| In order to study the influence of organic and chemical fertilizer combination on crop yields, nitrogen utilization, nutrient and microbial carbon in soil, a field experiment was carried out with wheat and maize growing in purple soil in Dianjiang County, Chongqing, from 2007 to 2009. The main results obtained were as follows:1. Application of chemical fertilizer plus manure or straw decreased greatly N fertilizer by 5%～63%, P by 4%～62%, and K by 16%～98%, respectively, resulting in less environment pollution.2. Combination of organic and chemical fertilizers also enhanced significantly the yields of wheat and maize year by year. FM3 obtained highest crop yields compared to the traditional fertilization treatment, namely with the yield increment of 806 kg·hm-2s for wheat and 810 kghm-2for maize during 2008-2009. The direct reasons for high grain output were mainly due to the enhancement of grain weight per a thousand grain, grain numbers per ear, and available spikelet for both wheat and maize.3. Plant N uptake enhanced significantly by adding organic fertilizer together with chemical fertilizers. Compare to traditional fertilization, FM1 and FM3 resulted in the largest increment of plant N absorption by 0.5%～47.6% and by 4.4%～37.1%. Agronomic N efficiency increased by 72.8%(FM1),72.5%(FM2), and 96.6%(FM3), respectively, on the base of Fl. Similarly, FM1, FM2 and FM3 increased N content in grain was by 34.1%,13.1%, and 34%. However, there were no significant differences in grain N contents among with balance fertilization treatments.4. Contents of organic matter, total N, total P and available N, P, K in the farming layer(0～20cm) were significantly higher than in the plow pan(20～40cm) except total K, which show that K in soil is being exhausted by the original fertilization systems in local. Balanced fertilization or combined application of organic and chemical fertilizers and increase the content of K in soil, the effection is especially significant when applied with straw (FM2, FM3).5. Concentration of NH4+-N in soil varied from 4.21 mg·kg-1～11.78 mg·kg-1 on main growth stage of wheat, and NO3--N in soil varied from 3.70 mg·kg-1～31.85 mg·kg-1. And on maim growth stage of maize, NH4+-N in soil varied from 4.15 mg-kg-1-14.34 mg·kg-1,NO3--N varied from 1.65 mg·kg-1～33.40 mg·kg-1. Jointing stage of wheat, trumpet stage and tasseling stage of maize are the period of strength for N uptake. Changes of NH4+-N and NO3--N concentration in soil in treatments applied with organic and chemical fertilizers are consistent with the demand, and the supply is gradual and sustainable, which is probably one of the reason for high outputs.6. Soil microbial activity can be increased significantly and reach the highest point in vigorous growth stages when organic and chemical fertilizers were applied together, and SMBC is higher than F1 by 122.6%～151.4% and 44.3%～97.5% at filling stage of wheat and tasseling stage of maize when the soil microbial activity is the strongest. It is beneficial to supply consistent nutrition in the middle-late growth stage when more organic matter is decomposed by stronger microbial activity.
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