Effects of Organic Manure and Biochar-Based Fertilizer Application on Soil Water and Salt Transport in Brackish Water Irrigated Soil Profile

It is uncertain how soil amendments (organic manure and biochar-based fertilizer) affect soil water and salt movement below the root zone under brackish water irrigation. Two-year field experiments (freshwater irrigation + chemical fertilizer (CK), twice alternative irrigation with brackish water + chemical fertilizer (BCF), twice alternative irrigation with brackish water + organic manure (BOM), and twice alternative irrigation with brackish water + biochar-based fertilizer (BBF)) were carried out to investigate the effects of organic manure and biochar-based fertilizer on soil water and salt dynamics and crop yield. In BOM, soil pH decreased and soil aggregate content and stability increased, resulting in higher soil water (average water content 1.92% higher than BCF) and lower electrical conductivity of soil solution (0.45% lower than BCF) in the root zone (0–200 cm) during the second rotation. Soil salt excreted from the root zone and accumulated in the deep zone (200–400 cm), and the salt leaching amount in 0–400 cm was lowest (55.71 t·hm −2 ) in BOM. Soil pH, sodium adsorption ratio and total porosity increased in BBF, and micro-aggregate development was promoted. Additionally, soil salt was transported to a deeper layer (with the lowest salt storage of 38.50 t·hm −2 in the root zone) and the salinity of the entire profile was reduced (with the highest leaching amount of 69.17 t·hm −2 ). Organic manure and biochar-based fertilizer reduced soil salt in the root zone through different soil amendment mechanisms, resulting in different results of soil salt accumulation and leaching. In BOM, soil salt accumulated in the deep zone, while salt leaching occurred through 0–400 cm in BBF. These findings are beneficial for the safe utilization of brackish water and sustainable agriculture development.