Ridge-Till Corn and Urea Hydrolysis Response to NBPT

Ammonia volatilization from surface-applied, urea-based fertilizers can result in reduced fertilizer N availability to the crop, particularly in minimum tillage systems with substantial surface residue. The objective of this study was to evaluate the potential for the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) to minimize ammonia loss and preserve fertilizer N availability of urea and urea-ammonium nitrate (UAN) solution applied to ridge-till, irrigated corn (Zea mays L.). The study was conducted at three sites over 3 yr, 1990 to 1992. The soil for all three site-years was a Crete silt loam (fine, montmorillonitic, mesic Pachic Argiustoll). Urea and UAN solution (28% N) were applied following planting via two methods, broadcast (BR) and surface band (SB), at two N rates, 100 and 200 lb/acre, with and without NBPT at the rate of 1.0 lb/acre. A nonvolatile check of UAN knifed into the soil, with and without NBPT, was applied each year. Grain yield, stover yield, ear leaf N and chlorophyll, basal stalk nitrate, grain N, grain moisture, and stover N were evaluated. A lab incubation study was also conducted to evaluate the effect of subsurface band application of UAN with and without NBPT on the rate of urea hydrolysis and fertilizer N availability to the crop. Nitrogen did not limit yield in 1990. Precipitation soon after fertilization in 1990 and 1991 resulted in little or no benefit from the use of NBPT. Limited precipitation and low humidity for an extended period following fertilization in 1992 resulted in a 56.6 bu/acre increase in yield when NBPT was applied with urea (averaged over rates and application methods), but no yield increase when NBPT was applied with UAN. No differences in yield were observed between BR and SB application methods. Parameters which are indicative of N use efficiency, such as grain N, basal stalk nitrate, leaf N and chlorophyll, responded to source, rate, and application method variables even when grain or stover yield did not. The lab incubation found that urea hydrolysis was reduced when NBPT was added to UAN in a subsurface band, but not slowed to the extent that N availability to the crop would be reduced over the growing season. This study found the yield response of corn to urea or UAN solution containing NBPT to be highly variable, dependent mainly on the climatic conditions following fertilization. The use of NBPT can be an effective management tool to minimize the potential for yield loss due to ammonia volatilization from urea fertilizers applied to high residue conditions in years when climatic conditions are conducive to N loss. Research QuestionThe use of conservation tillage systems is increasing nationwide as one method to address concerns regarding soil erosion and to meet conservation compliance requirements. Surface application of urea-based N fertilizers is common in such systems, The potential exists for significant N loss via ammonia volatilization from urea if soil, residue, and climatic conditions are favorable for such loss. This study evaluated the use of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) to minimize the potential for ammonia volatilization from urea and yield loss from ridge-till, irrigated corn. Literature SummaryPrior research has found that the volatile loss of ammonia from surface-applied urea-based fertilizers can vary significantly, depending on a variety of soil and climatic conditions. Reported N losses have ranged as high as 30 to 40% in field studies. Slight tillage or precipitation usually will move urea deep enough into the soil to minimize ammonia loss. The potential for ammonia loss is accentuated by crop residues, which can have urease activities several times that of the soil. Urease inhibitors are compounds that can block the activity of the urease enzyme, reducing the rate of urea hydrolysis and increasing the chance that precipitation or tillage will move urea into the soil before significant ammonia loss occurs. Prior studies have evaluated urease inhibitors primarily in no-till, rainfed systems. Study DescriptionField studies were located on three irrigated sites in south-central Nebraska from 1990 to 1992. All sites were ridge-tilled, in which no tillage occurred following harvest until planting. Urea or urea-ammonium nitrate (UAN) solution, with and without the urease inhibitor NBPT, were applied following planting with two methods, broadcast and surface band, at two rates, 100 and 200 lb N/acre. A subsurface band application method was also used for UAN solution as a nonvolatile check application method. Grain yield, stover yield, ear leaf N and chlorophyll, basal stalk nitrate, grain N, grain moisture, and stover N were evaluated. A lab incubation study was also conducted to evaluate the effect of subsurface band application of UAN with and without NBPT on the rate of urea hydrolysis and fertilizer N availability to the crop. Applied QuestionsWhat is the potential effect on yield due to ammonia loss from urea surface-applied to a ridge-till system, and what benefits, if any, can be gained from urease inhibitors? In 1990, factors other than N were yield limiting. Consequently, no significant effect of N source or NBPT was observed on yield (Fig. 1). In 1991, there was a yield response to applied N, but no benefit from NBPT due to rain, which moved the fertilizer into the soil soon after fertilization. In 1992, grain yield was almost doubled when NBPT was included with urea, while NBPT had little effect on yield with UAN solution. These years represent the range that might normally be observed in yield effects due to the use of a urease inhibitor —from no effect at all to a significant increase in yield. Evaporative loss of soil moisture and precipitation were primary factors influencing this range of impact from a urease inhibitor. Figure 2 illustrates precipitation and relative humidity for the 3 yr of the study following fertilization. In 1990, rain soon after application moved urea into the soil and minimized ammonia loss. In 1991, rain occurred on the third day after fertilization, while moist soils and low evaporative demand minimized loss potential earlier. In 1992, relatively little rain coupled with low humidity and high evaporative demand resulted in urea and hydrolysis products remaining near the soil surface for an extended period. Data from hydrolysis rate cylinders placed in the soil in the field suggested that most or all of the ammonia loss fiom urea occurred in the first 2 d after application. RecommendationYield reduction due to ammonia volatilization of the magnitude observed in 1992 is uncommon. However, it is indicative of how much loss is possible with certain climatic conditions. Ammonia volatilization from urea and UAN solutions can be minimized by incorporating the fertilizer, either mechanically or by imgation. The potential for loss can also be reduced by application to dry soils, since moisture is needed for urea hydrolysis, or by attempting to anticipate when rains might occur and fertilize just prior to rain. Urease inhibitors such as NBPT can be effective management tools to reduce the potential for ammonia loss, particularly in high residue, minimum tillage systems. Fig. 1Summary of grain yield as influenced by NBPT, 1990–1992. Fig. 2Cumulative precipitation and relative humidity after fertiliition, 1990–1992.