Spartina (Spartina Alterniflora) Salt Marsh Soil Organic Carbon Component Source Research And Structural Characteristics

The carbon cycle is one of the hot spots in the global change research. Spartina alterniflora salt marsh is a freshly-established ecosystem with rapid succession in a relatively short period of time in mudflats, which has attracted high attention to its contribution on soil carbon sequestration, soil organic matter and fertility. The study on the soil carbon cycle impacted by S. alterniflora is therefore of great importance to systematically recognize the ecological role of this species in the soil improvement and in carbon cycle in salt marshes.In order to study components, sources and structural characteristics of soil organic carbon (SOC) pool of S. alterniflora saltmarsh, field observations and sampling work had been fulfilled at permanent study sites including mudflat (MF), S. alterniflora salt marsh of1-year (SAF2011),5-year (SAF2007),12-year (SAF2000),23-year (SAF1989) and Suaeda salsa saltmarsh (SSF1989), landwards along coastal wetland of Yancheng. Soil samples were collected, and the0-3m soil SOC, recalcitrant carbon (RC), labile carbon(LC), and δ13C value,0-20cm soil dissolved organic carbon (MBC), microbial biomass carbon (MBC) and13C NMR were determined. Results showed that:1. The presence of Spartina alteniflora exhibited significant influence on SOC pool. Except1-year S. alteniflora saltmarsh, SOC of all sampling sites showed a decreasing trend with increasing depth from mudflat surface in the vertical direction. SOC levels of all groups stabilized at an approximate depth of50cm. In the horizontal direction, SOC levels in associations of different periods had significant difference in all vertical levels (P<0.05). The RC content of MF was among0.44～2.47g/kg in the vertical direction with an average of1.13g/kg, while the LC content showed a decreasing trend with increasing depth, ranging from0.10to0.91g/kg with an average of0.30g/kg. The RC content of23-year S. alterniflora saltmarsh was higher than MF, and the average of RC was2.00g/kg. The LC content of23-year S. alterniflora saltmarsh fluctuated from0.02to0.93g/kg, which was higher than that of MF. The DOC occurred as the order of S. alterniflora saltmarsh (36.08mg/kg)> S. salsa saltmarsh (17.43mg/kg)> mudflat (6.92mg/kg); the MBC occurred as the order of S. alterniflora saltmarsh (52.51mg/kg)> S. salsa saltmarsh (18.27mg/kg)> mudflat (13.56mg/kg). It was indicated that the soil active organic carbon pool had been enhanced by the establishment of S. alterniflora. 2. The sources of SOC were also significantly different in response to different periods of S. alterniflora establishment. The proportion of S. alterniflora-sourced topsoil SOC among total SOC showed an increasing trend over the period of establishment. The sample plot of1-year,5-year,12-year,23-year S. alterniflora saltmarsh recorded the contribution rates of6.90%、43.21%、47.92%、62.44%to the total SOC respectively. The contribution rates showed a downward trend as the depth increases within0-100cm, then the contribution rates of1-year,5-year,12-year,23-year S. alterniflora saltmarsh reduced to0.18%、1.26%、1.56%、0.65%between the depth of100to150cm. The topsoil RC and LC were of the highest rates from S. alterniflora, as the depth increased RC and LC showed firstly reduced and then stabilized.3. The SOC were mainly composed of alcoxyl carbon and aromatic carbon rather than other carbon fractions. The average proportion of SOC fractions showed apparently the order as:aromatic carbons (35.85%)> the alcoxyl carbon (32.83%)> carboxyl carbon (20.62%)> alkyl carbon (11.76%). The highest value of aromatic carbons, alcoxyl carbon and alkyl carbon occurred at the5-year S. alterniflora saltmarsh (SAF2007), while the highest value of carboxyl carbon occurred at the23-year S. alterniflora saltmarsh. The ratio of alkyl carbon/alcoxyl carbon in topsoil occurred as the order of SAF2007(0.44)> SSF1989(0.43)> SAF2000(0.28)> SAF2011(0.27)> SAF1989(0.22). The ratio of hydrophobic carbon/hydrophilic carbon occurred as SAF2007(0.97)> SSF1989(0.87)> SAF2000(0.85)> SAF2011(0.83)> SAF1989(0.81). The highest ratio of alkyl carbon/alcoxyl carbon (0.44) and hydrophobic carbon/hydrophilic carbon (0.97) occurred at the5-year S. alteniflora saltmarsh (SAF2007), indicating the highest degree of alkylation and stability of SOC pool.