The role of fluvial sedimentary areas as organic carbon sinks
remains largely unquantified. Little is known about mechanisms of organic
carbon (OC) stabilization in alluvial sediments in semiarid and subhumid
catchments where those mechanisms are quite complex because sediments are
often redistributed and exposed to a range of environmental conditions in
intermittent and perennial fluvial courses within the same catchment. The
main goal of this study was to evaluate the contribution of transport and
depositional areas as sources or sinks of CO2 at the catchment
scale. We used physical and chemical organic matter fractionation techniques
and basal respiration rates in samples representative of the three phases of
the erosion process within the catchment: (i) detachment, representing the
main sediment sources from forests and agricultural upland soils, as well as
fluvial lateral banks; (ii) transport, representing suspended load and
bedload in the main channel; and (iii) depositional areas along the channel,
downstream in alluvial wedges, and in the reservoir at the outlet of the
catchment, representative of medium- and long-term residence deposits,
respectively. Our results show that most of the sediments transported and
deposited downstream come from agricultural upland soils and fluvial lateral
bank sources, where the physicochemical protection of OC is much lower than
that of the forest soils, which are less sensitive to erosion. The protection
of OC in forest soils and alluvial wedges (medium-term depositional areas)
was mainly driven by physical protection (OC within aggregates), while
chemical protection of OC (OC adhesion to soil mineral particles) was
observed in the fluvial lateral banks. However, in the remaining sediment
sources, in sediments during transport, and after deposition in the reservoir
(long-term deposit), both mechanisms are equally relevant. Mineralization of
the most labile OC (the intra-aggregate particulate organic matter
(Mpom) was predominant during transport. Aggregate formation and OC
accumulation, mainly associated with macroaggregates and occluded
microaggregates within macroaggregates, were predominant in the upper layer
of depositional areas. However, OC was highly protected and stabilized at the
deeper layers, mainly in the long-term deposits (reservoir), being even more
protected than the OC from the most eroding sources (agricultural soils and
fluvial lateral banks). Altogether our results show that both medium- and
long-term depositional areas can play an important role in erosive areas
within catchments, compensating for OC losses from the eroded sources and
functioning as C sinks.
Introduction
Soil erosion, a complex process that causes the transport and deposition of
sediments with accompanying soil organic carbon (SOC) (Gregorich et al.,
1998; Wang et al., 2014), affects the dynamics of the terrestrial carbon (C)
cycle and has important implications for the rate of C inputs into the soil
(i.e., plant net primary productivity), as well as for the accumulation and
stability of organic matter in soil (Berhe and Kleber, 2013). According to
some authors (Van Oost et al., 2007), lateral C fluxes are key to determining
the fate of soil organic carbon (SOC) at the landscape scale. However,
current estimations of those fluxes (based on soil erosion and the associated
SOC content) show high variation among studies (Quinton et al., 2010;
Doetterl et al., 2012). The fate of the redistributed organic carbon (OC)
depends on multiple factors: (i) the nature of the soil organic matter being
detached from different “sources” within a catchment (Nadeu et al., 2011,
2012; Kirkels et al., 2014); (ii) its turnover rates during transport;
(iii) the type of erosion processes (selective or nonselective); (iv) the
connectivity and distance of travel between eroding sources and the streambed
(Boix-Fayós et al., 2015; Wang et al., 2010); and (v) the
microenvironmental conditions under which the OC is stored in sedimentary
settings (Van Hemelryck et al., 2011; Berhe and Kleber, 2013). All these
factors, which affect the protection of OC against decomposition through
physical and chemical mechanisms, remain considerably uncertain. Despite the fact that a
combination of different techniques (isotopic, spectroscopic, and traditional
wet chemistry) has been used (Wang et al., 2014; Kirkels et al., 2014; Liu et
al., 2018) to determine if the eroded OC is lost after erosional
redistribution, a full understanding of the dynamics and interactions
between OC sources and sinks, in relation to soil erosion and redistribution,
is still absent (Doetterl et al., 2016; Hoffman et al., 2013).
Determination of how the different OC pools are transported by erosion from
different sources, as well as the effect of new soil structure formation by
aggregation and OC stabilization after deposition, can partially contribute
to answering this question (Doetterl et al., 2016). In fact, it is already
known that soil aggregates physically protect OC from rapid decomposition by
microorganisms (Razafimbelo et al., 2008; Six et al., 2000), and aggregate
formation appears to be closely linked to soil C storage and stability
(Barreto et al., 2009; Golchin et al., 1995; Salomé et al., 2010).
Related to this, Wang et al. (2014) found that soil erosion and transport
result in disaggregation and, consequently, SOC mineralization (Lal, 2005;
Boix Fayos et al., 2015), while depositional and burial processes promote the
formation of macroaggregates that contribute to the physical stabilization or
protection of soil organic matter from decomposition, and enhance
sorptive interactions of soil minerals with organic matter protecting OC from
microbial decomposition (Berhe et al., 2013) The protection provided by
macroaggregates as an important mechanism for SOC stability in soils affected
by soil erosion processes has also been reported (Berhe et al., 2018; Nie et
al., 2018). However, to the best of our knowledge, this is the first study to
include the assessment of microaggregates contained within macroaggregates
and their associated OC, widely acknowledged as an indicator of the physical
protection of SOC (Six et al., 2004; Denef et al., 2007; Six and Paustian,
2014), during the different phases of soil erosion at the catchment scale.
