Positive effect of sorghum grown as a soil-improving crop on the CO ₂ environment in greenhouses and apparent photosynthesis of strawberry

The contribution of freshly applied sorghum grown as a green manure crop, to the CO₂ environment of greenhouses was determined for soils containing different amounts of organic carbon. Two growers using different sou management practices for more than 10 years were selected for study. Grower A applied 20 t ha year^-1 of commercial farmyard manure, while grower B had grown sorghum in the summer and also applied the same amount of manure as grower A. Soil carbon content was significantly higher in greenhouses of B (2.3%) than those of A (1.5%). In June 1993, sorghum was sown in one greenhouse of each grower and grown for 2 months, while adjacent greenhouses were left without sorghum. The rate of CO₂ evolution from the sou was higher in greenhouses of B than those of A. Use of sorghum as green manure in the current season resulted in an increased CO₂ evolution rate by ca. 100 ml CO₂ m^-2 hr^-1 in A throughout the measurement period, and consequently an increase in the pre-dawn CO₂ concentration of 300 ppm. Although the effect of freshly applied sorghum was smaller in B compared to A, the pre-dawn CO₂ concentration in greenhouses of B was 500-1000 ppm higher than that of A. Consequently, net CO₂ assimilation and light conversion efficiency of strawberry plants were highest in the greenhouse of B with sorghum, and lowest in that of A without sorghum.