Blue carbon storage variability in a hyper-arid evaporitic environment: A comparative study of inland and coastal sabkhas ecosystems

1. Introduction

Salt marshes are among the most productive coastal habitats and serve as significant blue carbon sinks by capturing atmospheric CO₂ in biomass and sediments (Kang et al. 2024; Maxwell et al. 2024; Sarika and Zikos 2021; McLeod et al. 2011). Their high primary productivity and slow organic matter decomposition of carbon sequestration rates that surpass those of most terrestrial forests. In coastal sabkhas, elevated carbon accumulation and low methane emissions further enhance their storage capacity (Al Disi et al. 2023; Neubauer and Megonigal 2021; Drake et al. 2015).

McLeod et al. (2011) outlined a detailed framework for understanding how coastal ecosystems, particularly salt marshes, mangroves, and seagrasses, contribute to global carbon storage, emphasizing the urgent need to conserve these habitats for climate regulation. Morris et al. (2012) assessed the amount of carbon sequestered in various coastal wetlands, concluding that significant amounts of carbon can be sequestered over long-time scales. Kelleway et al. (2016) investigated the sediment characteristics that influence carbon retention in southeastern Australian salt marshes, finding that grain size was a key determinant, while vegetation cover showed no significant effect. Chmura (2013) emphasized the necessity of comprehensive assessments to ascertain the stability of carbon stores over time in tidal marshes. Similarly, Osland et al. (2013) compared the effects of winter climate shifts on marshes and mangroves in the southeastern coast of the United States, revealing the resilience of marshes in colder climates and their notable carbon storage potential in areas with dense vegetation.

Quantifying carbon stored in tidal and non-tidal marsh soils and vegetation is essential both for effective ecosystem management and for assessing their role in climate change mitigation (Maxwell et al. 2024; Hengl et al. 2017; Granek et al. 2010; Chmura et al. 2003). While localized studies have documented SOC in tidal marshes and mangrove soils, such as in Saudi Arabia (Eid et al. 2019; Cusack et al. 2018) and Egypt (Youssef et al. 2024; Shaltout et al. 2020), there remains a lack of large-scale analyses that integrate extensive field data, especially for arid inland marshes like those in Saudi Arabia (El-Sheikh et al. 2018). Without such data, researchers and practitioners must rely on resource-intensive field sampling or on generalized global estimates that may not reflect the specific conditions of these ecosystems.

In Saudi Arabia, particularly in the inland and eastern coastal regions, salt marshes are common features of the landscape. These environments experience extreme climatic stress, which strongly shapes their ecological processes, yet studies on their carbon storage remain limited. For example, salt marshes along the western Arabian Gulf have been found to store an average of 81 Mg C/ha (Cusack et al. 2018). However, much of the carbon research in Saudi Arabia has concentrated on mangroves. In one study, carbon sequestration was compared between rehabilitated mangroves in Yanbu and those grown without intervention, with significant storage observed in all sites (Al-Guwaiz et al. 2021). Mangrove biomass carbon stocks along the coast of the Red Sea vary between 87.6 to 412.5 Mg C/ha (Shaltout et al. 2021), with soil carbon levels varying by species (Eid et al. 2020), indicating the influence of plant species and vegetative cover on storage potential.

In salt marshes, vegetation predominantly comprising halophytic plants plays a crucial role in stabilizing sediments, minimizing erosion, and promoting organic matter build-up. Although numerous studies have addressed the role of vegetation in enhancing the carbon storage capacity of sabkhas, few have investigated the sequestration potential of coastal salt marshes, and no work has compared inland and coastal marshes directly. Such a comparative study can illuminate the environmental drivers, ecological adaptations, and biogeochemical processes that shape these habitats, as well as their unique biodiversity and hydrological dynamics. Insights gained are valuable for environmental monitoring, sustainable resource management, and climate change impact assessments.

This study addresses this gap by assessing soil and plant carbon stocks in hyper-arid inland and coastal sabkhas of Saudi Arabia, investigating biogeochemical drivers of spatial variability, and identifying the main factors influencing blue carbon storage in these unique ecosystems.

