Nitrogen Loss Potential by Nitrification/Denitrification In Delayed-Flood Rice.

Nitrogen uptake efficiency can approach 70% of the applied N under ideal weather conditions and proper water management.  In the production environment, this is not always the case.  Research has documented that rice grain loss can reach 170 lb acre^-1 day^-1 when urea fertilizer is applied to clay soil and the flood is not established timely.  A loss mechanism that has not been studied greatly for clay soils is nitrification/denitrification.  A laboratory incubation was conducted to quantify the nitrification potential of a Sharkey clay soil.  Furthermore, a field study was conducted to evaluate products that inhibit losses via nitrification/denitrification.  An incubation study was conducted with a Sharkey clay soil in 2010.  One hundred g of soil (dry weight basis) was added to polypropylene containers and brought to 80% field capacity moisture and placed in an incubation chamber for 14 d.  At the conclusion of 14 d, two urea granules were applied below the soil surface and placed in the incubation chamber at 25°C.  The soils were extracted at 2, 5, 9, 15, 20, and 26 d after fertilizer application with 500 ml of 1.0 M KCl.  Extracts were analyzed on an auto-analyzer to determine NH₄⁺ and NO₃^- contents.  Total N recovery ranged from 60 to 100%.  Within 2 d after incubation, 37% of the recovered N was in the NO₃^- form.  This increased to 79 and 93% for 9 and 15 d, respectively.  In a field study, N treatments were applied at 10 days before permanent flood establishment (dbf) and 2 days dbf.  Urea applied at 168 kg N ha^-110 dbf produced 841 kg ha^-1 less than when applied 2 dbf.  A sulfur coated urea product (43%N) applied 10 dbf produced yields equal to urea applied 2 dbf.  Further studies are on-going looking at the differences among soils nitrification potentials.