Several authors have suggested that alluvial settings are important
nonquantified OC sinks (Wisser et al., 2013; Hoffmann et al., 2013; Ran et
al., 2014) with mechanisms of OC stabilization that can be quite complex in
semiarid and subhumid areas. In these areas sediments are often
redistributed and exposed to a wide range of environmental conditions (e.g.,
rewetting–drying cycles, high temperatures, and solar radiation) and
sub-catchments with ephemeral, intermittent, and perennial fluvial courses,
sometimes within the same catchment (Boix-Fayos et al., 2015). Ephemeral and
intermittent fluvial courses represent more than 50 % of the global river
network, being the dominant type of water course in dry climates (Datry et
al., 2014). In addition, it is expected that this percentage will increase
worldwide in response to climate change and increased water extraction for
human use (Schewe et al., 2014) because intermittent and ephemeral rivers
will suffer more severe, sustained, and more frequent droughts (De Girolamo
et al., 2017; Vadher et al., 2018). Thus, understanding the OC stabilization
mechanisms within these fluvial systems might have important implications for
the stability of soil OC stocks as affected by soil erosion and in response
to climate change. Moreover, the quantification of mineralization rates
and the assessment of the stabilization mechanisms of OC induced by soil
erosion, together with the identification of source materials contributing to
the sediment OC dynamics within a catchment, are key to determining C budgets,
to feed and develop prediction models, and to handle soil conservation
strategies at catchment scales (Liu et al., 2018). From previous works,
in which
the importance of soil disturbance in determining the way that OC associated
with sediments is transported and deposited within the catchment were
highlighted (Nadeu et al., 2011; Boix-Fayos et al., 2015), new research
questions arose regarding the main stabilization mechanisms of this OC during
transport and in several sedimentary deposits within the catchment. To answer
these questions, here we take a step further, studying in depth how OC is
mobilized across the catchment using a combination of physical and chemical
organic matter fractionation techniques and comparing the different
sedimentary deposits with three main sediment sources (forest and
agricultural upland soils, fluvial lateral banks) to determine their
relevance in OC sediment transport and deposition. Furthermore, we estimate
basal respiration rates of upland soils, fluvial lateral banks, sediments in
transit, and the different deposits. Combining information on the OC
associated with soil aggregates and mineral particles, as an indicator of OC
physicochemical protection, and basal respiration rates can shed light on
whether OC is being accumulated or lost by erosion at the catchment scale.
The main objective of this work was to evaluate the potential of the
transport and depositional areas as sources or sinks of atmospheric
CO2 in a subhumid Mediterranean catchment. The specific objectives
were to (i) assess the changes in aggregate size distribution and
associated OC in the sediments mobilized by soil erosion processes;
(ii) identify the main stabilization mechanisms of OC in eroded soils and
different sedimentary deposits; and (iii) determine the potential sources of
eroded OC in the sedimentary deposits through the catchment. We performed the
study in a sub-catchment (111 km2) in the headwaters of the Segura
catchment in southeast Spain.
MethodsStudy area
The study area is located in the headwaters of the Segura catchment (Murcia,
southeast Spain), which drains to the Taibilla reservoir (Turrilla
catchment) and is formed by three adjacent sub-catchments (Rogativa, Arroyo
Blanco, and Arroyo Tercero) covering a total area of 111 km2 (Fig. 1a).
The dominant lithology of the catchment consists of marls, limestones, marly
limestones, and sandstones of the Cretaceous, Oligocene, and Miocene. The
dominant soils in the area are Lithosols, Regosols, and Cambisols (IUSS
Working Group WRB, 2015). The catchment is representative of the
environmental conditions in Mediterranean mountainous areas of medium
altitude. This catchment receives 530 mm of precipitation per year and has
an average temperature of 13.5 ∘C. It has experienced important land
use changes since the 1970s involving reforestation, including the
construction of a dense network of check dams for hydrological and sediment
control. Socioeconomic changes in the region resulted in the abandonment of
agricultural activities and the recovery of the shrubland and forest.
Nowadays, forests and shrublands represent approximately 80 % of its
area,
while agricultural land represents 20 %. The hydrological and
geomorphological effects of these catchment changes were studied in
Boix-Fayos et al. (2008), Quiñonero-Rubio et al. (2016), and
Pérez-Cutillas et al. (2018). The main fluvial course studied is
ephemeral with water only flowing a few times per year during intense rain
events. A deeper description of the study area is given in Boix-Fayos et
al. (2015).
(a) Location of the Turrilla catchment and main sampling areas
selected for sampling of soils and sediments. (1) The Taibilla reservoir at
the outlet of the catchment (as representative of long-term depositional
areas) where reservoir sediments were sampled; (2) permanent stream (in blue;
Turrilla sub-catchment) and intermittent stream (in orange; Rogativa
sub-catchment). In this area alluvial bars (as bedload), suspended load,
soils, and lateral fluvial banks were sampled; (3) middle and upstream areas
where forest soils and alluvial wedges (as representative of medium-term
depositional areas behind check dams) were sampled. (b) Sediment cascade of the Turrilla catchment, with representation
of sampling areas according to Table 1. Please note that some depositional
areas (mainly alluvial wedges behind check dams) were also sampled in the
middle and upstream areas. Some soils were also sampled downstream.
Field experimental design and sampling
The experimental design combines a fluvial geomorphological perspective and
soil science analysis. Previous geomorphological analyses of the channel and
adjacent areas and of the dynamics of the fluvial morphology in the last
60 years (Boix-Fayos et al., 2007; Nadeu et al., 2011) were used to identify
the main sources and sinks of sediments in the Turrilla basin. This previous
analysis was the background to the combination of the sediment cascade
(Hoffman et al., 2013; Boix-Fayos et al., 2015) and the erosion cycle as the
experimental approach (Fig. 1b). To accomplish it a sampling design was
established (Fig. 1a, b, Table 1) that represented the following: (i) the eroding areas
(source of sediments) and detachment phase; (ii) the transport areas (main
channel) and the main transport processes (suspended sediments and bedload);
and (iii) the depositional areas (along the channel and downstream),
representing the sedimentary phase with medium-term depositional areas
(alluvial wedges) and long-term depositional areas (reservoir sediments).
Characteristics and representativeness of soil and sediment samples
at the different morphological positions representing the different phases of
the erosion process across the catchment.
