Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/18/2011
Publication Date: 9/9/2011
Citation: Herrera, J.M., Delgado, J.A., Dillon, M., Barbarick, K., Mcmaster, G.S. 2011. Accumulation of late-applied nitrogen and root dynamics during grain filling in irrigated spring wheat. Communications in Soil Science and Plant Analysis. 42:2235-2249. Interpretive Summary: The genotype Jerome had a higher grain and N yield than Snowcrest when grown in the field. The higher 33 kg N ha-1 accumulated by Jerome was achieved mainly during grain filling. Therefore, these two genotypes differed in their capacity to take up N during advanced growth stages. From anthesis to physiological maturity, the root length density of Jerome decreased an average of 38% and the root mass density decreased 47%, whereas the same values for Snowcrest decreased 64% and 69%. Although the root length density of both genotypes tended to decrease, greater maintenance of a functioning root system was associated with higher N uptake. The positive and negative relationships between 15N accumulation and the changes in root length density in the top soil (0.20 m) for Snowcrest and Jerome may be associated with competition for carbohydrates between N accumulation and grain growth during grain filling. Jerome produced more biomass, and therefore, N may have been more limiting during grain filling than carbohydrates. In contrast, Snowcrest had a lower production of biomass, so carbohydrates may have been more limiting than N. Therefore, for the latter with a lower demand for N during grain filling, the higher production and maintenance of roots functioning and competing for assimilates with the spike might have resulted in lower N yield. Nitrogen demand after flowering is a critical genotypic difference that determines N uptake and N use efficiency. The results of this study demonstrate that the capacity to use available N in advanced growth stages differed among two spring wheat genotypes and this difference indicates that there are genotypes with superior potential to be used under split N management scenarios. The greater capacity to take up N during grain filling was associated with greater growth and maintenance of the root system in the soil layers where N availability was higher.
Technical Abstract: One of the major goals of modern agriculture is to maximize nitrogen use efficiency. Although considerable advances have been achieved in N management, little progress has been made in identifying root traits that may enhance N uptake under modern N management practices. Wheat genotypes can differ in root growth and root distribution in the soil. However, little is known about whether or not these differences occur during advanced growth stages, such as grain filling and if they are relevant for N uptake under a split N fertilization management. The objective of this study was to investigate the relationship between root growth and N uptake during grain filling under the currently considered best N management practices. In addition, we wanted to assess the effect that genotype has on this relationship. Two spring wheat genotypes (Triticum aestivum L.) were grown in the field under three levels of N supply (180, 210 and 240 kg N ha-1) in a San Arcacio sandy loam (Typic Haplargid).. In addition, we conducted three experiments under controlled conditions in a glasshouse to simulate variations in N availability across both time and soil depth. The fate and distribution of N was traced in both environments by means of using labeled N (15N). The genotype Jerome had a higher grain and N yield than Snowcrest. Overall, the higher N yield (33 kg N ha-1) by Jerome was achieved mainly during grain filling. Post heading uptake of N, as indicated by the 15N content, was clearly higher for Jerome than Snowcrest, and Jerome also had the greater increase in root length density during grain filling. The N uptake during grain filling was significantly correlated with the changes in root length density from 0 to 0.6 m (r2 = 0.45), and the 15N accumulation in the biomass was significantly correlated with changes in root length from 0 to 0.2 m (R2 = 0.33 and R2 = 0.36 for Jerome and Snowcrest, respectively). 15N uptake under controlled conditions was 53% higher with 15N supplied at a 15 cm depth than at a 35 cm depth and depended on a three-factor interaction among genotype, N supply, and the time relative to anthesis, in which the onset of N availability was the highest.