Leaching and migration characteristics of nitrogen during coastal saline soil remediation by combining humic acid with gypsum and bentonite

Coastal saline soil is an important resource that is widely-distributed and available for mitigation, development and utilization. Adding exogenous materials to improve saline soil has been shown to be an effective measure to increase its agricultural productivity. However, there is limited information about the risk of nitrogen (N) leaching during coastal saline soil improvement process. Therefore, the leaching and migration characteristics of N in saline soils of the Yellow River Delta (Shandong Province, China) with the addition of weathered coal humic acid (HA), gypsum and bentonite, were examined using soil columns and 15N isotope tracers. The results showed that co-application of HA and gypsum could significantly increase the soil penetration rate (SPR) and reduce the pH of saline soil but did not increase N leaching, specifically an application ratio of 1:3 (HA: gypsum) based on weight. The co-application of HA and bentonite could not increase the SPR and reduce the pH, but could inhibit nitrification and better conserve 15N of saline soil. Therefore, we believe that bentonite cannot replace gypsum but could be used as an auxiliary agent to achieve better N retention. The mixed addition of HA and gypsum can achieve the dual effect of improving saline soil and maintaining N, which is beneficial to environmental protection and high-quality development of agricultural production in the Yellow River Delta.