Abstract: 【Objective】 Assess the effects of different types and ratios of organic fertilizer substituting chemical fertilizers on soil fertility and sunflower yield in the Hetao Irrigation Area,to select for the organic and inorganic fertilizer combined application models that could simultaneously improve the soil organic carbon and sunflower yield.【Methods】 Eight treatments were set up including no fertilization（CK）,conventional chemical fertilizer nitrogen application（MN）,20% sheep manure organic fertilizer nitrogen+80% chemical fertilizer nitrogen（SN1）,40% sheep manure organic fertilizer nitrogen+60% chemical fertilizer nitrogen（SN2）,20% cow manure organic fertilizer nitrogen+80% chemical fertilizer nitrogen（CN1）,40% cow manure organic fertilizer nitrogen+60% chemical fertilizer nitrogen（CN2）,20% pig manure organic fertilizer nitrogen+80% chemical fertilizer nitrogen（PN1）,and 40% pig manure organic fertilizer nitrogen+60% chemical fertilizer nitrogen（PN2）. The effects of different types and ratios of organic fertilizer substituting chemical fertilizer on soil organic carbon,total nitrogen content,ammonium nitrogen concentration,nitrate nitrogen concentration,inorganic nitrogen concentration,aggregate composition,as well as sunflower plant traits,aboveground biomass,yield,and nitrogen use efficiency were analyzed.【Results】Compared with MN treatment,application of sheep manure organic fertilizer nitrogen had no significant effect（P>0.05） on soil organic carbon content. Application of cow and pig manure organic fertilizer nitrogen significantly increased soil organic carbon content（P<0.05）,and the improvement effect increased with the increase of organic fertilizer application rates;CN1 treatment significantly increased（P<0.05） soil ammonium nitrogen,nitrate nitrogen,and total inorganic nitrogen concentrations by 9.7%,6.0%,and 7.6%,respectively;PN2 treatment significantly increased （P<0.05） the mass proportion of soil macroaggregates by 29.3%. Compared with MN treatment,CN1 and PN1 treatments significantly increased（P<0.05） sunflower disc diameter and thousand grain weight by 5.8% and 4.0%;the aboveground biomass of sunflowers significantly increased（P<0.05） by 4.6% and 12.8%;the yield and nitrogen use efficiency were significantly improved（P<0.05）,with CN1 treatment having the highest increase at 14.3% and 66.1%,respectively.【Conclusion】20% cow/pig manure organic fertilizer nitrogen+80% chemical fertilizer nitrogen treatments were beneficial for improving soil organic carbon content and increasing sunflower disc diameter,thousand grain weight,yield,and nitrogen use efficiency,achieved simultaneously improvement of soil organic carbon content and sunflower yield,can be used as suitable organic and inorganic fertilizer combined application models for low fertility farmland in Hetao Irrigation Area.

Key words: Organic fertilizer, Substitution ratio, Soil organic carbon, Sunflower yield, Nitrogen use efficiency