Inorganic carbon sequestration and its mechanism of coastal saline-alkali wetlands in Jiaozhou Bay, China

Geoderma 351 (2019) 221–234

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Inorganic carbon sequestration and its mechanism of coastal saline-alkali wetlands in Jiaozhou Bay, China Xiaotong Wang, Zhixiang Jiang, Yue Li, Fanlong Kong , Min Xi ⁎

T

⁎

College of Environmental Science and Engineering, Qingdao University, No. 308 Ningxia Road, Qingdao 266071, China

ARTICLE INFO

ABSTRACT

Handling Editor: Junhong Bai

The problem of “missing sink” in the carbon cycle has not been properly explained. To reveal the effects of coastal saline-alkali wetlands on global inorganic carbon sequestration, soil samples with different salinity and alkalinity were collected in Jiaozhou Bay coastal wetlands, and the main work were: split the soil CO2 flux through inactivation treatment; simulate the CO2 absorption process of saline alkali soils; and analyze the mechanism of the above process through stable isotopic labeling. The inorganic carbon sequestration process is common in coastal saline-alkali soil. The increasing salinity and alkalinity of soil can improve its CO2 uptake capacity, and the soil alkalinity showed greater effects on the capacity than soil salinity. The leaching/absorption of atmosphere CO2 and the dissolution/precipitation of carbonate system are confirmed as the two mechanisms of inorganic carbon sequestration. The former mechanism dominates the sequestration process in coastal wetlands. We conclude that the abundant water sources and the high alkalinity of soils contribute to the advantages of carbon sequestration processes in coastal saline-alkali wetlands.

Keywords: Inorganic carbon sink Coastal wetland Saline-alkaline soil Simulated leaching Stable isotopic labeling

1. Introduction The increase of atmospheric CO2 is widely considered to be the primary cause of global warming after the industrial revolution. Regarding to the studies of global carbon (C) balance and estimates, it has been found that nearly 20% of the CO2 absorption are unaccounted, which is known as the “missing sink” in the global change and C cycle field (Boucot and Gray, 2001; Goddéris et al., 2012; Zeng et al., 2017). Researches in forests, farmlands, oceans and other areas indicate that the C exchange between soil and atmospheric CO2 through leaching, adsorption, or other ways may be a potential pathway to find this mystery of missing C sink (Pacala et al., 2001; Zhao et al., 2006; Hopkinson et al., 2012). Soil CO2 flux (Rs) is an integrated value, which include the soil organic CO2 flux (Ro) and the soil inorganic CO2 flux (Rio) (Raich and Schlesinger, 1992; Rayment and Jarvis, 2000). Unfortunately, the latter process was not paid attentions sufficiently for a long time (Casals et al., 2000; Acosta et al., 2018). In recent years, it was found that a large amount of CO2 could be adsorbed by saline-alkali land and halophyte desert in abiotic ways (Jasoni et al., 2005; Chapin et al., 2006; Stone, 2008; Wohlfahrt et al., 2008), and the C exchange between saline-alkali soil and atmospheric CO2 through abiotic process has been concerned and discussed widely. But the relevant reports have

been questioned by some scientists to be lacking reliable mechanism analysis and model support (Schlesinger et al., 2009). Saline-alkali soil is widely distributed around the earth, occupying about 7% of the total land area (Zhao et al., 2006; Wohlfahrt et al., 2008). At present, most studies of inorganic C sinks in saline-alkali soil were conducted in desert areas, while few studies were carried out in similar soil of meadow, coastal wetland and inland arid ecosystem (Li et al., 2005; Xie et al., 2008; Ma et al., 2013; Faimon and Lang, 2013). Coastal saline-alkali wetlands have both aquatic and terrestrial characteristics, and the special geographical location makes it become an important link among the three state C cycles of solid, liquid and gas (Livesley and Andrusiak, 2012; Zhao et al., 2017). It had been found that the CO2 could be adsorbed by coastal saline-alkali soil in abiotic process (Han et al., 2014; Taillardat et al., 2018), but the evidences to support this process are limited. The underlying mechanisms still remain unclear. Overviewing previous studies, three potential mechanisms may contribute to the inorganic C sequestration in saline-alkali soil: (i) dissolution/precipitation process of carbonate system (Stone, 2008; Zeng et al., 2017); (ii) leaching/absorption of atmospheric CO2 (Kowalski et al., 2008; Serrano-Ortiz et al., 2010); (iii) underground hole ventilation process (Kowalczk and Froelich, 2010; Sanchez-Cañete

Abbreviations: C, carbon; Rs, soil CO2 flux; Ro, soil organic CO2 flux; Rio, soil inorganic CO2 flux; pCO2, CO2 partial pressure; St, salt content; ESP, exchange sodium percentage; SAR, sodium adsorption ratio ⁎ Corresponding authors. E-mail addresses: [email protected] (F. Kong), [email protected] (M. Xi). https://doi.org/10.1016/j.geoderma.2019.05.027 Received 16 January 2019; Received in revised form 15 April 2019; Accepted 16 May 2019 Available online 31 May 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.

