A icle
Pedogenic P ocesses in a Posidonia oceanica Ma
Ne ea Piñei o-Juncal 1,2,* , Ca men Lei a-Dueñas 2, Osca Se ano 3, Miguel Ángel Ma eo 2,3
and An onio Ma ínez-Co ízas 1
1
EcoPas (GI-1553), Depa amen o de Eda olox
í
a e Qu
í
mica Ag
í
cola, Facul ade de Biolox
í
a, Uni e sidade de
San iago de Compos ela, San iago de Compos ela, 15782 A Co uña, Spain; [email p o ec ed]
2
Cen e d’Es udis A ança s de Blanes, Consejo Supe io de In es igaciones Cien
í
icas, Blanes, 17300 Gi ona,
Spain; [email p o ec ed] (C.L.-D.); [email p o ec ed] (M.Á.M.)
3
School o Science and Cen e o Ma ine Ecosys ems Resea ch, Edi h Cowan Uni e si y, 6027 Joondalup, WA,
Aus alia; [email p o ec ed]
*Co espondence: ne ea.pinei [email p o ec ed]
Recei ed: 25 Feb ua y 2020; Accep ed: 24 Ma ch 2020; Published: 26 Ma ch 2020
Abs ac :
Scien is s s udying seag asses ypically e e o hei subs a um as sedimen , bu ecen ly
esea che s ha e begun o e e o i as a soil. Howe e , he logis ics o sampling unde wa e
subs a a and he agili y o hese ecosys ems challenge hei s udy using pedological me hods.
P e ious s udies ha e epo ed geochemical p ocesses wi hin he hizosphe e ha a e compa ible
wi h pedogenesis. Seag ass subs a um accumula ed o e he Recen Holocene and can each se e al
me e s in hickness, bu s udies abou deepe laye s a e sca ce. This s udy is a i s a emp o ind
sound e idence o e ical s uc u ing in Posidonia oceanica deposi s o se e as a basis o mo e
de ailed pedological s udies. A p incipal componen analysis on X-Ray Fluo escence-elemen al
composi ion, ca bona e con en and o ganic ma e con en da a along a 475 cm co e was able o
iden i y ou main physico-chemical signals: humi ica ion, accumula ion o ca bona es, ex u e and
o ganic ma e deple ion. The esul s e ealed a highly s uc u ed deposi unde going pedogene ical
p ocesses cha ac e is ic o soils a he han a me e accumula ion o sedimen s. Fu he esea ch is
equi ed o p ope ly desc ibe he subs a um unde nea h seag ass meadows, decide be ween he
sedimen o soil na u e o seag ass subs a a, and o he e en ual inclusion o seag ass subs a a in
soil classi ica ions and he mapping o seag ass soil esou ces.
Keywo ds: seag ass; ma ine sedimen s; subaqueous soils; geochemis y; X- ay luo escence
1. In oduc ion
Subaqueous soils we e i s ecognized in he second edi ion o he Soil Taxonomy (ST) [
1
].
The Wo ld Re e ence Base (WRB) has acknowledged soils o occu unde wa e as well, o a maximum
dep h o 2 m [
2
]. Un il ecen ly, he subs a e below ma ine phane ogams has been s udied as
sedimen by ma ine biologis s and chemis s. The ecen in e es om a soil science pe spec i e has
esul ed in a ew publica ions in which seag ass subs a es a e en isioned as soils [
3
–
6
], igge ing
discussion abou hei classi ica ion as sedimen o soil [
7
]. Rega dless o he e minology used, he
plan /subs a e in e ac ions lead o physico-chemical changes ha d ama ically modi y he ex u e [
8
,
9
]
and composi ion [
10
–
13
], inc ease o ganic ma e con en (OM) [
14
], mic obial popula ions and
di e si y [
15
–
18
], while changing edox po en ial [
19
]. All hese p ocesses a e ypical o pedogene ical
e olu ion in con inen al soils. Al hough his opic may seem i ial, soils a e conside ed impo an
esou ces wo ldwide, suppo ing ood and ag icul u e, biodi e si y and he p ese a ion o cul u al
he i age, among o he s, and a e p o ec ed by en i onmen al laws acco dingly.
Seag asses a e a polyphyle ic g oup o ma ine angiospe ms which can be ound in coas al wa e s
o all con inen s excep An a c ica [
20
]. One o he seag ass species ha la gely al e s he subs a e
Soil Sys . 2020,4, 18; doi:10.3390/soilsys ems4020018 www.mdpi.com/jou nal/soilsys ems
Soil Sys . 2020,4, 18 2 o 15
whe e i g ows is Posidonia oceanica [
12
,
21
,
22
], an endemic species om he Medi e anean Sea ha
inhabi s coas al a eas up o 48 m wa e dep h [
23
]. Posidonia oceanica inc eases sedimen a ion h ough
lowe ing hyd odynamic ene gy and by he di ec apping o suspended pa icles by he canopy [
24
].
