Introduction
The sedimentary organic carbon (Corg) stores of seagrass meadows –
often referred to as “blue carbon” – can vary among seagrass species and
habitats, with reports of up to 18-fold differences (Lavery et al., 2013).
Ambiguity remains in the relative importance of the depositional environment
and species characteristics contributing to this variability. Seagrasses
occur in a variety of coastal habitats, ranging from highly depositional
environments to highly exposed and erosional habitats (Carruthers et al.,
2007). Since seagrass species differ in their biomass and canopy structure,
and occur in a variety of habitat types, this raises the question of whether
mud content can be used to predict Corg content within coastal
sediments, or whether the species composition will significantly influence
the soil Corg stores independently of the geomorphological nature of
the habitat.
Geomorphological settings (i.e., topography and hydrology), soil
characteristics (e.g., mineralogy and texture) and biological features (e.g.,
primary production and remineralization rates) control soil Corg
storage in terrestrial ecosystems (Amundson, 2001; De Deyn et al., 2008;
Jonsson and Wardle, 2009) and in mangrove and tidal salt marshes
(Donato et al., 2011; Adame et al., 2013; Ouyang and Lee, 2014). While it is clear that
habitat interactions have a large influence on stores of soil Corg, our
understanding of the factors regulating this influence in seagrass meadows
is limited (Nellemann et al., 2009; Duarte et al., 2010; Serrano et al., 2014).
The accumulation of Corg in seagrass meadows results from several
processes: accretion (autochthonous plant and epiphyte production, and
trapping of allochthonous Corg; Kennedy et al., 2010), erosion
(e.g.,
export; Romero and Pergent, 1992; Hyndes et al., 2014) and decomposition
(Mateo et al., 1997). Previous studies demonstrate that both autochthonous
(e.g., plant detritus and epiphytes) and allochthonous (e.g., macroalgae,
seston and terrestrial matter) sources contribute to the Corg pool in
seagrass soils (Kennedy et al., 2010; Watanabe and Kuwae, 2015). Plant net
primary productivity is a key factor controlling the amount of Corg
potentially available for sequestration in seagrass ecosystems (Serrano et al., 2014), but the depositional environment is an important factor
controlling Corg storage in coastal habitats (De Falco et al., 2004;
Lavery et al., 2013).
Previous studies have shown a large variation in Corg stores among
morphologically different seagrass species (Lavery et al., 2013;
Rozaimi et al., 2013). Also, that Corg accumulates more in estuaries compared to
coastal ocean environments (estimated at 81 and 45 Tg Corg yr-1,
respectively; Nellemann et al., 2009). This is due
largely to estuaries being highly depositional environments, receiving
fine-grained particles from terrestrial and coastal ecosystems which enhance
Corg accumulation (i.e., silt and clay sediments retain more Corg
compared to sands; Keil and Hedges, 1993; Burdige, 2007) and preservation
(i.e., reducing redox potentials and remineralization rates; Hedges and Keil,
1995; Dauwe et al., 2001; Burdige, 2007; Pedersen et al., 2011). The inputs of
seagrass-derived Corg in the sedimentary pool could break the linear
relationship among mud (i.e., silt and clay particles) and Corg contents
typically found in terrestrial (Nichols, 1984; McGrath and Zhang, 2003) and
marine sedimentary environments (Bergamaschi et al., 1997; De Falco et al.,
2004). However, the amount of Corg that can be associated with mud
particles is limited (Hassink, 1997), which could lead to a poor
relationship between mud and soil Corg contents. Also, other factors
found to play a key role in controlling soil Corg accumulation in
terrestrial and coastal ecosystems, such as chemical stabilization of
organic matter (Percival et al., 1999; Burdige, 2007), carbon in microbial
biomass (Sparling, 1992; Danovaro et al., 1995), and soil temperature
(Pedersen et al., 2011), could also influence Corg storage in seagrass
meadows.
A significant relationship between mud and Corg contents would allow
mud to be used as a proxy for Corg content, thereby enabling robust
scaling up exercises at a low cost as part of blue carbon stock assessments.
Furthermore, since most countries have conducted geological surveys within
the coastal zone to determine sediment grain size, a strong, positive
relationship between mud and Corg contents would allow the development
of geomorphology models to predict blue carbon content within seagrass
meadows, dramatically improving global estimates of blue carbon storage. The
purpose of this study was therefore to test for relationships between
Corg and mud contents within seagrass ecosystems and adjacent bare
sediments.
