Abstract: The study on the effects of long-term continuous cropping of cotton and returning cotton stalk into field on the quantities of soil microbes and the activities of soil urease, invertase, catalase and diastase showed that the soil biological activities in cotton field were negatively affected by continuous cropping of cotton, but positively affected by the returning of cotton stocks into filed. The obstacle of short-term (5-10 years) continuous cropping was more obvious. After 5 and 10 years continuous cropping and stalk returning, the total quantity of soil microbes was decreased by 3654% and 25.66%, and the quantities of bacteria and actinomyces were decreased by 61.12% and 70.16%, and 67.91%, 145.89%, respectively, while the quantity of fungi was increased, compared with those after 1 year cropping. The activities of soil urease and invertase after 5 and 10 years continuous cropping and stalk returning were decreased by 0.76% and 2.52%, and 28.12% and 11.44%, respectively, and the soil catalase activity after 5 years continuous cropping and stalk returning was decreased by 9.21%, compared with those after 1 year cropping. Under long-term continuous cropping and stalk returning, soil biological properties improved, and soil biological diversity index increased. Compared with those after 5 years continuous cropping and stalk returning, the quantities of bacteria and actinomyces after 15 and 20 years continuous cropping and stalk returning were increased by 71.52% and 61.03%, and 141.74%, 240.83%, respectively, with the proportion of fungi declined and the activities of urase, catalase and invertase increased. Soil amylase activity after 5, 10, and 20 years continuous cropping and stalk returning was increased by 11.38%, 10.41%, and 31.34%, respectively, compared with those after 1 year cropping.

Key words: intercropping system, water use efficiency, forage crop, root/shoot, crude protein yield