The inc eased sedimen a ion, oge he wi h he accumula ion o la ge amoun s o belowg ound issues
om he seag ass i sel , esul s in he o ma ion o well-s uc u ed deposi s known as ma s [
25
].
The anoxic en i onmen p e ailing wi hin he ma , owing o a high OM con en and low oxygen
a ailabili y, and he e ac o y na u e o he seag ass de i us [
26
], hampe he apid mine aliza ion
cycles [
27
] and consequen ly a o he accumula ion o o ganic ca bon ha con ibu es o clima e
change mi iga ion [28]. The ma s unde nea h P. oceanica meadows ha e also been ound o se e as a
habi a o ee ish species [29], con ibu e o coas al p o ec ion agains e osion [30], ac as il e s and
sinks o pollu an s [
31
] and cons i u e secu i y aul s o a chaeological he i age [
32
]. Fu he mo e,
like pea lands, hey p o ide long and ch onologically well-o de ed eco ds o en i onmen al change
and human ac i i ies in coas al a eas [
31
,
33
,
34
]. Nowadays, he mos ex ended me hods o seag ass
ecosys ems’ eg ession and impac s’ e alua ion a e measu emen s o su ace co e , shoo densi y
and plan biomass [
35
,
36
]. No moni o ing is being ca ied ou o he seag ass deposi i sel o o
“dead meadows” deposi s whe e he plan co e has been los bu he deposi emains, unde going
di e en deg ees o e osion. The classi ica ion o hese deposi s as soils will con ibu e o p omo e
hei conse a ion unde speci ic soil egula ions schemes, as well as hei mapping and moni o ing,
and o aise public awa eness abou hei agili y and sensi i i y o coas al managemen policies.
The logis ics equi ed o in es iga e hese ecosys ems make sampling a duous and impede he use o
common pedological me hodologies, such as ield ho izon desc ip ion. Fu he mo e, like pea lands,
deposi s unde seag asses ake housands o yea s o accumula e [
21
,
37
], and he an h opic p essu e
has s ongly diminished hei ex ension in he Medi e anean Sea [
38
]. Eu opean egula ions ha e been
implemen ed o help p o ec hese ecosys ems, bu seem o only be slowing he plan co e loss [
35
].
Due o he agili y o he ecosys em, minimizing he impac o sampling is undamen al.
Al hough biogeochemical p ocesses wi hin he hizosphe e o P. oceanica ha e been p o usely
in es iga ed [
39
], biogeochemical s udies o he laye s below i a e a e. Some o he ew s udies
cha ac e izing P. oceanica deposi s below he hizosphe e come om a meadow in Po lliga bay
(NE Spain) whe e long ma co es ha e been used as en i onmen al eco ds [
31
,
34
], o cha ac e ize he
deposi [
5
] and o s udy he long- e m ca bon cycle associa ed o he ecosys em [
26
,
40
]. The esul s
om hese s udies poin o an ac i e physico-chemical en i onmen , abo e and below he hizosphe e,
a om a me e accumula ion o sedimen s. The p esen s udy is in ended as he nex s ep o unde s and
he long- e m p ocesses leading o he o ma ion and e olu ion o P. oceanica deposi , looking o
e idence o ho izona ion o e ical s uc u ing be o e conside ing la ge scale pedogene ical s udies.
To his end, we analyzed he elemen al composi ion (using X- ay luo escence; XRF), as well as he
ca bona e and OM con en s along a 475 cm-long P. oceanica co e encompassing he las 4000 yea s
and con ex ualize i wi hin he a ailable in o ma ion abou he Po lliga seag ass ma , aiming a
unde s anding he o ma ion p ocesses.
2. Ma e ial and Me hods
2.1. S udy A ea and Sampling Me hods
The co e s udied was collec ed in 2000 in Po lliga Bay (42
◦
17
0
32” N, 3
◦
7
0
28” E; NE Spain),
a shallow embaymen (<10 m dep h) connec ed o he sea by a 213 m-wide opening o he NE
(Figu e 1). P. oceanica meadows co e 41% o he bay a ea, and sandy bioclas ic pa ches and ocky
bo om a e dispe sed h oughou [
41
]. The bay ecei es e en ual e es ial inpu s om a c eek in
i s NW sho e, modula ed by he Medi e anean clima e o he a ea. P ecipi a ion mainly occu s
om Oc obe o Decembe , wi h alues anging be ween 200 and 1300 mm (a e age ange o he
pe iod 1950–2016, me eo ological s a ion o Roses, Se ei Me eo ol
ò
gic de Ca alunya). Po lliga bay is
embedded wi hin Roses Gul , which o igina ed 400 million yea s ago wi h he Py enees, belonging o
Soil Sys . 2020,4, 18 3 o 15
a migma i ic complex ha consis s o an associa ion o sillimani e schis s, g ani oids, qua z-gabb os
and pegma i es [42].
