Nitrogen Speciation and Concentration Dynamics from Agricultural Fields to Indian River Lagoon.

Surface runoff water from agricultural field and urban area has been considered as nonpoint sources of pollution to the Indian River Lagoon (IRL) in South Florida. However, influence of waterway biology and chemistry on the speciation and bioavailability of nitrogen is not yet accounted for. In this study, surface water samples were collected along the waterways from agricultural fields to the discharge point at the IRL and analyzed for nitrogen speciation and related water quality properties. The concentrations of total N (TN), organic N (ON), dissolved N (DN), particulate-N (PN), NO₃-N (NN), and NH₄-N (AN) in the surface water samples were in the range of 2.97-19.8, 2.61-19.4, 2.67-17.6, 0-6.09, 0-6.89, and 0-0.54 mg L^-1, respectively, with corresponding mean values of 6.86, 6.29, 6.13, 0.73, 0.43, and 0.13 mg L^-1. Organic N was the dominant form of N, which accounts for >90% of the total N while DN dominated over PN, indicating that dissolved organic N is likely the major N form lost from agricultural soils to the environment. The concentration of total N was highest in runoff water from agricultural fields, sharply decreased in ditch and canal water, implying that waterways such as farm ditch and canals can, to a certain degree, attenuate N flux from sources to the receiving waters. The concentration of TN in agricultural water was high in April-June (~15 mg L^-1), decreased in July-September (~10 mg L^-1), and relatively lower in October-December and January-March period (5 mg L^-1). Which agree with the pattern of fertilization and rainfall in south Florida.