## 1 Introduction
Additionally, they have some secondary beneficial effects like their use as hedges or ornamental plants [19; 27] and natural herbicides [29], and for their medicinal or immunological value [30]. Gorse was historically used as fuel in the British Isles; in northwest Spain and northern Portugal, it has been extensively mixed with animal waste to make fertilizer. In addition, gorse has been used as fodder and broom for cleaning purposes in several regions worldwide. Nowadays, these traditional uses, however, have been abandoned, leading to increased biomass availability [15; 30]. The harmful effects of gorse and Scotch broom are associated with their ability to form dense impenetrable monospecific stands that affect the dynamics of the landscape; modify the edaphic composition; reduce or even eliminate native grasslands, forests, rangelands, and agricultural lands areas; and create fire hazards [21; 31]. Moreover, serious fire hazards can affect both the invasive and native ranges of gorse and Scotch broom, because these shrubs are highly flammable and produce high residual biomass [31; 32; 33]. For example, the shrub communities in the European region of Galicia, Spain, and northern Portugal, belonging both to the Euro-Atlantic and Mediterranean biogeographic domains, are abundant. In this region, gorse and broom are prone to severe wildfires, which have intensified in recent decades [34; 35; 36]. The surface areas of gorse and broom in this European region have been estimated at about 530,000 and 470,000 ha, respectively [37]. Fuel loads have been measured in Galicia, yielding aerial biomass values of 33.6 \(\pm\) 14.5 and 53.7 \(\pm\) 36.0 t ha\({}^{-1}\) for gorse and broom, respectively. Additionally, the total measured fuel load amounts, including litter, are 40.0 \(\pm\) 22.8 for gorse and 61.9 \(\pm\) 47.3 t ha\({}^{-1}\) for broom. In addition, frequently gorse, but also broom, occurs under Pinus and Eucalyptus forests, which are also subjected to fire hazards. The risk of wildfires could be drastically reduced if this available and abundant low-cost aerial biomass is used as feedstock to synthetize renewable sources of energy, including biochar. Characterizations of BC from woody feedstocks including trees [38], tree by-products such as wood chips [39], wood industry residues such as sawdust [40], or pruning waste [3; 9] are widely available. Similarly, several invasive herbaceous species have been broadly used as biochar feedstock in several applications [41]. Selected examples are (i) _Ambrosia artemisinin_ L., used as a metal-accumulating biochar in order to treat potentially toxic elements (PTEs) [42]; (ii) _Eupatorium adenophorum_ biochar, which reduced the Pb and Cd contents from aqueous solutions, leading to possible use in heavy metal remediation [43]; and (iii) _Ambrosia trifida_ L., converted to biochar and used as an effective adsorbent for the treatment of Trichoroethylene (TCE)-contaminated groundwater, which is the predominantly occurring chlorinated hydrocarbon in groundwater in industrial/urban areas around the world [44]. For fuel production, the ideal invasive species is _Spartina alterniflora_ due to its high productivity and heat value [45]. However, despite the wide distribution of gorse and broom across much of the temperate and subtropical agroecological landscapes of the world, research on the valorization of its biomass produced via modern thermochemical conversion techniques has been rather limited. The valorization of gorse as a potential renewable energy source (fuel or electricity) has been addressed [46; 47], and the use of broom [48] and gorse [49] for fuel production has been also explored. Notably, a characterization of torrefraction products from both gorse and broom was reported [50]. Overall, the characterization of gorse- and Scotch broom-derived BC has received comparatively less attention than that of BC from forest species or invasive herbaceous plants. As a result, the former two biochars have been only partially described, and their full characterization still is lacking.