Spatial and Temporal Distributions in Sandy Soils with Seepage Irrigation: I. Ammonium and Nitrate

Abstract

Tomato ( Lycopersicon esculentum Mill.) in southwest Florida is typically grown on fumigated, raised beds with polyethylene mulch under seepage irrigation. An average grower's N rate currently exceeds the University of Florida's Institute of Food and Agricultural Sciences (UF‐IFAS) recommendation (224 kg N ha −1 maximum). Excess nutrients are subject to runoff and leaching, which raises environmental concerns in Florida watersheds. A field study was conducted to elucidate N spatial distribution in tomato beds during the 2006 spring and winter growing seasons with 224 and 358 kg N ha −1 rates. Ammonium N was highest for both seasons during 0 to 1 wk after planting (WAP) at the 0‐ to 10‐cm depth in the fertilizer band, but rarely moved vertically. Lateral movement of NH 4 + –N was observed through 7 WAP, however, probably due to diffusion. Nitrate N at 0 to 10 cm in the band peaked during 3 to 4 WAP, indicating about 3 wk of nitrification in the spring; however, this process was shortened to 1 to 2 wk in the winter, probably due to greater bacterial activity with warmer temperatures in the beginning of the winter than the spring (average 8°C in the first 3 wk). Both lateral and vertical movements of NO 3 − –N were observed in the spring, even at the 20‐ to 30‐cm depth. A rise and subsequent fall of the water table level during 5 to 8 WAP particularly enhanced NO 3 − –N downward movement, which was attributed to greater diffusivity of NO 3 − –N than NH 4 + –N. Minimizing water table fluctuations and applying a reduced N rate are critical to reduce NO 3 − –N leaching loss.