Influence of Vegetation Type on Soil Organic Nitrogen Composition and Mineralization in Subtropical Wetlands.

The major source of bioavailable nitrogen (N) in soils is organic N. Despite being approximately 95% of total soil N, less than half of soil organic N (SON) forms are identified. Previous studies in upland systems indicate SON composition is not influenced by vegetation type. However, it is unknown if shifts in microbial metabolic pathways from anaerobiosis will result in similar SON composition in wetlands. Therefore, the objective of this study was to determine the influence of vegetation type on SON composition and mineralization in subtropical wetlands. A constructed wetland located in south Florida was chosen based on similar nutrient status and soil age, yet contains two different vegetation types, emergent aquatic vegetation (EAV) and submerged aquatic vegetation (SAV). We hypothesized that SON will not differ in response to different vegetation types. Soil cores were obtained from each vegetation type and separated into floc (unconsolidated detrital material), recently accreted soil, and antecedent soil. Composition of SON was determined by the diffusion method to separate gross soil N pools into total hydrolyzable-N, NH₄-N, amino sugar-N, amino acid-N, and acid insoluble-N. In addition, mass spectrometry (MS) was used to qualitatively identify the molecular composition of SON. Absence of significant differences between amino sugar-N and amino acid-N, as a percent of total N, between vegetation types indicate that vegetation type is not the main driver of SON gross pools in subtropical wetlands. Differences in the molecular composition of proteins indicate a slight decrease in the relative abundance of proteins in EAV with depth, while the relative abundance of proteins in SAV increased with depth. Although gross N pools indicate vegetation is not the main driver, the molecular composition suggests differing relative abundance of proteins and amino sugars with vegetation type, which may be important for understanding the stability of SON in subtropical wetlands.