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et al., 2011; Lang et al., 2017; López-Ballesteros et al., 2017). Disputes on which mechanism may dominate the inorganic C sequestration have been going on, but it's a consensus that the leaching was an important part of the absorption process. In this study, we aim to confirm the existence of the inorganic C sequestration in the coastal saline-alkali wetlands and reveal its mechanism(s), which may contribute to the cognitions of coastal wetland abiotic C cycle. Thus, a laboratory simulated leaching experiment was conducted, in order to (a) verify whether the negative flux of abiotic CO2 exists in saline-alkali soil of coastal wetlands; (b) explore the effects of soil salinity and alkalinity on inorganic C sequestration process and (c) quantify the changes of C form and content during CO2 inorganic absorption process.

conditions of wetland and the distribution of vegetation in January 2018 to acquire the samples with different salinity and alkalinity (Fig. 1). Area 1 was located in the Dagu River estuary where the soil exhibits high alkalinity due to the presence of a large number of aquaculture ponds and the few growth of plants. Area 2 was located in the Yanghe estuary where the salinity and alkalinity of soil are lower than other two areas due to the S. alterniflora invasion. Area 3 was located in the middle and lower reaches of the Yanghe River where the water and salt gradient of soil are relatively high due to human activities. Three sampling sites were set in each sampling area, and the point-centered quarter method was used to obtain equivalent soil samples at a depth of 30 cm at each sampling site.

2. Materials and methods

2.3. Simulation of inorganic C sequestration process

2.1. Study area

2.3.1. Separation of Rs The Rs is defined as the sum of Ro and Rio. In order to separate the Rio from the Rs, the method of high pressure sterilization was adopted, which has been frequently used in similar studies (Xie et al., 2008; W. Wang et al., 2013). Soil samples were ground, air-dried, and sieved (20-mesh) to remove gravel and roots, and then placed in metal barrels with 40 cm height. The top of the barrels were sealed by layers of filter and brown paper to minimize water infiltrating into the soil. The soils were sterilized in a medical autoclave for 24 h at 120 °C. After sterilization, the soils in barrels were placed in a UV-sterilized room for 7 days to restore soil stability.

Jiaozhou Bay, with an area of nearly 500 km2, is located to the south of Shandong Peninsula, Shandong Province, China (Zang et al., 2018). Jiaozhou Bay's geomorphic type is the estuarine alluvial plain, and the surface slope is north-south with the altitude being high in north and low in south (Gao et al., 2014; K. Zhao et al., 2015). The experimental area was determined at the estuarine wetland of Dagu River and the lower reaches of Yanghe River. With the growing development of the aquaculture, the aquaculture ponds have become the main land use type of the Dagu estuary, and made the soil here be of high salinity and alkalinity. The Yanghe estuary has gradually formed a typical Spartina alterniflora beach since 1970 due to the species invasion, and the salinity and alkalinity of soils are relatively low. The changes of land use or vegetation cover have caused more complex C cycle processes, both for organic and inorganic ways (Lu et al., 2016).

2.3.2. Design of soil salinity and alkalinity All soils from the same sampling area were mixed due to the similarity of salinity and alkalinity. The original soil samples have only 3 salinity gradients, which is far from meeting the requirements of experiment. Therefore, two neutral salts (NaCl and Na2SO4) and two alkali salts (NaHCO3 and Na2CO3) were mixed in various proportions described by Shi and Wang (2005) to acquire different saline solutions

2.2. Sampling collection Three sampling areas were selected based on the hydrological

Fig. 1. Location map of sampling areas. 222

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Table 1 Physicochemical properties of saline-alkali soils and type classification. Soil number

pH

Conductivity (mS·cm−1)

ESP (%)

St (g·kg−1)

Soil type

Na-Ns Na-Ls Na-Ss La-Ns La-Ls La-Ms Ma-Ss Sa-Ms Sa-Ss

7.15 6.99 7.02 7.79 7.7 8.09 9.32 10.39 10.48

0.59 3.57 11.69 1.27 5.69 6.73 12.14 7.39 11.53

2.11 4.59 4.25 6.82 6.97 8.41 11.27 17.13 16.97

4.5 7.99 17.06 4.74 8.35 13.71 17.84 11.49 19.05

Non-alkalization-non-salinization Non-alkalization-low salinization Non-alkalization-severe salinization Low alkalization-non-salinization Low alkalization-low salinization Low alkalization-moderate salinization Moderate alkalization-severe salinization Severe alkalization-moderate salinization Severe alkalization-severe salinization

with five levels of pH (Table A.1) (Li et al., 2010; Y. Wang et al., 2013). Soaking three disinfected soils in these five saline solutions for 24 h would theoretically produce 15 soils of different salinity and alkalinity. After dried, the pH, conductivity, salt content (St), exchange sodium percentage (ESP), salinity and alkalinity of soils were re-determined, and the soils with similar salinity and alkalinity were mixed. According to the standards of soil salinization and alkalization grade classification (Table A.2) (Liu et al., 2012; Wang et al., 2015), nine kinds of salinealkali soils were finally obtained. Physical and chemical properties of the nine soils were listed in Table 1.

2.3.4. Isotope labeling experiment Based on the simulated leaching experiment, the CO2 in the air of the leaching column is replaced by isotope labeled CO2 to quantify the amount of atmospheric CO2 that be absorbed by soil. CO2 isotope labeling process is as follows: The gas in the sealed leaching column is circulated through a filter to absorb the background CO2. When the background CO2 concentration dropped to about 1 ppm, the filter is removed and the CO2 generator is connected. 13C-CO2 was produced by the H2SO4 (1 mol·L−1) dropping into 13C-Na2CO3 (99%) slowly and uniformly. Remove the CO2 generator when the 13CO2 concentration in the sealed column reached approximately 1200 ppm, and the C absorption process was simulated at 12 h after each mark. The CO2 concentration in the sealed column was monitored in real time by portable infra-red gas analyzer. At the same time, the control group was set up to determine the natural abundance of soil 13C.

