Improved effect of saline soil by grading condensate of cotton straw pyrolysis fluid

Abstract: A huge area of saline-alkali land cannot be cultivated in China, due to the salt stress and low nutrient. Pyrolysis fluid with high acidity has been used to improve the saline soils. Among them, phenolic substances are adverse to the growth of plants. Therefore, the aim of the study was to explore the improvement effect of pyrolysis fluid on the saline soils, in order to efficiently reduce the phenols during pyrolysis. Firstly, the pyrolysis fluid was prepared at different temperatures (300, 400, 500, 600, and 700 ℃), and then refined under the three-stage condensation mode. Secondly, an analysis was made to clarify the effect of graded condensation on the product composition. Taking the Chinese cabbage as an indicator, a pot experiment was performed on the saline soils using the tertiary product under different dilution folds (50, 200, 400 and 600). Finally, a systematic investigation was implemented to determine the effects of the tertiary product on the growth of Chinese cabbage, soil pH, total salt, sucrase, urease, alkaline phosphatase, catalase, organic matter, alkaline digested nitrogen, fast-acting phosphorus and potassium. The results showed that: 1) No new substance was found in the tertiary product from the pyrolysis fluid phase during three-stage condensation, compared with the single-stage. The graded condensation was used to realize the rapid and efficient separation of phenols and acids from the fluid phase products of cotton stalk pyrolysis. The concentrations of phenols and acids in the secondary product were 2.18-2.24 and 0.07-0.11 times higher than those by the single-stage, respectively. In the tertiary product, the concentrations of phenols and acids were 0.50-0.55 and 1.21-1.35 times higher than those by the single-stage, respectively. 2) The tertiary product significantly promoted the germination and growth of the Chinese cabbage (P<0.05). This promotion also depended mainly on both pyrolysis temperature and dilution ratio. The medium pyrolysis temperature and the larger dilution folds were more favorable for the Chinese cabbage seed germination. The best growth of Chinese cabbage was observed in the T500-D600 treatment (pyrolysis temperature of 500 ℃, dilution fold of 600), with 32.12%, 95.11%, and 120.03% increase in the germination rate, plant height, and fresh weight per plant, respectively, while 48.39% decrease in the malondialdehyde content. 3) The soil enzyme activity was dominated by the irrigation of the tertiary product with different dilution ratios (P<0.05). Interestingly, the activities of urease and alkaline phosphatase were reduced, only when the tertiary product was diluted by 50 times into the saline soil. Additionally, the activities of urease, alkaline phosphatase, sucrase and catalase in the saline soil increased by 218.13%, 123.65%, 64.78%, and 36.87% respectively, compared with the control. 4) The tertiary products significantly reduced the pH and total salt amounts in the saline soil, whereas, there was a significant increase in the nutrient contents (P<0.05). The contents of organic matter, alkaline soluble nitrogen, fast-acting phosphorus, and fast-acting potassium in the saline soil increased by 64.86%, 33.07%, 64.65% and 17.29%, respectively. Anyway, the tertiary-stage condensation products of cotton stalk pyrolysis greatly contributed to the high fertility and enzyme activity of saline soil, as well as the better growth of Chinese cabbage. This finding can provide a sound theoretical basis for the biomass pyrolysis fluid to improve the saline-alkali land.