2.1. Experimental design
A field experiment was established in 2015 in a farming field (47°53′ N, 153°33′ E), Heilongjiang Province, northeast China. The soil was a typical Mollisol with glacial parent material. The field site had 40 years of cultivation history with a 5° slope and 30 cm of A horizon. The annual average temperature in the region is 1.5°C, ranging from -37°C in the winter to 32°C in the summer. The average temperature is 18.1°C and rainfall during the growing season (from May to September) 424 mm.
The experiment deployed a split-block design with three simulated topsoil depths (10, 20 and 30 cm) as the main plot and crop species (maize and soybean) as sub-plot treatments. Each treatment had three replicates. Each plot had an area of 10.4 m × 7.5 m. We first removed all topsoil of experiment plots, mixed, and then back-filled up to 10, 20 and 30 cm in depth with a bulk density of 1.1 g cm-3. As the average depth of topsoil in the Mollisol region is 32.7 cm, this study treated the topsoil depth of 30 cm as the control, and 10-cm and 20-cm topsoil depths as erosion treatments. The reconstructed soil layer was naturally incubated for one year. Maize and soybean crops were grown in a rotation module, in which both maize and soybean were grown in the same season. Zero-tillage was deployed. The maize and soybean stalks were returned to the surface of field plots after harvesting.
Maize (Zea mays L. cv. Xinggen 5) and soybean (Glycine max(L.) Merr. cv. Dongsheng 1) were sown in early May 2016, 2017 and 2018. For maize, 150 kg ha-1 of diammonium phosphate, 75 kg ha-1 of urea, and 50 kg ha-1 of potassium sulfate were applied before sowing, and 225 kg ha-1 of urea was applied at the five-leaf collar (V5) stage. For soybean, the fertilizer application rates were 150 kg ha-1, 50 kg ha-1 and 50 kg ha-1 for diammonium phosphate, urea and potassium sulfate, respectively. Sowing rates were 5 and 25 seeds m-2 for maize and soybean, respectively. Maize plants were harvested at the jointing stage (V7) and milk stages (R3), and soybean plants were at the initial flowering (R1) and mid-seed-filling stage (R6).