Integrated biochar and fertilizer management for improving phosphorus availability in saline soils of coastal Bangladesh

Soil salinity severely limits phosphorus (P) availability and agricultural productivity in coastal Bangladesh, where over 1.056 million hectares of arable land are currently affected, with projections indicating a median 26–39% increase in salinity levels by 2050 due to climate change and sea-level rise. Biochar has been increasingly recognized as a multifunctional soil amendment that improves soil fertility, enhances P availability, alleviates salinity stress, and promotes plant growth. This short-term (60-day) controlled laboratory experiment evaluated the effects of P-rich pig manure-derived biochar (BC), triple superphosphate (TSP), and their 1:1P-equivalent combination (BC + TSP) on P availability, soil pH, and electrical conductivity (EC) in naturally saline clay soil (EC = 7.72 dS m⁻1) from Khulna, Bangladesh. TSP yielded the highest available P (152.51 mg kg⁻1 at 60 days; 59.24% apparent phosphorus recovery efficiency), followed by BC + TSP (53.87%) and BC (117.96 mg kg⁻1; 41.97%). Notably, BC + TSP reduced EC by 24.10%, indicating a slightly greater salinity reduction than other treatments, although the difference relative to the control was marginal. While the use of P-rich pig manure biochar provides mechanistic insights into the dynamics of slow-release P and salt immobilization, these findings are highly relevant to Bangladesh’s Ganges Tidal Floodplain, where salinity intrusion threatens smallholder farming. Future applications should prioritize locally abundant, culturally appropriate feedstocks (e.g., rice husk or coconut shell) to enhance scalability, nutrient efficiency, and resilience in saline-prone coastal zones. This integrated approach supports sustainable P management and aligns with national priorities under the Bangladesh Delta Plan 2100 and SDGs 2, 13, and 15.