The wetland of the Yellow River estuary is a typical new coastal wetland in northern China. It is essential to study the carbon pool and its variations for evaluating the carbon cycle process. The study results regarding the temporal-spatial distribution and influential factors of soil organic carbon in four typical wetlands belonging to the Yellow River estuary showed that there was no significant difference in the contents of the surface soil TOC to the same season among the four types of wetlands. For each type of wetlands, the TOC content in surface soils was significantly higher in October than that in both May and August. On the whole, the obvious differences in DOC contents in surface soils were not observed in the different wetland types and seasons. The peak of TOC appeared at 0–10 cm in the soil profiles. The contents of TOC and DOC were significantly higher in salsa than those in reed, suggesting that the rhizosphere effect of organic carbon in salsa was more obvious than that in reed. The results of the principal component analysis showed that the nitrogen content, salinity, bulk density, and water content were dominant influential factors for organic carbon accumulation and seasonal variation.
The accumulation and decomposition of soil organic carbon in wetlands influence the stability of soil pools and CO2 emission, which plays an important role in the global terrestrial carbon cycle and climate change. Meanwhile, maintaining the stability of carbon pools and the function of high carbon sinks is significant in mitigating concentrations of CO2 in the atmosphere and increasing the wetlands’ primary productivity [
Estuary wetland ecosystems, especially, are ecotones located between terrestrial areas and marine areas with high net primary productivity and carbon sequestration, playing an important role in alleviating coastal eutrophication and reducing CO2 emissions [
Currently, regional carbon cycle research is focused on the estimation of the distribution of soil stocks, carbon sequestration, and CO2 emission reduction potential. To determine the uncertainty of the temporal and spatial distribution in regional carbon sources and sinks is of great importance for accurate decisions and a comprehensive understanding of the intensity, process, and mechanism of global carbon cycle [
The wetland of the Yellow River estuary, as a new ecosystem, is a typical coastal wetland in north China. The accumulation and decomposition of soil organic carbon in wetlands directly affect the primary productivity and the regional carbon balance; the dynamics of the soil carbon pool are essential to accurately evaluate the process of carbon cycling and the potential of carbon sequestration [
The aims of this study include three aspects as follows: to determine the temporal and spatial distribution characteristics of total organic carbon (TOC) and dissolved organic carbon (DOC) in the Yellow River delta wetlands with different plant communities; to compare the soil organic carbon distribution and rhizosphere effects between rhizosphere soil and nonrhizosphere soil that cover different typical plants, including reed and salsa; to discuss the relationship among soil organic carbon, soil texture, bulk density, water content, pH, total nitrogen, and inorganic nitrogen content.
The Yellow River delta estuary wetlands (118°48′–119°08′E; 37°34′–38°09′N) are located in Dongying city in the Shandong province of China. The climate is a temperate humid northern continental monsoon climate with four distinct seasons, and the seasons alternate significantly. The mean temperature is 12.1°C and the evaporation capacity is 1962 mm. The average annual rainfall is 551.6 mm with rain and heat over the same period. With the uneven distribution of precipitation and the large evaporation capacity, the drought index reached 3.56 [
The predominant natural wetland vegetation is reed, tamarix, and salsa, of which distribution is mainly restricted by soil salinity, groundwater level, salinity, topography type, and human activities. Due to the impact of human activities, such as agricultural reclamation and artificial breeding, the salinization of wetlands was aggravated and the natural wetland was severely reduced and degenerated [
The four typical wetlands of bare soil (without vegetation, salsa wetland, tamarix wetland, and reed wetland) were chosen in the offshore estuary of the Yellow River delta. The four sampling points were selected in each type, with 16 total sites. There were 48 surface layer (0–20 cm) samples collected in October 2008, May 2009, and August 2009. There was one soil profile (YRW5, YRW9, YRW11, and YRW12) with a sampling depth of 60 cm in each type of wetland, and one sample was collected in 10 cm intervals. In the four soil profiles from the three seasons, 72 total samples were taken.
To discuss the rhizosphere effect of the plants on the accumulation of soil organic carbon, the typical reed and salsa communities were chosen and 11 (R1–R11) soil samples were collected from each rhizosphere and nonrhizosphere of reed soil (Figure
Location of studied region and sampling sites.
The soil samples were naturally air-dried and hand-picked to remove obvious plant debris and roots, sieved (<2 mm) in the laboratory, and subsequently the soil was analysed. The total organic carbon was determined using an automated carbon analyzer (EA2000 elementary analyzer, Germany). The quality control used the standard soil samples GBW07403 as internal control samples, and the recovery was controlled at 98%–104% [
The 10 g fresh soil was taken and extracted in 50 mL distilled water, and then it was shocked continuously for 5 h at normal temperature (25°C). Next, it was centrifuged at high speed for 5 min. The supernatant was filtered from the 0.45
The soil moisture was determined by the oven-drying method. The bulk density was determined by the cutting ring method and the pH was determined by the potentiometry method in situ. After air drying and fully mixing, the soil particle size was measured by the hydrometer sieve analysis method. The total nitrogen (TN) was determined by the selenium-copper-sulfate-acid-digestion method [
The statistics analysis and disposal of data were conducted by Suffer 8.0, Origin 8.5, and SPSS19.0. Significant differences in soil DOC and TOC were examined using one-way ANOVA at the 5% level of significance. To test effects of the soil physic-chemical properties on soil carbon principal components 1 (PC1) and 2 (PC2) of PCA, PCA was performed by SPSS19.0.
