Temporal and spatial variation of nutrient stoichiometry in coastal salt marshes dominated by Spartina alterniflora in eastern China: comprehensive effects of vegetation and tidal flooding.

To better understand carbon, nitrogen, and phosphorus dynamics within small-scale gradients of tidal and vegetation interaction, 13 study sites were established along three transects (near, far, and along a tidal creek) within a salt marsh in northern Jiangsu, China, and soil samples were collected during both the growing and non-growing season; the soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents were measured. Spatiotemporal variation in SOC, TN, and TP in coastal salt marsh soils was influenced not only by the physical and chemical properties of the elements themselves, but also by the distribution of local vegetation and repetitive tidal action. Complex interactions between the vegetation and tidal flooding determined spatiotemporal patterns in SOC, TN, and TP. Compared to tidal effects, vegetation effects were more stable and persistent. The spatial distributions of SOC and TN in salt marsh soils (sediment) were relatively consistent, with both SOC and TN levels being higher in vegetated zones as compared to the mudflat. Nutrient levels were also higher further from the tidal creek, and the SOC/TN content was maximized in the S. alterniflora marsh. The TP content was higher in the tidal-creek transect versus the other two transects; among sites, the TP content was higher in the mudflat than the vegetated sites. Within the vegetated sites, the S. alterniflora marsh had the highest TP levels. Seasonal variation in SOC, TN, and TP was minimal compared to differences among vegetated sites within each transect. Differences between transects were also minimal compared to those among vegetated sites within transects. Significant spatial variation was identified in C:N:P stoichiometry across the study area; the comprehensive effects of vegetation and tidal flooding on nutrient stoichiometry in coastal salt marshes dominated by S. alterniflora in eastern China are elevation-dependent. The N/P ratio measured 0.62 ± 0.41, suggesting that plant growth in the study area may be nitrogen limited. Together, the local vegetation and repetitive tidal flooding shaped SOC, TN, and TPs in salt marsh soils, producing similar patterns in nutrient element accumulation, constraining the geochemical behavior and fate of elements, and controlling the spatiotemporal distribution of soil elements. Therefore, it is critical to consider interactions between vegetation and tidal effects, as well as the nonlinear relationship between nutrient input and ecological response in salt marsh ecosystems when managing coastal zones, supporting ecosystem functioning, and conducting ecological risk assessments.