Soil organic matter stabilization varies with nitrogen enrichment and hydrogeomorphic setting in a marsh-mangrove...

As average global temperatures rise, coastal wetlands play a crucial role in carbon (C) sequestration and storage, primarily as soil organic matter (SOM). However, the poleward migration of mangroves into salt marsh habitats, along with nitrogen (N) pollution, may alter SOM formation and stability, and thus the capacity of coastal wetlands to preserve soil C long-term. Wetland SOM is composed of mineral-associated organic matter (MAOM), which is considered highly stable due to physicochemical protection, and particulate organic matter (POM), which has a more rapid turnover rate. This study evaluated the effects of mangrove presence, N enrichment, and hydrogeomorphic setting on SOM pools and biogeochemical soil properties in NE Florida, USA, to understand implications for long-term soil C and N preservation. A factorial field experiment (40 plots) tested interactions between hydrogeomorphic location (creekside vs. interior), vegetation (marsh vs. mangrove), and N treatment (fertilized vs. control), across two depths (0-15 and 15-30 cm) on MAOM and POM pools. Interior plots held 58% more MAOM-C and 61% more MAOM-N by mass than creekside plots, with control marshes MAOM-C and MAOM-N exceeding control mangroves. However, when normalized, MAOM accounted for 55.5% of total C and 73.8% of total N in creekside plots, compared to 44.3% and 61.2% in interior plots- suggesting greater C stabilization efficiency. N-fertilization effects varied by vegetation and location. In interior plots (0-15 cm), N-fertilization increased MAOM by 28-30% in mangroves, while decreasing MAOM by 20-21% in marshes. In creekside plots, N-fertilization had no main effect, though fertilized marshes held more MAOM than fertilized mangroves. Collectively, these findings underscore the need for site-specific assessments (vegetation, nutrient inputs, and hydrogeomorphic setting, etc.) of blue C potential as coastal vegetation and nutrient regimes continue to shift.