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).