The Effects Of Starter-N Plus Topdressing N On N Absorption Of Soybean Plants With Different Densities

In order to research the effects of starter-N plus topdressing N on N accumulation, distribution,translocation and yield of soybean under different densities, a field experiment was conducted withsoybean cultivar Dongnong52, using a split-plot design with three densities–200000,250000, and300000plants·ha^-1，and three N application patterns: a single application of N60kg·ha^-1as basalfertilizer; N18kg·ha^-1as basal fertilizer followed by N42kg·ha^-1at early pod stage （R3）; and N18kg·ha^-1as basal fertilizer followed by N42kg·ha^-1at early pod stage （R4）. N content at different growthstages in different soybean organs were determined, N accumulation in soybean organs were calculatedas well. Main results are shown as following:Under the same density, comared to using N only as basal fertilizer, starter-N plus topdressing Nproduced increases in N contents of leaves, petioles and stems at stage R6; N contents in leaves foradding N at stage R3increased by12%and17%（P<0.05）, for adding N at stage R4increased by7%and14%（P<0.05）, N contents in stems for adding N at stage R4increased by13.3%and18.8%（P<0.05）. Comparing the various N application patterns, the single application of N as basal fertilizerhas led to the highest N contents in pods after stage R5. At stage R8,250,000plants·ha^-1led to higher Ncontents of pods than other densities. Experimental results indicated that the use of topdressing Nincreased N contents in vegetative organs during seed-filling and slowed premature aging of leaves.Within the same N application pattern, N accumulation of various organs per plant increased as thedensities increased greatly. Under the same density, N accumulation per ha under250,000plants·ha^-1was higher than those under200,000plants·ha^-1and300,000plants·ha^-1at stage R7or R8. Compared tousing N only as basal fertilizer under200,000plants·ha^-1, N accumulation of single plant or per ha forstart-N plus topdressing N at R4under250,000plants·ha^-1increased by16.0%and45.0%（P<0.05）,respectively. Experimental results showed that using starter-N plus top dressing N optimized N nutritionduring late growth stage, enhanced N uptake and accumulation of250,000plants·ha^-1. Under the samedensity, using starter-N plus topdressing N led to higher accumulation of N at stage R6in various organscompared to using N only as a basal fertilizer. Compared to using N only as a basal fertilizer under200,000plants·ha^-1, starter-N plus topdressing N under250,000plants·ha^-1showed the followingincreases: N accumulation increased by59.2%（P<0.01） in leaves, increased by42.8%（P<0.01） instems, and increased by15.6%（P<0.05） in pods, N distribution of leaves and stems increased by6% and9%, respectively. The results indicated that starter-N plus topdressing N under250,000plants·ha^-1slowed premature aging in leaves during seed-filling stage, promoted capacity of N assimilation inleaves and stems, accelerated transportation of N into pods.Compareing to using N only as a basal fertilizer, using starter-N plus topdressing N increased Naccumulation and contribution rate, reduced contribution rate of N translocation to seed after stage R5.N assimilation after stage R5and contribution rate of N assimilation significantly and positivelycorrelated with yield. N accumulation after stage R5for using starter-N plus topdressing N under250,000plants·ha^-1increased by113.2%（P<0.05）. Under300,000plants·ha^-1density, N accumulationafter stage R5for adding N at stage R4was higher than those adding N at stage R3. Experimentalresults indicate that topdressing N at R4delayed premature aging, enhanced capacity of N accumulationafter stage R5under high density.When density increased from200,000plants·ha^-1to250,000plants·ha^-1, yield increased. Within thesame N application pattern, the yield under250,000plants·ha^-1increased by16.7%，15.2%and16.7%（P<0.05）, respectively, than that under200,000plants·ha^-1. Compared to using N only as basal fertilizerunder200,000plants·ha^-1, seeds per m2for adding N at stage R4under250,000plants·ha^-1increased by22.7%（P<0.05） as the density increased， while seeds of single plant haven’t decreased, resulting inyield increase by33.3%（P<0.05）. When the density was300,000plants·ha^-1, the yield showed adecreasing trend.