Received: 7 December 2018 Revised: 29 April 2019 Accepted: 18 May 2019

DOI: 10.1002/ldr.3375

R E S E A R CH AR T I C L E

Impact of geomorphic disturbance on spatial variability of soilCO2 flux within a depositional landform

Neda Mohseni1 | Amir Mohseni2 | Alireza Karimi3 | Farzin Shabani4,5

1Department of Geography, Ferdowsi

University of Mashhad, Mashhad

9177948974, Iran

2Department of Soil Science, University of

Tehran, Tehran 1417614418, Iran

3Department of Soil Science, Ferdowsi

University of Mashhad, Mashhad

9177948974, Iran

4ARC Centre of Excellence for Australian

Biodiversity and Heritage, Global Ecology,

College of Science and Engineering, Flinders

University, GPO Box 2100, Adelaide 5001,

South Australia, Australia

5Department of Biological Sciences,

Macquarie University, Sydney 2109, New

South Wales, Australia

Correspondence

Neda Mohseni, Department of Geography,

Ferdowsi University of Mashhad, Azadi

Square, Mashhad 9177‐948‐974, Iran.Email: nedamohseni@um.ac.ir

Funding information

Ferdowsi University of Mashhad, Grant/Award

Number: 2‐46534

Land Degrad Dev. 2019;30:1699–1710.

Abstract

Landform structure‐dependent erosive soil processes impact on the physical, chemi-

cal, and biological properties of arid and semiarid areas soils through the redeposition

of erosional soils and, consequently, play a role in changes to the global carbon cycle.

Understanding the biophysical mechanisms that influence the balance of soil C flux is

important for predicting the responses of dryland soilscapes to many forms of envi-

ronmental change. This study was done along an upper‐to‐lower alluvial fan gradient,

where debris flows, representing one of the most common erosive processes of dry-

land soils, cause local‐scale changes in soil biotic–abiotic patterns. In this empirically

founded study, we assessed (a) the variations in soil CO2 flux and some related phys-

iochemical variables such as soil organic–inorganic C, soil C storage, exchangeable

cations, pH, electrical conductivity, and volumetric water content and (b) whether

these variations in soil CO2 flux and its association with the above‐mentioned soil

factors remained constant at three different positions in altitude along the slope of

the alluvial fan. Our findings indicated significant differences according to slope posi-

tion in terms of the flux rate of soil CO2 and associated physiochemical properties.

The application of multiple regression demonstrated that the higher soil CO2 flux

rates in the upper and middle fan positions are significantly positively correlated with

organic C content. Despite the development of biological crusts on the lower fan sed-

iments, they exhibited the lowest rate of soil CO2 flux. Further, low CO2 flux in the

lower fan soils was found to be significantly negatively related to pH and inorganic

C. This anomaly may be attributed to the alkalinity of the environment formed by

the deposition of fine particle sediments on the lower fan position and the fact that

CO2 generated by the respiration of mosses may favor the exchange of organic C

to carbonate production. Our findings underline the paradoxical impacts of debris

flow disturbances on soil C dynamics. This erosive soil disturbance, together with

the asymmetric resource redistribution and subsequent variations in the functioning

of adjacent alluvial fan positions, can simultaneously provide ‘hot spots’ of reservoir

and flux of soil C within different positions of a depositional landform.

KEYWORDS

alluvial fan, geomorphic disturbance, ‘hot spot’, soil C reservoir, soil CO2 flux, soil erosion

© 2019 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/ldr 1699

1700 MOHSENI ET AL.

1 | INTRODUCTION

Soil is directly linked to climate through the exchange of carbon (C)

between the atmosphere and pedosphere (Bellamy, Loveland, Bradley,

Lark, & Kirk, 2005; Berhe, Harte, Harden, & Torn, 2007; Muñoz‐Rojas,

Doro, Ledda, & Francaviglia, 2015). Approximately 2,400 Pg organic

carbon (OC) is stored in the upper 2 m of soil, which, thus, makes it

an important factor in the global carbon cycle (Schomakers et al.,

2017). The C contained in soils is more than three‐times the amount

stored in terrestrial plants (560 Pg) and in the atmosphere (760 Pg;

Houghton, 2007; Lal, 2003; Liu et al., 2018; Schomakers et al.,

2017). Therefore, any change in soil biotic and abiotic properties

governing soil organic carbon (SOC) storage, may stimulate related

instability in the CO2 concentrations of the atmosphere and impact

on climate change (Parras‐Alcántara, Lozano‐García, Brevik, & Cerdá,

2015; Parras‐Alcántara, Lozano‐García, Keesstra, Cerdà, & Brevik,

2016; Xiao et al., 2017; Yigini & Panagos, 2016).