2. Materials and Methods

2.1 Study site

The study area included different salt marshes (sabkhas) in the inland and coastal eastern regions of Saudi Arabia. The first study sites are situated in the central region’s inland sabkhas of Saudi Arabia, which are in the northern part of the central Arabian Peninsula. Three inland sabkhas were selected in the central region. Aushazia sabkha is the first site that is situated east of Unayzah, approximately 20 km away (26° 07’ 55” N, 44° 16’ 34” E). The area of the sabkha is approximately 18 km². In general, it extends in a longitudinal direction from northwest to southeast (Fig. 1a). Ghuwaymid sabkha is the second site, located at 26° 17’ 52” N and 44° 17’ 27” E. The sabkha is situated 30 km before it reaches the city of Buraidah on the Riyadh-Qassim main road (Fig. 1b). Al Qasab salt marsh (25° 18’ 47” N, 45° 39’ 01” E) in Shaqra Governorate (Fig. 1c) is located 150 km northwest of Riyadh city. It extends in a northerly direction toward Al-Nafud Al-Gharbi (Ariq Al-Buldan) and reaches eastward to the road. Al Qasab sabkha is characterized by the presence of salt-tolerant flowering plants.

Close

Fig. 1.

Map illustrating the study locations and sampled stands. Insets show the six sabkha sites: (a) Aushazia, (b) Ghuwaymid, (c) Al Qasab, (d) Asfar Lake, (e) Al Uqayr, and (f) Al Dnan. Colored dots mark sampling stands in each area.

Export to PPT

The second area is situated along the eastern coast of Saudi Arabia, bound by Dhahran to the north, the Arabian Gulf to the east, Al-Ghuwaybah village to the south, and the Riyadh region to the west (Fig. 1). The first coastal Sabkha site is Asfar Lake, which is approximately 20.8 km² in area and is situated in the Javora Sands area (Fig. 1d). The second coastal Sabkha site, Al Uqayr Sabkha (Fig. 1e), is situated near the Arabian Gulf and occupies an area of 10 km². Al Dnan Sabkha is the third coastal site, located at 25° 50’ 30.1\” N and 50° 05’ 30\” E (Fig. 1f). It is in the Dammam region and stretches along the Arabian Gulf coast between the governorates of Dammam and Abqaiq.

There are six study sites, ‘Sabkhas,’ in all. At each studied site, four stands were selected to collect soil and plant samples (Figs. 1a-f). From each of the four selected stands in one region, one stand without any vegetative cover was selected as a control to examine the differences in the levels of carbon sequestration in the presence or absence of vegetative cover, i.e., the total number of stands was 6 sites × 4 stands = 24 stands, with each stand area measuring 50 m × 50 m. The coordinates of the sampling stands were recorded (Table 1) and drawn on a geographical map.

Table 1. The geographical coordinates of the 24 stands (50×50 m) that were sampled across the six studied salt marshes, with notes on the characteristics of each stand.

