2.2 Experimental design
The field experiment was established in a randomized complete block design with three triplicates, and each plot cover an area of 40 m2 (5 m×8 m). The summer maize variety was Zhengdan 958. Six treatments were (1) BF0: 100% NPK chemical fertilizer; (2) BF30 (70% NPK chemical fertilizer + 30% bio-fertilizer); (3) BF50 (50% NPK chemical fertilizer + 50% bio-fertilizer); (4) BF70 (30% NPK chemical fertilizer + 70% bio-fertilizer); (5) BF100 (100% bio-fertilizer). All these treatments received same amounts of fertilizers (225 kg N ha-1, 80 kg P ha-1 and 80 kg K ha-1) during the growing season. Both the chemical fertilizer and bio-fertilizer were spread evenly on the surface of the soil in each plot and were thoroughly mixed with the top 0-20 cm of the soil by rotary cultivator, a week prior to maize sowing. In each plot, summer maize was planted at a density of 65000 plants ha-1 about 5 cm deep in early June, and harvested in early October each experimental year.
Bio-fertilizer used in this experiment was aerobically fermented from wheat straw The bio-fertilizer (organic matter 29.7%, total N 12%, total P 0.5%, total K 5%) contains approximately 0.2×109 CFU g-1 fungus (Trichoderma asperellum ACCC30536), the fungus were previously isolated from the rhizosphere soil of healthy soybean roots in a field severely affected byFusarium oxysporumin in Zhoukou, Henan province, China.