Agro-based biochars combined with nitrogen fertilizer improve soil nutrient status and rice performance in contrasting soils of southern Nigeria

Abstract Agro-based biochars used together with mineral nitrogen fertilizer are increasingly recognized as climate-smart soil amendments for improving soil fertility and sustaining crop productivity. However, field information on the effects of rice husk biochar, sawdust biochar, and urea on post-harvest soil nutrient status across contrasting rice ecosystems in Nigeria remains limited. This study evaluated the effects of rice husk biochar, sawdust biochar, and urea on post-harvest soil nutrient status and rice performance in upland and lowland rice ecosystems established on Alfisol in Akure and Ultisol in Abakaliki, southern Nigeria. The experiment was conducted as a 2 × 2 × 4 factorial in a randomized complete block design with three replicates, involving two soil types, two rice ecologies/varieties, and four nutrient treatments: control, urea, rice husk biochar + urea, and sawdust biochar + urea. Soil pH, soil organic carbon (SOC), total nitrogen (TN), available phosphorus (P), exchangeable potassium (K), ammonium-N (NH 4 + -N), nitrate-N (NO 3 - -N), and rice yield components were assessed and analysed by factorial ANOVA, with mean separation using Tukey’s HSD at P < 0.05. Biochar-urea combinations improved post-harvest soil nutrient status and rice performance relative to the control and, in most cases, sole urea, with the strongest responses occurring in the 0–15 cm soil layer. Sawdust biochar + urea produced the highest grain yield of 4202.59 kg ha -1 , straw yield of 4999.21 kg ha -1 , and 1000-grain weight of 30.25 g, while rice husk biochar + urea produced statistically comparable grain and straw yields. These results indicate that locally available agro-based biochars can improve fertilizer effectiveness and post-harvest nutrient status under the conditions of this study; however, longer-term and rate-response studies are required before broad field recommendations are made.