Results
The soil organic carbon (Corg) and mud contents varied within the
seagrass meadows and bare sediments studied in Australia and Spain. The soil
Corg and mud contents were higher in seagrass meadows
(average ± SE, 1.5 ± 0.2 % and 18 ± 2.4 %,
respectively) compared to bare sediments (0.6 ± 0.1 % and
10.8 ± 1.2 %, respectively; Table 2). On average, seagrass meadows
of the genera Amphibolis and Posidonia contained higher
soil Corg (1.6 ± 0.1 %) and lower mud (7.2 ± 0.4)
than meadows of Halophila, Halodule and Zostera
(1.2 ± 0.2 % and 34.9 ± 5.4 %, respectively; Table 2).
Overall, carbon isotopic ratios from sedimentary organic matter
(δ13C) were similar between seagrass soils and bare sediments
(-17.6 ± 0.3 ‰ and -17.3 ± 0.2 ‰,
respectively). The Corg in soils from Posidonia and
Amphibolis meadows were 13C-enriched
(-15.5 ± 0.3 ‰) compared with seagrass soils from
Halophila, Halodule and Zostera meadows
(-20.7 ± 0.4 ‰; Table 2). The Corg content in
soils from estuarine and coastal habitats were similar, while mud content in
estuarine sediments was higher and δ13C values depleted when
compared to coastal habitats (Table 2).
The relationships between the variables studied (i.e., %Corg,
%mud, and δ13C signatures of sedimentary Corg) among
different species and habitat geomorphologies, and among different soil
depths were explored in Figs. 1 to 3, and Table 3. When accounting for the
whole data set (up to 475 cm long cores), the Corg content increased
with increasing mud content in bare sediments (R2= 0.78) and at
species level, except for Posidonia oceanica (i.e., Corg content decreased with increasing
mud content; R2= 0.15) and Amphibolis griffithii (i.e., no relationship was found, R2 = 0.05;
Table 3). Although most of the correlations at species level were
significant, they only explain 2 to 39 % of the variance in trends
described, except for Halophila ovalis (91 %; Table 3). In particular, Posidonia meadows (P. australis, P. sinuosa and
P. oceanica) had the lower correlation values (R2 ranged from 0.02 to 0.15). When
combining mud and Corg contents in seagrass meadows of the colonizing
and opportunistic genera Halophila, Halodule and Zostera (Kilminster et al., 2015), a relatively high
correlation was found (R2= 0.56; Fig. 1), while soil Corg and
mud contents in persistent genera were only slightly positively correlated
in combined Amphibolis spp. and not correlated in Posidonia spp. meadows (Fig. 1).
Pearson correlation analyses to test for significant
relationships among soil Corg and mud contents, and soil Corg and
δ13C signatures in up to 475 cm long cores; based on (a) species
identity and (b) habitat geomorphology. ns, non significant
correlation.
(a)
Habitat
Organic carbon (%) vs. mud (%)
Organic carbon (%) vs. δ13C (‰)
(species)
Formula
R2
P value
Formula
R2
P value
Posidonia oceanica
Corg= -0.26 × mud + 6.95
0.15
***
Corg= 1.59 × δ13C + 27.61
0.13
***
Posidonia australis
Corg= 0.02 × mud + 1.69
0.02
*
Corg= 0.18 × δ13C + 4.73
0.30
***
Posidonia sinuosa
Corg= 0.07 × mud + 0.61
0.09
***
Corg= 0.12 × δ13C + 2.44
0.23
***
Amphibolis (mixed spp.)
Corg= 0.17 × mud + 0.61
0.26
***
Corg= 0.14 × δ13C + 3.53
0.09
**
Amphibolis antarctica
Corg= 0.08 × mud + 0.47
0.32
***
Corg= 0.14 × δ13C + 3.10
0.29
***
Amphibolis griffithii
ns
0.05
0.18
Corg= 0.06 × δ13C + 1.79
0.21
**
Halodule uninervis
Corg= 0.02 × mud + 0.37
0.34
***
ns
0.00
0.89
Zostera muelleri
Corg= 0.02 × mud + 0.54
0.39
***
ns
0.08
0.07
Halophila ovalis
Corg= 0.04 × mud + 0.12
0.91
***
ns
0.00
0.89
Bare
Corg= 0.06 × mud - 0.03
0.78
***
ns
0.01
0.24
(b)
Habitat
Organic carbon (%) vs. mud (%)
Organic carbon (%) vs. δ13C (‰)
(geomorphology)
Formula
R2
P value
Formula
R2
P value
Coastal
ns
0.01
0.85
Corg= 0.17 × δ13C + 4.14
0.03
***
Estuarine
Corg= 0.02 × mud +1.01
0.14
*
Corg= 0.17 × δ13C + 4.52
0.22
**
Relationships among soil Corg and mud contents, and soil
Corg and δ13C signatures in all habitats and all soil depths
studied: bare sediments, combined Halodule, Halophila and
Zostera species, and combined Amphibolis and
Posidonia species. Only correlations with R2 > 0.5
are shown. The grey shaded areas showed the range of δ13C
signatures of plant detritus (based on literature values; see main text). The
white circles indicate the samples obviating the expected correlation between
soil Corg and mud contents.
