It is usually during the use phase of biochar products that many of the biochar effects take place: e.g. improved water holding capacity of a soil, reduced emissions from fertiliser applications, or improved stormwater management.
The use phase of biochar products often lasts several decades (e.g. urban soils, green roof), but can also be limited to less than a few years (e.g. filters). In the case of application to agriculural soils, the use phase is theoretically infinite (and undistinguished from the end-of-life of the biochar product).
A challenge when assessing new biochar products with long service life is to describe, understand, and quantify (over time) the biochar effects that take place during the use phase.
The effects arising from biochar use in agriculture are diverse. Most effects relate to biochar use in soils, as opposed to effects relating to, for instance, uses in animal husbandry, manure management, or horticulture.
In the LCA literature, several of the environmental impacts of these biochar use-phase effects have been modelled. We list them below, from most to least commonly modelled:
More details are presented in Azzi et al. (2021).
There are several challenges associated with modelling of biochar agricultural effects:
effects vary with time: some effects can last multiple years before they fade out, e.g. liming effect, crop yield increases; while other effects may only last one agricultural season or persist indefinitely. In general, there is a lack of long-term studies. This hinders modelling of multi-annual effects in biochar LCAs.
effects are multiple and interrelated: as illustrated in the figure below, biochar can affect soil water processes, various biological processes, the soil structure, the availability of nutrients and contaminants. The interaction of these effect, in combination with external factors like climate and human interventions, is what leads to an observed crop productivity or environmental impact. LCA often seek to quantify those final effects rather than describe the complex intermediary effects.
effects vary with the agroecosystem considered: each agroecosystem, with its given soil type, climate or agricultural practice, may respond differently to a given biochar use. This diversity adds a layer of complexity in LCA modelling. Biochar agricultural effects require the context of the study to be very precise, which limits applicability of results to other contexts.
Urban uses of biochar are also associated with various effects. A few will be illustrated here.
Biochar used for urban tree plantings
Biochar is nowadays routinely used in Stockholm and other cities in Sweden to plant trees in urban paved areas. The constructed soil is made of 80% of macadam (stones) and about 20% of a blend of biochar and compost (in equal parts), in volume. The constructed soil is very porous, and rainwater is chanelled through the planting beds. This tree planting technique is studied for its effects on tree growth, tree mortality, but also stormwater quality and quantity. This said, the data available to date is to scarce to be able to quantify the environmental benefits of these effects.
Biochar used for green roof
Biochar is also used for production of sedum green roofs. Biochar is one of the component of the mineral soil used to grow. A claim made by the manufacturer is that biochar-based green roofs require fertilisation only every second year, while conventional green roofs would require it annually. The green roofs, due to increase water holding capacity, are expected to reduce runoff of stormwater.
The first effect described (fertiliser use reduction) is simple to include in LCA. The second effect is more challenging to model as data is more scarce and will depend on the local climate.
To explore the topic further, we recommend the following references:
Azzi, E. S., Karltun, E., & Sundberg, C. (2021). Assessing the diverse environmental effects of biochar systems: An evaluation framework. Journal of Environmental Management, 286, 112154. https://doi.org/10.1016/j.jenvman.2021.112154
Azzi, E. S., Karltun, E., & Sundberg, C. (2022). Life cycle assessment of urban uses of biochar and case study in Uppsala, Sweden. Biochar, 4 (1), 18. https://doi.org/10.1007/s42773-022-00144-3