2nd UF Water Institute Symposium Abstract

Submitter's Name Kamaljit Banger
Session Name Poster Session: Hydrologic, Biogeochemical and Ecological Processes 2
Category Hydrologic, biogeochemical and ecological processes
Poster Number 204
Author(s) Kamaljit Banger,  Soil & Water Quality Laboratory, Gulf Coast Research & Education Center, University of Florida, Wimauma, FL 33598
  Gurpal Toor,  Soil & Water Quality Laboratory, Gulf Coast Research & Education Center, University of Florida, Wimauma, FL 33598
  Patrick Inglett, Soil & Water Science, University of Florida, Gainesville, FL 32611
  Craig Stanley, Soil & Water Science, Gulf Coast Research & Education Center, University of Florida, Wimauma, FL 33598
  Nitrogen Forms in Streams Draining Agricultural, Urban, Mining, and Pasture Sub-Basins of the Alafia River Watershed, Florida
  Non-point source pollution is the dominant pathway of nitrogen (N) loss in agricultural but also in urbanized watersheds. Nitrogen is the limiting nutrient for phytoplankton production in the Tampa Bay estuary, Florida, therefore understanding the contribution of different land uses to N loss is essential to devise strategies to prevent coastal eutrophication. In this study we determined the concentrations of inorganic and organic N forms in stream waters draining six sub-basins, ranging in size from 19 to 320 km2, of the Alafia River watershed (total drainage area: 1085 km2). The sub-basins were classified as: one urban (60% residential, 18% forest, 5% cropland), one pastureland (47% pastureland, 27% forest, 3% residential), two agricultural (32–42% cropland, 14–18% residential, 12–13% forest), and two mined land (43–48% mined lands, 2–16% residential, and 12–16% forest). We collected water samples at weekly intervals during April–October 2009 from streams draining these six sub-basins. No significant variation in the pH (7.0–7.3), EC (0.2–0.5 dS m-1), and dissolved oxygen (5–6 mg L-1) were observed during this period in all sub-basin stream waters. Total N (TN) concentrations ranged from 2 to 3 mg L-1 and were greatest in the agricultural sub–basins (2.8–3.0 mg L-1), followed by the urban (2.6 mg L-1), mined (2.3–2.8 mg L-1), and pastureland (2.0–2.3 mg L-1). Organic N forms dominated in all sub-basin stream waters (1.7–2.0 mg L-1; 58–88% of TN) while inorganic N ranged from 0.25 to 1.25 mg L-1 (11–42% of TN). Organic N was greater in pastureland (86–88% of TN) as compared to other sub-basins (58–76% of TN). Nitrate-N was the major inorganic N form in all the sub-basin stream waters except for the pastureland and was greatest in agricultural (0.7–1.0 mg L-1; 85–90% of inorganic N) followed by urban (0.6 mg L-1; 85%), and mined (0.5–0.6 mg L-1; 83–90%) and pastureland (0.1–0.2 mg L-1; 41–65%). In contrast, the proportion of ammonium-N was greater in the pastureland sub-basin (0.11–0.15 mg L-1; 35–59% of inorganic N) than either the agricultural, urban, or mined sub-basins (0.07–0.19 mg L-1; 10–17% of inorganic N). The greater nitrate-N concentration in agricultural sub-basin may represent runoff losses from inorganic fertilizers used in crop production while the increased abundance of ammonium-N in the pastureland may reflect more transformation and loss of mineralized N. Alternatively, lower nitrate-N concentrations in the stream waters may be attributed to high temperature and moisture availability in Florida, which accelerates vegetation uptake and turnover of N, as well as nitrate removal via denitrification. The overall high concentration of organic-N in stream waters is likely the result of abundant organic matter and animal waste in pastureland sub-basin, leaves and grass cuttings in urban sub-basin, and the wastewater disposal in one of the mined sub-basin. The lower loss of N as nitrate-N in urban sub-basin is surprising and contradicts the common assumption that high fertilizer N use results in greater N losses. However, it is important to note the urban hydrology in this watershed is significantly altered due to the presence of a series of retention ponds in urban areas and clay settling ponds in mined sub-basins, so it likely that N forms undergoes a series of transformations before their delivery to streams.