1 Includes a variety
of soils representing the mainland covers in the
catchment: forests, shrublands, and grasslands (three, four, and two samples,
respectively). 2 Includes alluvial bars and channel (18 and 6 samples,
respectively).
In each area and erosion phase, a representative number of samples was taken
to cover the spatial variability of soils and sediments within the catchment.
At each sampling point paired undisturbed (core of 100 cm3) and
disturbed soil and sediment samples were collected for bulk density
estimations and chemical analyses, respectively. All soil and sediment
samples were stored in a freezer (4 ∘C) until they were processed
for chemical analyses. A total of 89 samples were analyzed.
In eroding areas (Fig. 1a, b), representing the detachment phase of the
erosion cycle, two sources of sediments were sampled: (i) surface soils under
the two mainland uses of the catchment area (forest and agricultural land)
and (ii) fluvial lateral banks well connected to the channel (Fig. 1b,
Table 1). Surface soil (0–10 cm) samples were taken from different
locations distributed from upstream to downstream in the catchment,
representing the mainland uses and covers (high- and low-density forests,
pastures, shrublands, and croplands), but were finally pooled into two main
groups: forest (less eroded) and agricultural soils. Fluvial banks
(considered as subsoils) were sampled at an average depth of 80 cm.
In transport areas, suspended sediments and bedload samples were collected
in two fluvial reaches with different flow regimes, representing an ephemeral
and a permanent stream. Suspended sediment samples were taken using a siphon
sampler device designed to collect suspended sediment samples at different
depths immediately after each flooding event (Dielh, 20018). Six samplers
(1 L bottles) were spaced vertically (7.5 cm apart) and connected to an
intake tube and an air vent. A limited number of samples, for which
sufficient material was available to carry out all the analyses, were
selected. The suspended sediment samples were taken over a period of 2 years.
Moreover, the bedload in the fluvial bars and the channel of both ephemeral
and permanent reaches were sampled at 0–10 cm of depth as representative of
the bedload sediments, which are considered to be ephemeral at these dynamic
positions. Both bare and vegetated fluvial bars were sampled in two periods
of the year, representing dry and wet conditions.
At depositional sites, samples were taken at 0–40 and at 40–80 cm
depths in four alluvial wedges behind check dams that were installed in the
catchment in the 1970s. These alluvial wedges had a depth of between 1 and
3 m and many of them were covered by vegetation. They were considered
sedimentary areas of medium-term residence times. At the outlet of the
catchment the reservoir sediments were sampled down to a depth of 3 m at
several points. The different depths were pooled into surface layer (0–40 cm
depth) and deep layer (40–300 cm). These sediments were considered to have
long-term residence times. The samples used in the analysis, and their
depths, are shown in Table 1.
Soil and sediment analysesWater-stable soil aggregate size distribution
Water-stable soil aggregate size separation was carried out using a modified
wet-sieving method adapted from Elliott (1986). Briefly, a 100 g sample of
air-dried soil, disaggregated by hand, was placed on top of a
2000 µm sieve and submerged for 5 min in deionized water at room
temperature. The sieving was performed manually by moving the sieve up and
down 3 cm, 50 times in 2 min, to achieve aggregate separation. Two sieves
(250 and 63 µm) were used to obtain three aggregate classes:
(i) > 250 µm (macroaggregates; M),
(ii) 63–250 µm (microaggregates; m), and
(iii) < 63 µm (silt plus clay-sized particles; min). The
aggregate size classes were oven-dried (50 ∘C), weighed, and stored
in glass jars at room temperature (21 ∘C) (Fig. 2). From this, the
mean weight diameter (MWD) was obtained as an indicator of aggregate
stability.
Description of the main steps and physicochemical analysis
(wet-sieving and oxidation) used to obtain the different aggregate size
fractions (square in red), the intra-aggregate organic matter, the occluded
microaggregates and the occluded mineral fractions within macroaggregates
(square in green), and fractions resistant to oxidation (square in blue).
Secondly, and in order to quantify the protected microaggregates contained
within macroaggregates, the procedure described by Six et al. (2000) and
Denef et al. (2004) was carried out (Fig. 2, square in green). A subsample
(10 g) of the macroaggregates was immersed in deionized water on top of a
250 µm mesh screen inside a cylinder. The macroaggregates were
shaken together with 50 glass beads (4 mm diameter) until complete
macroaggregate disruption was observed. Once the macroaggregates had been
broken up, microaggregates and other material < 250 µm
passed through the mesh screen, with the help of a continuous water flow to
the sieve. The material retained on the 63 µm sieve
(silt + clay; min) was wet-sieved to ensure that the isolated occluded
microaggregates were water-stable (Six et al., 2000). These microaggregates
obtained from macroaggregates (Mm) were oven-dried at 50 ∘C
(24 h) in aluminum trays and weighed. The material retained on top of the
250 µm mesh was considered the intra-aggregate particulate organic
matter (Mpom), representing the most labile fraction (active). It was
separated and weighed after drying in an oven at 50 ∘C (Fig. 2).
Oxidation of mineral fractions
The free and occluded mineral fractions (< 63 µm) obtained
in steps 1 and 2 (Fig. 2) were oxidized by NaOCl to obtain a
chemically resistant C fraction (rOC) (Zimmermann et al., 2007) representing
the passive pool. A total of 1 g of every mineral fraction was oxidized for 18 h
at 25 ∘C, with 50 mL of 6 % NaOCl adjusted to pH 8 with
concentrated HCl. The oxidation residue was centrifuged at 1000 g for
15 min, decanted, washed with deionized water, and centrifuged again. This
oxidation step was repeated twice. The residue was dried at 50 ∘C
and weighed.
