The observed increase in MBC under PM+ SL, SL and PM declined with time; for example, the average increase in MBC within the 60 cm soil layer under PM + SL reduced from 102.65 to 52.49% in two years, while that under SL reduced from 48.13 to 24.63% and that under PM from 38.63 to 12.92%. The reason may be the joint effects of water, heat and salt (Gao and Li, 2005). It has been found that a large number of microorganisms decreased due to changes in micro-climate under continous plastic mulching, and so the content of MBC also decreased yearly (Li et al., 2004). Also, as years passed, the stability of soil aggregates increased, offering better protection to particulate organic carbon, and making it less available for microbial use.