Effect of different irrigation systems on lateral movement of labeled urea fertilizer in nitrogen-15 microplot under lettuce and sugar beet cultivation

In order to evaluate the influence of different irrigation systems on lateral movement of urea fertilizer and its effect on nitrogen-15 microplot design, an experiment was carried out during 2005-2006 in a randomized complete block (RCB) design (split plot) on lettuce and sugar beet with three treatments and replications. The plot area was 144 m2 and in the middle of each plot, an area about 1 to 3 m2 was selected as an isotopic subplot. The treatments were different irrigation systems (main factor) consisted of trickle fertigation (T1), sprinkler fertigation (T2), and furrow irrigation (T3) and different plant sampling areas (sub factor) consist of central part, first and second guard and non isotopic area. The crops were harvested in different parts of microplots and 15N/14N isotopic ratio was determined by the means of emission spectrometer NOI7. The results showed that, in lettuce culture, central parts of isotopic subplots and first guards were in the same statistical groups and 48% of fertilizers were supplied with the labeled materials. For sugar beet cultivation, central parts of isotopic subplots, first guards and non isotopic parts were situated in three different statistical groups and decreasing slope of nutrient absorption were 31% and 82% respectively. Therefore, for suitable design of isotopic subplots, increasing fertilizer use efficiency is quite necessary and by means of this methodology, low enriched fertilizers can be applied through the field experiment. It is also important to leave one row of plants (as guard) between isotopic and non isotopic rows to prevent lateral movement of nitrogen fertilizer. In this condition, area of isotopic subplots can be shrinked up to 0.56 m2 (in lettuce) and 1.12 m2 (in sugar beet). Under this condition, the efficiency of system can be increased up to the level of plastic barrier isotopic subplots. On the other hands, in furrow treatment, by presence of active rooting system (in accordance with absorption of nitrogen), liner movement of nitrogen were adjusted (along with irrigation channel), and this is the main reasons for similarity of system with drip and sprinkler fertigation methods. In the furrow irrigation system, movements of nitrogen-15 were more complicated. Therefore, the best place of sampling is 25 cm ahead of irrigation water pathway.