Organic carbon and nitrogen analysis and pools ratios
The OC and total nitrogen (N) concentrations were
determined separately for each water-stable aggregate size class and for the
occluded microaggregates and occluded mineral fractions using an elemental
analyzer (LECO TruSpec CN, Michigan, USA), after carbonate removal using 2
M HCl. All the samples were analyzed in triplicate. When necessary, the OC
concentration of each water-stable aggregate size class, as well as that of
the intra-aggregate particulate organic matter (POM) and of the microaggregates occluded in the
macroaggregates, was expressed on a sand-free aggregate basis (Elliott et
al., 1991). The OC content was also expressed on a soil basis by
multiplying the C concentration in each fraction by the weight proportion of
that fraction:
OCcontent(gOCkg-1soil)=(OC)fraction×(proportionofthefraction)soil,
where (OC)fraction is the OC concentration in each fraction and (proportion
of the fraction)soil is the percentage of the OC that each fraction
represents in the bulk soil. The same procedure was followed to analyze total
N in each of the separated fractions.
The total OC and total N in the bulk soil were considered as the sum of the
OC or total N in each separated water-stable aggregate size fraction.
TotalOC(ortotalN)=macroaggregate(M)+microaggregate(m)+mineralfraction(min)
In addition, we used the macroaggregate stabilization ratio as an indicator
of the physicochemical stabilization of OC in macroaggregates (OC occluded
in microaggregates and occluded in mineral particles, compared to that
associated with the intra-aggregate particulate organic matter fraction, based
on the macroaggregate turnover index by Six et al., 2000):
StabilizedOCinmacroaggregates=(OC-Mm+OC-Mmin)/(OC-Mpom),
where OC-Mm and OC-Mmin refer to the OC associated with the
occluded microaggregates and the mineral fraction, respectively, and OC-Mpom
refers to the OC associated with the intra-aggregate particulate organic
matter fraction. The higher the ratio, the more stabilized the OC is within
macroaggregates (Fig. 2).
Soil and sediment incubations
Soils and sediments were incubated under controlled aerobic conditions for 32
days while keeping moisture (at 60 % of the water-holding capacity) and
temperature (28 ∘C) constant to estimate their potential OC
mineralization rates (mg CO2 kg-1 soil or sediment). Thus,
optimal conditions for microbial activity were mimicked in order to induce
the (natural) potential maximum heterotrophic CO2 soil respiration,
following the approach adopted by other authors (Paul et al., 2001; Doetterl
et al., 2012). We incubated three replicate samples of 30 g of fresh soil
and sediment material that was prepared for fractionation from the
different areas throughout the catchment and at different depth intervals
(see Table 1). Previously, the maximum water-holding capacity for each sample
was estimated in triplicate, following the procedure of Howard and
Howard (1993). Each sample was put in a hermetically sealed flask (125 mL)
with no further additives. The CO2 released was measured
periodically (every day for the first 4 days, every 3 days during the
second week, and then weekly) using an infrared gas analyzer (CheckmateII,
PBI Dansensor, Denmark), and the flasks were opened for 30 min after each
measurement to avoid the accumulation of CO2. The moisture content
of the samples was also checked periodically, but replacement of the
evaporated water was not necessary during the experiment. We used linear
interpolations between sampling dates and then summed them across all dates
to estimate the cumulative amount of CO2 released (mineralized)
after 32 days of incubation; basal soil or sediment respiration was expressed
as mg CO2–C kg-1 soil per day.
Statistical analysis
Statistical tests to detect differences between the means of the sediment
sources and sinks – representative of the eroding, transport, and deposition
phases of the erosion process – were performed separately for each erosion
phase and depth (when applicable) using the nonparametric Kruskal–Wallis
test for independent measurements. This nonparametric test was used because
the sampling design was not balanced due to the different number of samples
(n) in the eroding, transport, and depositional areas (Table 1). Significant
differences were identified at the 0.05 probability level of significance.
Spearman correlations were performed to explore the relationships between
most of the studied variables within each erosion phase. All statistical
analyses were carried out using SPSS 24.0 (SPSS Inc., Chicago, IL, USA).
ResultsWater-aggregate size distribution and associated OC: M, m, and
min
Eroding areas. On average, the forest soils had the highest percentage of
total macroaggregates (M) and MWD values (Table 2) when compared to the
agricultural soils and fluvial lateral banks. The agricultural soils and
fluvial lateral banks showed the same distribution trend – a decrease in the
percentage of aggregates with increasing aggregate size – while in the
forest soils a predominance of macroaggregates existed (Fig. 3a).
(a) Water-stable aggregate size distribution (g aggregate
100 g-1
soil) for > 250 µm (macroaggregates: M),
63–250 µm (free microaggregates, m), and < 63 µm
(free mineral fraction: min) in different geomorphological positions within
the catchment: eroding, transport, and depositional areas. Numeral values are
means ±SE. Bars with different lowercase letters indicate
significant differences in the percentage of each water-stable aggregate size
among the different soils and sediments (p<0.05) according to
the Kruskal–Wallis test. (b) Organic carbon content (g 100 g-1 aggregate) for > 250 µm (macroaggregates: M), 63–250 µm (free
microaggregates, m), and < 63 µm (free mineral fraction:
min) in different geomorphological positions within the catchment: eroding,
transport, and depositional areas. Numeral values are means ±SE. Bars with different lowercase letters indicate significant differences in the
percentage of each water-stable aggregate size among the different soils and
sediments (p<0.05) according to the Kruskal–Wallis test.
Soil texture, MWD, and total organic carbon (OC) in different
morphological positions across the catchment: eroding, transport, and
depositional areas.
Numeral values are means ±SE. Columns with different
lowercase letters indicate significant differences among sites or processes.
Columns with different uppercase letters mean significant differences
between big pooled groups (p<0.05), according to the Kruskal–Wallis
test.
