Nitrogen Loss Inhibitors Evaluated for Humid-Region Wheat Production

Applied N fertilizer can be lost from the crop root zone by several pathways, reducing agronomic efficiency and causing 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 KCI), 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 inhibitors. 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. Research QuestionSeveral researchers have found evidence that certain compounds can reduce N losses from urea-ammonium nitrate (UAN) solution when mixed with the UAN solution prior to application. Our objective was to evaluate whether these compounds increased yield or N uptake when mixed with UAN solution topdressed on winter wheat in a humid region. Literature SummaryApplied N fertilizer can be lost before crop uptake by ammonia volatilization, by leaching, and by denitrification. Only the nitrate form of N is susceptible to leaching and denitrification, so compounds that inhibit the conversion of urea to ammonium (ammonium thiosulfate, phenylphosphorodiamidate [PPD], N-[n-butyl]thiophosphoric triamide [NBPT]) or the conversion of ammonium to nitrate (nitrapyrin, dicyandiamide) have been looked at for their potential to reduce leaching and denitrification losses. Several compounds have also shown promise for reducing ammonia volatilization, including ammonium polyphosphate, calcium, and potassium. Some fertilizer dealers are, on the basis of very limited research, recommending inclusion of a small proportion of ammonium thiosulfate in all UAN applications. Study DescriptionA total of four experimental locations were established over two study years—two locations in the Coastal Plain of Virginia during the first year and two locations in the Ridge and Valley region of Virginia during the second year. Soil surface textures ranged from fine sand to silty clay loam. Inhibitors evaluated were ammonium polyphosphate, ammonium thiosulfate, dicyandiamide, monoammonium phosphate, and KCl. Some combinations of inhibitors were also used as treatments. All treatments were topdressed midway between spring green-up (Zadoks growth stage 25) and prejointing (Zadoks growth stage 30) to supply a total of 60 lb spring N/acre. Applied QuestionDid any of these nitrogen loss inhibitors increase wheat yield? None of the inhibitors or combinations of inhibitors that we tested increased wheat yields at any of the four locations. Conditions in the first year were ideal for ammonia volatilization (warm, sunny, breezy, soil was moist prior to N application, light rain following N application) and in the second year were ideal for leaching and denitrification (numerous heavy rainfalls within 3 wk of N application), so all inhibitors got a fair test. Did any of these nitrogen loss inhibitors increase wheat N uptake? None of the inhibitors or combinations of inhibitors that we tested increased wheat N upfake at any of the four locations. Detecting small yield differences is very difficult, but if a treatment gave a true yield increase that was too small to be statistically significant, we might expect to find increased crop N uptake with that treatment. Could an inhibitor still be cost-effective if it gave a yield increase below the limits of experimental precision for this study? The cost of the additional N required to obtain the minimum detectable yield increase for the three N-responsive experiments was less than the cost of most of the treatments included in the study. This proves that applying additional N was more cost-effective than using an inhibitor in these cases. Is routine addition of ammonium thiosulfate to UAN justified? Our research did not indicate any benefit to this practice, nor do there appear to be any other research reports of yield gains due to N loss inhibition by ammonium thiosulfate.