Biochars have been successfully used to treat wastewater and contaminated soils.

Urea is the most used fertilizer because of its significance on world food security but it is also the toughest fertilizer to manage. It is readily available to the plant and it is vulnerable to loss in various ways, causing environmental pollution and huge economic losses. Urea application requires a sound knowledge for its effective management, which will increase its availability to plants and reduce possible losses. Ammonia (NH₃) and oxides of nitrogen (N) pollute the environment, and nitrate ( $${\text{NO}}_{3}^{ - }$$NO 3 - ) leaching alters the aquatic ecosystem, which lowers the nitrogen use efficiency (NUE) of applied urea. Nitrate-contaminated drinking water causes human and animal health risks, whereas the emission of nitrous oxide (N₂O) in the atmosphere is significant to ozone layer depletion and climate change. This review discusses the processes in the soil after urea application in the soil–plant system, which includes the loss mechanisms, and the significant factors affecting the N availability and losses. This review also shows that the judicial management of urea in soil–plant systems and maintaining the best management practices and technologies ensure sustainable agricultural development and decrease the risk of environmental contamination. Finally, the review summarizes the potential mechanisms of the applied urea in the soil with their mineralization and loss pathways and delivers the scientific reference to achieve sustainable crop production and reduce the risk of N losses. Graphical Abstract

Soil acidity is a major problem of agriculture in many parts of the world. Soil acidity causes multiple problems such as nutrient deficiency, elemental toxicity and adverse effects on biological characteristics of soil, resulting in decreased crop yields and productivity. Although a number of conventional strategies including liming and use of organic and inorganic fertilizers are suggested for managing soil acidity but cost-effective and sustainable amendments are not available to address this problem. Currently, there is increasing interest in using biochar, a form of biomass derived pyrogenic carbon, for managing acidity while improving soil health and fertility. However, biochar varies in properties due to the use of wide diversity of biomass, variable production conditions and, therefore, its application to different soils can result in positive, neutral and or negative effects requiring an in-depth understanding of biochar-acid soil interactions to achieve the best possible outcomes. Here, we present a comprehensive synthesis of the current literature on soil acidity management using biochar. Synthesis of literature showed that biochars, enriched with minerals (i.e., usually produced at higher temperatures), are the most effective at increasing soil pH, basic cation retention and promoting plant growth and yield. Moreover, the mechanism of soil acidity amelioration with biochar amendments varies biochar types, i.e., high temperature biochars with liming effects and low temperature biochars with proton consumption on their functional groups. We also provide the mechanistic interactions between biochar, plant and soils. Altogether, this comprehensive review will provide guidelines to agricultural practitioners on the selection of suitable biochar for the reclamation of soil acidity.

Biochars have been successfully used to reduce bioavailability and leaching of heavy metals in contaminated soils. The efficiency of biochar to immobilize heavy metals can be increased by reducing the particle size, which can increase the surface area and the cation exchange capacity (CEC). In this study, the empty fruit bunch biochar (EFBB) of oil palm was separated into two particle sizes, namely, fine (F-EFBB < 50 µm) and coarse (C-EFBB > 2 mm), to treat the contaminated soil with Cd and Pb. Results revealed that the addition of C-EFBB and F-EFBB increased the pH, electrical conductivity and CEC of the contaminated soil. The amounts of synthetic rainwater extractable and leachable Cd and Pb significantly decreased with the EFBB application. The lowest extractable and leachable Cd and Pb were observed from 1% F-EFBB-treated soil. The amount of extractable and leachable Cd and Pb decreased with increasing incubation times and leaching cycles. The application of F-EFBB to Cd and Pb-contaminated soil can immobilize the heavy metals more than that of C-EFBB. Therefore, the EFBB can be recommended for the remediation of heavy metal-contaminated soils, and a finer particle size can be applied at a lower application rate than the coarser biochar to achieve these goals.