A 475 cm-long, 8.5 cm in inne diame e co e was eco e ed om he seag ass ma a a ound 3 m
wa e dep h using a loa ing d illing pla o m wi h a sel -powe ed o ope cu o hamme . The sampling
de ails a e u he desc ibed elsewhe e [5].
Soil Sys . 2020, 4, x FOR PEER REVIEW 3 o 16
o a migma i ic complex ha consis s o an associa ion o sillimani e schis s, g ani oids,
qua z-gabb os and pegma i es [42].
A 475 cm-long, 8.5 cm in inne diame e co e was eco e ed om he seag ass ma a a ound
3 m wa e dep h using a loa ing d illing pla o m wi h a sel -powe ed o ope cu o hamme . The
sampling de ails a e u he desc ibed elsewhe e [5].
Figu e 1.
(
a
) Co ing si e o he P. oceanica soil co e in Po lliga Bay (Gi ona, Spain); (
b
) de ail o he uppe
and lowe sec ions o he co e, whe e plan ibe s can be ound. Modi ied om
Lei a-Dueñas (2018) [43].
Soil Sys . 2020,4, 18 4 o 15
2.2. Geochemical Analysis
The co e was cu in o 1 cm slices which we e o en-d ied a 70
◦
C un il cons an weigh . The bulk
samples we e g ound o ob ain homogeneous powde . Samples we e analyzed by X- ay luo escence
(EMMA-XRF) a he In as uc u e Ne wo k o he Suppo o Resea ch and Technological De elopmen
(Uni e sidade de San iago de Compos ela), o ob ain he concen a ions o he ollowing elemen s:
sul u , b omine, silicon, po assium, phospho us, chlo ine, aluminum, ubidium, s on ium, calcium,
ch omium, magnesium, a senic, zi conium, lead, i on and i anium [
44
,
45
]. The equipmen was
calib a ed using s anda d e e ence ma e ials. The quan i ica ion limi s we e 2 g
·
kg
−1
o S, 1 g
·
kg
−1
o Cl and Mg, 0.5 g
·
kg
−1
o Ca and K, 0.1 g
·
kg
−1
o Ti and P, 0.05 g
·
kg
-1
o Si and Al, 0.01 g
·
kg
−1
o
Fe, 15 µg·g−1 o C , 1 µg·g−1 o Pb and As, 0.5 µg·g−1 o B , 0.3 µg·g−1 o Rb and S , and 0.2 µg·g−1
o Z . Coa se (>1 mm) and ine (<1 mm) o ganic ma e (COM and FOM, espec i ely); he ca bona e
con en s used in his s udy we e p e iously published; and analyses p ocedu es and de ails can be
ound elsewhe e [5].
2.3. S a is ical Me hods:
The co ela ion o he elemen s is analyzed by p incipal componen analysis (PCA) using psych
package 1.8.12 [
46
] in R so wa e e sion 3.4.4 [
47
]. Da a we e p e iously ans o med by cen e ed log
a io (CLR) o a oid spu ious ela ionships ela ed o composi ional da a [
48
]. A a imax o a ion was
used o maximize he a iance o he a iables in he componen s. All componen s wi h eigen alues
>1 we e conside ed. Change phases (CP) in each p incipal componen sco es eco d we e de e mined
by change-poin modelling, CPQ R1.0.3 [49].
3. Resul s
3.1. O ganic Ma e , Ca bona es and Elemen al Composi ion
A i s sigh , mos o he a iables s udied s a ed wi h a maxima o minima a he op o he co e,
hen inc eased o dec eased sha ply o g adually along he i s me e , and inally s abilized down
he co e (Figu e 2). A no able in e up ion o his end occu ed a a ound 300 cm o all he mino
and majo elemen s, and o mos o he ace elemen s (wi h some excep ions o o a lesse ex en ).
Ano he dis up i e e en in his gene al end was obse ed a a ound 450 cm.
Among he majo and mino elemen s de e mined (Si, Al, Fe, Ti, Ca, K, Mg, P, S and Cl),
concen a ions o Ca and Si we e highe ( anging be ween 10–20%) han Al and Cl (3–4%), Fe, Mg, S
and K (1–2%) and Ti and P (0.10–0.15%). Among he ace elemen s (Rb, S , Z , C , As, Pb and B ), S
showed he highes concen a ion (a ound 1000
µ
g
·
g
−1
), ollowed by Z , Mn and B (100–180
µ
g
·
g
−1
),
C and Rb (30–75
µ
g
·
g
−1
), and Pb and As (10–20
µ
g
·
g
−1
). Ca bona e con en ( m, 45.4
±
11%) was
highe han OM con en (3.1%
±
2.8% FOM and 6%
±
9.4% COM; Figu e 2) along he co e. Howe e ,
he ca bona e con en wi hin he op 110 cm o he ma was lowe (28.3%
±
4.8%) han om cm 110 o
cm 475 (50.5%
±
5.9%). FOM con en was highe in he i s 30 cm (10.8%
±
3.4%), and COM con en
was highe wi hin he op 50 cm (23.5%
±
16.9%) compa ed o he es o he co e (2.5%
±
1.3% and
3.3% ±2.3%, espec i ely).
