A Study On N Accumulation And Distribution In Soybean Plants With¹⁵N Labelling Technique

15N isotope tracer technique was used in this pot experiment to study the effects of starter-N combined with topdressing has on N accumulation, distribution and soybean yield.5treatments under the same N application rate of50kg/ha and different N application time were set:all N as basal fertilizer (N50), all N as topdressing at R3/R4stage (N50R3, N50R4), N15kg as basal fertilizer and35kg as topdressing at R3/R4stage (N15+35R3, N15+35R4). N content in different growth stages and15N atom excess of different soybean organs were determined, N accumulation and Ndff (N derived from fertilizer) accumulation in soybean organs were calculated as well. Main results are shown as following:1. N accumulation in leaves, stems and nodules of N15+35R4were12%(p<0.05),29%(p<0.01) and18%greater, respectively, than N50at R6. N accumulation in pods of N15+35R4was16%(p<0.01) greater than that of N50, yet N accumulation in stems was15%(p<0.05) less in N15+35R4, indicating that starter-N combined with topdressing delayed the senescence of soybean leaves after pod filling effectively and increased N accumulation in soybean organs after R6significantly. There was a significant positive correlation between N accumulation in R4-R8stages and soybean yield (r=0.795**), and the increasement of N accumulation after R4stage improved soybean yield significantly.2. N accumulation of N15+35R3/R4was7.6%and14%(p<0.05) greater, respectively, than N50, and N accumulation in pods of N15+35R3/R4was9%and16%(p<0.05) greater, respectively, than N50. This significant enhancement of N accumulation in pods showed that N transportation into pods and seeds after R5stage were promoted by N15+35R3/R4, which is beneficial to soybean yield.3. Results of15N determination showed that more fertilizer N would be stranded in vegetative organs under all-basal N application. While seeds were filling up rapidly at R6, fertilizer N accounted for37%and51.6%of N accumulation in vegetative organs and pods, respectively, of N50, yet the proportions in N15+35R4were21.9%and71.8%, suggesting that the transportation of fertilizer N into pods was accelerated by topdressing N.4. Seeds per plant of N15+35R3/R4were15and16(p<0.01) greater, respectively, than N50, and yield increased by17.6%and19.9%(p<0.05), respectively, compared to N50, yet there’s no significant difference among N50, N50R3and N50R4. N use efficiency (NUE) of N50(13%) was far less than other treatments (24%-28%). NUE of N50R4/N15+35R4was10.7%and16.6%(p<0.05) greater than N50R3/N15+35R3, suggesting that the uptake of fertilizer N can be promoted by starter-N combined with topdressing, and topdressing at R4brings up a better effect than topdressing at R3.