Abstract:During 1977-1980,the field experiments have been conducted with radio-isotope tracer technique to Study the effecf of mulched wheat straw on yield of summer soybean.Experimental design was as follows:( 1 ) mulched wheat straw with 300kg/mu at the early-flowering stage of summer sown soybean and (2) control.Seeds yield of soybean was determined and the transition and accumulation of nutritive material in soybean plant were monitored and recorded by 14C and 32P technique.They are as follows:1.Under mulched wheat straw in the field the yield the yield of soy bean was increased about 16.2% or 22.lkg/mu The result is statistically significance as P0.05 level,L.S.D=±15.0% or ±20.3kg/rnu.2.Under mulched wheat straw in the field,the number and weight of nodules and nitrogen fixing activity of soybean were increased 39.2%;81.7% and 20.0% respectively over control during branching-early flowering stages.3.The experimental data with 14C and 32P showed that respiratory intensity of soil was increased.Average amount of CO2 released from mulched wheat straw was 5.60 kg/mu/day,and 5.21kg/mu/day of wheat straw was mineralized Intensity of photosynthesis by soybean plant were increased.The accumulation of 14C-photosynthate in pods and nodules were increased by 30.1%-70.5% and 4.5 times respectively over control.32p in shoot and pods under mulched wheat straw is increased by 44.8% and 22.1% respectively in comparison with the control.The results indicated that the amount of nutritive material in plant of soybean was increased by mclched wheat straw.It laid a material foundation for increasing the seed yield of soybean,4.Under mulched wheat straw in the field the organic matter of soil was increased 0.06% in every year.And 20.8% of total humus in soil was obtained from mulched wheat straw.It will be advantageous to maintain and improve soil fertility.