Soil samples collected from continuous croplands with 2

7

11

15and 20year gravel-sand mulching in Gaolan Research Station

Gansu Province were analyzed for soil microbial population and soil enzyme activities. The results show that with increase of mulching time

soil microbial population decreased from the beginning to a low stable state in about 7to 11years

and then the Shannon diversity index and Mclntosh diversity index continued to decrease.The enzyme activities of soils showed a parabola curve over time.Most of them increased and reached their peaks in 10to 11years

and then declined.The soil enzyme activity of the cropland covered for more than 15years was so low that the yield of the crops decreased.At this stage

farming practices should be stopped

and compost should be added with renewal of soil sand to recover the productivity of the land.The results of the study can provide a theoretical basis for the sustainable utilization and management of the gravel-sand mulched croplands.