Nitrogen Loss Inhibitors Evaluated for Humid-Region Wheat Production

Abstract

Applied N fertilizer can be lost from the crop root zone by several pathways, reducing agronomic efficiency and cansing environmental problems. One approach that has been suggested to reduce N losses is to mix compounds with the N fertilizer that inhibit some steps in the loss pathway. Our objective was to evaluate the effectiveness of five potential inhibitors (ammonium polyphosphate [APP], ammonium thiosulfate [ATS], dicyandiamide [DCD], monoammonium phosphate [MAP], and KCl), singly and in combinations, on yield and N uptake of winter wheat (Triticum aestivum L.) in a humid climate. A total of four experiments were established over 2 yr in which spring N topdress applications were made either as urea-ammonium nitrate (UAN) solution or as UAN solution mixed with inhihitors. All treatments gave a total spring application rate of 60 lb N/acre. Yields exceeded 90 bu/acre in three of the experiments and reached 70 bu/acre in the fourth. Nitrogen rate treatments were also applied in each experiment to determine the N-responsiveness of each site. Large yield responses to applied N occurred at three of the experimental locations. Treatments that reduced fertilizer N losses would thus be expected to translate into increased yield at those three locations. However, none of the inhibitors tested increased yield or N uptake in any experiment despite ideal conditions for N loss (ammonia volatilization the first year, leaching and denitrification loss the second year), good uniformity in our experiments, and our use of eight replications for selected treatments in the second year. In one experiment with 100 bu/acre yields, we could have detected a 3.3 bu/acre yield increase, which would have required only 8 lb/acre additional N to achieve. This implies that no inhibitor reduced N loss by as much as 8 lb/acre in this experiment, which was conducted on a sandy soil and received 8 in. of rain within 4 wk of treatment application. While it is possible that some treatments gave yield increases below our limits of detection, even if this happened it would be cheaper in almost all cases to achieve this yield increase by applying a small amount of additional N than by using an inhibitor