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Abstract: The degree of hydrological connectivity in wetlands plays a vital role in determining the flux of energy, material, and nutrients across these wet landscapes. During the last century, compartmentalization of hydrologic flows in the Florida Everglades by canals and levees has had a profound impact on the natural timing and supply of freshwater and nutrients across the southern Everglades. Nitrogen (N) is an understudied nutrient in the phosphorus-limited Everglades; it plays an important role in many Everglades processes. To gain a better understanding of the overall N-dynamics in southern Everglades’ marshes and the role that canals play in the distribution of N across this landscape, we analyzed δ15N natural abundance data for the primary ecosystem components (the macrophyte Cladium jamaicense, marl soils, peat soils, and periphyton). Three sample transects were established in the three main basins of the southern Everglades: Shark River Slough, Taylor Slough, and the C-111 basin. Each transect included sample sites near canal inflows, in interior marshes, and at the estuarine ecotone. Natural abundance δ15N signatures provided insights into processes that may be enriching the 15N content of ecosystem components across the marsh landscape. We also conducted a combined analysis of δ15N data, tissue N concentrations, and water column N data to provide a broad overview of N cycling in the freshwater marshes of the southern Everglades. The primary trend that emerged from each basin was a significant 15N enrichment of all ecosystem components at near-canal sites, relative to more downstream sample sites. These data suggest that the phosphorus-limited marshes of the southern Everglades are not inactive conduits for N. Rather, these marshes appear to be actively cycling and processing N as it flows from the canal–marsh interface through downstream freshwater marshes. This finding has important implications to downstream coastal estuaries, including Florida Bay, and to nearshore coastal ocean ecosystems, such as coral reefs, where N is the limiting nutrient. [Copyright &y& Elsevier]

Compared to phosphorus (P), nitrogen (N) has received little attention across the Everglades landscape. Despite this lack of attention, N plays important roles in many Everglades systems, including being a significant pollutant in Florida Bay and the Gulf of Mexico, the limiting nutrient in highly P-impacted areas, and an important substrate for microbial metabolism. Storage and transport of N throughout the Everglades is dominated by organic forms, including peat soils and dissolved organic N in the water column. In general, N sources are highest in the northern areas; however, atmospheric deposition and active N2 fixation by the periphyton components are a significant N source throughout most systems. Many of the processes involved in the wetland N cycle remain unmeasured for most of the Everglades systems. In particular, the lack of in situ rates for N2 fixation and denitrification prevent the construction of system-level budgets, especially for the Southern mangrove systems where N export into Florida Bay is critical. There is also the potential for several novel N processes (e.g., Anammox) with an as yet undetermined importance for nitrogen cycling and function of the Everglades ecosystem. Phosphorus loading alters the N cycle by stimulating organic N mineralization with resulting flux of ammonium and DON, and at elevated P concentrations, by increasing rates of N2 fixation and N assimilation. Restoration of hydrology has a potential for significantly impacting N cycling in the Everglades both in terms of affecting N transport, but also by altering aerobic-anaerobic transitions at the soil-water interface or in areas with seasonal drawdowns (e.g., marl prairies). Based on the authors' understanding of N processes, much more research is necessary to adequately predict potential impacts from hydrologic restoration, as well as the function of Everglades systems as sinks, sources, and transformers of N in the South Florida landscape. [ABSTRACT FROM AUTHOR]

The article presents a study which investigates the feeding preferences and the habitat use of the endangered West Indian manatee Trichechus manatus species in Florida, Belize and Puerto Rico. It mentions the use of stable isotope analysis in interpreting the feeding behavior of the species and the multivariate analysis of variance (MANOVA) to analyze the differences of the manatee skin from the values of nitrogen and carbon isotopes calculated with the use of statistical analyses. Results show that the species are fed from sea grasses or freshwater vegetation.

• Higher rates on applied N above 168 kg N ha−1 did not show any positive responses with respect to plant and soil N fertilizer recoveries, fruit and whole plant dry matter yields, indicating that the recommended N rate of 224 kg N ha−1 is more than adequate. • Fertilizer N contributions to the tomato crop were approximately 62 %, making soil N contributions 38 %. • A 38 % reduction in recommended N application rate may help enhance N fertilizer recovery efficiency in fertigated tomatoes grown on sandy soils. Plant use efficiency of applied N is estimated to be around 50 % for most crops. In coastal plain sandy soils particularly, leaching along with volatilization in warmer climates may be a predominant pathway for N loss to the environment. A replicated field study to determine both crop N requirement and recovery efficiency of N (REN) in tomatoes (var. BHN 602) grown on sandy soils under a plastic-mulched bed system was conducted in north Florida. Isotope labeled-ammonium nitrate (15NH 4 15NO 3) was applied in spring at four different N rates (0, 168, 224, 280 kg N ha−1). A subsequent study in the fall was conducted in order to determine the recovery of residual N fertilizer in tomatoes. In spring, no appreciable response to applied fertilizer N rate above 168 kg N ha−1 (lowest rate) were observed, indicating recommended N rates may be more than what is required. On average, approximately, 62 % of N accumulation in the plant came from fertilizer, whereas 38 % came from soil N. In the fall, recovery of residual 15N fertilizer at harvest ranged from 1.9 to 5.4 kg N ha−1. At the end of both studies, a 15N balance was calculated to estimate total crop and soil recovery of N fertilizer, where approximately 15.4 % of applied 15N fertilizer was recovered. Unrecovered N in this study shows that optimizing N fertilizer management in warmer climates is critical in order to avoid unnecessary excess application of fertilizer and minimize loss of N to the environment. [ABSTRACT FROM AUTHOR]