| Effects of different dosages of biogas slurry and their combination with chemical fertilizers on the properties of substrate and plant growth, yield and quality of watermelon under soilless culture condition were studied. There were five treatments in this experiment: T1:no fertilization, T2: only the chemical fertilizer, T3: biogas slurry : chemical fertilizer=3:7(w/w), T4: biogas slurry : chemical fertilizer=7:3(w/w) and T5: only biogas slurry. All fertilizers treatments were applied at the same total content of N, P and K.The experiment was set up in a random block design with four replications and a plot volume of 10 liters. The physical and chemical properties; N, P and K content, microorganism quantity, enzyme activity of substrate; and N, P and K content of fruit and plant development, dry weight, yield and quality were determined in the experiment. The major results were as follows:1. The combined application of biogas slurry with chemical fertilizer reduced pH, EC and bulk density of substrate, increased the substrate porosity and improved the absorption of N, P and K in substrate. The pH value tended to be neutral after the application of biogas slurry, furthermore, the pH was decreased with the increased application of biogas slurry in all treatments. The EC value of the biogas slurry treatment was significantly higher than that of other treatments. The pH value of T4 was 12.0% ~ 13.2% lower than T1, and 7.8%~10.3% lower than T2.Compared with T1, the bulk density of T4 decreased by 6.6% ~8.2%. The EC of T4 was 0.8%~4% lower than T2. Furthermore, there were significant differences in the contents of N, P and K between fertilizing and non-fertilizing (T1) treatments. The substrate N, P, K content of T4 treatment decreased by 7.8%, 8.7%, 19.6% than T2, respectively.2. The combined application of biogas slurry with chemical fertilizers increased the microorganism quantity in substrate. The results showed that, the number of the fungi, bacteria, actinomycetes gradually increased with the increased application of biogas slurry. Apparently, T4 showed the most obvious effects on microorganism quantity. The number of the bacteria, actinomycetes and fungi of T4 was 38×107cfu/g, 53×106cfu/g and 70×104cfu/g respectively, which were the highest in all treatments. Whilst, the number of bacteria, actinomycetes and fungi of T4 were 2.53,4.82 and 2.06 times of T1, 2,2.94 and 1.49 times of T2 ,respectively.3. The combined application of biogas slurry with chemical fertilizers improved the activity of enzymes in substrate. The results showed that, the activities of the urease, cellulose and protease of T4 was significantly higher than that of other treatments. The urease, cellulose and protease activities of T4 increased by 159.45%, 66.88%, 128.6% than T1, and increased by 102.9%,46.59%, 55.7% than T2, respectively. Enzyme activities increased with the increased application of biogas slurry in substrate, whereas overfull biogas slurry application would reduce the activity of such enzymes.4. The combined application of biogas slurry with chemical fertilizers increased plant height, stem diameter, leaf number and root activity, and T4 showed the best effect. The combined application of biogas slurry with chemical fertilizers increased root activity of watermelon and delayed root senescence. Root activity of T4 was 82.5% higher than that of T2. 5. The combined application of biogas slurry with chemical fertilizers increased the single fruit weight as well as the N, P and K content in fruit, reduced water content in fruit and improved fruit quality. Compared with T2, the content of Vc, soluble solids, soluble sugar and protein of treatment T4 increased by 6.6%,9.9%,4.4% and 12.1%,individually, whilst the nitrate content decreased by 20.0%. The combined application of biogas slurry with chemical fertilizers increased the single fruit weight, reduced the water content of fruit, and increased N, P and K content, such T4 was the optimal treatment. The N, P, K contents of fruit of T4 increased by 21.81%, 29.3%, 32.2% than T1, and increased by 17.0%,15.0%, 25.4% than T2, respectively.In conclusion, under the condition of consistent total amount of N, P and K, the best treatment was T4 (biogas slurry and chemical fertilizer were applied at the mass ratio of 7:3) for cultivating watermelon in greenhouse.
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