Soil Sys . 2020,4, 18 5 o 15
Soil Sys . 2020, 4, x FOR PEER REVIEW 5 o 16
Figu e 2. Majo and mino elemen s (Si, Al, Fe, Ti, Ca, K, Mg, P, S and Cl), ace elemen s (Rb, S , Z ,
C , As, Pb and B ), ine o ganic ma e (FOM), coa se o ganic ma e (COM) and ca bona e
concen a ion along he P. oceanica co e s udied.
Among he majo and mino elemen s de e mined (Si, Al, Fe, Ti, Ca, K, Mg, P, S and Cl),
concen a ions o Ca and Si we e highe ( anging be ween 10%–20%) han Al and Cl (3%–4%), Fe,
Mg, S and K (1%–2%) and Ti and P (0.10%–0.15%). Among he ace elemen s (Rb, S , Z , C , As, Pb
and B ), S showed he highes concen a ion (a ound 1000 µg·g−1), ollowed by Z , Mn and B
(100–180 µg·g−1), C and Rb (30–75 µg·g−1), and Pb and As (10–20 µg·g−1). Ca bona e con en
(mean ± SD, 45.4 ± 11%) was highe han OM con en (3.1% ± 2.8% FOM and 6% ± 9.4% COM;
Figu e 2) along he co e. Howe e , he ca bona e con en wi hin he op 110 cm o he ma was lowe
(28.3% ± 4.8%) han om cm 110 o cm 475 (50.5% ± 5.9%). FOM con en was highe in he i s 30 cm
(10.8% ± 3.4%), and COM con en was highe wi hin he op 50 cm (23.5% ± 16.9%) compa ed o he
es o he co e (2.5% ± 1.3% and 3.3% ± 2.3%, espec i ely).
Figu e 2.
Majo and mino elemen s (Si, Al, Fe, Ti, Ca, K, Mg, P, S and Cl), ace elemen s (Rb, S , Z , C ,
As, Pb and B ), ine o ganic ma e (FOM), coa se o ganic ma e (COM) and ca bona e concen a ion
along he P. oceanica co e s udied.
3.2. Geochemical Signals
The PCA yielded ou p incipal componen s, accoun ing o 83% o he o al a iance (Table 1).
All a iables show high communali ies (>0.5). Fo each componen , se e al change poin s (CPs) we e
ound: wo o PC1, wo o PC2, wo o PC3 and six o PC4 (Figu e 3). CPs indica e he p obabili y
o a change o phase in he PC sco es, bu do no necessa ily encompass he whole phase o change
(e.g., CP1.1 be ween 16 and 25 cm and obse ed in lexion end in he sco es a 35 cm, Figu e 3).
The i s componen (PC1) explains 32.9% o he o al a iance, showing high posi i e loadings
(>0.7) o Si, K, Al and Rb, mode a e posi i e loadings (0.4–0.6) o Ti and Z , while high nega i e
loadings a e shown by S, P, B (<
−
0.7) and mode a e nega i e loadings by Cl and As (
−
0.4 o
−
0.7;
Table 1, Figu e 4). The componen sco es dec ease om he op o he co e un il 35 cm (CP1.1;
Figu e 3) and emain s able below his dep h, wi h he excep ion o a local inc ease a ound 300 cm
(CP1.2; Figu e 3).
Soil Sys . 2020,4, 18 6 o 15
Table 1.
Loadings o chemical elemen s, coa se o ganic ma e (COM), ine o ganic ma e (FOM)
and ca bona es o each componen as yielded by he p incipal componen s analysis (PCA). Com:
communali y ( o al a iance o each a iable explained by he ex ac ed componen s), Va . %: pe cen age
o a iance accoun ed o by each componen ; Ac. Va . %: cumula i e a iance. Loadings >0.35 in bold.
PC1 PC2 PC3 PC4 Com
Si 0.94 0.15 0.08 −0.11 0.93
K0.88 −0.01 0.04 −0.18 0.81
Al 0.88 0.24 0.20 −0.01 0.87
S−0.85 −0.15 0.03 −0.21 0.79
Rb 0.83 −0.09 0.22 −0.01 0.75
P−0.83 0.14 0.14 −0.01 0.72
B −0.81 −0.42 −0.07 0.14 0.85
Cl −0.67 −0.37 −0.25 0.16 0.66
S 0.18 0.91 0.11 −0.28 0.95
Ca −0.01 0.88 −0.02 −0.36 0.90
COM −0.15 −0.87 0.06 −0.00 0.78
Mg −0.02 0.84 −0.08 −0.02 0.71
C 0.24 −0.82 −0.27 −0.09 0.82
As −0.42 −0.82 −0.03 −0.04 0.85
CaCO30.19 0.76 −0.13 −0.53 0.91
Z 0.46 0.70 0.26 −0.16 0.79
Fe 0.13 −0.01 0.92 0.03 0.87
Ti 0.61 −0.01 0.70 0.13 0.88
Pb 0.14 −0.40 −0.50 0.62 0.81
FOM −0.13 −0.12 0.14 0.92 0.89
Va . % 32.9 30.5 9.9 9.5
Ac. Va . % 32.9 63.4 73.3 82. 8
Soil Sys . 2020, 4, x FOR PEER REVIEW 7 o 16
Figu e 3. Dep h eco ds o p incipal componen s (PCs) sco es, do s; mo ing a e age o PCs sco es,
g ey line. Change poin p obabili y, blue line; limi o conside a change poin (0.02), black dashed
line.