Stand no.	Salt marsh	Latitude	Longitude
Inland Sabkhas of the Central region
1	Aushazia	26° 03’ 44.6” N	44° 08’ 28.1” E
2	Aushazia	26° 03’ 16.8” N	44° 08’ 26.2” E
3	Aushazia	26° 03’ 59.2” N	44° 08’ 16.9” E
4	Aushazia	26° 03’ 39.6” N	44° 09’ 42.9” E
5	Ghuwaymid	26° 08’ 45.2” N	44° 12’ 10.2” E
6	Ghuwaymid	26° 08’ 29.1” N	44° 12’ 39.2” E
7	Ghuwaymid	26° 08’ 59.2” N	44° 13’ 45.6” E
8	Ghuwaymid	26° 08’ 36.2” N	44° 13’ 55.2” E
9	Al Qasab	25° 16’31.0” N	45° 31’ 27.7” E
10	Al Qasab	25° 16’ 35.5” N	45° 31’ 32.4” E
11	Al Qasab	25° 16’ 34.5” N	45° 31’ 24.1” E
12	Al Qasab	25° 16’ 28.5” N	45° 31’ 21.6” E
Coastal sabkhas of the Eastern region
13	Asfar Lake	25° 32’19.1” N	49° 46’ 32.3” E
14	Asfar Lake	25° 33’ 07.8” N	49° 46’ 25.1” E
15	Asfar Lake	25° 33’ 28.4” N	49° 46’ 53.5” E
16	Asfar Lake	25° 33’ 04.7” N	49° 47’ 20.5” E
17	Al Uqayr	25° 38’ 03.8” N	50° 12’ 41.2” E
18	Al Uqayr	25° 37’ 30.3” N	50° 12’ 48.3” E
19	Al Uqayr	25° 38’ 10.9” N	50° 12’ 29.3” E
20	Al Uqayr	25° 38’ 18.1” N	50° 12’ 22.0” E
21	Al Dnan	26° 02’ 15.1” N	49° 59’ 59.0” E
22	Al Dnan	26° 02’ 11.4” N	49° 59’ 49.2” E
23	Al Dnan	26° 02’ 54.2” N	49° 59’ 48.7” E
24	Al Dnan	26° 58’ 55.3” N	50° 00’ 08.4” E

3. Results

3.1 SBD analysis

SBD varied notably across the six salt marshes of inland central and coastal eastern Saudi Arabia. Values ranged between 0.6 and 1.5 g cm⁻³ depending on depth and location (Fig. 2). Two-way ANOVA indicated that both site (P ≤ 0.001) and depth (P ≤ 0.05) significantly influenced SBD, while their interaction was not significant. Among the sites, Aushazia exhibited the lowest average density (0.89 g cm⁻³), followed by Al Uqayr, whereas Al Dnan and Al Qasab recorded the highest values (1.24 and 1.22 g/cm³, respectively). Comparisons between inland and coastal sabkhas showed no significant differences in SBD (t = –0.72, P = 0.47).

Close

Fig. 2.

Mean of SBD across depth profiles in six sabkhas. Error bars = SE (N = 4). ANOVA significance: *** P ≤ 0.001; * P ≤ 0.05; ns = not significant.

Export to PPT

3.2 SOC concentration

SOC concentrations also differed significantly among sites (P ≤ 0.001; Fig. 3), ranging from 0 to 60 g C/kg. In contrast to SBD, neither depth nor the site × depth interaction influenced SOC. Aushazia had the highest mean SOC (43.62 g C/kg), while Al Dnan recorded the lowest (5.15 g C/kg). Inland sabkhas (58.51 g C/kg) contained significantly higher SOC levels than coastal sites (26.41 g C/kg; t = –8.22, P ≤ 0.001).

Close

Fig. 3.

Depth-dependent distribution of mean SOC across study areas. Error bars = SE (N = 4). Significance codes as in Fig. 2.

Export to PPT

3.3 SBD and SOC relationship

Nonlinear regression was used to assess the relation between SBD and SOC (Fig. 4, Table 2). Overall, the associations were weak in most sabkhas, with significant correlations observed only at Al Uqayr (R² = 0.315, P ≤ 0.01) and Al Dnan (R² = 0.387, P ≤ 0.01), both coastal sites. The regression results highlight site-specific variability in this relationship.

Close

Fig. 4.

Nonlinear relationship between SBD (Y-axis) and SOC (X-axis) concentration from 120 soil samples collected from 24 stands (four per Sabkha).

Export to PPT

Table 2. Nonlinear SBD-SOC relationships across the six sabkha sites.

Salt marsh	Equation	R2	Significance 1
Aushazia	y = 0.8994e-6E-04x	0.0008	ns
Ghuwaymid	y = 1.0978e-0.004x	0.0805	ns
Al Qasab	y = 1.192e0.001x	0.0088	ns
Asfar Lake	y = 1.0376e-0.01x	0.1334	ns
Al Uqayr	y = 1.071e-0.01x	0.3148	**
Al Dnan	y = 1.3079e-0.012x	0.3868	**

¹ ns: not significant, **: P ≤ 0.01.

