干旱区生态环境知识资源中心

Long- term interactions between vegetation succession and soil quality in alfalfa fields were investigated in the semi- arid Loess Plateau of China. The successional ecosystem is composed of six different planting- year alfalfa fields ( namely, 3 - 5 9 - 15 - 21 - 25 years). The yield, cover, and height of alfalfa increased from 194 g dry haym(-2), 55.6%, and 52.8 cm in the 3- year stage to 406 g dry haym(-2), 85.5%, and 72.5 cm in the 9- year stage, respectively. Subsequently, they tended to decrease and reached their respective minimum levels at the 25- year stage. The density of alfalfa continued to decrease but the plant species richness increased with successional age. Microbial biomass carbon ( MBC), microbial biomass nitrogen ( MBN), microbial biomass C- to- N ratio and soil basal respiration ( BR) at 0 - 20 cm soil depth decreased steadily from 3 years to 9 years, but increased from 15 years to 25 years. The values for metabolic quotient ( qCO(2)) were the opposite of the above four parameters along the succession gradient. Principal component analysis of soil microbial activity and respiration data in relation to soil physical and chemical properties demonstrated the variable trend in soil quality along the gradient. Generally, soil quality worsened within the 9- year stage while alfalfa production increased, but it tended to recover in the following stages while alfalfa production decreased. Soil quality and vegetation variables differed with growing seasons and soil depths. Our experiment showed that alfalfa fields should be plowed down after alfalfa has been grown for more than 9 years when soil water and fertility conditions are favorable for the production of follow- up crops in this area.