The forest soils also showed the highest content of OC associated with the
macroaggregates (OC-M), followed by the agricultural soils. The agricultural
soils showed higher OC in macroaggregates than the fluvial lateral banks,
despite their lower percentage of macroaggregates (Fig. 3a, b). A decrease in
OC with decreasing aggregate size was found in the forest and agricultural
soils, while no differences in the OC associated with different aggregate
sizes were observed in the fluvial lateral banks (Fig. 3b).
Transport areas. A higher percentage of total macroaggregates and higher MWD,
but a lower percentage of free mineral particles, were observed in the
bedload in comparison with the suspended sediments (Table 2). However, the
suspended sediments had a higher OC-M content, while the OC content in the
free mineral fraction (OC-min) was similar between the two types of transport
sediment (Fig. 3a, b). In addition, for the suspended sediments the
distribution of aggregates (min > m > M) and the OC
associated with the largest aggregates (OC-M) were similar to that of the
agricultural soils.
Depositional areas. In the alluvial wedges, the free mineral fraction
predominated over the macroaggregates and microaggregates, regardless of
depth. The opposite was observed when considering the OC content associated
with these fractions, the order being
OC-M > OC-m > OC-min (Fig. 3a, b), similar to the
trend in the forest and agricultural soils. In addition, a decrease in the
percentage of macroaggregates from the upper to the deepest layer of the
alluvial wedges was found. For the reservoir sediments, the percentage of
macroaggregates and the OC-M content were highest in the upper layer, while
the free mineral fraction represented the highest percentages among the
distinct fractions in the deep sediment layers and had the highest
percentages across the catchment (Fig. 3a, b).
Intra-aggregate particulate organic matter, occluded microaggregates, and
occluded mineral fraction within macroaggregates and associated OC: Mpom, Mm,
and Mmin
Eroding areas. The Mpom fraction represented about 14 % of the total in
the forest soil, followed by the fluvial lateral banks and agricultural soils
(lower than 5 %). The associated OC content in the Mpom (OC-Mpom)
oscillated between 3 % and 0.61 %, decreasing in the following order:
forest soils > agricultural soils > fluvial lateral
banks (Fig. 4a, b). Although no clear differences were observed in the
percentages of Mm and Mmin among the sediment sources (due to high spatial
variability), the contents of OC-Mm and OC-Mmin were highest in the forest
soils (Fig. 4b).
(a) Weight percentage (g 100 g-1 aggregate) of the fractions
occluded within macroaggregates (M) for intra-aggregate particulate organic
matter (Mpom), occluded microaggregates (Mm), and occluded mineral fraction
(Mmin) in different geomorphological positions within the catchment:
eroding, transport, and depositional areas. Numeral values are means ±SE. Bars with different lowercase letters indicate significant
differences in the percentage of each water-stable aggregate size among the
different soils and sediments (p<0.05) according to the Kruskal–Wallis
test. (b) Organic carbon content (g 100 g-1 aggregate) associated to
the occluded fractions within total macroaggregates (%) for intra-aggregate
particulate organic matter (Mpom), occluded microaggregates (Mm), and occluded
mineral fraction (Mmin) in different geomorphological positions within the
catchment: eroding, transport, and depositional areas. Numeral values are
means ±SE. Bars with different lowercase letters indicate
significant differences in the percentage of each water-stable aggregate size
among the different soils and sediments (p<0.05) according to
the Kruskal–Wallis test.
Transport areas. The percentage of the Mpom fraction in the sediments in
transit (suspended and bedload sediments) was about 10 %. The OC-Mpom
content in the suspended sediments and in the bedload sediments was similar
and lower, respectively, when compared to those of the soils and fluvial
lateral banks (Fig. 4b). The sediments in transit displayed a significant
decrease (about 50 %) in the Mm and Mmin percentages as well as in the OC
associated with these fractions (OC-Mm and OC-Mmin) with respect to the
forest soils. However, sediments in transit showed a lower percentage of Mm but
similar OC-Mm and OC-Mmin contents compared to the agricultural soils and
fluvial lateral banks (Fig. 4a, b).
Depositional areas. In the upper sediment layer of the reservoir the
percentage of the Mpom fraction was higher and the OC-Mpom content was lower
than those of the alluvial wedges (Fig. 4b). In the upper sediment layer of
the alluvial wedges the OC-Mpom content was higher compared to the forest
and agricultural soils (Fig. 4b), while the opposite happened at the
reservoir. In addition, the OC-Mpom content decreased with depth in both
depositional areas, although this decrease was more pronounced in the case of
the alluvial wedges sediments (Fig. 4b). The percentages of occluded
microaggregates (Mm) at the reservoir, and of the occluded mineral fraction
(Mmin) in the alluvial wedges, were higher than in the eroding areas (forest
and agricultural soils and fluvial lateral banks) (Fig. 4a). However, both
types of sedimentary deposit had slightly lower OC contents associated with
these occluded fractions (OC-Mm and OC-Mmin) than the forest soils, but the
values were similar to those obtained in the agricultural soils and fluvial
lateral banks (Fig. 4a, b).
The percentage of the total Mm and Mmin decreased significantly with depth in
both depositional areas, being more pronounced in the case of the reservoir,
where the Mm percentage in the deep sediment layer was reduced up to 75 %
when compared to that in the upper sediment layer (Fig. 4a). The alluvial
wedges had a higher OC-Mm content in the upper sediment layer, and higher
OC-Mm and OC-Mmin contents in the deep sediment layer, compared to the
reservoir (Fig. 4b).
OC quality
The contents of free and occluded mineral fraction resistant to NaClO
oxidation, denoted as rOC-min and rOC-Mmin, respectively, were higher in the forest and
agricultural soils than in the fluvial lateral banks, with values ranging
from 0.55 % to 0.13 %. Interestingly, the rOC-min in the suspended
sediments was comparable to that in the forest and agricultural soils and
was lower than that in the bedload sediments, which displayed contents
similar to those of the fluvial lateral banks (Table 3). It is noteworthy
that the rOC was higher in the occluded fraction than in the free mineral
fraction in the lateral fluvial bank, bedload, and deep layer of the
reservoir (Table 3).