3.5 Comparison of carbon sequestration between inland and coastal sabkhas

Comparison of inland and coastal sabkhas showed higher carbon sequestration in inland sites for both SOC and POC (Table 4). Inland sabkhas stored 5.80 t C/ha in total, compared with 2.81 t C/ha in coastal ones. In both ecosystems, vegetation cover stored more carbon than the soil.

Table 4. Comparison of soil and plant carbon stocks between inland and coastal sabkhas.

Location	Inland sabkhas	Coastal sabkhas	t 1	P value
Stock of SOC (t C/ha)	0.73 ± 0.07 a	0.29 ± 0.04 b	-2.98	0.041
Stock of POC (t C/ha)	5.07 ± 0.85 a	2.52 ± 0.15 b	-2.97	0.007

¹ t-values are from two-sample Student’s t-tests, with marsh location (inland vs. coastal) as the factor. Values represent means of 12 replicates ± standard error. Different letters within the same row denote significant differences at P ≤ 0.05.

PCA (Fig. 5) further distinguished inland from coastal marshes based on environmental and/or biological variables. The first principal component (PC1), which explains 69.8% of the total variance, largely separates the inland (cyan) and coastal (red) groups, indicating substantial carbon sequestration capacity differences between these localities. The second principal component (PC2) accounts for 28.3% of the variance, adding additional separation within each locality type. The key environmental variables affecting this separation are SOC, SOM, and density, which are comparatively elevated in coastal regions, as seen by the direction and length of their respective vectors. The aggregation of inland regions, including Aushazia and Ghuwaymid, adjacent to the vector for SOC and SOM, indicates that organic matter content is crucial in these ecosystems. In contrast, coastal regions (i.e., Asfar Lake, Al Uqayr, and Al Dnan) are more significantly affected by various soil or habitat attributes. The convergence of specific locations, such as Al Qasab, on PC1 and PC2 indicates transitional zones or variability in environmental conditions within these areas.

Close

Fig. 5.

PCA biplot separating inland and coastal sabkhas. AUS = Aushazia, GHW = Ghuwaymid, KAS = Al Qasab, ASF = Asfar Lake, UQR = Al Uqayr, AND = Al Dnan.

Export to PPT

4. Discussion

The results of this study highlight the notable spatial variability in SBD and SOC levels across salt marshes of central and eastern Saudi Arabia. The significant influence of site-specific factors on both SBD and SOC concentration underscores the heterogeneity of these ecosystems. These results suggest that differences in soil texture, hydrological regimes, and plant communities strongly influence soil properties and their capacity to store carbon, as has also been observed in other studies of coastal wetlands (Duarte et al. 2005; Ouyang and Lee 2014; Ouyang and Lee 2020).

Interestingly, inland sabkhas have shown a greater capacity for carbon sequestration in sediments and plants than coastal sabkhas. The inland sabkhas excel in carbon sequestration mainly because of their high plant productivity, slow rate of decomposition, and minimal physical disturbance, allowing carbon to accumulate more when compared to coastal sabkhas. This agrees with studies showing that vegetation traits such as plant species diversity, biomass production, and species composition can strongly regulate soil carbon cycling (Chmura et al. 2003; McLeod et al. 2011). Conversely, the relatively young vegetation in coastal sabkhas may explain their lower carbon storage, as immature plant stands may not yet have significantly altered soil properties (cf. Al-Guwaiz et al., 2021). On the other hand, the observed depth-related variations in the SBD indicate that soil compaction and density increase with depth, which is a common pattern in many wetland ecosystems (Craft et al. 1991). However, the lack of a significant depth effect on SOC suggests that the organic carbon distribution is more influenced by surface inputs and microbial activity than by soil compaction.

