Diurnal variation of soil respiration in soil-potted with soybean (Glycine max L.) at various growth stages and its affecting factors

The diurnal variation of soil respiration at various growth stages of soybean and the influencing factors are investigated with static dark chamber method and gas chromatograph technique in pot experiment. In the flowering, pod, and grain-filling stage, soil respiration rate was measured consecutively 3 day-nights at 2 hours internal in daytime (6:00～18:00) and 3 hours internal in night (18:00～6:00). In the course of sampling, the temperature in air, on surface, and at 5 cm depth soil were recorded. The soil respiration rate was also determined one day-night at three hours internal after watering in the pod stage. Results showed that there was a substantial diurnal variation of soil respiration rate with a single peaked rate occurring from 12:00 to 16:00 in soybean-planted soil. The maximum soil respiration rate and the averaged rate in soybean-planted treatment were higher in pod and grain-filling stage than in flowering stage. The cumulative CO_2 emission during daytime accounted up 61%～69% of total emission over a day-night. The daily average of soil respiration rate was most close to the rate between 18:00 and 21:00. Rhizosphere respiration made up 85% ～ 96% of soil respiration if the priming effect of soybean growth on decomposition of native soil organic matter were neglected. In the bare soil, the soil respiration rate was significantly correlated with temperatures exponentially, but in soybean-planted treatment, the relationships were poor and depending on growth stages. The correlation was most significant in grain-filling plumping stage and lowest in pod stage, and more significant with temperature on surface and at 5cm depth soil than with air temperature. The results suggested that the soybean growth as well as growth stage influenced soil respiration rate and the relationship with temperatures. Watering affected the magnitude and intensity of soil respiration rate and enhanced the diurnal variation of soil respiration. All the results indicated that soybean growth and growth stages, temperatures, and watering are important factors affecting soil respiration rate and its diurnal cycle.