Biochar properties: Transport, fate, and impact

Biochar is the name given to charcoal produced from pyrolyzed biomass with the purpose to add into soils to enhance soil properties and sequestrate carbon. Biochar has been a very popular research topic and a large amount of scientific literature has been produced in the last decade. Papers show biochar production from almost any type of biomass available using diverse types of thermochemical processes which have variations in technology, design, and conditions. All this variability in biomass feedstock and production has made biochar a very heterogeneous product, making it difficult to understand the biochar characteristics and qualities and what are the benefits and risks of its application in soils. This review presents a comprehensive vision of biochar properties, and their impact when biochar is applied to soils. To better understand this impact, biochar-collected data were classified in six types according to the main biomass used in its production, and divided into two thermochemical treatments: convectional pyrolysis methods and Hydrothermal Carbonization (HTC). Four biochar properties were studied: (i) physical and structural characteristics, (ii) chemical properties, (iii) agronomical properties, and (iv) contaminants. Outcomes of this review suggest that biochar can be a good soil amendment with capacity to enhance physical, chemical, and agronomic soil qualities. Nevertheless, results show the current lack of appropriate methodological analytical determination of some of the biochar characteristics. This can mislead to erroneous biochar characterization which could lead to future environmental issues. The review of the data suggested that some types of biomass could be not suitable for biochar soil application due to high levels of contaminants, although the information about solubility and availability of these contaminants in many cases is not clear. A consistent methodology, protocol, or index to measure biochar carbon stability in soils was not found; data suggest that a small proportion of biochars would not be suitable for carbon sequestration in soils but they could have good agronomical properties—this finding suggested that biochar production can be customized based on its final use. However, before proceeding to recommend massive use of biochar in soil, more research is necessary to have enough knowledge and understanding of biochar properties to develop models to predict biochar transport, fate, and impact.