Effects of nitrogen forms on the absorption and distribution of nitrogen in Populus tomentosa seedlings using the technique of 15N tracing.

In order to explore the characteristics of absorption,utilization and distribution of nitrogen,with clone 83 of Populus tomentosa seedlings as materials,the stable isotope~(15)N tracer was used to measure the absorption,nitrogen use efficiency(NUE) and distribution rate under the same nitrogen (N) input treatment from March to September in 2007 in the Nursery of Beijing Forestry University. The results indicated that:1) the rates of absorption and NUE for NO_3-~(15)N and NH_4-~(15)N fertilizers reached the peak 28 days after fertilization with 0.36 in contrast to 0.15 g/plant,and 35.98% to 14.53%,respectively.2) Based on the overall estimate of the average NUE,NO_3-~(15)N fertilizer was up to 19.75%,about 2.5 times of that of NH_4-~(15)N fertilizer,which was only 7.95%.3) At each stage after fertilization,the percent of nitrogen in total plant derived from NO_3-~(15)N fertilizer(Ndff) was significantly higher than that from NH_4-~(15)N fertilizer.The transfer ability from NO_3-~(15)N fertilizer by each organ was much stronger than that from NH_4-~(15)N fertilizer.The stem of P.tomentosa seedlings was the most competitive organ for transferring nitrogen(N) fertilizer,which was followed by leaf and root.4) For the same three organs,nitrogen distribution rate was significantly different(P0.05) with the trend of leafrootstem.In the leaf,the distribution of NO_3-~(15)N was higher than that of NH_4-~(15)N.The nitrogen accumulated in the root mainly supplied for the growth of aboveground organs and declined gradually with growth.Stem,the so-called "temporary storehouse" as the most competitive organ,mainly transports nitrogen to leaf and other organs in order to support vigorous growth of P.tomentosa seedlings.