The relationships between soil Corg and mud contents within different
core depths (from 1 to 10 cm thick deposits, and from 11 to up to 110 cm thick
deposits) for bare sediments and each group of seagrass species
were explored in Fig. 2. The Corg content increased with increasing
mud content in bare sediments for both 1 to 10 cm thick (R2= 0.74)
and 11 to 110 cm thick (R2= 0.81) soils. When combining mud and
Corg contents in seagrass meadows of the genera Halophila,
Halodule and Zostera, a higher
correlation was found for deeper core sections (11 to 110 cm-thick; R2 = 0.74)
compared to top core sections (1 to 10 cm-thick; R2= 0.17). For combined
Amphibolis and Posidonia species, soil Corg and mud contents were only
slightly positively correlated in deeper Amphibolis spp. sections (11 to 110 cm-thick;
R2= 0.23) and not correlated in Posidonia spp. meadows (Fig. 2). The
classification of habitats based on geomorphology (i.e., coastal and
estuarine) showed a lack of correlation between soil Corg and mud
contents in coastal ecosystems, and a poor correlation in estuarine
ecosystems (R2= 0.14; Fig. 3 and Table 3).
Relationships among soil Corg and mud contents in 1 to 10 cm
and 11 to 110 cm thick soils: bare sediments, combined Halodule,
Halophila and Zostera species, and combined
Amphibolis and Posidonia species. Only correlations with
R2 > 0.5 are shown. The white circles indicate the samples
obviating the expected correlation between soil Corg and mud contents.
Relationships among soil Corg and mud contents, and soil
Corg and δ13C signatures in the coastal and estuarine habitats
studied. The grey shaded areas showed the range of δ13C signatures
of plant detritus (based on literature values; see main text). The white
circles indicate the samples obviating the expected correlation between soil
Corg and mud contents.
The relationships between soil %Corg and δ13C
signatures were poor for all individual Amphibolis and Posidonia species studied (R2
ranging from 0.09 to 0.3; Table 3), and for combined Amphibolis spp. (Fig. 1), with a
tendency of Corg-rich soils being enriched in 13C (Fig. 1). In
contrast, %Corg and δ13C signatures were not correlated
in any of the small and fast-growing Halodule, Zostera, Halophila meadows studied (Table 3), neither
individually nor when combined (Fig. 1 and Table 3). A lack of correlation
between soil %Corg and δ13C signatures was also found
in bare sediments adjacent to seagrass meadows (Fig. 3 and Table 3).
Discussion
Overall mud content is a poor predictor of soil Corg in seagrass
meadows and care should be taken in its use as a cost-effective proxy or
indicator of Corg for scaling-up purposes in the emerging field of blue
carbon science. Although we describe some promise for opportunistic and
early colonizing Halophila, Halodule and Zostera
meadows (i.e., mud content explained 34 to 91 % of
variability in Corg content) and in bare sediments adjacent to seagrass
meadows (explaining 78 % of the variability), mud is not a universal proxy
for blue carbon content and therefore should not be applied generally across
all seagrass habitats. In particular, mud content only explained 5 to 32 %
of soil Corg content in Amphibolis spp. meadows and 2 to 15 % of soil Corg
content in Posidonia spp. meadows, and therefore, mud content is not a good proxy for
blue carbon content in these meadows.
