Abstract: Maize,sorghum and millet were used as reference crops in quantitatively estimating sym-biotic nitrogen fixation by soybean.With no nitrogen fertilizer application,the amounts ofnitrogen taken up by the three reference crops were closely similar,indicating that the nitro-gen taken up from soil by crops seemed to be controlled by the soil nitrogen release rate whi-ch was not related to the crop types.According to the difference of total nitrogen betweensoybean and reference crops,the nitrogen fixed by soybean was estimated to be 131 to 155kgN per hectare in 1983 and 143 to 163 kg N in 1985,which accounted for 50—55% of thetotal nitrogen in the harvest of soybean.

Key words: Municipal sludge, Resource and environment, Urban and agricultural sustainable development