The nonlinear correlation analysis between the SBD and SOC concentration revealed weak relationships at most sites, except for coastal Al Uqayr and Al Dnan. These significant correlations suggest that under certain conditions, denser soils might be associated with higher carbon concentrations, potentially due to the compaction of organic material and reduced decomposition rates. However, the low R-squared values imply that additional, unmeasured environmental factors likely influence SOC concentration. Comparable findings have been reported in mangroves and similar semi-aquatic ecosystems (Al-Guwaiz et al. 2021; Eid et al. 2016; Eid et al. 2020; Shaltout et al. 2021)

PCA biplot highlights the ecological differentiation between coastal and inland localities, driven primarily by soil- and habitat-related factors. Coastal localities are characterized by relatively high organic matter contents (SOC and SOM), which likely reflects the influence of proximity to marine systems, increased vegetative productivity, or deposition processes. These factors may promote nutrient-rich soils conducive to supporting diverse or specialized biological communities. The clear segregation along PC1 suggests strong environmental gradients that influence the carbon sequestration potential at the study sites. The clustering of sites within locality types also points to consistency in environmental conditions within these regions. However, the partial overlap of some sites suggests the presence of ecotones or zones of transition, which may be of particular interest for further ecological studies. Understanding the drivers of such transitional zones could provide insights into ecosystem connectivity and resilience under changing environmental conditions. Future studies should concentrate on measuring the specific contributions of these variables to biological diversity and investigating how these patterns are maintained or altered under anthropogenic pressures or climate change scenarios.

The overall higher carbon stocks in marshes such as inland Aushazia highlight the importance of site-specific characteristics in determining carbon sequestration potential. Factors such as soil type, water salinity, and tidal influence could all contribute to the observed differences. Such site-specific variability is widely acknowledged in salt marshes globally (Chmura 2013; Chmura et al. 2003; McLeod et al. 2011).

This study was limited by a restricted number of sampling locations (six sites-three from inland and three from coastal sabkhas). Nonetheless, the chosen sites were representative of the key characteristics of both inland and coastal sabkhas in the region, ensuring that essential biogeochemical processes were documented despite the small sample size. Another constraint is the lack of direct comparisons with other sabkhas in arid regions, which is due to the lack of published data on carbon sequestration in similar environments. Despite these limitations, the present study provides significant data on carbon sequestration in under-researched arid sabkhas, emphasizing their potential contribution to regional carbon budgets.

Future studies should emphasize long-term monitoring of plant cover and community dynamics to capture temporal trends in C-sequestration. More detailed assessments of plant diversity, hydrological patterns, and soil chemistry are also needed to clarify the mechanisms controlling carbon storage. Such work is essential for developing management strategies that enhance the C-sequestration potential of Saudi salt marshes and other arid-region wetlands.

Overall, these results highlight the urgent need for conservation strategies tailored to hyper-arid salt marshes. Since soil carbon storage is shaped by a combination of soil characteristics, vegetation, climate, and land use, integrated management that prioritizes high-performing sites could significantly strengthen regional carbon budgets and contribute to climate mitigation.

5. Conclusion

This study highlights the significant role of salt marshes in inland central and coastal eastern Saudi Arabia in terms of carbon sequestration. It also reveals considerable spatial variability in the SBD, SOC concentration, SOC stock, and POC stock across different sites. Inland Aushazia has emerged as the most effective carbon sink, with the highest SOC concentration, SOC stock, and POC stock, whereas coastal Al Dnan has the lowest values. These findings indicate that local environmental conditions and site-specific factors predominantly influence carbon sequestration potential. The complex interplay between soil properties and site characteristics necessitates tailored conservation and management strategies to optimize carbon storage in these ecosystems. Future research should prioritize long-term monitoring and a more comprehensive investigation of the interactions between vegetative cover, soil properties, and hydrological conditions. This information will facilitate a deeper understanding of and enhance the carbon storage potential of Sabkhas in these arid regions. These insights are crucial for informing effective conservation practices aimed at mitigating climate change through improved carbon management in salt marshes.