Biochar and Ammonium Nitrate Synergies: Enhancing Nitrogen Availability and Maize Growth in Oxisols

Effective nitrogen (N) management and the development of novel N fertilizers are essential for enhancing maize growth in tropical soils. One strategy to increase N use efficiency is the use of organic matrices as a source of N or their combination with the application of mineral N sources. Among these organic matrices, biochar emerges as a highly promising option for optimizing N use efficiency. Thus, the aim of this study was to evaluate the effects of different feedstocks, their respective biochars, and their combination with N on the dynamics and uptake of N by maize plants in two contrasting Oxisols. A 30-day greenhouse experiment was conducted using maize grown under treatments with four feedstocks (bamboo, sunflower cake, chicken manure, and shrimp carcass) and their respective biochars. The biochars were applied with or without ammonium nitrate (AN), alongside negative (no N) and positive (AN-only) controls. Ammonium and nitrate levels were analyzed in the soil solution at 1 and 15 days and in the whole soil before and after cultivation. Maize biomass production and shoot N accumulation were also evaluated at the end of the experiment. Among the main results, it was observed that soil type played a key role in available N, maize nutrition, and growth. In the medium-textured Oxisol studied, native soil organic matter partially met maize N requirements due to high content of available N observed. Biochars influenced N availability by increasing nitrate-N prevalence in the soil solution. Although whole-soil N levels were sufficient for robust maize growth, post-cultivation residual N remained low (<75 mg kg−1), indicating the need for supplemental N fertilization for plants grown in pots. In the medium-textured Oxisol, bamboo or sunflower cake biochar combined with AN increased biomass production by ~12% compared with AN alone. Similarly, in the clayey Oxisol, maize fertilized with sunflower cake or shrimp carcass biochar—regardless of AN addition—outperformed AN-fertilized plants by 19–30%. Thus, this study highlights the potential of integrating biochar with N fertilization to improve soil and solution N availability and increase N use efficiency by maize plants.