha−1y −1)(%y −1)(%y −1)(%y −1)
F ance (code=1)
30 N Cambic Podzol 3.79 16.8 3.96 1.55 567 11.9 7.28 4.25 0.10 −0.90 −1.00
41 N Mollic Andosol 23.9 16.6 4.23 7.47 842 10.6 4.43 4.15 0.00 −1.10 −1.30
84 N Cambic Podzol 4.09 22.8 3.39 4.07 774 10.5 7.66 3.77∗0.50 2.00 1.00
Belgium (code=2)
11 P Dys ic Cambisol 3.54 17.7 2.81 6.22 805 11.0 18.7 13.2 0.40 −11.0 −8.00
21 P Dys ic Podzolu isol 11.2 15.4 3.59 2.41 804 10.3 16.8 13.2 0.00 −9.00 −5.00
Ge many (code=4)
303 N Haplic Podzol 17.3 16.5 3.05 8.77 1180 9.10 17.5 0.40 −5.00 −2.00
304 N Dys ic Cambisol 21.3 17.7 3.63 6.14 1110 6.20 16.4 0.00 −3.00 −0.40
308 N Albic A enosol 3.80 16.5 3.41 1.63 816 9.20 14.2∗0.00 −5.00 −2.00
802 N Cambic Podzol 6.00 25.7 3.35 4.33 836 11.9 25.2 13.2 0.50 −2.40 −1.50
1502 N Haplic A enosol 4.40 23.8 3.78 2.35 593 9.40 9.79 5.66 −16.0 −14.0
306 P Haplic Calcisol 782 10.2 13.9 0.50 2.00 2.00
707 P Dys ic Cambisol 704 10.7 18.3 8.49 0.00 −10.0 −2.00
806 P Dys ic Cambisol 1349 8.30 23.0 6.81 0.30 −7.00 −6.00
903 P Dys ic Cambisol 905 9.60 0.20 −5.00 −3.00
920 P Dys ic Cambisol 908 8.90 −1.00 −6.00 −0.50
1402 P Haplic Podzol 8.65 26.2 3.24 9.04 805 6.90 13.5 24.3 1.20 −6.00 9.00
1406 P Eu ic Gleysol 15.9 23.1 3.59 6.67 670 8.80 15.3 6.23 1.11 −4.00 −3.00
I aly (code=5)
1 N Humic Ac isol 3.14 12.2 5.32 31.6 670 23.3 −0.30 −10.0 −10.0
Uni ed Kingdom (code=6)
922 P Umb ic Gleysol 34.8 15.6 3.31 10.8 1355 9.50 0.40 −9.00 2.00
Aus ia (code=14)
9 N Eu ic Cambisol 20.1 12.8 5.26 25.9 679 10.8 3.80* 0.40 −1.50 −0.60
Swi ze land (code=50)
15 N Dys ic Planosol 17.6 14.7 3.73 7.76 1201 8.90 15.1 4.67 −0.10 −13.0 −4.00
2 P Haplic Podzol 14.7 18.3 3.17 3.59 1473 4.40 −0.80 −5.00 −3.00
No way (code=55)
14 N Cambic A enosol 9.83 25.4 3.46 14.7 21.9 0.10 −1.70 −3.30
19 N 10.5 18.7 3.79 836 4.60 1.54 2.61 0.50 −7.00 −4.00
18 P 3.05 29.5 3.69 1175 0.35 2.40 −0.90 0.00 0.00
The S uc u al Equa ion Model accoun ed o 32.7% o he
a iance in DOC end slopes (Fig. 7a). Acco ding o his
model, lowe mean h ough all SO2−
4deposi ion esul ed
in inc easing end slopes o DOC in soil solu ion, and
highe mean h ough all NO−
3deposi ion esul ed in inc eas-
ing end slopes o DOC (Fig. 7a). When conside ing ends
in SO2−
4and NO−
3deposi ion, he e was no appa en spa-
ial co ela ion wi h soil solu ion DOC ends, wi h deposi-
ion mainly dec easing o no changing o e ime (Fig. 8)
and he DOC ends a ying g ea ly ac oss Eu ope (Figs. 3
and 4). Howe e , when SEM was un using he end slopes
in SO2−
4and NO−
3deposi ion ins ead o he mean alues,
we ound ha end slopes o DOC signi ican ly inc eased
wi h inc easing end in NO−
3and dec eased wi h inc easing
end in SO2−
4deposi ion, bu he la e was a non-signi ican
ela ionship (Fig. S3). Howe e , he pe cen age o a iance
in DOC end slopes explained by he model was mo e han
wice as low (16%).
Si es wi h low and medium N deposi ion
The a iables in he model ha bes explained he empo-
al changes in DOC we e he same o he o es s wi h low
and medium N deposi ion; o bo h g oups, NO−
3deposi ion
and SO2−
4deposi ion (di ec ly, o indi ec ly h ough i s in lu-
ence on plan g ow h) in luenced he end in DOC (Fig. 7b).
Lowe mean SO2−
4deposi ion again esul ed in a signi i-
can inc ease in end slopes, while inc easing NO−
3depo-
si ion esul ed in inc easing DOC end slopes. The pe cen -
age o a iance in DOC end slopes explained by he model
was 33%. The SEM un wi h he ends in SO2−
4and NO−
3
h ough all deposi ion o o es s wi h low and medium N
deposi ion explained 24.4% o he a iance in DOC ends,
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