Understanding the redistribution and fate of N is essential for justification of Best Management Practices (BMP). This project was conducted on a Hecla fine sandy loam (sandy, mixed, Aquic Haploboroll) soil at the BMP field site near Oakes, North Dakota. One objective of this investigation was to evaluate the residence times of N03- -N in 20 undisturbed lysimeters and its infiltration time through the soil profile to tile drains. Corn (Zea mays L.) was fertilized with 135 kg N ha -1 as ¹⁵N-enriched urea plus 13.5 and 48.1 kg N ha -1 preplant for 1993 and 1994, respectively. Urea-N was band applied to 20 and 10 undisturbed lysimeters at 2.0 and 5.93 atom percent (at %) ¹⁵N in 1993 and 1994, respectively. Average resident times of N03- -N in the lysimeters was 11.7 months. Lysimeter and tile drainage indicate the presence of preferential pathways. Residence times of N03- -N depend on frequency and intensity of precipitation events. Another objective was to determine what portion of the total N in the crop was from applied urea-N and what portion was from the native soil-N. Nitrogen plots received ¹⁵N enrichments of 4.25 and 5.93 at % ¹⁵N in 1993 and 1994, respectively. At the end of the 1993 and 1994 growing season, 41.5% and 35.7% of the labeled fertilizer N remained in the soil profile, while the total recovery of applied ¹⁵N in the soil-plant system was 86.2% and 75.4%, respectively. Low recoveries of applied N may have been the result of soil or aboveground plant biomass volatilization, or denitrification or preferential flow processes. Further research needs to be conducted with strict accountability of gaseous loss and the mechanism(s) responsible. / U.S. Bureau of Reclamation

One must consider the effects of both soil N and fertilizer N, if rates of N-fertilizer application are to be optimized and NO$ sb3 sp-$ leaching into groundwater be controlled. Objectives were (1) to determine soil $ rm NO sb3 sp-$-N and NH$ sb4 sp+$-N, soil total N, and N fertilizer levels and corn and wheat yields; and, (2) to determine soil sampling times and depths for N analyses that correlated with yields and fertilizer N response. Soil samples taken at seeding and postseeding were analyzed for NH$ sb4 sp+$-N and NO$ sb3 sp-$-N, and for total N in 29 wheat sites and 44 corn sites fertilized with four rates of N. N-Trak (quick test) and laboratory methods were employed to measure soil NO$ sb3 sp-$-N. A significant correlation was found between N-Trak and laboratory methods (R$ sp2$ = 0.61$ sp{**}$ for corn and 0.89$ sp{**}$ for wheat). Compared to the laboratory method, N-Trak overestimated soil NO$ sb3 sp-$-N. Soil NO$ sb3 sp-$-N levels increased upon drying. In some cases soils showed an increase in soil NO$ sb3 sp-$-N from seeding to postseeding. Wheat yields and wheat grain N uptake were better correlated with soil NO$ sb3 sp-$-N at seeding than at postseeding. For corn, yields and grain uptake showed a higher correlation with soil NO$ sb3 sp-$-N at postseeding. Corn regression models predicted 180 to 240 kg ha$ sp{-1}$ of N fertilizer to obtain optimum economic yields and 0 to 40 kg ha$ sp{-1}$ N for wheat. Soil total N was not correlated with corn yields as highly as soil NO$ sb3 sp-$-N. Soil NO$ sb3 sp-$-N can be used as an indicator of fertilizer N requirements for corn and wheat in Quebec.

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