Arid and semiarid terrestrial ecosystems account for over 40% of

global climatic systems (Housman, Powers, Collins, & Belnap, 2006;

Thomas, 2012) and approximately 25% of global SOC (Maestre et al.,

2013; Safriel & Adeel, 2006). Hence, any change of SOC stock in arid

and semiarid areas will have a substantial impact on global atmo-

spheric C concentrations. In dryland ecosystems, SOC preferentially

is concentrated at the top soil layers (Ciais et al., 2011; Thomas,

2012); on the other hand, sediment entrainment and transport pro-

cesses mostly affect the surface, (Turnbull, Wainwright, Brazier, &

Bol, 2010; Wainwright, Parsons, & Abrahams, 2000), which leads to

the SOC being extremely sensitive to erosive soil processes. As a

result, arid and semiarid areas soils are responsible for a greater pro-

portion in the exchange of C from biosphere to atmosphere compared

with other ecosystems (Liu et al., 2018; Rey et al., 2011).

Interactions between extrinsic environmental controls such as geo-

morphic factors (e.g., topography; Cammeraat, 2002; Parsons, Wain-

wright, Schlesinger, & Abrahams, 2003) and geomorphic processes,

such as erosive soil disturbances through the creation of localized var-

iations in soil biotic and abiotic components, can disturb the soil C flux

balance. Some dominant geomorphic processes in drylands are

strongly dependent on the landform structure. Debris flow—a variety

of landslide that transports a mass of rock fragments, mud, soil, and

water down a slope (Geertsema & Pojar, 2007)—is one of the most

common disturbances in dryland landscapes that is strongly depen-

dent on physical alluvial fan structure. Hence, local scale variations in

the slope of the alluvial fan (from the apex to apron) encourage signif-

icant variations in the sedimentation process of sediments transported

by debris flow on the surface of the alluvial fan (e.g., the difference in

proportions of coarse‐grained vs. fine‐grained sediment along alluvial

fan), stimulating the varying conditions for the development of local

habitats with different soil biological, chemical, and physical properties

within different alluvial fan positions (Geertsema, Highland, &

Vaugeouis, 2009; Geertsema & Pojar, 2007; Mohseni, Hosseinzadeh,

Sepehr, Golzarian, & Shabani, 2017; Pop & Chiţu, 2013; Walker &

Shiels, 2012). Such localized variations in edaphic properties can influ-

ence the storage and flux balances of SOC within dryland depositional

landforms (Goldsmith et al., 2008; Hilton, Meunier, Hovius, Belling-

ham, & Galy, 2011; Ramos Scharrón, Castellanos, & Restrepo, 2012).

To date, some studies have examined the role of landslide as a

geomorphic disturbance to SOC cycling, focusing on forest or tropical

ecosystems (Hilton et al., 2011; Ramos Scharrón et al., 2012;

Schomakers et al., 2017; Walker & Shiels, 2008). The majority of stud-

ies in this field investigated the impacts of landslide on the dynamics

of soil C along a hillslope or on the larger landscape scale. In the case

of arid and semiarid ecosystems, many studies have demonstrated

how soil C flux varies over time and space, relative to variations in

vegetation (Barron‐Gafford, Scott, Jenerette, & Huxman, 2011;

Maestre & Cortina, 2003; Vargas et al., 2010) and changes of surface

soil features (Almagro, Querejeta, Boix‐Fayos, & Martínez‐Mena,

2013; Cable et al., 2011; Cable, Ogle, Williams, Weltzin, & Huxman,

2008). Some studies have investigated the impacts of variations in

the seasonality, duration, and magnitude of rainfall events on the

dynamics of soil CO2 efflux (Thomey et al., 2011; Vargas et al.,

2012). Other works have examined the effects of climate and alter-

ations in land use on the SOC reservoir (Lozano‐García, Muñoz‐Rojas,

& Parras‐Alcántara, 2017; Munoz‐Rojas et al., 2012; Thomas, 2012;

Willaarts, Oyonarte, Muñoz‐Rojas, Ibáñez, & Aguilera, 2016). How-

ever, despite the high sensitivity of arid and semiarid areas soils to

erosive disturbances and, subsequently, its global impact on soil C

dynamics, there is very limited research on how local‐scale changes

in the functioning of landform in response to erosive soil processes

can affect soil C flux dynamics and thereby simultaneously create

‘hot spots’ of sources and sinks of C within different arid land land-

form positions.

We used a conceptual approach incorporating a simple empirical

model and our field data to test the following hypotheses: (a) The

asymmetric distribution of sediments transported by a debris flow

along gentle changes in slope of the alluvial fan can stimulate fine‐

scale variations in soil physical and biochemical properties within the

different alluvial fan positions, causing asymmetric dynamics of soil

CO2 flux, and (b) such localized variations in soil physical and biochem-

ical properties along the alluvial fan can vary the relationship patterns

between soil CO2 flux and edaphic variables, encouraging ‘hot spots’

of reservoir and flux of soil C within different positions of this

landform.