2.3.3. Simulation of atmosphere CO2 inorganic absorption process The sterilized soils were loaded into a sealed plexiglass column, and the deionized water (used as the leaching solution) was added through the leaching tube with a constant rate (Fig. A.1). Quartz sand was added in soils to enhance the porosity of the soil and facilitate leaching. The leaching process started as soon as the liquid leached out from the outlet pipe. Each leaching stage lasted for 1 h, twice a day for two days, thus total of four times. During the leaching process, the CO2 partial pressure (pCO2) of the air above the soil in the leaching column was measured by portable infra-red gas analyzer (model GXH-3011A) in real-time. During the measurement, the tightness of the device was ensured. 200 mL of leachate was collected for each leaching and stored in a volumetric bottle for later analysis. After each leaching, part of the soil was sampled from the leached soil of the sealed column and stored for later analysis. The CO2 flux can be used to reflect the rate of uptake or release of CO2 gas of soil, which can be calculated by the difference of CO2 content in the observation box per unit area per unit time. The positive value indicates that the system releases CO2 gas into the atmosphere, and the negative value indicates that the system absorbs CO2 gas from the atmosphere. The calculation of Rio is provided as follows:

F=

dc M P T × × × 0 ×H dt V0 P0 T

2.4. Analytical methods 2.4.1. Ion content Soil samples were air dried, then grinded and sieved through a 2 mm mesh for chemical analysis, including pH, conductivity, St, CaCO3 content and basic ion concentration (CO32−, HCO3−, Cl−, SO42−, Ca2+, Mg2+, K+, Na+). All tested samples were prepared by 1:5 soil water extracts according to the soil analysis methods described by Lu (2000). CO32−and HCO3− were measured by double indicator titration; Ca2+ and Mg2+ were measured by EDTA complexometric titration; K+ and Na+ were measured by flame photometry; Cl− and SO42− were measured by the methods of standard AgNO3 titration and EDTA indirect titration separately; Soil pH and conductivity were determined by a Sartorius PP-20 Professional Meter (Sartorius, Germany); St was calculated by the method of ion summation; CaCO3 was measured by gas quantity method. Total alkalinity is mainly related to the content of weak acid ions in soil, thus the contents of CO32−and HCO3− in this study was used to represent the total alkalinity of soil (Zhang et al., 2017; Li et al., 2018). ESP was calculated by empirical formula, in which the sodium adsorption ratio (SAR) was calculated by equivalent formula (Zhang et al., 2010). The formulas are provided as follows:

(1) −2 −1

where the F denotes Rio (μmol·m ·s ); the dc/dt denotes the slope of the two degree curve when the gas concentration varies with time during leaching; the M, P and P0, T and T0, V0 and H respectively denote the molar mass of CO2 (g·mol−1), the pressure of sampling point and standard atmospheric pressure (Pa), the absolute temperature at sampling time and standard temperature (K), the molar volumes of CO2 (mL·mol−1) and the height of sampling box (m). To rule out the possibility of instrumental error and make sure the negative fluxes are real, the instrument and measuring procedure were checked carefully by using inert quartz sand as the soil matrix to verify the flux, because the fluxes are always close to zero in the pure dry quartz sand condition. Result showed that a small CO2 flux rates ( ± 0.1 μmol·m−2·s−1) were detected when quartz sand was leached by distilled water. In contrast, the CO2 flux rates of approximately ± 1.3 μmol·m−2·s−1 were observed when quartz sand was leached by leachate of soil La-Ls. Hence, the negligible variations of CO2 flux for the quartz sand alone attest the instrument was reliable.

ESP = 100 ×

SAR =

0.0126 + 0.01475SAR 1 + ( 0.0126 + 0.01475SAR)

(2)

Na+

(

Ca2 + + Mg2 + 2

)

1 2

(3)

Dissolved inorganic carbon (DIC) consists of H2CO3, HCO3−, CO32− and dissolving CO2, of which HCO3− and CO32− account for about 99%. Therefore, the sum of HCO3− and CO32− is used to represent DIC in this paper. The proportion of these DIC components depends on the pH value. If pH ranges from 7 to 9, 95% of the DIC exists as HCO3−. If pH is higher than 9, CO32− content rose and HCO3− content dropped sharply, the CO32− dominant DIC components (M. Zhao et al., 2015).