OC resistant to NaOCl oxidation in the free mineral fraction
(rOC-min) and occluded mineral fraction (rOC-Mmin); contribution of
rOC-min and rOC-Mmin to the total mineral fraction in the different
morphological positions across the catchment: eroding, transport, and
depositional areas.
Numeral values are means ±SE. Columns with different
lowercase letters indicate significant differences among sites or
processes (p<0.05) according to the Kruskal–Wallis test. Rows with
different capital letters mean significant differences between OC fractions
within each site or process.
The degree of physicochemical stabilization of the OC in macroaggregates –
(OC-Mm + OC-Mmin) / OC-Mpom – could be divided into three groups: (i) the
reservoir, showing the highest ratios (more stabilized); (ii) the soils and
fluvial lateral banks, bedload sediments, and alluvial wedges, with medium
ratios; and (iii) the suspended sediments, with the lowest value (less
stabilized) (Table 4).
C:N ratios, basal respiration rate, and indicators of OC
stabilized in macroaggregates (OC-Mm + OC-Mmin / OC-Mpom) in the different
morphological positions across the catchment: eroding, transport, and
depositional areas.
Numeral values are means ±SE. Columns with different
lowercase letters indicate significant differences among sites or processes
(p<0.05) according to the Kruskal–Wallis test. * The
higher the value
the more stabilized the OC in macroaggregates.
The basal respiration (BR) rates ranged between 0.81 and 6.04 mg
CO2 kg-1 day-1 in the eroding sources, with the lowest
values occurring in the fluvial lateral banks (Table 4). In the sedimentary
deposits, BR ranged from 0.70 to 13.8 mg
CO2 kg-1 day-1, with the values being highest in the upper
sediment layers of the alluvial wedges and lowest in the deep layer of the
reservoir. In the transport areas, the suspended sediments had higher
respiration rates than the bedload sediments, being the second-highest rate
observed through the catchment (Table 4).
Higher C:N ratios were found in the upper sediment layers of the
alluvial wedges, forest soils, and suspended sediments. A decrease in the
C:N ratio with increasing depth occurred in soils and sediments of the
alluvial wedges, but no changes with depth were observed at the reservoir
(Table 4).
Correlations between the different physical, chemical, and biological
variables
Positive correlations among the labile fraction, OC-Mpom, and the total OC
associated with macroaggregates (OC-M) were observed in all areas. However,
correlations among OC-Mpom and the macroaggregates (M), micro within
macroaggregate (Mm) percentage, and OC-Mm content were found in the eroding
and depositional areas but not in transport areas. Moreover, in the eroding
and depositional areas (the reservoir), positive correlations were obtained
between OC-Mpom and the oxidable occluded OC (Table 5). The BR was highly and
positively correlated with the OC-Mpom content in eroding and depositional
areas but not in transport areas. On the other hand, negative correlations
between BR and the OC stabilized in macroaggregates (r=-0.40; p=0.01)
across the study areas were obtained.
Spearman correlation coefficients between the OC associated with
intra-aggregate POM (OC-Mpom) or basal respiration (BR) and aggregate
percentage and associated total OC, MWD, occluded oxidable OC, CN ratios, and
OC stabilized in macroaggregates in the different geomorphological positions
within the catchment: eroding, transport, and depositional areas.
OC-Mpom (g kg-1 soil sed-1)Eroding areasTransport areasDepositional areas Alluvial wedgesReservoirMacroaggregates (%)0.82**0.040.230.77**Micro within macro (%)0.520.390.040.27OC macroaggregates (g kg-1soil sed-1)0.85**0.57**0.96**0.71**OC micro within macro (g kg-1 soil sed-1)0.72**0.220.580.58*MWD0.78**0.130.300.81**Occluded oxidable OC (g kg-1 soil sed-1)0.76**0.280.090.61**Basal respiration (mg C-CO2 kg-1 soil sed-1 day-1)Eroding areasTransport areasDepositional areas Alluvial wedgesReservoirMicro within macro (%)0.270.110.070.61**OC macroaggregates (g kg-1soil sed-1)0.57*0.160.95**0.55**OC micro within macro (g kg-1soil sed-1)0.460.270.130.52*CN0.430.120.65*-0.02Occluded oxidable OC (g kg-1 soil sed-1)0.510.100.160.41OC-Mpom0.73**0.100.68**0.75**OC stabilized in macroaggregates-0.360.13-0.61*-0.38
Single asterisk, double asterisk, and bold font represent significance at p<0.05, p<0.001, and p<0.10, respectively.
DiscussionDynamics of OC in the eroding areas
The differences among the sources of the sediments, in terms of aggregation
and OC distribution within aggregates, determined the way in which the
sediment and associated OC moved across the catchment, which is in line with
results given by some authors reporting that aggregation considerably
reduces the potential transport distance of eroded OC and hence potentially
skews its redistribution in watersheds towards terrestrial deposition (Hu
et al., 2016).
The distribution of OC within aggregates (the OC content increased with
increasing aggregate size) observed in the forest soils, together with the
high percentage of occluded microaggregates within macroaggregates (Mm),
rich in OC indicates a hierarchical order of aggregation in which
macroaggregates are the nucleus for microaggregate formation (Oades, 1984).
