The effect of nitrogen fertilization on nitrogen use efficiency of irrigated and non-irrigated tobacco (Nicotiana tabacum L.).

Burley tobacco (Nicotiana tabacum L.) plants were grown in the field with or without irrigation and fertilized with 0, 120, 240 or 360 kg N ha^–1 over two growing seasons to assess nitrogen use under Mediterranean climate conditions. Kjeldahl-N and NO₃-N in leaves and stems and NO₃-N and NH₄-N in the soil at two depths (0–0.3 and 0.3–0.6 m) were determined. The effect of N fertilization on total N accumulated in the canopy biomass was markedly different between irrigated and non-irrigated plants. Under non-irrigated conditions N accumulated in the plant did not depend on the amount of N applied. In both years, the amount of N in irrigated plants increased in response to the amount of N applied, starting from 49 to 56 days after transplanting (DAT). The average amount of total N in the canopy of irrigated plants, measured across all sampling dates of both years, ranged from 30 kg ha^–1 of the unfertilized control to 88 kg ha^–1 of the 360 kg ha^–1 of N applied. The average amount of plant NO₃-N was 2.6 and 4.4 kg ha^–1 for non-irrigated and irrigated plots across all N treatments (means of 1996 and 1997). Nitrogen uptake rate (NUR) of non-irrigated plants was high between seedling establishment and the period of rapid stem elongation in 1996 (from 36 to 50 DAT) and until flowering in 1997 (from 42 to 71 DAT), but much less or negligible at later stages of plant development. Irrigation increased NUR dramatically in the second part of the growing season. Maximum NUR was estimated for plants receiving 240 or 360 kg N ha^–1 in both years. The year of study did not affect the recovery fraction (RF), physiological efficiency (PE) or agronomic efficiency (AE). Irrigation and N fertilization had significant effects on both RF and AE, but not on PE. Maximum values of RF were 45 and 22% for irrigated and non-irrigated treatments, respectively. In irrigated plots there was a negative relationship between RF and increasing N levels at all sampling dates.