Soil remediation of degraded coastal saline wetlands by irrigation with paper mill effluent and plowing

Combined with anti-waterlogging ditches, irrigation with treated paper mill effluent (TPME) and plowing were applied in this study to investigate the effects of remediation of degraded coastal saline-alkaline wetlands. Three treatments were employed, viz., control (CK), irrigated with 10 cm depth of TPME (I), and plowing to 20 cm deep before irrigating 10 cm depth of TPME (IP). Results show that both I-treatment and IP-treatment could improve soil structure by decreasing bulk density by 5% and 8%. Irrigation with TPME containing low salinity stimulated salts leaching instead of accumulating. With anti-waterlogging ditches, salts were drained out of soil. Irrigation with 10 cm depth of TPME lowered total soluble salts in soil and sodium adsorption ration by 33% and 8%, respectively, but there was no significant difference compared with CK, indicating that this irrigation rate was not heavy enough to remarkably reduce soil salinity and sodicity. Thus, irrigation rate should be enhanced in order to reach better effects of desalinization and desodication. Irrigation with TPME significantly increased soil organic matter, alkali-hydrolyzable nitrogen and available phosphorus due to the abundant organic matter in TPME. Plowing increased soil air circulation, so as to enhance mineralization of organic matter and lead to the loss of organic matter; however, plowing significantly improved soil alkali-hydrolyzable nitrogen and available phosphorus. Improvements of physicochemical properties in I-treatment and IP-treatment both boosted soil microbial population and activity. Microbial biomass carbon increased significantly by 327% (I-treatment) and 451% (IP-treatment), while soil respiration increased significantly by 316% (I-treatment) and 386% (IP-treatment). Urease and dehydrogenase activities in both I-treatment and IP-treatment were significantly higher than that in CK. Phosphatase in IP-treatment was significantly higher than that in CK. Compared to I-treatment, IP-treatment improved all of the soil properties except for soil organic matter. The key to remediation of degraded saline-alkaline wetlands is to decrease soil salinity and sodicity; thus, irrigation plus plowing could be an ideal method of soil remediation.