Vertical distribution of fertilizer nitrogen from surface water flooding of a silt loam and clay soil used for rice production

Nitrogen (N) fertilizer retention and short‐term vertical redistribution into soil affect N balance and N‐use efficiency in rice ( Oryza sativa L.) production systems. A series of experiments was conducted on intact soil cores obtained from rice production fields to track floodwater and soil ammonium‐ and nitrate‐N concentrations up to 96 hr after urea and ammonium sulphate fertilizer amendments to dry, silt loam soil (a) as compared to clay soil, (b) with and without a delay before floodwater establishment and (c) at two initial soil water conditions (dry and muddy). The impact of a 5‐day delay before flood establishment was also compared between untreated and N ‐( n ‐butyl)thiophosphoric triamide (NBPT)‐treated urea‐N concentrations. Urea‐derived N generally moved deeper into dry, silt loam soil than ammonium sulphate. Ammonium sulphate‐derived N concentration was greatest in the surface 2 cm of soil, decreased with depth, and was unaffected by floodwater duration. A 5‐day delay between fertilization and flooding resulted in urea behaving similarly to ammonium sulphate, although NBPT treatment resulted in vertical N distributions similar to those for untreated urea applied to dry, immediately flooded soil. When urea was applied to muddy soil, no fertilizer N was observed below 2 cm, while large concentrations of ammonium‐N remained in the floodwater. Results collectively demonstrate the importance of immediate downward movement of fertilizer‐applied N into dry soil for retention, and the use of a urease inhibitor to maximize urea‐derived N downward movement into soil when a delay in flood establishment cannot be avoided.