Zhigao Sun

Fluxes of nitrous oxide and methane in different coastal Suaeda salsa marshes of the Yellow River estuary, China

The spatial and temporal variations of the fluxes of nitrous oxide (N(2)O) and methane (CH(4)) and associated abiotic sediment parameters were quantified for the first time across the coastal marsh dominated by Suaeda salsa in the Yellow River estuary during 2009/2010. During all times of day and the seasons measured, N(2)O and CH(4) fluxes from coastal marsh ranged from -0.0147 mg N(2)O m(-2) h(-1) to 0.0982 mg N(2)O m(-2) h(-1) and -0.7421 mg CH(4) m(-2) h(-1) to 0.4242 mg CH(4) m(-2) h(-1), respectively. The mean N(2)O fluxes in spring, summer, autumn and winter were 0.0325 mg N(2)O m(-2) h(-1), 0.0089 mg N(2)O m(-2) h(-1), 0.0119 mg N(2)O m(-2) h(-1) and 0.0140 mg N(2)O m(-2) h(-1), and the average CH(4) fluxes were -0.0109 mg CH(4) m(-2) h(-1), -0.0174 mg CH(4) m(-2) h(-1), -0.0141 mg CH(4) m(-2) h(-1) and -0.0089 mg CH(4) m(-2) h(-1), respectively, indicating that the coastal marsh acted as N(2)O source and CH(4) sink. Both N(2)O and CH(4) fluxes differed significantly between times of day of sampling. N(2)O fluxes differed significantly between sampling seasons as well as between sampling positions, while CH(4) fluxes had no significant differences between seasons or positions. Temporal variations of N(2)O emissions were probably related to the effects of vegetation (S. salsa) during summer and autumn and the frequent freeze/thaw cycle of sediment during spring and winter, while those of CH(4) fluxes were controlled by the interactions of thermal conditions and other abiotic factors (soil moisture and salinity). Spatial variations of N(2)O and CH(4) fluxes were primarily affected by soil moisture fluctuation derived from astronomic tide, sediment substrate and vegetation composition. N(2)O and CH(4) fluxes, expressed as CO(2)-equivaltent (CO(2)-e) emissions, showed that N(2)O comprised the principal part of total calculated CO(2)-e emissions during spring and winter, while the contributions of CH(4) could not be ignored during summer and autumn. This study highlights the importance of seasonal N(2)O and CH(4) contributions, particularly during times of significant CH(4) consumption. For the accurate up-scaling of N(2)O and CH(4) fluxes to annual rates, a careful sampling design at site-level is required to capture the potentially considerable temporal and spatial variations of N(2)O and CH(4) emissions.

Nitrogen cycle of a typical Suaeda salsa marsh ecosystem in the Yellow River estuary

The nitrogen (N) biological cycle of the Suaeda salsa marsh ecosystem in the Yellow River estuary was studied during 2008 to 2009. Results showed that soil N had significant seasonal fluctuations and vertical distribution. The N/P ratio (15.73 +/- 1.77) of S. salsa was less than 16, indicating that plant growth was limited by both N and P. The N absorption coefficient of S. salsa was very low (0.007), while the N utilization and cycle coefficients were high (0.824 and 0.331, respectively). The N turnover among compartments of S. salsa marsh showed that N uptake from aboveground parts and roots were 2.539 and 0.622 g/m2, respectively. The N translocation from aboveground parts to roots and from roots to soil were 2.042 and 0.076 g/m2, respectively. The N translocation from aboveground living bodies to litter was 0.497 g/m2, the annual N return from litter to soil was far less than 0.368 g/m2, and the net N mineralization in topsoil during the growing season was 0.033 g/m2. N was an important limiting factor in S. salsa marsh, and the ecosystem was classified as unstable and vulnerable. S. salsa was seemingly well adapted to the low-nutrient status and vulnerable habitat, and the nutrient enrichment due to N import from the Yellow River estuary would be a potential threat to the S. salsa marsh. Excessive nutrient loading might favor invasive species and induce severe long-term degradation of the ecosystem if human intervention measures were not taken. The N quantitative relationships determined in our study might provide a scientific basis for the establishment of effective measures.

