Biochar restores potassium balance in rice-crab coculture by enhancing potassium retention and cycling

Rice-aquatic animal coculture enhances nutrient cycling via ecological synergies, yet its negative potassium (K) balance caused by leaching and long-term crop residue removal remains a bottleneck for maintaining paddy K sustainability. Here, maize straw-derived biochar was applied back to rice-crab coculture (RC) paddy to explore the feasibility of reversing the negative K balance. We measured soil K fractions, K leaching and utilization, and evaluated the K balances treated with rice monoculture (RM), RC, and RC with 20 t ha⁻ 1 maize straw-derived biochar (total K: 1.38%) application at once (RCB). RC significantly increased soil solution K (0–40 cm), exchangeable K, and K balances compared with RM, but elevated K leaching and failed to address the negative K balance. RCB increased soil solution K (0–20 cm), exchangeable and non-exchangeable K, boosted aboveground K uptake and crab K accumulation, and lowered K leaching by 15.92%, compared to RC, shifting total K balance from −77.27 kg ha −1 to 237.15 kg ha −1 . Partial least-squares path modeling further revealed biochar-originated K input and soil exchangeable K as the two dominant drivers governing total K balance. Although upfront biochar purchase costs lowered net income for RCB relative to RC, RCB still delivered higher net income than RM. Collectively, biochar integration within RC curtails K loss, strengthens soil K retention and transforms negative K budgets into positive ones, though these findings are limited to a single experimental site, biochar feedstock and fixed application rate, so targeted regional field trials are required to validate its universal practicability.