The second p incipal componen (PC2) accoun s o 30.5% o he o al a iance, wi h high
posi i e loadings o S , Ca, Mg, CaCO3 and Z , high nega i e loadings o COM, As and C , and
mode a e o low nega i e loadings o Pb, B and Cl (Table 1, Figu e 4). The sco es inc ease sha ply
om he op o he co e un il 35 cm (CP2.1; Figu e 3), ollowed by s able alues be ween
35 cm and 100 cm and an inc ease om 100 cm o 130 cm (CP2.2, Figu e 3). Values a e ela i ely
cons an below 130 cm dep h.
The hi d p incipal componen (PC3) explains 9.9% o he a iance, and i is mainly ela ed o Fe
and Ti, wi h high posi i e loadings, and o Pb wi h a mode a e nega i e loading (Table 1, Figu e 4).
The sco es inc ease sha ply wi hin he uppe 30 cm (CP3.1, Figu e 3), bu show low a ia ion below
30 cm, excep o a shi o nega i e alues be ween 200 cm and 300 cm (s a ing a 330, CP3.2, Figu e
3).
The ou h p incipal componen (PC4) accoun s o 9.5% o he a iance wi h posi i e loadings
o FOM (high) and Pb (mode a e) and nega i e, mode a e o low, loadings o CaCO3 and Ca
(Table 1, Figu e 4). The eco d o sco es shows a dec easing end om he su ace o he bo om,
wi h a sligh shi o mo e nega i e alues below 300 cm (Figu e 3). This is he componen wi h he
highes numbe o change poin s, mos o hem o low p obabili y and loca ed a he bo om pa o
he co e (6; Figu e 3).
Figu e 3.
Dep h eco ds o p incipal componen s (PCs) sco es, do s; mo ing a e age o PCs sco es,
g ey line. Change poin p obabili y, blue line; limi o conside a change poin (0.02), black dashed line.
The second p incipal componen (PC2) accoun s o 30.5% o he o al a iance, wi h high posi i e
loadings o S , Ca, Mg, CaCO
3
and Z , high nega i e loadings o COM, As and C , and mode a e o
Soil Sys . 2020,4, 18 7 o 15
low nega i e loadings o Pb, B and Cl (Table 1, Figu e 4). The sco es inc ease sha ply om he op o
he co e un il 35 cm (CP2.1; Figu e 3), ollowed by s able alues be ween 35 cm and 100 cm and an
inc ease om 100 cm o 130 cm (CP2.2, Figu e 3). Values a e ela i ely cons an below 130 cm dep h.
The hi d p incipal componen (PC3) explains 9.9% o he a iance, and i is mainly ela ed o Fe
and Ti, wi h high posi i e loadings, and o Pb wi h a mode a e nega i e loading (Table 1, Figu e 4).
The sco es inc ease sha ply wi hin he uppe 30 cm (CP3.1, Figu e 3), bu show low a ia ion below
30 cm, excep o a shi o nega i e alues be ween 200 cm and 300 cm (s a ing a 330, CP3.2, Figu e 3).
The ou h p incipal componen (PC4) accoun s o 9.5% o he a iance wi h posi i e loadings o
FOM (high) and Pb (mode a e) and nega i e, mode a e o low, loadings o CaCO
3
and Ca (Table 1,
Figu e 4). The eco d o sco es shows a dec easing end om he su ace o he bo om, wi h a sligh
shi o mo e nega i e alues below 300 cm (Figu e 3). This is he componen wi h he highes numbe
o change poin s, mos o hem o low p obabili y and loca ed a he bo om pa o he co e (6; Figu e 3).
Soil Sys . 2020, 4, x FOR PEER REVIEW 8 o 16
Figu e 4. PCA loadings (black a ows). The black do s ep esen he samples dis ibu ion acco ding
o hei sco es in he plo ed p incipal componen s.