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Fig. 2. Hydro-chemical index of the original saline-alkali soils in Jiaozhou Bay. 16 points in different shapes and colors represents 16 original soil samples in coastal wetland of Jiaozhou Bay. The coordinate axis indicates the percentage of ion(s). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.4.2. Analyses of C stable isotope ratios (δ13C) Stable isotopic compositions of inorganic matter were determined using Isotope Ratio Mass Spectrometer (Delta V Advantage), and the analytical precision was 0.05‰. The stable isotope ratios were expressed in delta notation in per mil (‰) unit (relative to an international standard) using the standard δ notation:

X (‰) =

Rsample Rreference

1 × 1000

3. Results 3.1. Basic physicochemical properties of the soil The major cations of original soil samples that were collected in Jiaozhou Bay were Na+ and K+, followed by Ca2+ and Mg2+ (Fig. 2). The major anion was Cl−, followed by SO42− and HCO3− (Fig. 2). Jiaozhou Bay wetland soil can be classified into the coastal sulfatechloride or chloride saline soil according to the salinity. The total alkalinities ranged from 0.68 to 0.99 g·kg−1. The ratios of HCO3− to total amount of anions in all soil samples were > 0.5%, and it was > 1% for about 91.67% samples. These data testified that the tested soils belonged to soda-alkali soil according to the alkalinity. The correlation analyses were used to understand the accumulation characteristics of soil compositions (Table A.3). St was significantly correlated with all measured soluble ions. There was a significantly negative correlation between soil pH and soil soluble ions except Ca2+ and HCO3−. Soil ESP was significantly positively correlated with St, K+, Na+ and Cl−, and negatively correlated with pH. In cations, the Na+ showed significantly positive correlations with others, and the correlation with Cl− was the highest. In anions, the Cl− was positively correlated with K+, Mg2+, SO42− and HCO3−. There was a very significantly positive correlation between most of the ions, but Cl− and Ca2+ showed no correlation.

(4)

where, X is the isotopic abundance, and R is the corresponding ratio of δ13C/δ12C. 2.5. Statistical analysis and data drafting The statistical analyses were carried out using SPSS version 22.0 and Origin 9.1. One-way analysis of variance (ANOVA) was used to analysis the difference between different treatments and the multiple comparisons (Dunnett's T3) was used to evaluate it. Pearson correlation was used to reveal the relationships between CO2 inorganic absorptive capacity and other soil composition contents. Origin 9.1 software, Adobe Photoshop CS6 and CorelDRAW12 software were used for drawing.

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3.2. Changes of atmospheric CO2, leachate and leached soil in leaching process

dropped during the first three leaching stages, and rose slightly in the fourth leaching stage, while the content of Ca13CO3 increased during all four leaching stages.

3.2.1. pCO2 and Rio All tested saline-alkali soils showed the capacity of absorbing CO2 from the air through abiotic process (Rio < 0) (Fig. 3). Soil Na-Ls exhibited the smallest capacity of abiotic CO2 uptake, in which the CO2 concentration was reduced by 219 ppm during four leaching stages. Soil Sa-Ss had the largest capacity of abiotic CO2 uptake, in which the CO2 concentration decreased by 680 ppm, about three times higher than soil Na-Ls treatment. All the CO2 in the column of soil Sa-Ss were absorbed after the leaching processes. Compared with non-leaching phase, leaching process could significantly improve the abiotic absorption capacity of CO2 in saline-alkali soil, but the capacity declined during leaching. Although Rio had the positive correlations either with salinity or alkalinity, the effects of these two factors were different (Table A.4). In four salinity groups (non-salinization, low salinization, moderate salinization and severe salinization), the positive correlations between Rio and alkalinity were all detected, while in three alkalinity groups (nonalkalization, low alkalization and severe alkalization), the positive correlations between Rio and salinity were detected only in low alkalization group and severe alkalization group.

4. Discussion 4.1. Changes and factors in leaching process Previous studies have not yet reached a consensus on the potential mechanism of inorganic C sequestration in saline-alkali soil, but all the conjectures were related to the physical and chemical properties of soil in the region (Mielnick et al., 2005; Xie et al., 2008; Serrano-Ortiz et al., 2010). Changes of each detected index during leaching were influenced by different factors. Because of the differences in initial content, leaching rate and migration ability of the ions, the trends of each component concentration changed in the leached soil were different. During the leaching processes, salt ions in the soil were gradually transferred into the leachate, so the diverse ion content and pH value were detected in the leachates. The concentrations of main ions in seawater are ordered by Cl− > Na+ > SO42− > Mg2+ > Ca2+ > K+ > HCO3− (Gislason et al., 2009). Similar distribution of main ions in texted soil samples indicates that seawater has a greater influence on coastal saline-alkali soil. Soil St content declined rapidly in the beginning of leaching and then slowed down in the later leaching phase, which is consistent with the results in most of previous studies (Chu et al., 2016). Salt ions in soil macro-pores can be easily moved by the water flow. When the leaching occurred, these salt ions can be carried away even by a small amount of leaching water, a rapid decrease of soil St was observed in the initial phase of leaching. In the later phase of leaching, soil salt ions also can be leached continuously, but the leached rates slowed down, which was contributed by the most of residual salt ions remained in soil concentrated in micro-pores. The leached rates were slower in the later phase of leaching, because the salt ions in micro-pores entered the convective zone under the slow water flow condition mainly by dispersion, which was much slower than convection (Tanton et al., 1995). The different trends of soil CaCO3 in different saline-alkaline soils can be explained by the theory of dissolution/precipitation of salinealkali soil carbonate system in coastal wetlands (Mielnick et al., 2005; Zeng et al., 2017). Influenced by tide, the material exchange between ocean and coastal wetlands is frequent, which constitutes a CO2-H2OCaCO3 three-phase unbalanced system. The alkaline property makes the coastal wetlands have the potential to absorb the CO2 through the reaction between carbonate and CO2 with the bicarbonate as product (Ma et al., 2013). However, the soil with low alkalinity could not provide sufficient alkaline conditions for the CaCO3 decomposition process, so the soil CaCO3 rose again after a short decline. Both Ca2+ and Mg2+ contents of leached soils and leachates in nonalkaline soils showed a downward trend, which were consistent with most of the related researches (Verrall et al., 2009; Gabriel et al., 2012). The downward trend was determined by the strength of interaction between ions and soil colloids (Verrall et al., 2009). In the initial phase, salt ions (include Ca2+ and Mg2+) contents of soils were high, thus the salt ions in macro-pores could be easily carried away by leaching water under convection at the beginning of leaching. As the leaching proceeds, the contents of salt ions decreased continuously, but the migration rate of the ions became slowly in later phase because the most of the remained salt ions concentrated in micro-pores. The contents of Ca2+ and Mg2+ after leaching for the high salinity gradient soils went up gradually, which was related to the change of soil CaCO3. The high saline-alkali of soils promoted the dissolution of CaCO3 and MgCO3, so the contents of Ca2+, Mg2+ and DIC increased. One mechanism contributes to the risen DIC in soils during the leaching is the equilibrium of ion exchange adsorption in the soil. Concretely, carbonate minerals begin to dissociate in order to balance the positive and negative charges in the soil, when the anions (such as