Comparison of arsenic and heavy metals contamination between existing wetlands and wetlands created by river diversion in the Yellow River estuary, China

Samples were collected at 71 sites in the Yellow River Delta Natural Reserve in December 2010 to represent soil conditions before and after the Yellow River (YR) diversion. The As, Cd, Cu, Pb, Zn, and Ni concentrations were measured to determine metal contamination levels. Results suggest that Cd concentrations were significantly higher after the YR diversion than before. The As, Cd, Cr, Cu, Ni, Pb, and Zn soil contamination indices did not exceed contamination levels, although the heavy metal content increased after the YR diversion. The mean concentrations of these heavy metals were lower than the Class I criteria. Correlation analysis shows significant correlations between As and Cr, Cu, Ni, Pb, and Zn concentrations both before and after the YR diversion. However, no significant correlations were observed between heavy metal concentration and pH before the diversion, and no heavy metal concentration was correlated with salinity. The principal component analysis indicates that these trace elements, including As, were closely correlated with each other and therefore likely originated from shared pollution sources before the diversion. These results are useful for assessing the heavy metal contamination and proposing feasible suggestions to improve soil quality.

Nitrogen Biological Cycle Characteristics of Seepweed(Suaeda salsa) Wetland in Intertidal Zone of Huanghe(Yellow) River Estuary

From April 2008 to November 2009, the nitrogen (N) cycle of plant-soil system in seepweed (Suaeda salsa) wetland in the intertidal zone of the Huanghe (Yellow) River estuary was studied. Results showed that soil N had significant seasonal fluctuations and vertical distribution, and the net N mineralization rates in topsoil were significantly different in growing season (p < 0.01). The N/P ratio (9.87 ± 1.23) of S. salsa was less than 14, indicating that plant growth was limited by N. The N accumulated in S. salsa litter at all times during decomposition, which was ascribed to the N immobilization by microbes from the environment. Soil organic N was the main N stock of plant-soil system, accounting for 97.35% of the total N stock. The N absorption and utilization coefficients of S. salsa were very low (0.0145 and 0.3844, respectively), while the N cycle coefficient was high (0.7108). The results of the N turnovers among compartments of S. salsa wetland showed that the N uptake amount of aboveground part and root were 7.764 g/m2and 4.332 g/m2, respectively. The N translocation amounts from aboveground part to root and from root to soil were 3.881 g/m2 and 0.626 g/m2, respectively. The N translocation amount from aboveground living body to litter was 3.883 g/m2, the annual N return amount from litter to soil was more than 0.125(−) g/m2 (minus represented immobilization), and the net N mineralization amount in topsoil (0–15 cm) in growing season was 1.190 g/m2. The assessment of N biological cycle status of S. salsa wetland indicated that N was a very important limiting factor and the ecosystem was situated in unstable and vulnerable status. The S. salsa was seemingly well adapted to the low-nutrient status and vulnerable habitat, and the N quantitative relationships determined in the compartment model might provide scientific base for us to reveal the special adaptive strategy of S. salsa to the vulnerable habitat in the following studies.

Responses of CH4 emissions to nitrogen addition and Spartina alterniflora invasion in Minjiang River estuary, southeast of China

The nitrogen (N) input and Spartina alterniflora invasion in the tidal marsh of the southeast of China are increasingly serious. To evaluate CH4 emissions in the tidal marsh as affected by the N inputs and S. alterniflora invasion, we measured CH4 emissions from plots with vegetated S. alterniflora and native Cyperus malaccensis, and fertilized with exogenous N at the rate of 0 (N0), 21 (N1) and 42 (N2) g N/(m2·yr), respectively, in the Shanyutan marsh in the Minjiang River estuary, the southeast of China. The average CH4 fluxes during the experiment in the C. malaccensis and S. alterniflora plots without N addition were 3.67 mg CH4/(m2·h) and 7.79 mg CH4/(m2·h), respectively, suggesting that the invasion of S. alterniflora into the Minjiang River estuary stimulated CH4 emission. Exogenous N had positive effects on CH4 fluxes both in native and in invaded tidal marsh. The mean CH4 fluxes of N1 and N2 treatments increased by 31.05% and 123.50% in the C. malaccensis marsh, and 63.88% and 7.55% in the S. alterniflora marsh, respectively, compared to that of N0 treatment. The CH4 fluxes in the two marshes were positively correlated with temperature and pH, and negatively correlated with electrical conductivity and redox potential (Eh) at different N addition treatments. While the relationships between CH4 fluxes and environmental variables (especially soil temperature, pH and Eh at different depths) tended to decrease with N additions. Significant temporal variability in CH4 fluxes were observed as the N was gradually added to the native and invaded marshes. In order to better assess the global climatic role of tidal marshes as affected by N addition, much more attention should be paid to the short-term temporal variability in CH4 emission.