Preparation of experimental plots
In this study, we used 10-m long and 1-m wide plots. The slope gradient of the plots was set to 5° to simulate the typical natural geomorphological conditions of the gently sloping farmland in the region (Zhao, 1986). The depth of the tested black soil was 0.3 m, similar to the average thickness of the A-horizon of black soil (Xu et al., 2010). The black soil layer was followed by a 0.3-m sand layer.
The soil used in this study was typical black soil with 22.01 g kg-1 of organic matter. It was collected from the top-30 cm soil layer in a local slope farmland. The agglomerate impurities were removed manually, but without passing the soil through a sieve, to maintain its natural state. The soil was packed into plots on the sand layer for a period of 1.5 years to ensure that the bulk density (1.20 g cm-3) reached the field level by natural deposition and the soil structure recovered to the natural cropland state before the experiment.
We used Xianyu 335 (DuPont Pioneer Ltd., USA), a widely cultivated maize variety in Northeast China. Seeds were sown at a distance of 0.4 m between rows and 0.2 m between plants distance and fertilized with urea (CO2(NH2)2) at 150 kg ha-1 in early June in 2013. All plots were plowed simultaneously at ~0.2 m depth. Ridges were stacked in all ridging plots one month after sowing, obeying the local routine in the region. Air seasoning maize stalks were chopped into approximately 5-cm fragments and mulched onto mulching plots at a dosage of 20 000 kg ha-1.