3.2.2. Leached soil The influence of soil alkalinity on leached soil is greater than soil salinity, which can be supported by the comparison among the soils in different salinity or alkalinity gradients (Table A.4). The contents of DIC showed the increasing trends in leaching process (Fig. 4). The contents of St in all soils, and the contents of Ca2+, Mg2+ in the soils with low salinity and alkalinity declined during leaching, while Ca2+, Mg2+ in the soils with high salinity and alkalinity rose gradually. The contents of CaCO3 was dropped and then edged up in the soils with low salinity and alkalinity, while that was decreased continuously in the soils with high salinity and alkalinity. The pH of soils remained stable or was slightly increased. The alterations of the measured indices in high alkalinity soil were more drastic than those in low alkalinity soil in the four leaching stages, especially the DIC and CaCO3. 3.2.3. Leachate In the leaching process, the influence of soil alkalinity on leachate is greater than soil salinity (Table A.4). Influenced by leached soil, the significant differences also existed for the changes of leachates during leaching (Fig. 5). The pH of all leachates dropped slightly in the leaching process. The Ca2+ and Mg2+ declined gradually and the change of DIC concentration presents like a “reverse U” in leachates of non-alkaline soils. Concentrations of Ca2+ and Mg2+ in leachates of soils with low alkaline were stable in all four leaching phases while the concentrations in leachates of soils with moderate and severe alkaline edged up. DIC of leachates in all samples consistently increased except in non-alkaline soils. 3.3. Changes of

13

C in leaching process

As a control group of quantitative analysis experiment, a total of 16 kinds of original soil samples were randomly selected to determine the 13 C abundances (Table A.5). The natural 13C abundance of soil in sampling area is −23.14 ± 3.70‰. Soil La-Ls has the moderate salinity and alkalinity in all tested soils and the highest salinity and alkalinity in all original soils of Jiaozhou Bay. Therefore, the soil La-Ls was taken as an example. The variation of CO2, DIC and CaCO3 content and 13 C abundance during four leaching stages (Fig. 6) testified that the soils always absorbed CO2 from air. The decrements of the 13CO2 content were decreased in leaching process, which means that the inorganic absorption capacity of soils was weaken gradually. The content of DIC and DI13C both increased during leaching. The content of CaCO3 225

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Fig. 3. Changes of pCO2 and Rio during leaching. Each soil was leached 4 times, each time for 1 h, record the number every 10 min. In the X-axis, I, II represents 2 counts before leaching, IV to VIII represents the 6 counts during the leaching process.

Fig. 4. Changes of Ca2+, Mg2+, DIC, CaCO3, St contents and pH in leached soils during leaching.

Cl−) were leached out of the soil with the leaching water. With the decreasing anions during the leaching process, more DIC are required to balance the lost charge in the soil correspondingly. Another possible mechanism contributing to the increased DIC is that the precipitation dissolution equilibrium of soil CaCO3 (Oren and Steinberger, 2008). Due to the domination by CaCO3 solubility product, Ca2+ and Mg2+ in soil decreased, which can promote the dissolution of CaCO3 precipitation, and thus more DIC will be produced. In addition, the dissolution of CO2 will also form DIC, which may lead to the similar outcomes (Silver et al., 2000). As in leached soil, the concentration of DIC in most leachates also increased, which is consistent with the results of other previous studies (Li et al., 2016; Wang et al., 2016). It can be inferred

that the increase of DIC in leachate mainly derived from the risen contents of the DIC in the soil. Soil pH, ESP and total alkalinity are important diagnostic indices to judge whether the soil alkalization occurs and what the degree of soil alkalinity is. The essence of soil alkalization was the accumulations of Na2CO3 and NaHCO3. Soil desalination is accompanied by soil alkalization, which is the same as the result of Bai et al. (2019). In the early phase of leaching, the stronger the leaching was, the more obvious the alkalization was. Soil alkalization was accompanied by a series of phenomena, such as deterioration of soil texture, soil consolidation and fertility decline (Wang et al., 2016). The alkalization performances were consistent with that of Li et al. (2016). 227

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Fig. 5. Changes of Ca2+, Mg2+, DIC contents and pH in leachate during leaching. In the X-axis, 1, 2, 3, 4, represents the 4 times leaching, I and II represents samples are collected twice in each leaching stage.