Figure
Soil TOC content in the four types of Yellow River estuary wetlands. The columns represent the average content of soil TOC, while the lines represent the variation range. The number at the right part is the range of TOC and the letters represent the difference of soil TOC in the different types of wetlands in the same season. The * means the significant difference at the 0.05 level.
The TOC contents of the other three types of wetlands in May were slightly more than that in August, except for the bare soil without vegetation. When the wetland vegetation comes to mature period in October, the vegetation root system can fix more carbon. In addition, the amount of litter decomposition return made the organic carbon accumulated in autumn [
Through the long process of soil freezing in the winter, the decomposition and mineralization interaction of the organic fragment leads to the decomposition of soil carbon exceeding its accumulation [
In October and May, the mean TOC contents of the surface soils for the salsa, tamarix, and reed wetlands were slightly higher than that in the bare soil without vegetation, because soils with vegetation could fix more carbon and make the accumulation of organic matter through the return of litter [
Figure
Soil DOC content in four types of Yellow River estuary wetlands. The columns represent the average content of soil TOC, the lines represent the variation range, the number at the right part is the range of TOC, the letters represent the difference of soil DOC in different types of wetlands in the same season, and * means the significant difference at the 0.05 level.
Compared with the different months in the same type of wetlands, the differences of DOC contents in the surface soils of bare soil, tamarix, and reed were not significant in May, August, and October; the differences of DOC contents in the surface soils of salsa were significant (
Figures
TOC contents in the soil profile of the Yellow River estuary wetland.
DOC contents in the soil profile of the Yellow River estuary wetland.
That form of root distribution determined the high organic material in the surface soil [
Figure
Rhizosphere is the interaction center of plants, soil, and microorganisms. The rhizosphere processes can induce change, through plant roots, in the physical structure of the surrounding soil. The release of root exudates and rhizosphere microorganisms can significantly influence the rhizosphere soil nutrients [
Figures
TOC contents in rhizosphere and nonrhizosphere soils of reed and salsa. The columns represent the average contents and the error bars in left part represent the uncertainty U of TOC contents and in the right part represent the variation range. The letters represent the differences of soil TOC contents at the 0.05 level.
DOC contents in rhizosphere and nonrhizosphere soils of reed and salsa. The columns represent the average contents and the error bars in left part represent the uncertainty U of TOC contents and in the right part represent the variation range. The letters represent the differences of soil TOC contents at the 0.05 level.
The mean value of the DOC content in the rhizosphere soil of reed was up to 21.22 mg kg−1, slightly higher than that in nonrhizosphere soil (Figure
The difference of TOC and DOC contents in the nonrhizosphere soil of reed and salsa was not obvious, but the TOC and DOC contents in the salsa rhizosphere soil were obviously higher than those in the reed rhizosphere soil (Figures
The close relationships between the soil organic carbon of the wetlands and the soil physic-chemical properties were observed in this study. The results of the principal component analysis (PCA) showed that the accumulative percentage of the three principal components reached 70.71%, and the eigenvalue of each component was more than 1.
All of the above could objectively reflect the main control factors of organic carbon in the Yellow River estuary. The eigenvalue of PC1 was 1.93. The DIN and TON, with larger load values, were 0.85 and 0.81, respectively. This principal component for soil organic carbon was a dominant influencing factor, with a contribution rate of 27.49%. The eigenvalue of PC2 was 1.58. Salt (S) and bulk density (B) with larger load values were 0.78 and 0.72, respectively. The eigenvalue of PC3 was 1.45. The moisture content with larger load value was −0.74. Therefore, nitrogen content, salinity, bulk density, and water content are dominant influencing factors of organic carbon [
The results of the principal component analysis showed that the DIN content was the main influencing factor of the organic pool in Yellow River estuary wetlands (Figure
The principal component analysis for soil properties, S-salt, B-bulk density, M-moisture content, clay (the content of grain with diameter <0.063
On one hand, the increase of nitrogen elements in wetland soils, especially the increase of inorganic nitrogen content, can be used directly by vegetation and can help plants to flourish. The high organic carbon storage was observed in the soil with high primary productivity [
Furthermore, the TOC content in surface soils exhibited a significant positive relationship with water content (
On the whole, the TOC content of surface soils in October was much higher than that in May and August, and the difference was significant (
The vertical distribution: TOC contents of soil profile presented near-surface characteristic (peaked at 0–10 cm soil layer). The TOC content changes little in soil profiles and remained low at a soil depth of below 20 cm. The TOC contents in the soil profile were higher in October than in both May and August. The DOC contents of salsa and reed in surface (0–10 cm) soil were higher than that in the soil beneath the surface in May and October. The slight changes of the DOC content were observed in bare soil and in the tamarix soil profile.
The TOC contents of the two plants (reed and salsa) in the rhizosphere soil were significantly higher than that of the nonrhizosphere soil (
The results of the principal component analysis showed that the nitrogen content, salinity, bulk density, and water content were dominant influencing factors of organic carbon. It suggests that bulk density, water content, and nitrogen content, which are significantly influenced by vegetation types and microtopography, were dominant factors of organic carbon accumulation.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This study was supported by the National Marine Public Welfare Research Project of China (Grant nos. 201305021, 201405007, and 201105005).