Other authors (Sodhi et al., 2009; Wang et al., 2011) also reported a higher
OC content in macroaggregates than in microaggregates in soils, which means
that organic matter could be the major binding agent in such soils (Oades and
Waters, 1991). Here, the agricultural soils displayed a lower aggregate
stability and total OC content than the forest soils, but the same pattern of
OC distribution within aggregates. Moreover, in the agricultural soils, the
OC content in the free mineral fraction (OC-min) was higher than that in the
occluded fractions (OC-Mm and OC-Mmin) (Fig. 4b). Altogether, this indicates
the perturbation of these agricultural soils by land use change, tillage, and
water erosion, which is also supported by the higher proportion of OC
resistant to oxidation (Table 3) compared to forest soils; it also indicates the
high capacity of Mediterranean calcareous soils for OC stabilization in
organo–mineral complexes, in which the OC is less susceptible to
mineralization (Courtier-Murias et al., 2013; Trigalet et al., 2014;
Garcia-Franco et al., 2015a). These stabilization mechanisms are common in
Mediterranean areas and have been found at other sites close to the study
area (Garcia-Franco et al., 2014, 2015b). From a geomorphological
perspective, the agricultural soils showed a more rigorous selection of the
detached OC produced by the perturbations cited above; as a consequence, the
most stable, passive pool remained in these soils.
In the fluvial lateral banks, a lack of hierarchical order of aggregation was
found, together with a lower OC content in all the aggregate size classes,
compared to the forest and agricultural soils, despite the fact that the
three sources had similar percentages of M and Mm aggregates.
This indicates a decoupling of aggregates and OC, as found for other soils
and land uses (Del Galdo et al., 2003; Denef et al., 2007), and contrasts
with Elliott (1986), who reported that the distribution of OC associated with
the aggregate fractions is primarily controlled by the amount of soil present
in the fraction. However, in our study, such decoupling is explained by the
fact that the fluvial lateral banks were sampled on average at 80 cm of depth,
which is equivalent to a C horizon with a lack of soil formation and slow OC
accumulation. This is consistent with the significantly lower BR rates observed
in this source of sediments compared to those in the forest and agricultural
soils, indicating very low microbial activity at this depth (Table 4). At the
fluvial reach scale, the lateral banks were well connected to the channel,
providing sediments to the main fluvial channels.
Sediments and associated OC dynamics during transport
During transport, a strong selection of texture and OC pools indicates
that the most resistant OC was bound strongly to the mineral fraction (free
and occluded), traveling the longest distances within the fluvial network.
Compared to the bedload, the suspended sediments displayed a lower percentage
of total macroaggregates (M) (30 % and 10 % for the bedload and
suspended sediments, respectively), but a greater amount of OC associated
with this fraction (decoupling), mainly as intra-aggregate particulate
organic matter (Mpom) (Fig. 3a, b), suggesting mobilization of the most
labile OC once the macroaggregates had been broken and resulting in less
protection by physical and chemical processes. Other authors have reported
how erosion enhances the release of easily mineralizable C encapsulated
within aggregates in the mineralization process (Six et al., 2004; Polyakov
and Lal, 2008; Van Hemelryck et al., 2010; Wang et al., 2014; Nie et al.,
2018) and how that labile OC is also more easily transported in suspension
(Starr et al., 2000). On the other hand, the high labile OC (OC-Mpom) content
in suspended sediments is consistent with the overall relatively higher BR
rate and C:N ratio, and lower aggregation and macroaggregate
stabilization index, compared to the eroding and depositional areas (Tables 2
and 4), indicating that mineralization might be predominant during sediment
transport.
Depletion of the total OC in suspended and bedload sediments of about
66 % and 80 %, respectively, was observed when compared to the
richest sediment source (forest soils). However, enrichments of OC in the
suspended sediments and depletion in the bedload were observed compared to
the agricultural soils and fluvial lateral banks. This, together with the
positive correlation between total OC and clay (r=0.48; p<0.05) and the higher clay content and lower aggregation observed in
sediments during transport, demonstrates the selectivity of erosion during
detachment and transport, as other authors have also reported (Starr et al.,
2000).
Textural comparison of the sediment sources and sinks indicates that coarse
material with low aggregation and a low OC content is selectively transported
as bedload, while the finest and most labile material with a high OC content
continues in transport as suspended load. Part of this OC might be
(i) mineralized before deposition, (ii) mineralized once it has been
deposited and before it is buried by following flooding events (Stallard et
al., 1998), or (iii) transported longer distances. The similar presence of
stable OC in macroaggregates (total occluded OC : OC-Mm + OC-Mmin), and
OC resistant to oxidation (the most passive pool) in the transported
sediments and the most erodible sources (agricultural soils and fluvial
lateral banks; Table 3, Fig. 4b), indicates that the sediment came mainly from
these eroding areas and that the more stable and resistant OC was not
mineralized during transport. These results highlight the important role of
both physical and chemical mechanisms in the protection of OC in transported
sediments in semiarid and subhumid climates, where erosion can be a
significant contributor to the regulation of catchment C budgets.
Sediments and associated OC dynamics in depositional areas
Compared to sediments coming from the poorest sources of OC within the
catchment (i.e., agricultural soils and fluvial lateral banks) and to the
transported sediments, a significant increase in the total OC, mainly
associated with macroaggregates (OC-M; > 250 µm) and
microaggregates within macroaggregates (OC-Mm), was observed at the surface
layer of the depositional areas. This increase, much greater in the alluvial
wedges than at the reservoir, supports the idea of new macroaggregate
formation following the breakdown of aggregates during transport. Moreover,
in the alluvial wedges, the OC concentration in these newly formed
macroaggregates was higher than that in the microaggregates within
macroaggregates in the agricultural soil and fluvial lateral bank sources,
suggesting that aggregate formation and OC sequestration processes are
occurring at the surface layer of these depositional areas. In fact, in these
depositional areas, the distribution of OC within aggregates was similar to
that observed in the forest soils, showing a decrease in the OC content with
decreasing aggregate size and reinforcing the idea that OC accumulation occurs
in the sediments of depositional sites. The formation of new aggregates,
providing physical stabilization for eroded soil organic carbon in
depositional positions, has been reported elsewhere (Berhe et al., 2012).
