Understanding the physiological basis of post-flowering nitrogen (N) dynamics in spring barley to improve nitrogen use efficiency (NUE)

Identifying ways to improve nitrogen use efficiency (NUE) in cereal production is a major international research priority. It has been estimated that globally only around 40% of N from fertilizer is recovered in the grain of cereal crops. Rates of post-anthesis N uptake (PANU) by crops are often low and high quantities of N can be found in the soil at harvest, apparently left unused by the crop. The low rate of PANU needs to be overcome to minimise the negative environmental impacts caused by the use of N fertilizers. A better understanding of the physiological control of PANU as an interrelated mechanism of soil N availability, plant N demand, N remobilisation and root growth and activity is needed to further improve NUE. The aim of this project was to investigate the dynamics of N uptake and remobilisation during the post-flowering period in relation to the N status at flowering, the soil N supply and plant N demand during grain filling. Controlled environment experiments were conducted on spring barley cv Westminster grown in sand-perlite using 15N labelling techniques to discriminate between the fate of N taken up before anthesis and that taken up during grain filling. The results showed that barley roots have the capacity to uptake N from repeat low concentration (1 mM) applications with high efficiency throughout grain filling (>90%). The efficiency of PANU was not affected by the plant N status at flowering. Higher N status plants remobilized a larger quantity of N from vegetative tissue for allocation to the grain, but the remobilisation efficiency was little affected. The temporal dynamics of remobilisation from each organ, however, showed some differences, with a slower onset of remobilisation from leaf sheaths and stems in plants of higher N status compared to plants of low status. The role of N supply and grain N demand in the control of PANU was tested by varying the concentration of N applied after flowering and by partial de-graining. Regardless of the N supply, de-grained plants accumulated significantly less N than the control plants between early and late grain filling. However, there was no significant difference between de-grained and control plants in their net N uptake measured by short term (24 hours) 15N labelling. De-graining and increased N supply increased the partitioning of 15N to vegetative tissues, mostly towards the tillers. These results indicate that there is a large sink demand for N post-flowering even after grain numbers are reduced. They also suggest that the observed reduction in the total N accumulation is not the result of reduced PANU, but the consequence of a different mechanism, potentially N losses from the plant to the atmosphere. However, this hypothesis was not supported by measurements of NH3 volatilization from leaves and ears of de-grained and control plants. Further experiments are needed to determine the cause of the reduced N accumulation in de-grained plants. A field experiment was conducted in 2019 to compare post-anthesis soil nitrate depletion by spring barley with that of spring oats, a species regarded as having a high N uptake efficiency. Spring oats depleted soil nitrate to a greater extent than spring barley and this effect was consistent across four varieties of each species tested at anthesis, but the soil N dynamics change during the grain filling period. The poorer depletion by barley was associated with its smaller average root length density in the topsoil, but variability in the data prevented the establishment of critical root length densities for nitrate uptake by each species. These experiments have shown that spring barley has a large demand for N during grain filling and maintains a high physiological capacity for its uptake. The relatively poor depletion of soil nitrate by field grown barley crops during this period may be associated with restricted access of roots to N rather than physiological controls over its uptake. Improvements in root distribution may be a suitable target to increase N uptake efficiency of spring barley