4. Discussion
Al hough in o ma ion abou he biogeochemis y o he subs a e o he Po lliga meadow is
sca ce, i is by a he bes -s udied seag ass deposi below 50 cm om he sedimen op, as i has been
used as a sou ce o paleo-en i onmen al in o ma ion [31,33,34,43,50]. The a e age hickness o he
deposi has been es ima ed o be 4.5 m, accumula ed in he las 6000 yea s [41]. The ma e ial used in
his s udy was a pea y-like da k b own o g ey ma e ial, eleasing an in ense smell o sul ide, which
con ains P. oceanica mac o emains along he en i e co e leng h [5]. The mine aliza ion o OM has
been s udied h ough py olysis echniques inding a selec i e p ese a ion o p-hyd oxybenzoic
acids and he deg ada ion o ca bohyd a es and sy ingyl lignin down co e [26,51]. Mic obial
me abolism is expec ed o ollow a chain o elec on accep o s (O
2
, NO
3–
, Mn, Fe, SO
4–2
and CO
2
) being
he ae obic ac i i y dominan in he a ea o in luence o he hizosphe e due o he di usion o O
2
h ough he oo s [40].
Ou esul s, suppo ed by he in o ma ion discussed abo e, indica e a clea e ical s uc u e in
he ma , mainly de e mined by he dynamics o OM mine aliza ion. The mine aliza ion p ocess is
e y likely con olled by he p esence/absence o oo s, which in luence mic obial me abolism by O
2
di usion and he ecalci ance o P. oceanica deb is, leading o wo mine aliza ion phases abo e and
below he hizosphe e. Due o he dec ease in OM con en wi h dep h, he ca bona e con en
inc eases. The wo ho izons ha can be de e mined by he ST (his ic and calcic) coincide wi h he wo
main p ocesses ound, he humi ica ion o OM and he accumula ion o ca bona es (nega i e PC1
and posi i e PC2). Only one ho izon has been de e mined ollowing he WRB, ela ed wi h high o
medium accumula ion o OM, whe e humi ica ion is cap u ed by he i s p incipal componen , and
he deposi ed ma e ial was classi ied as calcic due o he high concen a ion o ca bona es.
Fu he mo e, wo mo e componen s we e iden i ied in he PCA. The hi d p incipal componen is
ela ed o soil ex u e and he ou h o OM deple ion. Se e al me allic elemen s we e ound o co a y
wi h OM. The in e p e a ion o his co a iance is unclea and u he esea ch is ecommended o
be e unde s and he me als’ beha io and in e p e hei accumula ion.
4.1. O ganic VS Ino ganic Ma e Accumula ion
In he i s componen o he PCA (PC1), he opposi e ela ionship be ween elemen s ypical o
mine al ma e (Si, Al, K, Rb, Ti and Z ) and biophyle (S and P) [52] plus o ganically bound elemen s
(Cl and B ; Table 1), sugges s a mass balance be ween ino ganic and o ganic ma e con en wi hin
Figu e 4.
PCA loadings (black a ows). The black do s ep esen he samples dis ibu ion acco ding o
hei sco es in he plo ed p incipal componen s.
4. Discussion
Al hough in o ma ion abou he biogeochemis y o he subs a e o he Po lliga meadow is
sca ce, i is by a he bes -s udied seag ass deposi below 50 cm om he sedimen op, as i has
been used as a sou ce o paleo-en i onmen al in o ma ion [
31
,
33
,
34
,
43
,
50
]. The a e age hickness
o he deposi has been es ima ed o be 4.5 m, accumula ed in he las 6000 yea s [
41
]. The ma e ial
used in his s udy was a pea y-like da k b own o g ey ma e ial, eleasing an in ense smell o sul ide,
which con ains P. oceanica mac o emains along he en i e co e leng h [
5
]. The mine aliza ion o OM has
been s udied h ough py olysis echniques inding a selec i e p ese a ion o p-hyd oxybenzoic acids
and he deg ada ion o ca bohyd a es and sy ingyl lignin down co e [
26
,
51
]. Mic obial me abolism is
expec ed o ollow a chain o elec on accep o s (O
2
, NO
3–
, Mn, Fe, SO
4–2
and CO
2
) being he ae obic
ac i i y dominan in he a ea o in luence o he hizosphe e due o he di usion o O
2
h ough he
oo s [40].
Ou esul s, suppo ed by he in o ma ion discussed abo e, indica e a clea e ical s uc u e in
he ma , mainly de e mined by he dynamics o OM mine aliza ion. The mine aliza ion p ocess is e y
likely con olled by he p esence/absence o oo s, which in luence mic obial me abolism by O
2
di usion
and he ecalci ance o P. oceanica deb is, leading o wo mine aliza ion phases abo e and below he
hizosphe e. Due o he dec ease in OM con en wi h dep h, he ca bona e con en inc eases. The wo
ho izons ha can be de e mined by he ST (his ic and calcic) coincide wi h he wo main p ocesses
ound, he humi ica ion o OM and he accumula ion o ca bona es (nega i e PC1 and posi i e PC2).