The similar (at the reservoir) or even higher (in the alluvial wedges) BR
rates and enrichment of OC, compared to their main eroding sources
(agricultural soils and fluvial lateral banks), indicate that new aggregates
are being formed. Likewise, the positive correlations of the active pool
(Mpom) with the percentages of M and Mm at the reservoir, and with total OC-M
and OC-Mm in both types of deposits, stress the role of the labile material
in the activation of aggregate formation, as was also observed in the eroding
areas. The formation of new aggregates might be favored by the growth of
roots and the stimulation of microorganisms by root exudates of established
terrestrial and aquatic vegetation at these depositional areas (Boix-Fayos et
al., 2015). The formation of microaggregates within larger aggregates driven
by microorganisms protects the OC associated with these microaggregates by
increasing turnover time and leads to long-term C sequestration
(García-Franco et al., 2015). Strong positive correlations between the
OC-Mpom and the occluded oxidable mineral fraction (rOC-Mmin - OC-Mmin)
at the reservoir, similar to the correlations in the eroding areas (Table 5),
suggest that, at this site, fresh OC inputs are more rapidly transformed by
microorganisms in an oxidable pool, promoting aggregate formation and
supporting the idea that the absorption of OC into the mineral surfaces (silt
and clay) by chemical processes could enhance physical protection and thus
long-term OC preservation (Kennedy et al., 2002; Kirkles et al., 2014;
García Franco et al., 2015). Von Lützow et al. (2007) found that
organic molecules stabilized by strong molecular interactions with mineral
surfaces decomposed more slowly than OC stabilized by physical mechanisms
(e.g., occlusion in soil aggregates). A lack of difference between the OC-Mm
and OC-Mmin contents (Fig. 4b) indicates that both physical and chemical
processes might be important with regard to the reservoir sediments acting as
long-term residence deposits.
The higher percentages of Mpom and Mm, but lower OC content associated with
them at the reservoir, compared to the alluvial wedges at the surface layers
(Fig. 4a, b), suggest differences in the temporal dynamics of aggregate
formation between these two types of sediment deposit. The aggregate
formation is slower in the alluvial wedges than at the reservoir, where the
OC is also much more protected from decomposition by microorganisms than in
the alluvial wedges (higher stabilized OC in macroaggregates; Table 4). In
fact, at the reservoir, the OC occluded in aggregates (Mm) represented about
20 % more of the total OC, while BR rates were 66 % lower (Table 4)
than in the alluvial wedges. Altogether, these results indicate greater
physicochemical protection of the OC in long-term residence deposits
compared to medium-term ones.
In the deep layers of the depositional areas, the relatively lower OC,
mineralization rates, and Mpom were less favorable for aggregate formation
and the process unfolded very slowly compared to the upper layers, which
confirms the results reported by other authors (Xie et al., 2017). The
reduction in BR rates with depth suggests that the OC has less chance of
being released, which reinforces the C sink potential of these deposits at the
deep layers in which a higher concentration of OC resistant to oxidation in the
occluded fraction, relative to the free mineral fraction, indicates a higher OC
stabilization compared to the upper layers (Table 3). In summary, the medium-
and long-term depositional areas identified seem to be not only the main
sinks for OC coming from different sources, but also areas of aggregate
formation due to the microorganism activity stimulated by fresh OC inputs of
established terrestrial and aquatic vegetation. Our results are in agreement
with those of Doetterl et al. (2016), who indicated that depositional
landform positions are not only able to store large OC stocks but also
preserve OC more effectively when compared to eroding landscape positions.
Conclusions
Physicochemical mechanisms favoring OC stabilization in sources or
eroding areas and the connectivity between the latter and channel areas
determine both the redistribution of OC within catchments and the
C dynamics. Good physicochemical protection of OC in the original sediment
sources results in better protection of OC in sediments during fluvial
transport and deposition downstream. In ephemeral or intermittent
Mediterranean streams similar to our study site, sediments often originate
from agricultural soils and fluvial lateral banks, in which the
physicochemical protection of OC is much lower than that of forest soils
less sensitive to erosion.
Different stabilization mechanisms were detected in the different sediment
sources, transported sediments, and sedimentary deposits: (i) the
predominance of physical mechanisms of OC stabilization in forest soils and
alluvial wedges; and (ii) the predominance of chemical protection of OC in
fluvial lateral banks. In the other positions, both processes are equally
important in the stabilization of OC.
New processes of soil formation in the upper layers of the depositional
areas and highly protected and stabilized OC stored in the deeper layers
(mainly in the long-term deposits) strengthen the important role of
depositional areas in erosive catchments, compensating for OC losses from eroded
sources and functioning as C sinks.
These results underline the importance of studying soil erosion, soil
formation, and geomorphological processes together in semiarid and subhumid
catchments, where intermittent fluvial courses are predominant. Good
management of these environments will be a powerful tool for climate change
mitigation, given the high potential of alluvial settings as C sinks.
Data availability
Data used in this study are archived by the authors and are available on request.
Author contributions
CBF, MA, MMM, and JDV performed the field experimental design and field
sampling. MA designed and performed the laboratory incubations, including the
data processing and analysis; NGF and MMM developed the organic matter
fractionation techniques, EG performed them at the laboratory, and MA
performed data processing and analysis together with MMM; MMM wrote the
paper with contributions from all authors.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was financially supported by the project DISECO (CGL2014-55-405-R)
from the Spanish Government, National Plan of Science. We thank the members
of the Soil and Water Conservation Group (CEBAS-CSIC) who helped us in the
field and laboratory work. María Almagro was supported by the Juan de
la Cierva Program (grant IJCI-2015-23500). Carolina Boix-Fayos was also
supported by a project from the program “Salvador de Madariaga”
(PRXI7/00045) (Ministry of Education, Culture and Sport of Spain) and a
project (20186/EE/17) of the Fundación Séneca (Regional Agency of
Science of the Murcia Region) in the program “Jiménez de la Espada”.
Review statement
This paper was edited by Ji-Hyung Park and reviewed by one
anonymous referee.
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