Effects of Long-Term Nutrient Input on Progeny Seed Nutrient Contents, Germination and Early Growth Characteristics of Typical Coastal Wetland Plants

Seed reproduction is a vital stage in the life cycle of plants. In coastal wetlands, where nutrient regimes are increasingly altered by climate change and anthropogenic inputs, understanding how long-term nutrient enrichment affects progeny seed germination and early seedling establishment is essential for predicting vegetation shifts. In this study, we conducted a long-term nutrient addition experiment (2015-2024) in the Yellow River Delta with three nitrogen (N) and phosphorus (P) supply levels (Low, Medium, and High) and three N:P ratios (5:1, 15:1, and 45:1) to examine the effects of nutrient enrichment on seed nutrient contents, germination, and early seedling growth characteristics of <i>Suaeda salsa</i> and <i>Phragmites australis</i> measured in late 2024. Results showed that long-term fertilization increased the P content in <i>S. salsa</i> seeds by 17.1% to 146.0%, whereas N content was less influenced. In contrast, both N and P contents of <i>P. australis</i> seeds remained relatively stable across treatments. The seed germination and early seedling growth of the two species responded differently to various fertilization treatments. <i>S. salsa</i> seeds showed higher sensitivity to long-term fertilization, with improved behaviors under high nutrient level or 5:1 supply ratio. In contrast, <i>P. australis</i> performed better under medium nutrient level or higher N:P ratios (15:1 or 45:1). Correlation analysis indicated that P content in <i>S. salsa</i> seeds was significantly positively correlated with germination percentage, germination rate, germination index, and early seedling biomass, while N content was significantly correlated only with biomass. No significant correlations were observed between seed nutrients and germination or early seedling growth in <i>P. australis</i>. These findings underscore species-specific strategies in response to long-term nutrient enrichment and may ultimately influence species coexistence, community succession, and the resilience of coastal wetlands under ongoing global change.