To assess the effect of time and rate of fertilizer N applications on fate of the applied N, a 5‐yr field experiment was conducted with winter wheat (Triticum aestivum L.). Microplots confined in metal frames to a depth of 1.42 m received annual applications of (NH4)2SO4 containing 7.65 atom %15N. Treatments, replicated four times, consisted of 50 and 100 kg N/ha applied in fall and spring on a Pachic Argiustoll soil. In four of the years, spring applications gave better fertilizer use efficiency than fall treatments, probably because of greater immobilization of fall‐applied N. After 5 yr, 27 to 33% of the applied fertilizer N had been removed by the grain. Soil N use was increased by fertilizer in some years and in those cases was correlated highly with increased growth, suggesting that the cause was larger root systems rather than a priming effect. Amounts of fertilizer N in the 1.8‐m soil profiles increased each year and at the end of the experiment averaged 54% of that applied during the 5 yr for 50‐kg treatments and 47% for 100‐kg treatment. From 71 to 77% of the surface‐applied fertilizer N remaining in the profiles was in the 0‐ to 0.1‐m soil layers. Most of the residual fertilizer N was immobilized in organic forms, but late applications and dry conditions in 1980 greatly reduced both nitrification and immobilization. There was good agreement between 15N and difference measurements of crop removal of fertilizer N for the 5‐year period, although difference measurements were more variable and did not agree well with 15N measurements in individual years.
A field experiment was conducted for 2 years with ammonium sulfate tagged with 5.93 atom % 15N to determine the fate of N fertilizer applied to sprinkler‐irrigated corn (Zea mays L.). All areas of triplicate, 356‐cm square plots were treated with 50 or 150 kg tagged N/ha. N fertilizer used by the crop and that remaining in the top 240 cm of soil were measured. NH4+‐N and NO3‐‐N in the 0‐ to 10‐cm layers after the second harvest also were determined. Grain yields in 1976 did not differ significantly. In 1977 response to N was significant, but responses to two N rates did not differ significantly.Only about one‐fourth of the N fertilizer at either rate was removed by the grain. Amounts in the soil were in proportion to amounts applied and about three times more after 2 years than 1 year, reflecting the N in crop residue from the first crop. The N balance indicated losses from the system of 17 to 18% of the N applied during the 2 years, after correcting for apparent errors. Fertilizer did not significantly alter amounts of soil N used by the crop but total N uptake increased with each fertilizer increment. There was no priming effect on mineralization of indigenous soil N. Comparng the 15N and difference methods for calculating fertilizer N uptake showed that the difference method varied more and showed significantly less uptake in one case.From 65 to 73% of the N fertilizer remaining in the soil was in the 0‐ to 10‐cm layer. Depth of movement was related to amount applied, varying from 20 cm with 50 kg after 1 year to 240 cm with 150 kg applied each year for 2 years. Some of the N with the 150‐kg rate may have leached below 240 cm after 2 years, but there appeared to be no leaching from the measured zone in other cases so losses from the system must have been by denitrification.Most of the N fertilizer in the 0‐ to 10 cm‐layer after 2 years was immobilized with only 4.4 to 4.7% remaining in the NH4+ and NO3‐ forms. However, there was about seven times more inorganic N in the soil originating from fertilizer than came from indigenous soil N, which confirms that residual fertilizer N is more likely than indigenous N to be present in inorganic forms.Amounts of fertilizer N found in grain and lost were proportional to amounts applied. The 150 kg N/ha rate gave no higher grain yields than 50 kg N/ha rate, but resulted in much greater crop removal, gaseous loss, and downward movement into the soil profile.
A field experiment using ammonium sulfate tagged with 7.65 A% 15N was conducted to assess how rates and times fertilizer is applied influence the fate of N applied to winter wheat (Triticum aestivum L.). Bottomless metal boxes were pressed 142 cm into the soil to confine the tagged fertilizer. Treatments, replicated four times, consisted of two rates (50 and 100 kg N/ha) and two application times (fall and spring). Fertilizer N used by the crop and that remaining in the upper 180‐cm of the soil after harvest were measured. Amounts of ammonium‐N and nitrate‐N from the fertilizer in the 0 to 10‐cm layer also were determined.The N balance indicated 9.7 – 10.3 kg N/ha were unaccounted for at the 50‐kg N rate and 19.7 · 23.4 kg, at the 100‐kg rate. Losses did not differ significantly between application times. With 100 kg/ha the crop removed significantly more fertilizer N, and significantly less remained in the soil with spring than with fall applications. The crop used similar amounts of total N in each case, but with the spring applications more fertilizer N but less soil N was taken up. Fertilizer applications caused no priming effect on mineralization of indigenous soil N. Most of the fertilizer remaining in the soil was in the 0 to 10‐cm layer, with no evidence of N moving deeper than 50 cm, so losses were from gaseous loss rather than leaching.Most of the fertilizer N in the 0 to 10‐cm layer of soil after harvest was immobilized, with only 9.6 – 11.5% remaining in inorganic forms. Immobilization was the principal reason for differences in spring and fall applications and for limited leaching. The percentages of NH4+‐N and NO3‐‐N, which originated from the fertilizer were 4 – 8 times the percentages of total N from the fertilizer, so more fertilizer N was in inorganic forms than was the case with indigenous soil N. Since there was a direct relationship between amounts of NO3‐‐N in the surface soil from fertilizer and amounts of N unaccounted for, it was concluded that gaseous losses resulted from denitrification processes.
An experiment was conducted to ascertain the minimum size of unconfined plot that can be used to measure accurately residual fertilizer N and to measure N taken up by corn (Zea mays L.) from broadcast N fertilizer. Large plots were fertilized with 15N‐enriched (NH4)2SO4 and plant samples were taken from different places within plots. Samples of grain and stover taken 36 cm from two edges of the plots contained significantly less N derived from fertilizer than plants 107 or 154 to 202 cm from edges.Accurate values for plant uptake of fertilizer N could be obtained by sampling the center plant of 3‐row plots at least 214 cm long and 71 cm apart. Plots should be larger if several plants are to be sampled. It appears larger plots are needed also to determine residual fertilizer N in the soil.
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