through the inorganic absorption, which was evidenced by the reduced pCO2 and Rio. This outcome is a supplement to the study of global CO2 flux in saline-alkali soil, also confirms the researches of Xie et al. (2008) and Ma et al. (2013) in Xinjiang saline-alkali desert. Soil texture may exert a strong influence on nutrient retention and soil C storage (Silver et al., 2000; Zhao et al., 2018). As mentioned above, the underground hole ventilation process is one of the three potential mechanisms that may contribute to the inorganic C sequestration in saline-alkali soil. However, the sediments in the coastal wetland of Jiaozhou Bay are mainly sand and sandy clay, and the soil moisture content also maintain at a high level. The underground hole ventilation process mainly occurs in karst landform, extremely unlikely to happen in such soil texture of Jiaozhou Bay. Salt recharge of coastal saline-alkali soil in Jiaozhou Bay is caused mainly by the seawater immersion, backwater irrigation and artificial input of aquaculture ponds. Salt composition of the soil is same as that of the seawater, and the soil type is classified as the coastal sulfatechloride and soda-alkali soil. Based on the analysis of physical and chemical properties, and the elimination of the underground hole ventilation process, it can be inferred that the dissolution/precipitation of carbonate system and the leaching/absorption of atmosphere CO2 may be the two main mechanisms involving in the inorganic C sequestration process in coastal saline-alkali soil. These two processes include the following reactions, so that the leaching of carbonate and the absorption of CO2 are synchronous, and the changes of the contents of CO2, HCO3−, CaCO3 should influence each other:

Fig. 6. Variations of contents and 13C abundances of soil La-Ls during leaching. 1, 2, 3, 4, represents the 4 times leaching.

CO2 + H2 O = HCO3 + H+

4.2. Qualitative analysis of the mechanisms

CaCO3 +

Atmosphere CO2 absorbed into saline-alkali soil through abiotic way was verified by the simulated leaching experiment in laboratory. Coastal saline-alkali wetlands have the potential to become a C sink

H+

=

Ca2 +

+ HCO3

CaCO3 + CO2 + H2 O = Ca2 + + 2HCO3

(5) (6) (7)

In order to verify the above conjectures of synchronous 228

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Fig. 7. Variations of CaCO3, CO2 and DIC during leaching (a) and correlation analysis between CO2 and DIC (b), CO2 and CaCO3 (c) of different soils. 1, 2, 3 in (a) respectively indicates the variation of CaCO3, CO2 and DIC content in soil Na-Ns, and the same as soil Na-Ls to soil Sa-Ss. The significance level of each line in (b) and (c) was p < 0.01.

transformation, the changes of CO2, HCO3− and CaCO3 contents were integrated (Fig. 7a). The increment of alkalinity or pH may enhance the formation of abiotic carbonate, and it was further suggested that the soil alkalinity was linked to the carbonate formation (Liu et al., 2018). The DIC, including HCO3− and CO32−, in alkaline soils can be transformed continuously through the capture of atmospheric or microbemetabolic CO2 by the alkaline characteristics of soil, which was likely attributed by the continuously reacting between H+ and OH−. The increased alkalinity promoted the dissolution of carbonate proceeds, which promotes the absorption of CO2 by soil from the atmosphere (Portillo et al., 2009; Banks et al., 2010). Correlation results further certified that the saline-alkali properties of soil can promote the reactions above, and greatly improve the abiotic absorption capacity of soil to atmospheric CO2 (Fig. 7b, c, Table A.6). Within the same alkalinity soil, the soil CO2 absorption intensity was determined by the degree of salinity, and in different saline-alkaline soils, alkalinity seems to be the major determinant in their ability to absorb CO2, which is consistent with Xie et al. (2008). Generally, the process can be summarized as: the saline-alkali soil absorbs CO2, dissolves CaCO3 and produces HCO3−. The reduction of

CO2 and the dissolution of CaCO3 can basically meet the increase of HCO3−. It is confirmed that the transformations among CO2, CaCO3 and HCO3− were cyclic and continuous during leaching (Gislason et al., 2009), and the (i) dissolution/precipitation of carbonate system and (ii) leaching/absorption of atmosphere CO2 were the two mechanisms of inorganic C sequestration in saline-alkali soil. However, quantitative analyses of these two transformations are limited and the relationship between each process and total abiotic C cycle are also unclear. Thus, the studies regarding to these aspects are needed in the future. 4.3. Quantitative analysis of the mechanism Abundance of 13C is expressed in terms of ratios, but concentrations and contents of DIC and CaCO3 are different and constantly changing. Therefore, it is necessary to convert the abundance value to the specific content of 13C for analysis and comparison. Fig. 8 illustrated quantitative transformation of abiotic C transfer based on the results of soil La-Ls. In the first leaching stage, saline-alkali soil absorbed 0.2746 mmol of 13CO2 and produced 0.2059 mmol of H13CO3− through mechanisms (i) and (ii), which consumed 0.0228 mmol of Ca13CO3 229

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Fig. 8. Quantitative processes of atmospheric CO2 absorbed by coastal saline-alkali soil during successive four times leaching.

