Seed Yield and Nitrogen Efficiency in Oilseed Rape After Ammonium Nitrate or Urea Fertilization

Abstract

In agricultural plant production, nitrate, ammonium, and urea are the major fertilized nitrogen forms, which differ in root uptake and downstream signaling processes in plants. Nitrate is known to stimulate cytokinin synthesis in roots, while for urea no hormonal effect has been described yet. Elevated cytokinin levels can delay plant senescence favoring prolonged nitrogen uptake. As the cultivation of winter oilseed rape provokes high nitrogen-balance surpluses, we tested the hypotheses whether nitrogen use efficiency increases under ammonium nitrate- relative to urea-based nutrition and whether this is subject to genotypic variation. In a 2-year field study, 15 oilseed rape lines were fertilized either with ammonium nitrate or with urease inhibitor-stabilized urea and analyzed for seed yield and nitrogen-related yield parameters. Despite a significant environmental impact on the performance of the individual lines, which did not allow revealing consistent impact of the genotype, ammonium nitrate-based nutrition tended to increase seed yield in average over all lines. To resolve whether the fertilizer N forms act on grain yield via phytohormones, we collected xylem exudates at three developmental stages and determined the translocation rates of cytokinins and N forms. Relative to urea, ammonium nitrate-based nutrition enhanced the translocation of nitrate or total nitrogen together with cytokinins, whereas in the urea treatment translocation rates were lower as long as urea remained stable in the soil solution. At later developmental stages, i.e., when urea became hydrolyzed, nitrogen and cytokinin translocation increased. In consequence, urea tended to increase nitrogen partitioning in the shoot toward generative organs. However, differences in overall nitrogen accumulation in shoots were not present at the end of the vegetation period, and neither nitrogen uptake nor utilization efficiency was consistently different between the two applied nitrogen forms.

Keywords: ammonium nitrate; cytokinin translocation; nitrogen partitioning; nitrogen uptake; nitrogen uptake efficiency; nitrogen use efficiency; rapeseed; urea.

FIGURE 1

Seed yield of 15 oilseed rape genotypes after ammonium nitrate or urea fertilization in 2012/2013 and 2013/2014. Dots show means ± SD (n = 4). Asterisks *, **, and *** indicate significant differences or interactions at p < 0.05, p < 0.01, p < 0.001, resp., according to ANOVA on ranks; G, genotype; T, N treatment; E, environment; ns, not significant.

FIGURE 2

Mean translocation rates of total N (A), amino acids (AAs, B), N forms (C), and cytokinins (CKs, D) in the xylem sap of an oilseed rape panel at different developmental stages in dependence of ammonium nitrate or urea application. In the experimental years 2012/13 and 2013/14, 15 oilseed rape cultivars were grown for xylem sap analysis after fertilization with ammonium nitrate or stabilized urea (see Supplementary Figure 3). Boxes show median and first and third quartiles; whiskers show minimum and maximum of all data in a group (n = 60; A). Bars show means -SD (n = 60; B,C). Different upper- or lower-case letters show significant differences among the ammonium nitrate- or urea-treated variants, resp., within individual experimental years according to Tukey’s test on ranks at p < 0.05. Asterisks indicate significant differences between ammonium nitrate and urea treatment at individual developmental stages and years according to unpaired t-test (2012/13: Ammonium nitrate at BBCH57) or Mann–Whitney rank sum test (all other xylem compounds in both years and respective developmental stages) at p < 0.05. Letters and asterisks refer to the compound color code. AN, ammonium nitrate.

FIGURE 3

Individual N pools in aboveground organs and organ fractions as mean over all genotypes or in individual genotypes after ammonium nitrate or urea fertilization in 2012/13 (A) and 2013/14 (B). Pie charts show N contents (means of all lines ± SD) of the indicated organs and the corresponding percentage to total plant N at BBCH79 (n = 60). Asterisks in the right-hand chart indicate significant mean or median differences among ammonium nitrate and urea treatment according to unpaired t-test (A: stem pods, branch pods) or Mann–Whitney rank sum test (A: all other fractions, B: all fractions) at p < 0.05. Bars show means of single lines -SD (n = 4). ANOVA or ANOVA on ranks (- -) results *, **, and *** indicate significant differences or interactions at p < 0.05, p < 0.01, and p < 0.001, respectively; G, genotype; T, N treatment; ns, non-significant. Asterisks referring to the organ color code indicate significant mean difference among ammonium nitrate and urea treatment within a line according to unpaired t-test at p < 0.05. Different upper/lower case letters within a bar indicate significant mean or median differences among the lines within ammonium nitrate/urea treatment according to Tukey’s test (A—Ammonium nitrate: stem pods, stem, branches, branch leaves, stem senescent leaves; - Urea: stem pods, branch pods, stem, branches; B—Ammonium nitrate: stem pods, stem green leaves; - Urea: stem pods, branch pods, stem) or Tukey’s test on ranks (A—Ammonium nitrate: branch pods, stem green leaves; - Urea: branch leaves, stem green leaves, stem senescent leaves; B—Ammonium nitrate: branch pods, stem, branches, branch leaves, stem senescent leaves, - Urea: branches, branch leaves, stem green leaves, stem senescent leaves) at p < 0.05. AN, ammonium nitrate.