In order to understand the decomposing mechanism of the straw returning to field under rotary tillage

the different treatments based on amounts of straw returning were set up. Soil microbial biomass

soil enzymes activities and available nutrients contents were measured at maize jointing stage

male stage

middle maturity stage and mature stage respectively. The results showed that: (1) The straw returning could promote the soil microbial reproduction

increase the soil enzymes activities and the content of alkali hydrolyzable nitrogen

but decrease the contents of available phosphorus and available potassium. The number of fungi

bacteria

actinomycetes and sucrase and cellulase activities increased substantially under the straw returning amount of 11 250 kg/hm2. Urease and phosphatase activities and the contents of alkali hydrolyzable nitrogen increased under 7 500 kg/hm2

but the available phosphorus and available potassium decreased. (2) There were positive correlations between the number of soil fungi

bacteria

actinomyces and cellulase activity

but the fungi quantity was negatively related to phosphatase activity after planting for 40 days. Actinomycetes quantity was positively related to phosphatase and sucrase after 60 days

and related to sucrase after 140 days. (3) There were correlations among the soil enzymes activities. Urease was positively related to cellulose

and phosphatase was positively correlated with sucrase in 67 days. Sucrase was positively related to cellulose

and urease was negatively related to sucrase and cellulose in 140 days. (4) There were correlations between the soil enzymes activities and the available nutrients contents. Urease in 40 days and 112 days

cellulase in 67 days

phosphatase in 112 and 140 days had positive correlations with alkali hydrolyzable nitrogen contents.