A tenet of carbon cycling within the coastal ocean is that fine-grained
sediments (i.e., mud) have higher Corg contents. The positive
relationship found between mud and Corg contents in coastal bare
sediments (explaining 78 % of the variability) is in agreement with
previous studies (e.g., Bergamaschi et al., 1997; De Falco et al., 2004), and
is related to their larger surface areas compared to coarse-grained
sediments, providing more binding sites for Corg on the surface of
minerals (Keil and Hedges, 1993; Mayer, 1994a, b; Galy et al., 2007;
Burdige, 2007). In addition, the predominance of fine sediments reduces
oxygen exchange and results in low sediment redox potentials and
remineralization rates, contributing to the preservation of sedimentary
Corg after burial (Hedges and Keil, 1995; Bergamaschi et al., 1997;
Dauwe et al., 2001; Burdige 2007; Pedersen et al., 2011). However, the maximum
capacity of a given soil to preserve Corg by their association with
clay and silt particles is limited (i.e., mud-Corg saturation; Hassink,
1997). The results obtained showed that bare sediment samples with relatively
high Corg contents (i.e., > 4 % Corg) and relatively
low mud contents were also 13C-depleted (Fig. 1), suggesting
significant contributions of soil Corg from allochthonous sources
(e.g.,
terrestrial and sestonic; Kennedy et al., 2010). This could have disrupted
the correlation found between soil Corg and mud contents in the bare
sediments studied.
Mud is not a universal proxy for soil Corg content in seagrass meadows,
which could be mainly explained by additional inputs of seagrass-derived
Corg and/or allochthonous Corg to the sedimentary Corg pool,
obviating the linear relationship between mud and Corg contents found
in the absence of vegetation. The δ13C values indicated that
both seagrass-Corg and non-seagrass-derived Corg (i.e., epiphytes,
algae, seston or terrestrial matter) were buried in the soils of all studied
meadows, but are consistent with a model of increasing capture of
seagrass-derived Corg at meadows formed by persistent, high-biomass
seagrasses (i.e., genera Posidonia and Amphibolis) relative to opportunistic, low-biomass
seagrasses (i.e., genera Halophila, Halodule and Zostera).
On one hand, the soil δ13C signatures measured in these
long-living and large seagrass meadows (averaging -15 ± 0.2 ‰
in both cases) were closer to the δ13C
signatures of Posidonia and Amphibolis tissues (ranging from -8 to -14 ‰;
Hyndes and Lavery, 2005; Hindell et al., 2004; Cardona et al., 2007; Fourqurean
et al., 2007; Collier et al., 2008; Kennedy et al., 2010; Hanson et al., 2010;
Serrano et al., 2016) than to δ13C values of algae or
terrestrial organic matter (ranging from -18 to -32 ‰;
e.g., Smit et al., 2006; Cardona et al., 2007; Kennedy et al., 2010; Hanson et
al., 2010; Deudero et al., 2011). The poor relationship between mud and soil
Corg contents in Amphibolis soils could be explained by samples with
relatively high Corg contents (i.e., > 2.5 % Corg) and relatively
low mud contents, as a result of both the contribution of seagrass-derived
Corg (i.e., 13C-enriched) and Corg from allochthonous sources
(i.e., 13C-depleted; Fig. 1). In Posidonia soils, the poor relationship between
mud and soil Corg contents could be explained by samples with
relatively high Corg contents (i.e., > 10 % Corg) and relatively
low mud contents, as a result of the contribution of seagrass-derived
Corg (i.e., 13C-enriched; Fig. 1). The contribution of
seagrass-derived Corg (i.e., root, rhizome and sheath detritus) in
Posidonia soils play a much larger role than the accumulation of fine, organic-rich
allochthonous particles.
On the other hand, the soil δ13C signatures measured in
Halodule, Halophila and Zostera meadows (averaging -21 ± 0.4 ‰) were more
similar to δ13C values of algae or terrestrial organic matter
than to δ13C values of their seagrass tissues (ranging from -10
and -14 ‰; e.g., Hemminga and Mateo, 1996; Kennedy et al.,
2010; Hanson et al., 2010). The positive relationship between mud and soil
Corg contents in Halodule, Halophila and Zostera
soils could be explained by their relatively high
mud content and 13C-depleted Corg, indicating that allochthonous
Corg inputs and mud content play a major role in soil Corg
accumulation in these opportunistic and early-colonizing seagrasses.
However, the relatively high Corg contents found with relatively low mud
contents (i.e., mud-Corg saturation) disrupted the correlation found
between soil Corg and mud contents in these meadows (Corg > 1 %
in samples with 0–20 % mud; Corg > 2 %
in samples with 20–70 % mud and Corg > 3.5 in
samples with 70–100 % mud; Fig. 1).
