The use of co-composted biochar (COMBI) made by the addition of biochar at the beginning of the composting process has greatly increased in agriculture during the last decade. There are more benefits of using the co-composting end product COMBI than using compost and biochar separately or the mixture of the two products. We conducted an extensive review of the production of several COMBIs and their contribution to the composting process and biochar properties as well as the further use of COMBIs in agricultural lands to improve soil health and increase crop yields, and to remediate areas severely contaminated with potentially toxic metals (PTMs). Although the number of researches focused on COMBI production and its application is so far limited, there is enough evidence to elucidate the importance of creating such products to promote sustainable agriculture and environmental safety. Even if a few drawbacks or side effects are found, they are outweighed by the many benefits achieved with COMBIs production and application in comparison to other amendments. The quality of both biochar and compost is largely improved in so many ways during the co-composting process, which in turn improved soil health and crop yields of up to 300% in some particular cases. This work improved the overall understanding of COMBI production and application in agriculture. Based on the review, we suggested future researches to better understand the mechanisms of COMBI long-term application to promote awareness on its role over time through alterations in its surface chemistry, ionic nutrient adsorption, supply (aging effect), and environmental implications.

Biochar produced from pyrolysis of biomass such as wood, canopy, animal manure, and agricultural waste is recognized for its stability and for being a benefactor of soil health and plant growth. Its application in forestry is an area with growing research interest due to its ability to enhance soil physicochemical properties, including structure, water retention, and nutrient availability, thereby boosting plant growth, drought tolerance, and resistance to pests and diseases. However, the effectiveness of biochar varies based on factors like biochar type, application rate, soil type, and tree species. Potential risks associated with biochar use include nutrient immobilization, increased pH in alkaline soils, and enhanced leaching of toxic elements. Despite its promise, challenges such as knowledge gaps, lack of site-specific studies, and concerns of economic viability hinder widespread adoption of biochar in forestry. This qualitative review compiles over 150 published works from the past two decades on biochar application in forestry. It assesses the impacts of biochar on soil health and tree crops, highlighting its potential to improve soil fertility and promote tree growth. The review identifies significant findings, such as the positive influence of biochar on soil and plant health and outlines existing knowledge gaps that need addressing. By synthesizing current research, the review proposes future directions to optimize biochar use in sustainable forestry management, emphasizing the need for tailored approaches and economic assessments to facilitate broader adoption. The findings underscore the potential role of biochar in enhancing forestry practices while calling for further studies to resolve uncertainties and improve its practical implementation. Graphical Abstract

Introduction: Enhancing soil cation exchange capacity (CEC) is of paramount importance for sustainable agriculture and ecosystem health. This study investigated the pivotal role of biochar in altering soil CEC and challenges conventional assumptions about universal effects of biochar application.Methods: Contrasting biochar types, one with a low ash content of 4.4% (switchgrass-derived biochar, SGB) and the other with a high ash content of 45.9% (poultry litter-derived biochar, PLB) were used. Two experiments treated with increasing biochar application rates were conducted: one without plants and the other grown with ryegrass. Effective CEC (summation method) was determined by two extraction methods: Mehlich-3 (M3) and ammonium acetate (AA).Results and discussion: The SGB decreased CEC by 27% on average (from both experiments) from the lowest to the highest rate of biochar application, while the PLB significantly increased CEC by 91%. This highlights the critical role of biochar properties in influencing CEC dynamics. In the second experiment, ryegrass cultivation revealed the greater importance of the calcium and magnesium/potassium ratio ([Ca+Mg]/K) in the soil CEC than CEC only for plant growth in biochar-amended soils. The ratios for optimum ryegrass production ranged from 82‒86 (M3) and 69‒74 (AA), which was translated to 88:11:1 Ca:Mg:K percent base saturation ratios. Moreover, predictive models for estimation of soil CEC after biochar application were successfully developed based on initial soil and biochar CEC. However, M3 was more reliable than AA for such predictions mainly because it was more successful in predicting nutrient availability from biochar. These models offer a promising tool for informed soil management decisions.Conclusion: This research emphasizes the importance of biochar feedstock, elucidates nutrient ratio effects on plant growth, and provides a practical means to anticipate soil CEC changes post-biochar application.

Plant residues and certain fertilizers accelerate soil acidification and increase the levels of aluminum-Al+3 in soils under no-tillage (NT). Silicates act as acidity amendments and as a source of silicon as in H₄SiO₄. An increase in the pH of soil solution causes the deprotonation of H₄SiO₄ and generates the anionic form (H₃SiO₄⁻). The aim of this study was to evaluate the ionic speciation of Si, Al, Ca, Mg and K in aqueous extracts by means of a software calculation. Since 2006, a field experiment has been under way on an Oxisol under NT subjected to lime and calcium-magnesium silicate applications under four crop systems. The amendments were applied in Oct 2006 and in Oct 2011, aiming to raise base saturation to 70 %. Soil samples were collected in Oct 2013, at depths of 0-5, 5-10, 10-20, 20-40 and 40-60 cm. Both Ca and Mg formed complexes with dissolved organic carbon (DOC) whereas the same was not observed for potassium. These three basic cations were mostly in their free forms regardless of treatment, while Al was mostly complexed with DOC even at the lowest depths (40-60 cm). The highest value of free Al form was 15 %. Si was almost 100 % as H₄SiO₄, and its activity was similar to its concentration in solution for all crop systems and at all depths, regardless of amendment applied. The percentages of H₃SiO₄⁻ and Al-H₃SiO₄²⁺ were irrelevant, providing more phytoavailable H₄SiO₄ in soil solution.