Effects of biochemical fulvic acid on the water-salt transport characteristics of soda saline-alkali soil with different textures

Saline-alkali soil improvement is one of the important measures to enhance the production capacity in sustainable agriculture. Fulvic acid can serve as an ecologically friendly soil conditioner in the saline-alkali soil treatment, in order to improve soil fertility, crop yield, and quality. Biochemical humic acid extracted from the straw has demonstrated a superior improvement on the saline-alkali soil, compared with the mineral-derived humic acid, thus offering an effective way for the straw resource utilization. This study aims to explore the improvement mechanism of biochemical fulvic acid on the soda saline-alkali soil with different textures. A systematic investigation was also implemented to assess the impact on water-salt transport characteristics. The indoor one-dimensional vertical infiltration experiments were then conducted to explore the water-salt transport, infiltration model parameters, and eight major salt separators. Different amounts of biochemical fulvic acid 0 (CK), 1, 2, 4, and 8 g/kg were applied in the soda saline-alkali soil with various textures. The results showed that the cumulative infiltration amount decreased by 3.70%, 10.19%, 12.04%, and 25.00% in the sandy loam soil for the treatments with the biochemical fulvic acid of 1, 2, 4, and 8 g/kg, respectively, after 280 min of infiltration compared with the CK treatment. The wetting front depth also decreased by 9.33%, 17.00%, 24.00%, and 27.33%, respectively. In the loamy sandy soil, the cumulative infiltration amount of the treatments with the biochemical fulvic acid of 1, 2, 4, and 8 g/kg decreased by 1.36%, 10.51%, 18.98%, and 29.83%, respectively, after 40 min of infiltration compared with the CK treatment, with the wetting front depth reductions of 5.00%, 11.33%, 21.67%, and 31.33%, respectively. The infiltration rate of soil moisture decreased with the increase in the application amounts of biochemical fulvic acid. At the soil depths of 0, 5, 10, 15, 20, 25, and 30 cm, the sandy loam treated with 8 g/kg showed a higher soil moisture content than the CK treatment by 11.43%, 4.99%, 8.54%, 8.79%, 9.02%, 4.98%, and -7.76%, respectively. Similarly, the loamy sandy exhibited a higher soil moisture content than the CK treatment by 4.76%, 9.28%, 6.18%, 9.25%, 8.05%, 8.77%, and -0.06% at the corresponding depths, respectively. Both the Kostiakov and Philip models were used to effectively simulate the infiltration, indicating the gradual decrease in the infiltration rate with the increasing application amounts of biochemical fulvic acid. Soil infiltration also decreased the soil pH value. Specifically, the soil pH value increased with the increase of application amounts, with the most significant change in soil pH, compared with the CK treatment at 8 g/kg application amount. Moreover, the application of biochemical fulvic acid increased the soil salt content at various depths, potentially leading to elevated Na⁺ concentrations in soils with low Ca²⁺ and Mg²⁺ concentrations. Therefore, the application of biochemical fulvic acid significantly influenced the water and salt transport characteristics and then improved the water distribution for the reduced pH value in the soda saline-alkali soil with different textures. To avoid further secondary salinization caused by the introduction of salt ions via biochemical fulvic acid, the application rate of 2-4 g/kg was recommended for better improvement of soda saline-alkali soil with varying textures. Therefore, the finding can serve as the theoretical foundation and support for the green management of saline-alkali soil using biochemical fulvic acid.