The results obtained showed a tendency for high-biomass and persistent
meadows (i.e., Posidonia and Amphibolis) to accumulate higher Corg stores and
seagrass-derived Corg compared to ephemeral and low-biomass meadows
(i.e., Halophila, Halodule and Zostera), suggesting that factors (biotic and abiotic) affecting the
production, form and preservation of Corg within habitats exert a
significant influence on soil Corg content (Lavery et al., 2013; Serrano
et al., 2014, 2016). The above- and below-ground biomass in meadows of the
genus Posidonia (averaging 535 and 910 g DW m-2, respectively) is up to 2-fold
higher than in Amphibolis meadows (averaging 641 and 457 g DW m-2, respectively)
and 4 to 18-fold higher than in small and opportunistic seagrasses of the
genera Halophila, Halodule and Zostera (125 and 49 g DW m-2, on average; respectively; Duarte
and Chiscano, 1999; Paling and McComb, 2000). Indeed, larger seagrasses tend
to have larger and more persistent rhizomes, constituted by more refractory
forms of Corg, more prone to be preserved in soils than simpler, more
labile forms of Corg such as seston and algal detritus which are more
suitable to experience remineralization during early diagenesis (Henrichs,
1992; Burdige, 2007). In addition, the larger size of detritus within
Amphibolis and Posidonia meadows compared to Halophila,
Halodule and Zostera meadows could also contribute to the
larger accumulation of Corg in the former, since decay rates of
seagrass detritus increase with decreasing particle size due to larger
surfaces available for microbial attack (Harrison, 1989). Differences in
above- and below-ground biomass and recalcitrance between Posidonia and Amphibolis spp. could
explain the larger contribution of seagrass-derived Corg (i.e.,
13C-enriched) in the former, thereby obviating the linear relationship
between mud and Corg contents (Fig. 1).
The soil Corg content tends to decrease with soil depth and ageing in
seagrass ecosystems (e.g., Serrano et al., 2012), thereby the persistence of
discrete organic detritus within upper soil horizons could lead to organic
matter concentrations above those levels explained by the association with
clay and silt particles, as previously demonstrated for terrestrial soils
(Mayer and Xing, 2001; Gami et al., 2009). The organic matter preserved in
most marine sediments is intimately associated with mineral surfaces (i.e.,
selective preservation by sorption of organic matter into minerals; Keil et
al., 1994) and therefore the correlation between soil Corg and mud
contents in seagrass meadows could vary as a function of soil depth and
ageing. The results obtained show that soil depth is not an important factor
when attempting to predict soil Corg content based on mud content in
bare sediments (i.e., R2 > 0.74 for all core depths explored;
1 to 110, 1 to 10, and 11 to 110 cm thick; Fig. 2).
However, a clearer pattern appeared when exploring the correlation between
soil Corg and mud contents in top 10 cm and within 11–110 cm soil
depths of combined Halodule, Halophila and Zostera species (R2= 0.17 and R2= 0.74,
respectively). These results suggest that the relatively small below-ground
biomass of these species (i.e., organic detritus) only has an impact on the
expected positive correlation between soil Corg and mud content within
the top 10 cm, while the correlation for deeper soil depths (11–110 cm)
improved (R2= 0.74) compared to the whole data set (1 to 110 cm thick;
R2= 0.56). For combined Amphibolis and Posidonia species, the results
obtained show that soil depth is not an important factor when attempting to
predict soil Corg content based on mud content (i.e., R2 < 0.2
in all cases; 1 to 110, 1 to 10, and 11 to 110 cm
thick; Fig. 2). These results suggest that the relatively large
below-ground biomass of these species (i.e., organic detritus) has an impact
on the expected positive correlation between soil Corg and mud content
within all depths studied.