2 | METHODS AND MATERIALS

2.1 | Description of study site

Our study site was an alluvial fan in the southern Binaloud watershed

located in northeastern Iran (Figure 1) with an annual precipitation

between 200 and 250 mm and an average annual temperature

approximating 13°C. The extreme mechanical weathering due to the

geomorphic condition of the study site that includes a steep slope of

more than 3,000 m in elevation with thin soils lacking support from

the sparse vegetation cover, the semiarid climatic conditions and the

potential for extreme rainfall events, as well as the active faults of

FIGURE 1 (a1–a3) Representative images taken of the sampling positions within alluvial fan from the upper toward the lower down the slope. (b)Elevation‐related perpendicular views from the three positions of the study alluvial fan taken from Google earth; b‐1: upper fan position in

1,400 masl, b‐2: middle fan position in 1,375 masl, and b‐3: lower fan position in 1,350 masl. (c) Location of the study alluvial fan within KhorasanRazavi province [Colour figure can be viewed at wileyonlinelibrary.com]

MOHSENI ET AL. 1701

the Binaloud watershed, produce a high susceptibility of the region to

mass wasting and varieties of landslides, particularly debris flow. Dur-

ing heavy rainfall, especially after intense flooding, sediment‐laden

water flows from the fan watershed, carrying sediment varying in com-

position from particles of clay to boulders. The flow initially follows the

existing channels flowing down the slope, eventually traversing the sur-

face of the fan. Additionally, disturbances of the topography due to the

existence of active faults in the area have led to changes in the base level

of the alluvial fan and subsequent anomalies in slope gradient. The con-

dition indirectly affects the erosion intensity along the landform causing

the emergence of additional erosional‐depositional characteristics

within the landscape. Detachment and transportation of surface soils

along the slope as well as the erosional‐depositional nature of the land-

scape have resulted in the alluvial fan exhibiting a contradictory envi-

ronment in terms of ecological, hydrological, and edaphic patterns

along the small‐scale spatial intervals.

2.2 | Design and phase execution

Although alluvial fans are recognized as depositional landforms, the sed-

imentation process is not the same throughout the landform. Certainly,

gentle variations in elevation and slope from the apex toward the apron

of the alluvial fan can affect the nature of the sediment on the surface of

this landform, stimulating noticeable differences in sediment delivery

rate and type in small spatial scales. Such local scale variations create

three primary zones within an alluvial fan, described as the proximal

fan, medial fan, and the distal fan. Hence, each position can have differ-

ent sedimentary properties in terms of physical, chemical, and biological

conditions. The gradient of study alluvial fan forms a gentle slope from

the upper fan to the drainage lower fan ranging in elevation from

1,400 to 1,340 masl. Noticeable differences in the biological and phys-

ical surface soil characteristics of the upper fan and lower fan positions

were observed in the field survey. These can probably be explained by

the varied responses of the topographic positions to the heterogeneous

redistribution of sediment transported by debris flow on the surface of

alluvial fan. One of the most apparent differences was the localized var-

iations in the distribution of biological soil crust, together with the visi-

ble variations in the distribution of coarse fractions of different sizes

over small‐scale changes of topographical position on the alluvial fan.

Such physical and biological variations in the soil were systematically

classified to produce three surface study microzones along the slope

gradient from the alluvial fan apex toward apron based on variations

in elevation of 25 m, creating an upper fan of elevation at 1,400 m, a

middle fan at 1,375 m, and a lower fan at 1,350 m.

For the soil sampling process, each position was randomly dis-

sected into 10 replicate 1 × 1‐m plots and two samples were allocated

to each plot. The plots within each position were distributed in differ-

ent positions along upper‐to‐lower alluvial fan gradient. Because SOC

is concentrated on the surface layer of semiarid and arid areas soils

(Ciais et al., 2011; Thomas, 2012) and the detachment of soil particles

by erosion processes is predominantly from the surface layer (Turnbull

et al., 2010; Wainwright et al., 2000), samples were collected to a

depth of 10 cm. Because the crusts were only observed in the lower

fan position, consequently, our study could not be based on a compar-

ison of crusts in the different positions, the data related to lower fan

constituted a combination of soil crusts and the soil beneath them

(depth of 10 cm).

To determine the pattern of spatial variations in soil CO2 flux rate (g

CO2 m−2 day−1) within the landform, we employed the closed static

chambermethod using alkali traps (Syers,Williams, Campbell, &Walker,

1967). For each position, 10 replicate 1 × 1‐m plots were randomly

1702 MOHSENI ET AL.

dissected, and two chambers were allocated to each plot, thus, produc-

ing 20 points of observation per position. The spatial variability of soil

CO2 flux was measured daily, at the same time for all positions, at

06:30 h (UTC; 10:00 h local time) throughout May. Polyvinyl chloride

cylinders with a diameter and height of 10 cm were placed in the soil.

Polyethylene was used to seal the chambers and aluminum foil for insu-

lation. Glass vials containing 15ml of NaOHwere placed on the soil sur-

face beneath the closed chambers. CO2 emitted by the soil was trapped

by the vials containing NaOH for 24‐hr periods (Gregorich & Carter,

2007), and the excess of NaOH was titrated against HCl, with phenol-

phthalein used as an indicator (Rowell, 1994). Soil temperatures of the

cylindersweremeasured at a 10‐cm depthwith standard thermometers

at 06:30 and 13:30 h (UTC) and averaged to derive the mean daily soil

surface temperatures for all 20 observed points for each of the three

positions.

