Fate of Nitrogen-15 Enriched Ammonium Nitrate Applied to Corn

Nitrogen utilization by corn (Zea mays L.) is influenced by the form of inorganic N present in the root zone. A field experiment was conducted on Enon sandy loam (fine, mixed, thermic Ultic Hapludalf) to determine N use efficiency and its partitioning in various plant parts of corn. Ammonium nitrate labeled either as NH4-N or NO3-N and applied at 50, 100, or 200 kg N ha" was evaluated. Microplots (1.06 m) were established in the main N plots for N fertilizer application, and main plots were used to determine grain yield. After the crop harvest, soil N alone, fertilizer N used by the crop, and fertilizer N remaining in inorganic and organic forms in the top 75 cm of the soil were measured. Grain yield response to N application was significant (P < 0.01). Recovery of applied fertilizer N in corn and weeds ranged from 43 to 57% and 3 to 5%, respectively. Nitrogen sources showed no significant differences with respect to N accumulation in corn. Only 17 to 20% of the fertilizer N was recovered in the grain. The amounts of soil N utilized by the corn was about threeto sixfold higher than the fertilizer N, suggesting extensive turnover of soil and fertilizer N through immobilization and mineralization. More N was recovered (21-63% of added N) in the soil from NH4 than from NO, (6-38%). Loss of N (unaccounted for) ranged from 11 to 18% at 100 kg N ha-' and from 34 to 48% at 200 kg N ha-. Nitrogen loss was higher in the plots receiving NO, than NH4. Most of the fertilizer N remaining in the soil at the end of the growing season was in the organic fraction, suggesting immobilization into microbial and root biomass. N USE EFFICIENCY by corn is influenced by source, rate and time of fertilizer application, and hydrologic conditions such as rainfall distribution and water-table fluctuations during the growing season. The use of fertilizer labeled with the stable isotope N provides a meaningful way to differentiate between the total amount of fertilizer N utilized by the crop and N losses during crop production for both environmental and agronomic reasons. Many researchers (e.g., Carter et al., 1967; Westerman et al., 1972; Bijeriego et al., 1979; Olson, 1980) have studied the recovery of labeled fertilizers in corn and sudangrass [Sorghum x drwnmondii (Steudel) Millsp. & Chase] at the time of harvest. Kitur et al. (1984) reported that 71 to 75% of the fertilizer N was accounted for in harvested grain and stover of corn and soil. Nitrogen rate had no large effect on the fraction of fertilizer N unaccounted for and presumed to be lost by denitrification and leaching. Chabrol et al. (1988) found that, when a mixture of (NH4)2SO4 and KNO3 (122 kg N ha") was applied to corn 12 d after planting, 61% was present in the crop and 19% remained in the soil; the 20% loss observed was assumed to be due to a combination of leaching and denitrification. Hera (1977) observed that, when labeled NH4NO3 was • applied at the rate of 160 kg N ha ~ in split applicaG.B. Reddy, Dep. of Plant Science and Technology, North Carolina A&T State Univ., Greensboro, NC 27411; and K.R. Reddy, Dep. of Soil and Water Science, Univ. of Florida-IFAS, Gainesville, FL 32611. Joint contribution from North Carolina A&T State Univ. and Univ. of Florida. Received 21 Feb. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:111-115 (1993). tion, 47 and 14 kg N ha" were recovered in grain and stalk, respectively, and the efficiency of fertilizer utilization was 38%. Sanchez and Blackmer (1988) found that 13 to 33% of the labeled N was recovered in corn grain and 19 to 23% in soil after application of N-labeled anhydrous NH3. Inorganic N occurs in most soils as NHJ and NO^, and in upland soils NO^ is the predominant form of inorganic N available to plants. Therefore, it is important to know which form of inorganic N is highly available to corn and its partitioning in the soil-plant system. The studies reported above present very little information on the partitioning of applied fertilizer N in plant parts of corn at the end of the growing season. This information may provide insight on N use efficiency and translocation, especially for N-limited systems. The objectives of this study were to (i) quantify the fertilizer N uptake by corn, and its partitioning in various plant parts; (ii) determine the N use efficiency by corn; and (iii) estimate fertilizer N loss using mass balance. MATERIALS AND METHODS The field experiment was conducted in 1986 on an Enon sandy loam soil, at the North Carolina A&T State University farm in Guilford County, North Carolina. The surface soil characteristics were: pH, 6.0; CEC, 7.0 cmolc kg-; soil total N, 0.31 g kg-'; total inorganic N, 21 mg kg-; and organic matter content, 8.1 g kg-. The field plots were tilled before planting. The plot size was 4.57 by 7.62 m and was planted with six rows of corn =0.91 m apart. Two weeks prior to planting, 3.51 L habutylate (S-ethyl diisobutylthiocarbamate) and 2.35 L haatrazine (2chloro-4-ethylamino-6-isopropylamino-s-triazine) were mixed in an appropriate volume of water and applied to the plots. Pioneer 3369AR corn was planted on 8 May 1986, at a rate to obtain 43 078 plants ha-. Ammonium nitrate was applied at the rate of 0, 50, 100, or 200 kg N ha-. Recommended rates of P (60 kg ha-) and K (60 kg ha-) were applied to all the plots. Planting and fertilizer application were done by hand. Fertilizers were applied in bands ~5 cm deep and 8 cm away from the center of each row. Treatments were replicated three times in a randomized complete-block experimental design. Average ambient minimum and maximum air temperatures during the growing season were 16 and 27 °C, respectively. The total precipitation during the growing season was 170 mm. For determination of fertilizer N recovery, N-enriched fertilizer was applied to microplots (1.16 by 0.92 m) within each large plot. Two microplots were established permanently in the center of each main plot by inserting a galvanized metal (1-mm-thick) barrier into the soil to a depth of 0.31 m. Two microplots in each main plot were 2.14 m apart. One microplot was treated with NH4NO3 (10.387 atom %) and another with NH4NO3 (10.396 atom %), at respective N levels of each treatment. Enriched N fertilizer was placed in bands as described for the main plot. Due to the dry season, plots were irrigated (50 mm ha-) once in May and twice in June. Plots were hand weeded and weed plants were washed, dried, and saved for chemical analysis. At harvest, plants in the center rows were harvested for Abbreviations: CEC, cation-exchange capacity; Ndff, plant N derived from labeled fertilizer N; Ndfs, soil N plus nonlabeled fertilizer N; Nds, native soil N.