Phosphorus biological cycle in the different Suaeda salsa marshes of the Yellow River estuary, China

Much uncertainty exists in the phosphorus (P) cycle in the marshes of the intertidal zone. This study explored the P cycling in the two Suaeda salsa marshes [middle S. salsa marsh (MSM) and low S. salsa marsh (LSM)] of the Yellow River estuary during April 2008 to November 2009. Results showed seasonal fluctuations and vertical distributions of P in different S. salsa marsh soils, and variations in P content in different parts of plants due to water and salinity status. The N/P ratios of the different S. salsa were 9.87 ± 1.23 and 15.73 ± 1.77, respectively, indicating that plant growth in MSM was limited by N, while that in LSM was limited by both N and P. The S. salsa litter in MSM released P to the environment throughout the year, while that in LSM immobilized P from the environment at all times. The P absorption coefficients of S. salsa in MSM and LSM were very low (0.0010 and 0.0001, respectively), while the biological cycle coefficients were high (0.739 and 0.812, respectively). The P turnovers among compartments of MSM and LSM showed that the uptake amounts of roots were 0.4275 and 0.0469 g m⁻² year⁻¹ and the values of aboveground parts were 1.1702 and 0.1833 g m⁻² year⁻¹, the re-translocation quantities from aboveground parts to roots were 0.8544 and 0.1452 g m⁻² year⁻¹, the translocation amounts from roots to soil were 0.0137 and 0.0012 g m⁻² year⁻¹, the translocation quantities from aboveground living bodies to litter were 0.3157 and 0.0381 g m⁻² year⁻¹, and the annual return quantities from litter to soil were less than 0.0626 and −0.0728 g m⁻² year⁻¹ (minus represented immobilization), respectively. P was an important limiting factor in S. salsa marshes, especially in LSM. S. salsa was seemingly well adapted to the low-nutrient condition and the vulnerable habitat, and the nutrient enrichment due to the import of N and P from the Yellow River estuary would be a potential threat to the S. salsa marshes.