Soil Sys . 2020,4, 18 8 o 15
Only one ho izon has been de e mined ollowing he WRB, ela ed wi h high o medium accumula ion
o OM, whe e humi ica ion is cap u ed by he i s p incipal componen , and he deposi ed ma e ial
was classi ied as calcic due o he high concen a ion o ca bona es. Fu he mo e, wo mo e componen s
we e iden i ied in he PCA. The hi d p incipal componen is ela ed o soil ex u e and he ou h o
OM deple ion. Se e al me allic elemen s we e ound o co a y wi h OM. The in e p e a ion o his
co a iance is unclea and u he esea ch is ecommended o be e unde s and he me als’ beha io
and in e p e hei accumula ion.
4.1. O ganic VS Ino ganic Ma e Accumula ion
In he i s componen o he PCA (PC1), he opposi e ela ionship be ween elemen s ypical o
mine al ma e (Si, Al, K, Rb, Ti and Z ) and biophyle (S and P) [52] plus o ganically bound elemen s
(Cl and B ; Table 1), sugges s a mass balance be ween ino ganic and o ganic ma e con en wi hin he
P. oceanica ma . Chlo ine and B a e indica o s o ma ine OM [
52
–
55
], as hey a e known o enzyma ically
bind o OM du ing humi ica ion (e.g., halogena ion o alipha ic and a oma ic compounds) [
56
–
58
].
Nei he COM no FOM show high loadings in his componen . Coa se OM is composed by P. oceanica
mac o-deb is (i.e., shea hs, oo s and hizomes) ba ely deg aded in ou co e [
5
], and i is he majo pa
o he o al OM (59
±
14.5%). The ac ha COM is no co ela ed wi h PC1 suppo s he hypo hesis
ha he OM in he nega i e sco es o his componen ep esen s he highly humi ied ac ion, ins ead
o he o al OM, which includes he hizosphe e and mac o seag ass deb is. Howe e , he humi ied
OM would necessa ily belong o one o bo h o he wo OM ac ions (COM and FOM), and e y likely
o bo h, as hei con en s dec ease om he op o 35 cm (Figu e 2) a he same ime ha a dec ease in
he humi ied OM is de ec ed (inc ease in PC1 sco es wi h dep h, Figu e 3). The ac ha COM and
FOM do no ha e high loadings in PC1 may be due o he small p opo ion o samples e lec ing his
p ocess in he co e (9% o he samples), wi h S, P, B and Cl being quickly deple ed and no ollowing
COM and FOM a ia ions in he emaining eco d (91% o he samples).
The i s p incipal componen sco es dis ibu ion shows high nega i e alues in he uppe 35 cm
(CP1.1, Figu e 3), indica ing a high con en o humi ied OM in acco dance wi h ou in e p e a ion,
a cha ac e is ic inhe en o a soil epipedon. Based on he age–dep h model o his co e epo ed
by Se ano e al. (2012), he humi ica ion p ocesses may seem o ha e p e ailed du ing he las
≈
300 cal. yea s BP a e bu ial. A s udy o he molecula composi ion o he OM on he same co e ound
ha he main changes in o ganic compounds o COM occu ed be ween 200 and 400 cal. yea s BP [
26
].
Despi e he long- e m s abili y o OM wi hin P. oceanica ma s, which is ela ed o he ecalci ance
o seag ass OM and he anoxic condi ions p e ailing wi hin he soil [
59
,
60
], he O
2
pumped by he
oo s wi hin he op o he co e [
19
,
61
] esul s in high ae obic mic obial ac i i y [
40
] and he apid
decomposi ion o he labile ac ions o he OM [
26
,
51
], explaining he deple ion o OM in he op o
he co e (Figu e 2) and coinciding wi h he ele a ed humi ica ion signal o PC1. Changes in PC1 sco es
below he uppe 35 cm a e mino , excep o a maximum a 300 cm (CP1.2). These small changes likely
e lec pas changes in he p opo ion o mine al and o ganic inpu s o he soil o he meadow. The main
change obse ed is a 15- old excu sion o posi i e alues a 300 cm dep h (Figu e 3), concu ing wi h a
high e osion phase in he bay ca chmen led by an inc ease in looding e en s [
62
] and human ac i i y,
which is in ag eemen wi h en i onmen al paleo econs uc ions o he a ea [34,43].
4.2. Accumula ion o Ca bona es
The second p incipal componen clus e s S , Ca, Mg, ca bona es and Z wi h posi i e loadings,
agains COM, C , As and, o some ex en , Pb (Table 1), poin ing o a balance be ween ca bona e
accumula ion and OM. The apid decomposi ion o COM and FOM wi hin he op 35 cm esul ed
in an en ichmen o ca bona e con en , ollowed by u he OM mine aliza ion and en ichmen o
ca bona es, S , Ca, Mg and Z , un il 130 cm whe e bo h OM and ca bona e con en s s abilized.