To establish the variations in the relationship patterns between the

soil CO2 flux and the associated edaphic variables within the

microzones, the samples from each plot were collocated with the

CO2 flux measurements to reveal the spatial patterns of the soil phys-

ical, chemical, and biological properties in each microzone as described

below (an overall 20 observations per position).

The organic carbon content (SOC%) was measured using Walkley–

Black titration (Rowell, 1994), whereas the soil inorganic carbon con-

tent (SIC%, as calcium carbonate equivalent) was measured by hydro-

chloric acid neutralization and sodium hydroxide titration (Loeppert &

Suarez, 1996). As small variations in soil total carbon storage reflect in

relatively large changes of CO2 concentrations in the atmosphere, soil

total carbon (STC%) content was calculated based on the sum of SOC

and inorganic carbon. Finally, the soil total carbon stock (STCs; g cm−2)

calculation for each position was based on the equation:

STCs ¼ STC% × BD × 1 − RCð Þ × dð Þ;

where STCs represents the total carbon stock (g cm−2), STC total car-

bon content (%), and BD bulk density (g cm−3) of the soil. RC is the

percentage of rock fragments >2 mm (gravel) in the samples and d

the sampling depth (10 cm).

The Kjeldahl method was used to determine total nitrogen (TN)

content (g kg−1; Bremner, 1996) and the sodium carbonate and

fusion—Molybdenum blue photometric method for measuring total

phosphate (TP; g kg−1; Syers et al., 1967). Atomic absorption spectro-

photometry was employed to determine the exchangeable cation con-

centrations (cmoles kg−1; David, 1960), which included the Ca+, Mg+,

Na+, and K+ levels in the sediment of the debris flow. The pH and

EC (ds/m) levels in the saturated paste and paste extract were mea-

sured with a pH meter and an EC meter, respectively.

To establish the physical factors influencing soil infiltration and its

nutrient content, particle size distribution (PSD), including fine earth

particles of clay, silt, and sand less than 2 mm in diameter, was

assessed using the standard hydrometer method (ASTM 152H;

Bouyoucos, 1962). The distribution of coarse fractions greater than

2 mm in diameter was determined by means of a mechanical

sieve shaker, which passes dry soil samples through meshes of

different sizes.

As bulk density (g cm−3) can impact on the distribution of soil C

content, due to the potential for mechanical resistance of soil to the

expansion of organisms (Drewry, Cameron, & Buchan, 2008; Raheb,

Heidari, & Mahmoodi, 2017), it was determined using intact soil cores

(Blake & Hartge, 1986), established by the insertion of cylinders,

10 cm in diameter and depth. The extracted samples were dried in

an oven at 100°C for 24 hr.

As the availability of water can regulate the performance of

microbes and affect the soil C cycle, on the dates on which CO2 flux

was measured, soil samples were collected to a depth of 10 cm after

each measurement of flux to determine the volumetric water content

(VWC; m3 m−3). This variable was determined based on the difference

of wet and dry weights of the soil mass, using the oven‐dry method

and bulk density of the samples.

2.3 | Data analysis

To explore how a landform structure‐dependent geomorphic distur-

bance can impact on soil C flux dynamics, variations of soil CO2 flux

levels, as well as the variables impacting on soil C reservoir‐flux bal-

ance, including SOC, SIC, STCs, TN, TP, exchangeable cations, EC,

pH, PSD, and VWC, were compared for different alluvial fan

microzones (upper fan, middle fan, and lower fan) using the one‐way

analysis of variance in conjunction with the Tukey post hoc test. The

Kolmogorov–Smirnov method was used to verify the normality and

distributional adequacy of the data and log transformations were

made where necessary.

After determining the statistically significant differences in soil

CO2 flux rates and relevant physiochemical variables of the

microzones, a stepwise multiple regression was employed to examine

whether the landform structure‐dependent geomorphic disturbance

had a similar effect on the relationship of soil CO2 flux and relevant

physiochemical variables in different alluvial fan positions. After deter-

mining the predictor controls affecting variability in soil CO2 flux levels

in each microzone, Pearson correlation analysis was performed to

examine the causes of variations in factors controlling CO2 flux levels,

within the soils belonging to the different microzones. Overall, these

analyses can explain the sources and causes of local scale variability

in soil CO2 flux and storage within debris flow fan. R statistical soft-

ware (version 2.14.0 R Foundation for Statistical Computing, Vienna,

Austria) was used for the statistical analyses. The probability level (p

value) of .05 and .001 was set for all statistical analyses.

3 | RESULTS

3.1 | Variations of CO2 flux levels and relatedphysiochemical properties of soils within the landform

The analysis of variance and Tukey results indicated statistically signif-

icant differences in terms of CO2 flux level and physiochemical

MOHSENI ET AL. 1703

properties for the three classified positions within the fan, which may

be attributed to the heterogeneous redistribution of sediments

transported by the debris flow on the surface of the alluvial fan

(Table 1).

