Abstract: Abstract: China is an important agricultural country with an annual crop straw production of nearly 700 million tons, which represents a substantially large number of straw resources that can be utilized. The wheat-rice rotation cropping system in the Yangtze River delta area is always accompanied by high yield; however, the existing conventional method of straw returning would not completely solve the problems caused by the produced straw with large quantity and short processing time. Alternatively, the new method of returning straw into a ditch is also widely used under the practice of conservation farming with no tillage and direct seeding cultivation. This method not only achieves a fully returning of straw, but also is helpful to the multi-purpose ditch use and enhances soil carbon sequestration. However, long-term practice has shown that under the natural state, straws in the ditch will not decay rapidly, and therefore will not be able to be transferred into organic fertilizer in a timely manner for adsorption by the crops, which can negatively influence crop growth. In order to solve this problem, a microbial spray was used in this study, which contained organisms possessing characteristics that induced degradation of the rice and wheat straw under room temperature (i.e., yeast, bacteria, actinomycetes, Bacillus licheniformis, and Bacillus subtilis). To explore the effect of the microbes on straw decomposition, the microbial mixture was tested with a spray device equipped on a multifunctional machine with the functions of ditching and stalk-disposing while harvesting. For wheat straw in summer, the straw dry weight showed a decreasing trend over time regardless of whether or not the straw in the ditch was covered by soil. The rate of decline in dry weight of each treatment group was faster than that of the control, indicating that the microbes could promote the decomposition of the wheat straw. During the observation period, the rates of decline in dry weight of wheat straw were the fastest under the YJ-G treatment (using a liquid microbial suspension with high concentration), which were 44.11% and 74.60% without and with soil covering, respectively, whereas the corresponding degradation rates in the control group were only 10.34% and 12.80%, respectively. Therefore, the YJ-G treatment could promote the decomposition rate by 326.6% and 482.8% compared to the control without and with soil covering, respectively. By contrast, the rates in decline of dry weight of wheat straw for the FJ (using powdered microbes) treatment groups were much lower than those in the YJ groups (using liquid microbes), and were more similar to the control, and the microbial concentration had no significant effect on the change of straw dry weight for the FJ groups. For the rice straw in winter, the microbial activities were considerably inhibited due to the low temperature; thus, although the straw dry weight still declined with time, the change was not as significant as observed for wheat straw and the difference among treatments was not significant. During the sampling period, the difference in the rate of decline in straw dry weight between each treatment group and the control group was similar and less than 20%. However, the degree of decomposition was always better under the soil-covered treatments. The same results were obtained over two-year experiment, illustrating the reliability of the above conclusions. Therefore, the use of microbes to promote decomposition is not suitable for rice straw; however, it is useful for wheat straw, and can provide guidance to wheat straw returning in summer for the ditch straw-returning method.