Abstract: Soil respiration is an important factor in carbon cycle in ecosystem, which influences the concentration of CO₂ in atmosphere. To understand effects on soil respiration in the leguminous and non-leguminous crops, we conducted a pot experiment planted soybean (Glycine max) and cotton (Gossypium spp), and the soil respiration rate during crop growing season was sampled by static closed chamber method and analyzed by gas chromatography. The results showed that seasonal changes of soil respiration rate in cropped soil were in accord with crop growing and there were significant correlations of quadratic function between soil respiration rates and growing days of crops. While the seasonal changes of soil respiration rate in bare soils were not significant and their correlations to days after sowing were weak. The maximum soil respiration rate in soybean planted soil was 2.4 times as high as and appeared earlier than that in cotton planted soil. In the soil planted soybean, the respiration rate for growing stages was seedling <branching <ripening <flowering-podding <filling stages, the soil respiration during filling and flowering-podding stages contributed 82% of the total soil respiration, but the days of the growing stages accounted for only 38.7% of the entire growth period, while in cotton planted soil, that was seedling <boll opening <budding <flowering and boll forming stages, soil respiration during budding, flowering and boll forming stages contributed 77.8% of the total soil respiration, but the days of the growing stages accounted for only 44.7% of entire growth period. The total soil respiration in soybean and cotton planted soil was 11.5 and 4.9 times as high as that in its corresponding bare soil. The total respiration, the average soil respiration rate, the total rhizosphere respiration, and the average rhizosphere respiration rate in soybean planted soil were 1.77, 2.34, 2.03 and 2.68 times as high as those in cotton planted soil, respectively. The contribution of rhizosphere respiration to soil respiration during whole growing period was 91.3%, ranging from 3.2% to 95.8% in soybean planted soil, and 79.6%, ranging from 21.8% to 88.0% in cotton planted soil. The exponential correlation between soil respiration rates and air temperatures was significant in growing plants soil, but not in bare soils. The N fertilizer had no significant effect on bare soil respiration. In summary, in the system of soil-crops, crop types and their growing stages are the main factors affected soil respiration, rhizosphere respiration is the main component of soil respiration, and by reason of symbiotic nitrogen fixation, soil respiration and the contribution of rhizosphere respiration were significantly higher in growing soybean soil than in growing cotton soil.

Key words: Soybean, Cotton, Soil respiration, Rhizosphere respiration, Growing stages