The average soil CO2 flux level indicated its lowest value in the

lower fan and higher values in the middle and upper fan. In terms

of physical properties, despite the consistency of the mainly loamy

clay soil texture over the whole alluvial fan, local‐scale differences

in particle size distribution were evident. The higher values of fine

fractions ≤2 mm and coarse fractions≥2 mm were observed in the

lower fan soils and the upper‐middle fan soils, respectively. In the

lower fan position, bulk density decreased in inverse proportion to

increases of fine earth fractions. Further, the greatest and least

values for VWC were observed in the lower fan and upper fan posi-

tion, respectively.

As shown in Table 1, the microzones within the debris flow fan

exhibited significant differences in levels of some soil chemical

variables including pH, inorganic C, storage of SIC, storage of STC,

SICs/SOCs ratio, TN, TP, and exchangeable cations. In a parallel

decrease in elevation and slope, average pH value exhibited an increas-

ing trend indicating a greater presence of alkaline soils in the lower fan

TABLE 1 Results of one‐way ANOVA and Tukey post hoc test of the codifferent microzones within alluvial fan

Variable

Position

Upper fan

Soil CO2 flux (g cm−2 day) 2.99 ± 1.14a

Soil temperature (°C) 27.23 ± 3.89

Fine fractions <2 mm (%) 56.86 ± 5.24b

Coarse fractions >2 mm (%) 41.23 ± 9.07a

Bulk density (g cm3) 1.55 ± 0.18a

VWC (m3 m−3) 0.07 ± 0.02b

pH 7.10 ± 1.13b

EC (ds/m) 1.03 ± 0.31

Inorganic carbon (%) 15.66 ± 2.14b

Organic carbon (%) 0.43 ± 0.14

SOCs (g cm−2) 5.08 ± 2.17

SICs (g cm−2) 141.38 ± 21.53b

STCs (g cm−2) 146.46 ± 22.34b

SICs/SOCs ratio 33.52 ± 16.02b

Total N (g kg−1) 0.34 ± 0.13b

Total P (g kg−1) 0.006 ± 0.002b

Ca (cmol kg−1) 0.23 ± 0.07b

Mg (cmol kg−1) 0.10 ± 0.03b

Na (cmol kg−1) 0.66 ± 0.06b

K (cmol kg−1) 0.65 ± 0.10b

Note. All values are defined based on mean ± SD, n = 20. Different lowercase l

within the study landform.

Abbreviations: ANOVA, analysis of variance; EC, electrical conductivity; SICs, s

carbon stock; Total N, total nitrogen; Total P, total phosphate; VWC, volumetr

position compared with other positions. Although an increased level of

SOC was expected in the lower fan soils due to the extensive develop-

ment of biological soil crusts, there was no statistically significant

difference in SOC between the three positions. Conversely, the lower

fan soils indicated a significant increase in carbonate‐based inorganic C

content, and consequently SIC storage, in comparison with other

positions. Further, the highest and the lowest values of STC storage,

SICs/SOCs ratio, amount of TN, TP, and exchangeable cation concen-

trations were displayed in the lower fan soils and the upper

microzones, respectively. As shown in Table 1, the different positions

exhibited no significant differences for soil temperature.

3.2 | Analysis of factors affecting spatial variabilityof soil CO2 flux within alluvial fan

Simple linear models can contribute to a better understanding of bio-

physical mechanisms affecting soil CO2 flux levels within depositional

landforms. Table 2 displays the results of the multiple linear stepwise

regressions, which indicate that the independent variables controlling

soil CO2 flux level, as well as the relationship between soil CO2 flux

mparison of soil CO2 flux and related physical–chemical properties at

Middle fan Lower fan

3.10 ± 1.38a 1.25 ± 0.55b

27.18 ± 3.80 26.50 ± 3.47

61.75 ± 10.53b 71.24 ± 11.34a

42.26 ± 8.31a 27.63 ± 10.81b

1.47 ± 0.33ab 1.33 ± 0.24b

0.08 ± 0.03ab 0.10 ± 0.04a

7.43 ± 0.8b 8.14 ± 0.72a

1.15 ± 0.26 1.25 ± 0.32

16.12 ± 2.69b 18.29 ± 2.45a

0.52 ± 0.47 0.53 ± 0.22

4.43 ± 3.63 3.93 ± 1.38

137.72 ± 43.00b 173.14 ± 30.01a

142.15 ± 44.50b 177.08 ± 30.69a

41.93 ± 19.66ab 49.54 ± 20.91a

0.40 ± 0.17b 0.67 ± 0.34a

0.007 ± 0.002ab 0.009 ± 0.003a

0.39 ± 0.08a 0.40 ± 0.08a

0.12 ± 0.04b 0.19 ± 0.03a

0.70 ± 0.08b 1.06 ± 0.26a

0.62 ± 0.10b 0.95 ± 0.08a

etters show statistically significant differences (p < .05) among microzones

oil inorganic carbon stock; SOCs, soil organic carbon stock; STCs, soil total

ic water content.

