2.1.Field experiment
The experiment was performed at the Crop Institute (Jiamusi) of the
Heilongjiang Academy of Land Reclamation Science (latitude, 46 °17.02’
N; longitude, 46° 53.16 ’E, northeast China). Mean elevation is 81.2 m
above sea level, average annual precipitation is 460 mm, and nearly 59%
of the total rainfall is received during the northwest monsoon from July
to September. The active accumulated temperature (≧10°C) is 2,600°C per
year, with a frost-free period of 137 d. The soil is classified as
meadow soil, with the following properties: pH 6.5, available nitrogen
89.2 mg kg-1, available phosphorus 128.2 mg
kg-1, available potassium 106 mg
kg-1. Mulberry (Morus alba L.c v. qiuyu )
seedlings with high yield, cold- and drought-resistant, were provided by
the Heilongjiang Institute of Silkworm breeding; alfalfa (Medicago
sativa L.c v. zhaodong ) seeds with a strong wintering ability
and a high yield were obtained from Institute of Crop development the
Heilongjiang Academy of Land Reclamation Science.