Responses of soil water, nitrogen, and organic matter to the alfalfa crop rotation in semiarid Loess area of China

Alfalfa (Medicago sativa L.) crop rotation is regarded as a key procedure for ecological restoration in the semiarid areas of China's Loess Plateau. This study was designed to clarify the soil water restoration in alfalfa crop rotation, to examine the responses of soil nitrogen and organic matter to alfalfa crop conversion in two years, and to assess the adaptability of various rotations for soil water restoration and soil fertility's sustainability. Soil moisture in the millet fields converted from alfalfa grassland was always higher than in alfalfa grassland in the two years after the conversion, which indicates rotation as an efficient way to restore soil water after continuous planting of alfalfa. However, planting millet (Setaria italica Beauv) as it is usually done as local practice, is not helpful for soil water restoration in the first year. In the second year after the conversion, planting potatoes (Solanum tuberosion L.), spring wheat (Tritician aestivitin L.), corn (Zea mays L.), and fallow following millet restored soil water by 36.0 mm, 11.3 mm, -1.3 mm and 4.9 mm, respectively, from April to October, and potatoes resulted the best choice in alfalfa crop rotation for soil water restoration. Compared to the continuous alfalfa planting, rotation treatments resulted in lower soil total nitrogen (by 9.9% in 2001 and 21.5% in 2002), lower soil organic matter (by 46.8% in 2001 and 28.1% in 2002), and higher soil mineral nitrogen (by 27.2% in 2002). Among the rotation treatments, soil total nitrogen and organic matter decreased greatly in cornfields, while they remained the same or increased slightly in spring wheat fields. This indicated that if there is no fertilizer application, spring wheat might be the best choice to keep soil fertility in balance for the farmland converted from perennial alfalfa stands in this region, whereas planting corn and other crops with large consumption of soil nutrients should be avoided.