Biochar application to soils is being considered as a means to sequester carbon (C) while concurrently improving soil functions. The main focus of this report is providing a critical scientific review of the current state of knowledge regarding the effects of biochar application to soils on soil properties, processes and functions. Wider issues, including atmospheric emissions and occupational health and safety associated to biochar production and handling, are put into context. The aim of this review is to provide a sound scientific basis for policy development, to identify gaps in current knowledge, and to recommend further research relating to biochar application to soils. See Table 1 for an overview of the key findings from this report. Biochar research is in its relative infancy and as such substantially more data are required before robust predictions can be made regarding the effects of biochar application to soils, across a range of soil, climatic and land management factors.
In this report, biochar is defined as: “charcoal (biomass that has been pyrolysed in a zero or low oxygen environment) for which, owing to its inherent properties, scientific consensus exists that application to soil at a specific site is expected to sustainably sequester carbon and concurrently improve soil functions (under current and future management), while avoiding short- and long-term detrimental effects to the wider environment as well as human and animal health.' Biochar as a material is defined as: 'charcoal for application to soils'. It should be noted that the term 'biochar' is generally associated with other co-produced end products of pyrolysis such as 'syngas'. However, these are not usually applied to soil and as such are only discussed in brief in the report.
Biochar is an organic material produced via the pyrolysis of C-based feedstocks (biomass) and is best described as a ‘soil conditioner’. Despite many different materials having been proposed as biomass feedstock for biochar (including wood, crop residues and manures), the suitability of each feedstock for such an application is dependent on a number of chemical, physical, environmental, as well as economic and logistical factors. Evidence suggests that components of the carbon in biochar are highly recalcitrant in soils, with reported residence times for wood biochar being in the range of 100s to 1,000s of years, i.e. approximately 10-1,000 times longer than residence times of most soil organic matter (SOM). Therefore, biochar addition to soil can provide a potential sink for C. It is important to note, however, that there is a paucity of data concerning biochar produced from feedstocks other than wood. Owing to the current interest in climate change mitigation, and the irreversibility of biochar application to soil, an effective evaluation of biochar stability in the environment and its effects on soil processes and functioning is paramount. The current state of knowledge concerning these factors is discussed throughout this report.
Pyrolysis conditions and feedstock characteristics largely control the physico-chemical properties (e.g. composition, particle and pore size distribution) of the resulting biochar, which in turn, determine the suitability for a given application, as well as define its behaviour, transport and fate in the environment. Reported biochar properties are highly heterogeneous, both within individual biochar particles but mainly between biochar originating from different feedstocks and/or produced under different pyrolysis conditions. For example, biochar properties have been reported with cation exchange capacities (CECs) from negligible to approximately 40 cmolc g-1, C:N ratios from 7 to 500 (or more). The pH is typically neutral to basic and as such relatively constant. While such heterogeneity leads to difficulties in identifying the underlying mechanisms behind reported effects in the scientific literature, it also provides a possible opportunity to engineer biochar with properties that are best suited to a particular site (depending on soil type, hydrology, climate, land use, soil contaminants, etc.).