Biologically media ed p ecipi a ion by calci ying o ganisms is he mos plausible sou ce o
ca bona es o seag ass deposi s a ou s udy si e, s imula ed by he augmen ed wa e pH esul ing om
Soil Sys . 2020,4, 18 9 o 15
in ense pho osyn he ic ac i i y [
5
,
12
,
63
,
64
] and he low hyd odynamic ene gy en i onmen p e ailing
wi hin he bay [
65
]. The co ela ion o S wi h Ca is also indica i e o biogenic ca bona es [
52
]. Howe e ,
a ecen s udy sugges s ha alloch honous ca bona e sou ces could be he main o igin o ca bona es
in seag ass meadows [
66
] and in ou co e Ca also co a ies wi h Z and Mg, elemen s associa ed o
li hogenic sou ces, so his sou ce canno be disca ded. The ca bona e concen a ion in ou co e is lowe
han he a e age ca bona e concen a ion epo ed o bo h he op 10 cm and op me e o P. oceanica
deposi s (28 and 122 mg
·
cm
−3
, espec i ely) [
12
], which may be explained by he lack o ca bona e
ma e ials in he Po lliga ca chmen [42].
The lowe concen a ion o ca bona es in he op laye s has been obse ed be o e o seag ass
ma s [
12
]. This may be due o a dilu ion e ec by he la ges OM abundance in he op sec ions o he
subs a e (due o he p esence o he hizomes and oo s o he li ing plan ). The co ela ion be ween
COM and me al elemen s can e lec he biological p ocess o me al accumula ion o so p ion a e
bu ial [
67
,
68
]. Howe e , he inc ease in As and Pb accumula ion in he op-mos o he co e can be
ela ed o a mosphe ic me al pollu ion [
31
]. Sco es om PC2 showed nega i e alues in he uppe pa
o he co e, inc easing downco e in wo s eps o posi i e alues a 35 cm and 150 cm (CP2.1 and CP2.2;
Figu e 3). These wo change phases can be a esul o di e en OM mine aliza ion s ages. The i s one
coincides wi h he humi ica ion end de ec ed in PC1, du ing which he coa se de i us is physically
deg aded and seag ass labile compounds, such as a y acids and polysaccha ides, a e consumed
as e [
26
,
51
]. The second decay s ep is slowe (CP2.2, Figu e 3;
≈
1050 yea s a e bu ial) because he
emaining COM ac ion, mainly composed o P. oceanica deb is, is now en iched in mo e ecalci an
compounds such as phenols and a oma ics [
26
]. Fu he mo e, he mic obial communi y below he
hizosphe e shi s o a p edominan anae obic me abolism [
40
], which is slowe and less e icien han
ae obic me abolism [
69
]. Bo h he OM composi ion and he sedimen en i onmen al condi ions can
explain he second, slowe OM mine aliza ion phase.
4.3. En ichmen in Fine Soil Pa icles
The hi d p incipal componen accoun s o he dis ibu ion o Fe and Ti, and i is likely ela ed o
changes in e es ial uno and/o dus deposi ion [
52
]. I on and Ti anium a e usually en iched in he
clay ac ion o con inen al soils and sedimen s [
70
,
71
]. Al e na i ely, Fe in solu ion can p ecipi a e
a he sedimen wa e in e ace, due o he alkaline pH o he seawa e [
72
] and he oxygen pumped
by P. oceanica oo s [
19
,
61
], bu his does no explain Ti accumula ion, as i is mos ly hos ed in highly
esis an mine als (such as u ile and i ani e), he e o e suppo ing he o me hypo hesis.
The hi d p incipal componen sco es show ai ly cons an alues below 35 cm, wi h he excep ion
o ela i ely lowe alues be ween 200 cm and 300 cm (CP3.2), sugges ing a coa se g ain size
composi ion (Figu e 3). I is in e es ing o no e ha his dec ease in ine mine al pa icles is coe al
wi h he peak in ino ganic mine al con en (PC1 sco es, CP1.2, Figu e 3), ein o cing he occu ence
o an in ense ‘ e es ial e en ’ al eady discussed elsewhe e [
34
,
43
]. Again, a high accumula ion o
plan emains in he uppe pa o he soil and could accoun o he la ge dec ease in Fe and Ti
concen a ions (CP3.1).
The nega i e co ela ion ound be ween Pb and Fe and Ti in his componen can be ela ed o
his o ical p ocesses (e.g., inc eased Pb luxes om human ac i i ies in he las 300 yea s, [
73
]; coinciding
wi h he dec ease o Fe and Ti by OM dilu ion), o may be poin ing o di e en sou ces (e.g., land
e osion s. a mosphe ic deposi ion). Thus, he inc eased Pb concen a ions may be esponding o
egional o global p ocesses [
31
] and no o soil geochemis y. Fu he esea ch would be needed o
cla i y his p ocess.
4.4. Fine O ganic Ma e (FOM)
The ou h p incipal componen sco es end o dec ease wi h dep h h oughou he co e, which
is likely linked o OM mine aliza ion (Figu e 3). FOM concen a ion would be he esul o a
balance be ween ou pu s due o mic obial mine aliza ion and inpu s om he agmen a ion o COM.
