Effects of organic amendments on severe saline soil nutrient content and reed growth under wastewater irrigation

The effects of organic amendments on the nutrient state of severe saline soil and reed growth under wastewater irrigation were investigated in this study. The results obtained were critical for bioremediation of degraded saline-alkali wetlands in coastal regions. Experiments were performed in a severely degraded coastal saline-alkali wetland (with salinity level of 16.7 g·kg^-1) in the Binzhou County of Shandong Province, China. The experiments were started in the spring and four treatments were tested - tillage (CK) as the control; tillage + wastewater irrigation (FF), tillage + wastewater irrigation + grass straw application (FFJ) and tillage + wastewater irrigation + sludge application (FFW). For each treatment, soil organic matter increased significantly compared with CK. At the end of October, organic matter contents of FFJ, FFW and FF were 1.34, 1.29 and 1.22 times that of CK. Compared with CK, soil available nitrogen and phosphorus also increased with the order of FFW > FFJ > FF. The salinity of the top soil layer (0~20 cm) decreased in all the treatments, and especially in FFJ and FFW which decreased by 22.6% and 16.3%, respectively, compared with that of CK. At the end of August, reed height under FFW, FFJ and FF were 3.1, 2.7 and 2.2 times that of CK. Total biomass, root mass/crown mass and average leaf area under FFW and FFJ increased significantly (P < 0.05) compared with CK. Reed heights under FF, FFJ and FFW treatments significantly correlated with soil organic matter/available nitrogen with respective correlation coefficients (R²) of 0.897/0.963, 0.999/0.837 and 0.998/0.778. Total biomass under FF, FFJ and FFW treatments significantly correlated with soil organic matter/available nitrogen with respective R² of 0.863/0.937, 0.971/0.876 and 0.996/0.799. Plant height and biomass significantly correlated with soil organic matter under FFJ and FFW, but only significantly correlated with soil available nitrogen under FF. The results showed that wastewater irrigation provided ample water in severe saline soil systems and organic amendments supplied massive amounts of soil nutrient. This relieved water and nutrient stress in severe saline soil and promoted reed growth, which combined effect was beneficial to the restoration of severe saline soils. No significant differences were found between soils amended with grass straw and those with sludge. This study showed that combined treatment of wastewater irrigation and organic amendment sufficiently reduced soil salinity in severe saline soils. Furthermore, the combined treatment of wastewater irrigation and organic amendments improved soil nutrient (organic matter and available nitrogen and phosphorus) content and soil microbial biomass (respiration intensity). The above-noted improvements were ultimately reflected in the improvements in reed plants - reed height, biomass and allocation, etc.