Abstract: In a three-year field trial, this paper studied the dynamics of biological fertility of degraded cultivated mountain shrubby steppe soil in the lower reaches of Lisa River as affected by fertilization. The results showed that under nonfertilized condition, the organic matter (OM) content in soil surface layer declined slightly, but after fertilization, the OM in this layer accumulated with a relatively high rate, humus structure improved obviously, soil bacteria had a significant multiplication, while soil microbial community was still in unharmonious state. With the increasing application rate of organic manure or chemical fertilizers, the OM content in 0～30 cm and 30～60 cm soil layers showed a significant increasing trend, with an annual increment being 1.35 and 0.67 g·kg^-1, respectively, and the ratio of humus carbon (C) to organic C and of humic acid C to humus C was also increased. Fertilization had a significant promotion effect on the multiplication and activity of soil bacteria. There was a significant positive correlation between the numbers of bacteria and OM content in 2～30 cm (r=0.7194**) and 30～60 (r=0.6042*) cm soil layers, while soil fungi and actinomycetes were not affected by fertilization. The numbers of azotobacter and cellulosedecomposing bacteria in different soil layers were lower in most fertilization treatments than in the control, and azotobacter in soil surface layer had a negative correlation with cellulosedecomposing bacteria (r = - 0.4799).

Key words: Thrips palmi, Population growth, Distribution pattern, Sampling technique