| The objective of an urease inhibitor (UI) is to retard urea hydrolysis, thereby reducing loss of nitrogen (N) as ammonia (NH{dollar}sb3{dollar}). A nitrification inhibitor (NI) is intended to slow the biological oxidation of NH{dollar}sb4sp+{dollar} to NO{dollar}sb3sp-{dollar}, and reduce potential leaching and denitrification of NO{dollar}sb3sp-{dollar}-N.; Laboratory studies confirmed that N-(n-butyl) thiophosphoric triamide (NBPT) was effective as an UI for up to two weeks, and that dicyandiamide (DCD) was effective as a NI for up to six weeks in loamy sand soils.; A field study for three location-years evaluated the effectiveness of NBPT (0.25 or 0.50% of N rate) as an UI applied with urea sidedressed to corn (Zea mays L.) at 45, 90, and 134 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}. Urea without NBPT and ammonium nitrate were comparison treatments. At Headland, AL, in 1987, there were no differences among N source or NBPT in earleaf N or grain yield. A 1-cm rain three days after N sidedressing probably prevented NH{dollar}sb3{dollar} volatilization. At Headland, and in Shorter, AL, in 1988, no rain fell for 10 days after N sidedressing. At Shorter, NH{dollar}sb3{dollar} losses were measured in semi-closed chambers. Ammonia volatilization from urea without NBPT was 20.5 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}, while only 1.5, and 0.25 kg N ha{dollar}sp{lcub}-1{rcub}{dollar} were measured for urea plus NBPT, and ammonium nitrate, respectively. Higher earleaf N was found at both locations with the inhibited urea vs. urea alone. Grain yields, however, were not affected by NBPT at either location. Drought limited yields and probably masked any effects due to NBPT.; A one-year field study using 2.24% atom {dollar}sp{lcub}15{rcub}{dollar}N urea was conducted to evaluate DCD (as 10% of N rate) as a NI for fall-applied N on winter wheat (Triticum aestivum L.). Fall application with and without DCD and splitting N between fall and spring were compared. With fall N application, higher levels of NH{dollar}sb4sp+{dollar}-N in the upper 30 cm of soil were maintained with DCD for three months, compared to fall N without DCD. Grain yield increased with N rate (67, 101, and 134 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}), but was not affected by DCD or timing of N application. Recovery of {dollar}sp{lcub}15{rcub}{dollar}N (mean 53%) was not affected by DCD or timing of N application. Immobilization of {dollar}sp{lcub}15{rcub}{dollar}N was 21, 20, and 15% with the fall N plus DCD, split applied N, and fall N treatments, respectively. Although denitrification was apparently reduced, significantly higher immobilization with DCD probably explains the absence of a yield response to DCD. |
