Relative contribution of soil N availability and grain sink demand to the control of post-anthesis N uptake by field-grown spring barley

Context: Increasing nitrogen (N) use efficiency of cereal production requires the optimisation of the source-sink relationships governing N accumulation by the grain, post-anthesis N uptake (PANU) and remobilization of N from vegetative tissue. There is evidence that N uptake may be regulated by both plant demand, of which grain N demand is the major component after flowering, and by N availability in the soil but the relative contribution of each to the control of PANU in field-grown barley crops is not understood. Objective: The objectives were to investigate the control of PANU by determining its response to variations in N supply and grain N demand in crops differing in N status and soil mineral N content at flowering. Methods: Field experiments were conducted on spring barley (Hordeum vulgare L.) over three years between 2011 and 2014. N fertilizer application at anthesis was used to vary post-anthesis N supply whilst partial degraining and barley variety were used to vary grain N demand. These treatments were imposed independently or in factorial combination depending on the experiment. Contrasting fertilizer regimes before or at the start of tillering, sites and seasonal weather generated a range of crop and SMN contents at flowering. Measurements were made of above-ground crop N content and soil mineral N (SMN) through the season. Results: By, or shortly after, anthesis the SMN content of the root-zone had depleted to a relatively steady minimum value of around 60–115 kg N ha ⁻¹ depending on the site and year. PANU of control plants (non-degrained and without additional N fertilizer at anthesis) ranged from − 20–70 kg N ha ⁻¹. Additional N fertilizer increased (P < 0.05) PANU in all experiments, whilst degraining reduced it significantly only in 2012. The response to degraining and anthesis N application were unaffected by crop N status at anthesis. There was no relationship between PANU and the unsatisfied grain N demand (that not met by retranslocation alone) at any level of anthesis N-fertilizer application in 2014. In 2012 there was a weak relationship accounting for only a small amount of the variation in PANU. Conclusions: PANU of spring barley is limited mostly by N availability to the root system and not by a low grain N demand. High residual SMN contents at harvest and the poor relationship between SMN at anthesis and PANU suggest that transfer of N from bulk soil to the sites of active uptake at the root surface is a major limitation to PANU. Implications: Increasing N uptake efficiency during grain filling will require improvements at the root-soil interface and not an increase in grain N demand.