Abstract: A field experiment was conducted on a paddy field to study the effects of returning straw to soil and different tillage methods (no-tillage + returning straw, no-tillage, tillage + returning straw, and tillage) on the fertility level and microbial quantities of different soil layers. The results showed that in upper soil layer, the organic matter content in treatment ‘no-tillage + returning straw’ was 533, 279, and 537 g·kg-1 higher than that in treatments ‘no-tillage’, ‘tillage+returning straw’, and ‘tillage’, respectively, and the contents of total and available N, P and K in treatment ‘no-tillage+returning straw’ were also the highest, followed by in treatments ‘no-tillage’ and ‘tillage+returning straw’, and in treatment ‘tillage’. In deeper soil layer, all the fertility indices were higher in treatment ‘tillage+returning straw’. Treatments of ‘returning straw to soil’ had the highest quantities of soil microbes. The quantities of bacteria, fungi, and actinomycetes in upper soil layer were the highest in treatment ‘no-tillage + returning straw’, and thus, the cellulose decomposition intensity in this treatment at maturity period was 2644%, 7901%, and 9815% higher than that in treatments ‘tillage+returning straw’, ‘no-tillage’, and ‘tillage’, respectively. In deeper soil layer, the quantities of bacteria, fungi, and actinomycetes were the highest in treatment ‘tillage+returning straw’. Treatment ‘no-tillage+returning straw’ had the features of high fertility and abundant microbes in surface soil layer. The quantities of soil bacteria and actinomycetes and the decomposition intensity of soil cellulose were significantly positively correlated with soil fertility level.

Key words: Pinus yunnanensis forest, fuel, vertical distribution, forest fire behavior, terrain.