Effect of soil type on nitrogen balance at maturity in wheat characterized using 15N-labelled NH4NO3-fertilizer and sink-source modifications at flowering

Wheat plants were grown in a greenhouse in containers filled with chalky or loamy soil. Single-labelled ammonium nitrate fertilizer (NH4NO3)-N-15 or (NH4NO3)-N-15 (5 atom% N-15) was applied in a spit-dose after the third leaf-stage. Initial soil nitrate levels were lower in chalky soil. At maturity, the amount of N recovered in shoots was lower in chalky plants (CP) than in loamy plants (LP) but N fertilizer recovery was higher in CP than in LP. There was a greater (NO3)-N-15 recovery, in CP: (NO3)-N-15 /(NH4)-N-15 being 1 in LP and 1.3 in CP seeds. This was explained by efficient N mobilization enriched in (NO3)-N-15 in CP. Leaf excision or shoot shading at flowering changed the seed (NO3)-N-15 /(NH4)-N-15 ratio; it increased in CP and decreased in LP for plants with excised leaves whereas it was not modified in CP but decreased in LP for shaded plants. This indicated that grain filling was predominantly via mobilization in CP, whereas a late assimilation was involved in LP. The flag leaf in CP was the site for early and transient storage of NO3- and later a main source of assimilated N for seeds. Benefits previously observed in vegetative wheat plants grown on chalky soils compared to loamy soils with respect to enhanced NO3 utilisation, are also manifest at grain-filling and maturity.