TABLE 2 The results of multiple linear stepwise regressions of the extent of influence of independent variables on spatial variability of soil CO2

flux along positions of erosional to depositional within the study alluvial fan

Model/position

Unstandardized coefficient Standardized coefficient

B Std. error Beta t Sig.

Upper fan

1 (Constant) −0.13 0.58 — −0.22 .82

VWC 44.34 7.95 0.79 5.57 .00

2 (Constant) −0.35 0.48 — −0.71 .48

VWC 30.35 8.05 0.62 3.76 .002

Organic C 2.24 0.74 0.65 3.02 .008

Middle fan

1 (Constant) 0.34 0.53 — 0.42 .67

VWC 41.11 5.93 0.80 5.75 .000

2 (Constant) 0.23 0.62 — 1.79 .09

VWC 35.89 6.30 0.71 1.42 .000

Organic C 4.41 1.28 0.63 3.48 .003

Lower fan

1 (Constant) 3.98 0.40 — 9.91 .00

CaCO3 −0.21 0.030 −0.94 −2.14 .00

2 (Constant) 3.14 0.82 — 7.68 .00

CaCO3 −0.22 0.048 −0.82 −0.96 .00

SICs −0.21 0.002 −0.75 −6.88 .00

3 (Constant) 3.10 — 5.34

CaCO3 −0.18 0.12 −0.72 −0.51 .00

SICs −0.16 0.014 −0.68 −3.09 .00

pH −0.45 0.11 −0.65 −0.99 .00

4 (Constant) 2.94 — — 2.51 .022

CaCO3 −0.11 0.18 −0.67 −2.14 .026

SICs −0.12 0.17 −0.67 −0.15 .001

pH −0.40 0.25 −0.63 −3.43 .003

VWC 4.60 1.55 0.35 2.44 .026

Abbreviations: Organic C, organic carbon; SICs, soil inorganic carbon stock; VWC, volumetric water content.

1704 MOHSENI ET AL.

and the variables, differed significantly within the study landform

positions (upper fan, middle fan, and lower fan). It is noteworthy

that the differences between the upper fan and middle fan position

were insignificant. The fitted equations and predictors related to

variability in soil CO2 flux levels for each microzone were estimated

as follows:

Soil CO2 Flux Upper Fan ¼ −0:35þ 30:35VWCþ 2:24OC; (1)

R2 ¼ 0:51; p < :001� �

;

Soil CO2 Flux Middle Fan ¼ 0:23þ 35:89VWCþ 4:41OC; (2)

R2 ¼ 0:56; p < :001� �

;

Soil CO2 Flux Lower Fan ¼ 2:94 − 0:11CaCO3 − 0:12SICs − 0:40 pHþ 4:60VWC;

(3)

R2 ¼ 0:79; p < :001� �

:

In the case of the upper fan and the middle fan position, inde-

pendent controls that affected soil CO2 flux level included VWC

and OC, whereas a combination of variables of CaCO3, SICS, pH,

and VWC affected CO2 flux rate in the lower fan (Figure 2). The

regression established that the linear relationship between soil CO2

flux and VWC was positive and significant for all positions. This

implies that the other independent controls are important determi-

nants of heterogeneity in soil C reservoirs and soil CO2 flux within

the three landform positions. Similarly, the positions at the upper

FIGURE 2 Relationships between the soil CO2 flux rate and the independent variables in the microzones of the upper fan (left column), themiddle fan (middle column), and the lower fan (right column) within the alluvial fan, predicted by multiple regression. The r values are definedbased on the partial correlations between soil CO2 flux as dependent variable and soil physiochemical variables as independent controls. p > .05indicates that the independent variable has no significant impact on the soil CO2 flux. Organic C, organic carbon; VWC, volumetric water content

MOHSENI ET AL. 1705

and middle elevation indicated that the linear relationship between

soil CO2 flux and SOC content is positive and significant

(Figure 2). Contrary to expectation, despite the development of moss

biological crusts on the lower fan soils, SOC content was not a var-

iable impacting CO2 flux in the lower fan. The regression results

showed the highly significant negative linear relationships of CaCO3,

SIC storage, and pH, with CO2 flux in the lower fan soils. The results

of the Pearson correlation analysis explain how these independent

variables, according to the regression analysis, affect soil CO2 flux

level within landform (Figure 3). As shown in Figure 3, the lower

fan sediment indicated that the correlation between inorganic and

organic C content is significant and negative, whereas the correlation

of the content of inorganic C with exchangeable cations is significant

and positive.