fourth leaching stage, the contents of soil CaCO3 and Ca13CO3 simultaneously went up, testified that the mechanism (i) continued to inhibit the inorganic C sequestration, and only mechanism (ii) occurred in the later phase of leaching. According to the results, the leaching adsorption process may be the dominated process for the inorganic C sequestration by the saline-alkali soil in coastal wetlands, and the chemical reaction process is CO2 + H2O = HCO3− + H+. The contribution of dissolution/precipitation of carbonate system is much smaller than that of the leaching and absorption process to the total amount of inorganic C sequestration. Soil abiotic C was stable in alkaline soils, as it is formed by carbonate minerals, which are stable in alkaline conditions. These abiotic C are difficult to dissolve to product CO2, but easy to migrate with leaching (Zhao et al., 2018). It should be noted that the release of CO2 into the

through mechanism (i). The analysis of C isotope data indicated that the two mechanisms of inorganic C sequestration occurred during the first leaching stage, and the contribution of mechanism (i) to total inorganic C sequestration was about 11.94%, while 73.19% for mechanism (ii). Similarly, it can be deduced that the two mechanisms also occurred in the second leaching stage, but the contribution of mechanism (i) to total inorganic C sequestration was reduced to 9.65% and that of mechanism (ii) was 77.22%. It is notable that the content of CaCO3 in the soil decreased in the third leaching stage, but the content of 13C labeled Ca13CO3 increased. This proved that the reaction of H13CO3− producing 13CO2 and Ca13CO3 occurred simultaneously along with the reaction of CaCO3 and 13CO2 producing the H13CO3−. The contribution of mechanism (i) dropped to 0 in the third leaching stage, and even began to show obvious reactions for releasing CO2 into the atmosphere. In the 230

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atmosphere may occur because of the CaCO3 precipitation in the soil with low salinity, which may result in that the soil become the inorganic C sources (W. Wang et al., 2013; Zeng et al., 2015). Ma et al. (2013) also revealed that approximately 90% of DIC-C were from soil pCO2 rather than carbonate salts by partitioning the sources of DIC using the isotopic mixing model. They suggested that the contribution of C transferred from atmospheric CO2 to the CaCO3 precipitation in soil may be underestimated in some previous studies, because the atmospheric CO2 dissolution in the alkali matrix occurred before the stable C isotopic tracing experiment was conducted. Ignoring mechanism, there is no doubt to the phenomenon of CO2 inorganic absorption by saline-alkali soil, but whether this phenomenon can be recycled and accumulated continually to eventually form an important C sink needs more research. Combining with the results of present studies and this research, an integrated cyclic process of abiotic C in coastal wetland system can be speculated: Due to the high salinealkaline property of coastal wetland soil, the atmospheric CO2 can be absorbed by the saline-alkaline soil solutions to form DIC, which can be washed downward into the groundwater by the rise or fall of groundwater table and eventually enter the ocean. Groundwater is regarded as the main geological agent for transportation, accumulation, and discharge of soil salt (Fan et al., 2012). The surface soil of coastal wetland is characterized by the intensive fluctuations between fresh-water and seawater, as well as between groundwater and surface water, which may potentially alter the soil C dynamic (Fan et al., 2012; Han et al., 2014), and thus may promote the transportation of DIC from surface to subsoil (Maher et al., 2013; Taillardat et al., 2018). In addition, the character of abundant rainfall and tidal resources of coastal wetland makes the leaching process in this area is much more intense, which makes the salinity of estuarine wetland change frequently (Weston et al., 2011; Neubauer, 2013; Chambers et al., 2014). It was expected to hamper both aerobic and anaerobic C mineralization rates because of the increase of soil salinization by seawater intrusion, thereby promoted the formation of carbon sinks (Setia et al., 2011; Wen et al.,

2019). Hence, we can reasonably speculate that the salinity and alkalinity of the leached soil can be recovered immediately by the regular tide and rainfall, and the absorption of atmospheric CO2 by leaching/ absorption and dissolution/precipitation of carbonate system can be maintained successively. However, the fate of these absorbed and leached abiotic C downward into the deep soil sediments or seawater is not determined and the eventual potentials of C sequestration through these abiotic processes are also uncertain. Thus, the extensive studies include the interfaces of land-sea or air-sea are still needed in the future. 5. Conclusion The potential abiotic absorption of CO2 is common in coastal salinealkali wetlands. The salinity and alkalinity of soil promote its CO2 uptake capacity, and the soil alkalinity shows greater effect on the capacity than soil salinity. The leaching/absorption of atmosphere CO2 is the dominate mechanism of the inorganic C sequestration process in coastal wetlands. The specific mechanism can be speculated as: The atmospheric CO2 is absorbed by the saline-alkaline soil solutions and then form DIC, which can move to deep soil or eventually enter the ocean through the transport of groundwater. In addition, the salinity and alkalinity of the leached soil can be recovered immediately by the regular tide and rainfall, and the absorption of atmospheric CO2 by leaching/absorption can be maintained successively and eventually form the carbon sink. However, the eventual inorganic C sequestration capacity of coastal wetland system is still uncertain, due to the lack of studies regarding to the fate of absorbed and leached abiotic C downward into the deep soil sediments or seawater. Acknowledgement The authors acknowledge financial support from the National Science Foundation of China (NSFC) (Grant No. 41771098, 41703084).

Appendix A

Fig. A.1. Diagrammatic sketch of the simulated leaching unit.

