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

Abstract: From April 2008 to November 2009, the nitrogen (N) cycling of plant-soil system in Suaeda salsa wetland in the intertidal zone of the Yellow River estuary was studied with a compartment model. Results showed that the N in soil had significantly seasonal fluctuations and vertical distribution, and the net N mineralization rates in topsoil were significantly different in growing season (p＜0.01). The N contents in root and stem of S. salsa generally decreased, while those in leaf fluctuated significantly. The N/P ratio (9.87±1.23) of S. salsa was less than 14, indicating that the growth of plant was limited by N. The litter production and the N content in litter changed significantly, and the change trends were just the opposite. The mass loss and N content increased at all times during litter decomposition, and the C/N ratio controlled the N dynamics of S. salsa litter. The N accumulated in S. salsa litter at all times, which was ascribed to the N immobilization by microbes from the decomposition environment. The N in the plant subsystem was mainly stored in aboveground living body. 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 cycling coefficient was high (0.7108). The N turnovers among compartments of S. salsa wetland showed that the N uptake amount of aboveground part and root were 7.764 and 4.332g/m2, respectively. The N translocation amounts from aboveground part to root and from root to soil were 3.881 and 0.626g/m2, respectively. The N translocation amount from aboveground living body to litter was 3.883g/m2, the annual N return amount from litter to soil was larger than 0.125(-)g/m2 (minus expressed immobilization), and the net N mineralization amount in topsoil (0-15cm) in growing season was 1.190g/m2. The assessment of N cycling status of S. salsa wetland indicated that the N was a very important limiting factor, and the ecosystem was situated in unstable and vulnerable status. The S. salsa seemed to be well adapted to the low-nutrient status and vulnerable habitat, and the N quantitative relationships in the compartment model might provide some scientific bases for us to reveal the special adaptive strategy of S. salsa to the vulnerable habitat in the following studies.