Effect of Integrated Biochar and Seaweed Extract on Chemical Soil Properties, N-Use Efficiency Indices and Wheat Production Under Different Nitrogen Levels in Saline Soil

Wheat ( Triticum aestivum L.) productivity in saline soils is often constrained by nutrient imbalance, water scarcity, and ionic stress, particularly in arid regions such as the Nile Delta of Egypt. This study evaluated the combined effects of biochar (2.4 t ha −1 ) and 1% foliar seaweed extract under varying nitrogen application levels on soil chemical properties, wheat growth, yield, nutrient uptake, and N-use efficiency indices over two consecutive winter seasons (2023/2024 and 2024/2025). A factorial field experiment with three replicates was conducted using four nitrogen rates: 0%, 50%, 75%, and 100% of the recommended application (168 kg N ha −1 ), combined with four treatments: control, seaweed extract, biochar, and their integration. Combined analysis showed that the highest grain yield was obtained under full nitrogen with biochar and seaweed extract (7085.75 kg ha −1 ), although this was not significantly different from several integrated treatments, particularly those involving 75% nitrogen with amendments. The 75% N + biochar + seaweed extract treatment achieved comparable yield while significantly improving nitrogen-use efficiency indices, including recovery efficiency, agronomic efficiency, and partial factor productivity. Biochar and seaweed extract improved soil organic carbon, cation exchange capacity, and nutrient availability, while electrical conductivity was not significantly affected. These results indicate that nitrogen input can be reduced by up to 25% without yield loss when combined with these amendments, while enhancing nutrient-use efficiency. However, conclusions regarding salinity stress mitigation remain indirect due to the absence of physiological measurements. Overall, this integrated approach supports more sustainable wheat production in saline soils.