Habitat conditions in seagrass meadows not only influence the amount of
Corg accumulation through detrital plant inputs, but the capacity of
the plant canopies to retain particles (Gacia et al., 1999). The amount of
fine suspended particles available for burial varies among sites, driven by
geomorphological features (e.g., run-off, hydrodynamic energy and water
depth), while meadow structure (i.e., density, cover and morphology of the
canopy) constrains their capacity to accumulate sediment particles (Hendriks
et al., 2010; Peralta et al., 2008). Although the number of cores and species
studied in coastal and estuarine ecosystems was unbalanced (i.e.,
Amphibolis and Posidonia dominate in coastal habitats
and Halophila, Halodule, Zostera dominate in estuarine habitats),
the lack of, or poor correlations found within estuarine and coastal
ecosystems, precludes the general use of mud as a predictor of blue carbon
content based on habitat geomorphology (Fig. 3). Seagrass meadows and bare
sediments in environments conducive for depositional processes (i.e.,
estuaries) accumulated up to 4-fold higher amounts of mud compared to other
coastal ecosystems, but the saturation of mud with Corg and the large
contribution of seagrass detritus into the sedimentary Corg pool
(13C-enriched soils) in some study sites disrupted the positive
relationship expected between mud and soil-Corg contents. In estuarine
ecosystems, soil Corg originated from both mud inputs linked to
allochthonous-Corg via deposition from upstream transport (e.g., Aller,
1998) and seagrass inputs (i.e., in samples with Corg > 5 %;
Fig. 3). The insignificant relationship between mud and soil
Corg contents in coastal habitats could be explained by their
relatively low mud content and the accumulation of seagrass-derived
Corg, in particular in samples with Corg > 5 %
(Fig. 3).
In sum, mud is not a universal proxy for blue carbon content in seagrass
ecosystems and should not be applied generally across all habitat and
vegetation types. Overall, the positive relationship between mud and
Corg contents found in bare sediments and in opportunistic and/or low
biomass seagrass meadows (i.e., genera Zostera, Halodule
and Halophila) allow mud to be used as a
proxy for Corg content in these ecosystems, thereby enabling robust
scaling up exercises (i.e., benefiting from existing geological surveys and
models) at low cost as part of blue carbon stock assessment programs.
However, mud content is not a good predictor of Corg content in highly
productive meadows such as those constituted by P. oceanica in the Mediterranean Sea and
P. australis, P. sinuosa and Amphibolis spp. in Australia. Analyses of soil grain size (i.e., %mud) could
constitute a relatively cheap method to estimate soil organic carbon content
in seagrass ecosystems, particularly dry and wet sieving using standard
geological sieves (Erftemeijer and Koch, 2001). These could be used to
cheaply quantify mud content as a proxy for carbon, particularly in student
projects, citizen science and in countries where funding for science is
limited and they do not have access to higher technology methods or cannot
afford to pay for analysis. In addition, since most countries have conducted
geological surveys within the coastal zone to determine sediment grain size
(e.g., Passlow et al., 2005), a strong, positive relationship between mud and
Corg contents could allow the development of geomorphology models to
predict blue carbon content within seagrass meadows, dramatically improving
global estimates of blue carbon storage. Indeed, maps of soil grain-size
could be obtained using remote sensing (Rainey et al., 2003; De Falco et al.,
2010), opening new opportunities for scaling exercises.
Previous studies suggested that the relationship between organic matter and
the sediment matrix is best seen with clay-sized fractions (< 0.004 mm;
Bergamaschi et al., 1997; De Falco et al., 2004). However, the grain size
cut-off selected in this study (mud, < 0.063 mm) is more
representative of the bulk soil and their Corg content
(Pedrosa-Pàmies et al., 2013) and therefore a higher correlation is
expected when comparing bulk soil Corg with a larger and more
representative fraction of the sediment (i.e., including the silt fraction,
0.004–0.063 mm, also provides binding sites for Corg; Burdige, 2007).
Other biological, chemical and geological factors not explored in detail in
this study may also play a key role in Corg storage, and ultimately in
the relationship between soil Corg and mud contents. For example, the
effects of habitat geomorphology (e.g., hydrodynamic energy, terrestrial mud
and Corg inputs, export of seagrass biomass) and species identity
(e.g.,
variation in terms of productivity, oxygen exposure and recalcitrance of
Corg stores, and plant influence on sediment retention) within both
coastal and estuarine environments, are among the factors identified in this
study which might explain significant variation in the Corg stores of
meadows in relatively similar exposure conditions (Serrano et al., 2016).
Other factors found to play a key role in controlling soil Corg
accumulation in terrestrial ecosystems, such as chemical stabilization of
organic matter (Percival et al., 1999; Galy et al., 2008) and microbial
biomass carbon (Danovaro et al., 1994), could also influence Corg
storage in seagrass ecosystems. Further studies are needed to identify the
influences of these other factors on Corg storage in seagrass meadows,
and in addition to the mud content, other characteristics should be taken
into account when attempting to obtain robust estimates of Corg stores
within coastal areas.