2.1 Experimental site and management
The Exp. 1 and Exp. 2 was conducted under a half-electric rain shelter at the Shangzhuang Experimental Station of China Agricultural University, Beijing, China, (40° 08’N, 116° 10’E) in 2019. The half-electric rain shelter contained 9 big ponds (2 m long, 4 m wide, and 1.8 m deep) and 9 small ponds (2 m long, 2 m wide, and 1.8 m deep) (Figure 1). Each pond was cemented on the four sides and the bottom, and each pond was filled with soil. The soil was classified as loam. The chemical properties of top 20 cm soil layer sampled prior to planting was total nitrogen 0.81 g kg-1, organic matter 18.88 g kg-1, rapid available phosphorus 30.31 mg kg-1, rapid available potassium 100.47 mg kg-1, soil bulk density 1.35 g cm-3.
Three seeds (Zhengdan 958) per hole were sowed on May 17, 2019 in each pond and the seedlings were thinned to 7.5 plants m-2at the 3-leaf stage with a row spacing of 60 cm and plant spacing of 20 cm. Prior to sowing, 120 kg N ha-1, 50 kg P2O5 ha-1, and 150 kg K2O ha-1 was applied evenly to the soil surface. The additional 120 kg N ha-1 was applied by furrowing at the 12-leaf stage. Weeds, pests, and diseases were well controlled during the two seasons.
As a pot experiment, Exp. 3 was also conducted at the Shangzhuang Experimental Station in 2016. The same hybrid (Zhengdan 958) was grown in a 120 pots (diameter was 30 cm at the top and 25 cm at the bottom, 29 cm deep). In each pot, 20 kg naturally air-dried soil that was 1 cm sieved per pot was filled. The bottoms and inside of pots were perforated with small holes which allow free drainage. The soil was classified as loam. The chemical properties were total nitrogen 0.99 g kg-1, organic matter 20.95 g kg-1, rapid available phosphorus 24.03 mg kg-1, rapid available potassium 87.33 mg kg-1, soil bulk density 1.28 g·cm-3. All the pots were buried in the 25 cm ditch avoiding high soil temperature. At the commencement of the experiment, fertilizer was applied prior to grown with the amount of 3.9 g N pot-1, 12.5 g P2O5pot-1, and 5.8 g K2O pot-1, and then 2.6 g pot-1 was top-dressed at V12 stage. Four seeds were sown in the central of each plot, at the V3 stage, the five maize seedlings were thinned to one plant per pot.