FIGURE 3 (a) Pearson's correlation between the alkaline cations (Ca+and Mg+) and storage of inorganic carbon in lower fan soils. (b)Correlation between carbonate content and organic carbon content inlower fan soils

1706 MOHSENI ET AL.

4 | DISCUSSION

Despite extensive research on environmental factors affecting the

temporal–spatial variation of CO2 flux in semiarid and arid areas soils

(Ahlström et al., 2015; Delgado‐Baquerizo et al., 2016; Leon et al.,

2014; Maestre et al., 2013; Maestre & Cortina, 2003; Morillas et al.,

2017; Rey et al., 2011), there is no dedicated study showing how

debris flow, as a dominant geomorphic process in many dryland allu-

vial fans characterized by localized variations in biophysical controls,

affects soil C flux dynamics within different landform positions. The

findings displayed the variability of soil C flux within microhabitats

generated by debris flow along alluvial fan. Our measurements of sed-

iment physiochemical properties confirm that the heterogeneous

redistribution of debris flow‐related resources along different slope

positions has an important role in the localized distributions of soil

inorganic and organic carbon stimulating local‐scale variability in the

storage of C and CO2 flux.

One of the key properties of the study debris flow fan was hetero-

geneity in the spatial distribution of biological soil crusts (mainly short

mosses). Notwithstanding the small spatial intervals of distribution of

the classified positions, the crusts only extended significantly on the

lower fan sediments (Figure 4), as confirmed in the field survey. There

was no evidence of the distribution of the crusts in the positions with

higher elevation and steeper slopes. In the lower fan position, the

topographic conditions (i.e., less slope in comparison with other

positions) provided a stable environment for increasing infiltration of

water into soil. On the other hand, a significant accumulation of fine

fractions increased the water holding capacity (PSD and VWC in

Table 1). The condition that can stimulate the development of short

mosses in this position. The absence of these physical conditions (fine

fractions of earth and soil moisture) at other positions does not sup-

port the formation of the biological crusts for the upper fan and the

middle fan. These asymmetric biophysical conditions explain the sig-

nificant differences in chemical patterns and soil nutrients between

the positions (increased STC storage, TN, TP, and multiple macronutri-

ents such as Ca, Mg, Na, and K in the lower fan position compared

with the other positions).

The results of the multiple regression illustrate clearly how small‐

scale spatial variations of landscape functioning induced by the debris

flow affect the relationship pattern between soil C flux and related

physiochemical factors. The findings highlight significant differences

in the factors impacting on soil CO2 flux over adjacent landform

microzones. Apart from the VWC factor that impact CO2 flux in all

positions, the increased CO2 flux in soils belonging to the upper posi-

tions is significantly positively correlated with SOC content. However,

contrary to the expected outcome, despite the development of biolog-

ical crusts of moss that may facilitate increased SOC and, subse-

quently, increased CO2 flux, the sediments concentrated in the

lower fan position represent the lowest value of soil CO2 flux. This

result contradicts the findings of previous studies that show soil CO2

flux is attributable to a combination of microbial functioning and

SOC content (Baldock 2007; Schmidt et al. 2012). Considering the

extensive development of mosses on the lower fan position, our find-

ings demonstrate that lower rates of CO2 flux in the lower fan soils

cannot be explained by a lack of microbial activity. Rather, soil biogeo-

chemical changes, induced by the interaction of the heterogeneous

redistribution of debris flow sediment and geomorphic factors such

as elevation and slope, account for the local‐scale variations in factors

impacting on soil CO2 flux within the landform.

Decreased soil CO2 flux in the lower fan position demonstrates a

significantly negative relationship to the increased inorganic C concen-

tration and pH, which may be attributed to the alkalinity of the envi-

ronment formed by the lower fan deposition of sediment favoring

the exchange of SOC to carbonate production (Thomas, Dougill,

Elliott, & Mairs, 2014). This implies that the fine particle concentra-

tions on the lower fan soils, as well as the CO2 generated by moss res-

piration (or SOC decomposition), bond with alkaline cations such as

Ca, Mg, Na, and K (factors that exhibited the highest values in the

lower fan soils) to contribute toward carbonate production. Addition-

ally, this process may explain the negative correlation between SIC

and SOC content in the lower fan soils (Figure 3), whereas in the

upper positions, no significant relationship exists between these vari-

ables. As this condition caused the SICs/SOCs ratio to increase signif-

icantly in the lower fan soils, consequently, SOC manifested no

significant differences between the three positions. The results illus-

trate the high propensity of lower fan soils for capturing mineral forms

of C, other than the organic forms, due to runoff–runon mechanism

generated by the debris flow (runoff sediment‐laden water from the

FIGURE 4 Typical images from the development of biological soil crusts (mainly short mosses) on sediments belonging to the lower fan followedby accumulation of fine earth fractions related to the heterogeneous redistribution of debris flow's sediment along slope [Colour figure can beviewed at wileyonlinelibrary.com]

MOHSENI ET AL. 1707

upper positions and runon onto of the lower positions) encouraging

the emergence of ‘hot spots’ of soil C storage within the alluvial fan

than the upper fan and the middle fan position as ‘hot spots’ of CO2

emissions from soil to atmosphere.

