While some biochar properties varied between species and location, seaweed as a feedstock was relatively consistent in the sense that all species yielded high amounts of biochar per unit biomass and the resulting biochars were relatively low in C, but rich in nutrients (N, P, K, Ca and Mg) and with a basic pH. There was, unsurprisingly, some variability in the characteristics of seaweed biochar produced from species collected from different locations. Further research should clearly consider environmental factors that might affect the spatial and temporal variability in seaweed biochar properties if large-scale production from cultivated seaweed is to occur. Nevertheless, the overriding conclusion of our work is that seaweed biochar is fundamentally different to biochar produced from ligno-cellulosic feedstock. Seaweed yields more biochar than ligno-cellulosic biomass during pyrolysis and the resulting biochar has a relatively low C content, but high concentrations of N, P and exchangeable K and Na relative to ligno-cellulosic feedstock. The seaweed biochars also have a much lower BET surface area than ligno-cellulosic biochar10. Seaweed biochar is consistently different to ligno-cellulosic biochar with respect to each of these properties, regardless of species or location and these findings are consistent with previous results for biochar from non-commercial macroalgae2 and microalgae11.

The combination of the low C content but high mineral content of seaweed biochar results in a low HHV of seaweed biochar in comparison to ligno-cellulosic biochars, which can exceed 30 14MJ kg−112,13. While the elemental profile of seaweed biochar limits its HHV, it also makes seaweed biochar a unique substrate that may be tailored to agricultural applications3. Seaweed biochars have a relatively consistent elemental composition that was more similar to biochar from manure than from ligno-cellulosic feedstocks2 and the low C/N ratio is particularly important in this context. The C/N ratio estimates the ability of organic substrates in the biochar to mineralize and release inorganic N when applied to soils. Typically, a C/N > 20–30 suggests N will not be available to plants14. As ligno-cellulosic biochars have high C and low N contents (and consequently high C/N ratios), the benefits of ligno-cellulosic biochar application to soils is indirect through improved nutrient retention14. In contrast, Gracilaria, Saccharina and Undaria biochars have a C/N ratio < 20, indicating they could directly contribute bioavailable N and P to soils, in addition to enhancing the retention of supplemental nutrients provided in the form of fertilizer. This prediction is supported by the known beneficial effects of other seaweed biochars on crop production3.

The main limitation of seaweed biochars that requires consideration is the high concentration of exchangeable Na which could increase soil salinity. Previous research suggests that the Na component of seaweed biochar is leachable, but levels are within biosolids limits and overall positive short-term effects on crop productivity have been described following the application of seaweed biochar to low fertility soils3. Nevertheless, it may be necessary to apply the biochar to soils well in advance of cropping to allow exchangeable Na to be leached from the biochar15. An additional approach to the production of biochar from seaweed biomass could be to blend it with ligno-cellulosic biomass to dilute the Na content of the resulting biochar. Indeed, the targeted blending of seaweed and ligno-cellulosic biochar may be a particularly strategic approach as it would yield a biochar that combines the nutrient- and mineral-rich properties of seaweed biochar with C-rich ligno-cellulosic feedstock. This blended biochar could be more suited to delivering stable C accrual in agricultural soils. For example, under the Carbon Farming Initiative (CFI) in Australia land owners are given financial incentives to adopt land management practices that result in soil C accrual16. However, when one factors in the cost of N addition that is required to stabilize C accrual, the costs of achieving C accrual typically outweigh any profits that may be realized through C credits16. The production of N- and C-rich blended biochar from seaweed and ligno-cellulosic feedstock could yield a C-rich soil ameliorant that also includes a significant component of exchangeable N to stabilize soil C accrual.

Increased seaweed cultivation has been proposed as a sink of “Blue Carbon” in climate change mitigation strategies17,18. If one assumes that the 19 14Mt wet (landed) weight of cultivated seaweed equates to 1.9 14Mt dry weight (a 10:1 wet to dry ratio)18 and a biochar yield of 59% with a mean C Org content of 30%, the scope for C sequestration from seaweed biochar derived from commercially cultivated seaweeds is 0.33 14Mt C yr−1. These figures do not take into account energy costs in the production process as the energy balance of biochar production varies greatly with the scale of the production system19. Regardless, the C sequestration potential for biochar produced from cultivated seaweeds is small relative to anthropogenic C emissions. However, seaweed biochar could deliver significant improvements in soil C accrual indirectly as a soil ameliorant to enhance crop production. Soil accounts for 20% of the global capture of anthropogenic CO 2 emissions each year, but is also a non-renewable resource that is being increasingly degraded20. The unique properties of seaweed biochar provide an opportunity for it to be blended with ligno-cellulosic biochar to produce targeted products for broad-spectrum agricultural applications. Blending of mineral-rich seaweed biochars with C-rich ligno-cellulosic biochar could yield unique soil ameliorants that are specifically created to match the requirements of specific types of soil21.

In conclusion, we found that biochar can be produced from a range of commercially cultivated seaweeds to yield unique ameliorants that could be applied to improve soil fertility. Biochar produced from red species of seaweed has higher concentrations of K and S and lower C, H and pH than biochar produced from brown species of seaweed. However, while some properties of seaweed biochar unsurprisingly vary between seaweed divisions and origin of seaweed feedstock, seaweed biochar is consistently different to biochar produced from ligno-cellulosic feedstock with respect to its key characteristics, having low C content but high concentrations of exchangeable nutrients (particularly N, P, K, Ca and Mg). Therefore, opportunistic harvesting of seaweed blooms such as those that regularly occur in China22 and France23, as well as the production of seaweed in bioremediation processes24,25 could be appropriate and sustainable feedstock to support expansions in the production of seaweed biochar, in addition to the species of seaweed that are currently cultivated at large scales world-wide. Targeted blending of seaweed and ligno-cellulosic biochar could produce “designer” biochars that could be matched to specific soil types for broad-spectrum agricultural applications.