Demonstration of a new indicator for studying upwelling in the northern South China Sea**This research was supported by the projects of knowledge innovation program of the Chinese Academy of Sciences (No. KZCX2-YW-Q07-02, No. KSCX2-SW-132 and No. KSCX2-YW-Z-1024), the National Natural Science Foundation of China (41076070) and the National 908 project (No. 908-02-04-04).

Abstract

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/m 2 and 4.332 g/m 2 , respectively. The N translocation amounts from aboveground part to root and from root to soil were 3.881 g/m 2 and 0.626 g/m 2 , respectively. The N translocation amount from aboveground living body to litter was 3.883 g/m 2 , the annual N return amount from litter to soil was more than 0.125(-) g/m 2 (minus represented immobili-zation), and the net N mineralization amount in topsoil (0-15 cm) in growing season was 1.190 g/m 2. 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.