Carbon and nitrogen biogeochemistry in mangrove physiognomies along a salinity gradient in the Eastern Amazon

The Brazilian Amazon has one of the largest continuous areas of mangroves in the world, playing an essential role in carbon (C) sequestration. However, the productivity of this ecosystem is often limited by nitrogen (N) availability, which is influenced by various environmental factors, including soil salinity. On the coast of the Pará State, geomorphological complexity and variations in flooding regimes establish salinity gradients that shape contrasting mangrove physiognomies. Here we evaluated how salinity affects C and N content, C/N ratio, and C and N stable isotope composition (δ¹³C and δ¹⁵N, respectively) in the soil-plant system of mangrove forests in the Caeté River estuary, municipality of Bragança, PA, Brazil.

Soil samples (0-20 cm) and leaves were collected in mangrove physiognomies across three salinity zones (low, intermediate, and high), for determining their nutritional and isotopic composition. Variations among mangrove physiognomies were tested using generalized linear mixed models.

When compared to lower salinity levels, the vegetation from the high-salinity zone had higher leaf N concentration and lower C/N ratios, suggesting higher metabolic investment in osmotic adjustment. Vegetation δ¹³C values confirmed reduced stomatal conductance in response to higher salinity. The soil from the high-salinity zone had the lowest δ¹⁵N values, contrasting with the vegetation, which had enriched ones.

Increased soil salinity is reflected in a reorganization of nutrient partitioning, where vegetation sustains its biomass through a more closed and highly efficient N cycle, reducing losses. These findings are of great importance for predicting the responses of Amazonian mangroves to climate change and sea-level rise.