Previous studies demonstrated that soil water content is the pre-

dominant source of soil CO2 flux, due to the impacts of soil moisture

in increasing metabolic activity (Feng et al., 2014; Leon et al., 2014;

Maestre & Cortina, 2003). Conversely, our results show how a geo-

morphic disturbance, with local scale variations in the structuring

and functioning of landscape, can disturb the soil water content—soil

CO2 flux equation via the occurrence of other regulatory controls at

points with higher moisture content. The condition could stimulate

the emergence of ‘hot spots’ of soil C reservoirs, as opposed to those

with high C emissions of the other positions.

The fragmentation of soil aggregates and subsequent losses of

SOC in the erosional positions can be an inhibitor of plant growth

and biological crusts in those positions (Wei et al., 2016), as noted in

the field survey. Conversely, the deposition of nutrient‐rich sediments,

following heterogeneous resource redistribution along the slope (run-

off mechanism), could increase the proportion of the microbial com-

munity at the depositional position (Huang et al., 2013; Li et al.,

2015; Xiao et al., 2018). Although the microbial abundance and spe-

cies diversity that are influential factors of soil CO2 flux (Xiao et al.,

2018) were not measured in the current study, the significant

enhancement of soil nutrients and the development of biological

crusts that formed only on depositional positions, could be indirect

indicators of greater microbial activity in the lower fan position than

in the higher positions.

Xiao et al. (2018) pointed out that removal of SOC from eroding

positions can lead to the emergence of an environment of OC‐rich

sediment at depositional positions if soil moisture and limited oxygen

is sufficient to inhibit microbial activities. In such conditions, deposi-

tional locations can be a source of SOC storage (Vanden Bygaart,

Gregorich, & Helgason, 2015). However, our findings showed that

despite the increasing microbial activity in lower fan soils facilitating

SOC decomposition and consequently its loss, other edaphic variables

(preferential moisture condition and the presence of alkaline cations)

provided an environment for the exchange of organic C to carbonate

production (i.e., storage of other forms of C).

To date, many studies have focused on the role of soil erosion pro-

cesses in SOC cycling (Smith, Renwick, Buddemeier, & Crossland,

2001; McCarty & Ritchie, 2002; Lal, 2003; Post et al., 2004; Van Oost

et al., 2005; Van Hemelryck, Fiener, Van Oost, Govers, & Merckx,

2010; Van Hemelryck, Govers, Van Oost, & Merckx, 2011; Nadeu,

Gobin, Fiener, Van Wesemael, & Van Oost, 2015; Xiao et al., 2018).

The majority have compared the impacts of soil redistribution on the

dynamic of soil C within the erosional and depositional position along

a hillslope or on a larger landscape scale. However, there is minimal

research on how the heterogeneous soil redistribution induced by

geomorphic processes along the slope gradient of an exclusively depo-

sitional landform can affect the relationship pattern between edaphic

variables and soil C flux at the local scales, causing the simultaneous

1708 MOHSENI ET AL.

emergence of hot spots of sources and sinks of C within different arid

land landform positions.

Although drylands contribute a high proportion of CO2 emissions

from soil to the atmosphere due to large‐scale heterogeneity in the

distribution of vascular plants and surface concentrations of SOC,

some dominant geomorphic disturbances in dryland regions may pro-

vide an opportunity for capturing C via changes in the structure and

functioning of landscapes. Our research reinforces the findings of pre-

vious studies that erosive soil disturbances, such as specific categories

of landslide, result in a net carbon loss from soil to atmosphere in trop-

ical and forest ecosystems (Ramos Scharrón et al., 2012; Schomakers

et al., 2017; Walker & Shiels, 2008). Additionally, our findings highlight

that changes to the biochemical mechanisms by landform structure‐

dependent geomorphic disturbances, such as debris flows, can exhibit

paradoxical impacts on soil C cycling, via the simultaneous emergence

of ‘hot spots’ of soil C reservoir and CO2 flux in adjacent landform

positions. It can be assumed that if the relationship of CO2 flux and

biotic–abiotic variables of the soil is consistent throughout the land-

form, erosive soil disturbances would create only a net carbon loss.

5 | CONCLUSIONS

The present study has provided an insight into how different

responses to heterogeneous resource redistribution along an upper‐

to‐lower alluvial fan gradient can cause heterogeneous dynamics of

soil C flux within a depositional landform. The comparison of three

slope positions of an alluvial fan, in terms of the patterns of relation-

ship between soil CO2 flux and the related physicochemical variables,

illustrates that debris flow disturbances can alter the variables control-

ling soil CO2 flux on a local scale causing the simultaneous emergence

of ‘hot spots’ of soil C reservoir and flux. The findings showed how

interactions between extrinsic and intrinsic environmental factors

could provide an opportunity for storage of other forms of C (carbon-

ate production) in lower fan position, despite a high CO2 flux due to

the higher microbial activity, creating ‘hot spots’ of C pools, as

opposed to other slope positions.

ACKNOWLEDGMENTS

The work was supported by Grant 2‐46534 from the Ferdowsi Uni-

versity of Mashhad.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ORCID

Neda Mohseni https://orcid.org/0000-0003-0691-9408

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