Abstract: Rice bean (Phaseolus calcaratus) is an important summer green manure crop for improvement of soil quality. In the current study, rice bean was sown on three dates (May 2, May 7, and May 12) and was ploughed in soil on August 4, August 18 or September 2 in Chongqing, China (9 treatments; 3 replicates). Data collected included the above- and below-ground biomass, the plant nutrient contents at different growth stages, the decay rate of matter after burial using a nylon bag method, and levels of soil available nutrients (N, P, and K) after the rice bean was buried. With later sowing dates, the aboveground biomass was significantly decreased, while the root∶shoot ratio was reduced with earlier sowing, but tended to be increased with later sowing. Delayed sowing increased the rice bean plant total nitrogen (TN) content when compared with the control. Plant total potassium (TK) content was significantly reduced by delayed sowing date, whereas differences between sowing dates for total phosphorus (TP) showed no consistent pattern. Dry matter decay rate and plant nutrient levels showed similar trends for the different sowing dates. The values for residual dry matter in nylon bags, TN, TP, and TK were significantly higher than those of the control when the burial date was delayed 15 days in August. After the rice bean was buried, soil available N levels increased during the 21 days, whereas soil available K and P levels were variable. Because of the higher rice bean biomass and nutrient contents from earlier sowing, 10 days after canola harvest is the recommended sowing date for rice bean in Chongqing. The burial date should be in early August, when temperatures are still high enough for the buried plant material to decompose.

Key words: rice bean, optimum sowing date, overturning, nutrient decomposition