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Table A.1

Saline solutions with five pH levels obtained after different treatments. Saline solution number

1

2

3

4

5

NaCl:Na2SO4:NaHCO3:Na2CO3 pH

1:1:0:0 6.7 ± 0.2

1:2:1:0 7.9 ± 0.2

1:9:9:1 8.9 ± 0.2

1:1:1:1 9.8 ± 0.2

9:1:1:9 10.7 ± 0.2

Table A.2

Soil salinization and alkalization grade classification. Alkalinity degree

ESP (%)

Salinization degree

St (g·kg−1)

Salinization type

Cl−/SO42−

Non-alkalization Low alkalization Moderate alkalinization Severe alkalization Alkaline soil

< 5.0 5.0–10.0 10.0–15.0 15.0–30.0 > 30.0

Non-salinization Low salinization Moderate salinization Severe salinization Salt soil

< 5.0 5.0–10.0 10.0–15.0 15.0–20.0 > 20.0

Sulfate type Chloride sulfate type Sulfate chloride type Chloride type

< 0.5 0.5–1.0 1.0–4.0 > 4.0

Table A.3

Correlation matrix of soil salinization indexes in Jiaozhou Bay.

St pH ESP CaCO3 K+ Ca2+ Na+ Mg2+ Cl− SO42− DIC ⁎⁎ ⁎

St

pH

ESP

CaCO3

K+

Ca2+

Na+

Mg2+

Cl−

SO42−

1 −0.652⁎⁎ 0.512⁎⁎ −0. 671⁎⁎ 0. 853⁎⁎ 0.351⁎⁎ 0.952⁎⁎ 0.756⁎⁎ 0.976⁎⁎ 0.693⁎⁎ 0.474⁎⁎

1 −0.427⁎⁎ −0.573⁎⁎ −0.550⁎⁎ −0.140 −0.671⁎⁎ −0.382⁎⁎ −0.628⁎⁎ −0.446⁎⁎ −0.224⁎

1 0.371⁎ 0.466⁎⁎ −0.269⁎ 0.677⁎⁎ −0.053 0.539⁎⁎ −0.043 0.126

1 0.55⁎ 0.786⁎⁎ 0.671⁎⁎ 0.592⁎⁎ 0.692⁎⁎ 0.599⁎⁎ 0.596⁎⁎

1 0.484⁎⁎ 0.853⁎⁎ 0.769⁎⁎ 0.802⁎⁎ 0.625⁎⁎ 0.395⁎⁎

1 0.286⁎ 0.366⁎ −0.0753 0.5065⁎⁎ 0.4265⁎

1 0.656⁎⁎ 0.904⁎⁎ 0.598⁎⁎ 0.377⁎⁎

1 0.722⁎⁎ 0.424⁎⁎ 0.515⁎⁎

1 0.545⁎⁎ 0.537⁎⁎

1 0.282⁎⁎

p < 0.01. p < 0.05.

Table A.4

Correlation between Soil St/pH and other indexes in leaching process. Adj. R-square

Soil St Soil pH

CO2

Leached soil

0.344 0.791

Leaching solution

DIC

CaCO3

Mg2+

Ca2+

DIC

Mg2+

Ca2+

0.238 0.883

0.371 0.763

0.423 0.772

0.293 0.928

0.256 0.882

0.371 0.952

0.252 0.915

Table A.5

The natural

13

C abundance of soil in coastal wetlands of Jiaozhou Bay.

Number

ppm

d13CV-PDB (‰)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2.15 12.42 19.56 22.70 55.42 10.41 23.66 61.99 20.25 12.49 15.82 23.81 11.58 16.01 23.42 15.90

−17.04 −20.84 −19.53 −20.22 −24.67 −25.03 −24.31 −22.82 −24.94 −22.80 −22.20 −20.34 −23.09 −25.24 −24.67 −24.46

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Table A.6 Regression equation and R2 of CaCO3, DIC and CO2 variations in different soils. Soil number

CO2-CaCO3

CO2-DIC

Equation Na-Ns Na-Ls Na-Ss La-Ns La-Ls La-Ms Ma-Ss Sa-Ms Sa-Ss

y = (1.97 y = (2.42 y = (1.40 y = (2.48 y = (1.43 y = (1.77 y = (3.09 y = (2.33 y = (2.34

± ± ± ± ± ± ± ± ±

0.49) 0.43) 0.18) 0.29) 0.13) 0.11) 0.21) 0.14) 0.14)

x + (0.10 ± 0.03) x + (0.11 ± 0.02) x + (0.06 ± 0.01) x + (0.15 ± 0.03) x + (0.05 ± 0.01) x + (0.06 ± 0.01) x + (0.15 ± 0.03) x + (−0.02 ± 0.03) x + (0.02 ± 0.03)

R2

Equation

0.573 0.737 0.839 0.866 0.916 0.956 0.953 0.961 0.964

y = (−6.33 y = (−3.76 y = (−2.81 y = (−3.87 y = (−2.25 y = (−1.94 y = (−2.84 y = (−2.85 y = (−1.58

R2 ± ± ± ± ± ± ± ± ±

0.86) 0.63) 0.32) 0.59) 0.12) 0.16) 0.28) 0.21) 0.11)

x + (−0.30 ± 0.05) x + (−0.13 ± 0.03) x + (−0.12 ± 0.02) x + (−0.22 ± 0.05) x + (−0.06 ± 0.01) x + (0.05 ± 0.02) x + (0.01 ± 0.03) x + (0.34 ± 0.04) x + (0.45 ± 0.02)

0.828 0.760 0.875 0.793 0.971 0.927 0.906 0.941 0.952

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