## 1 Introduction  
Aluminium (Al) is a natural constituent of all soils, comprising approximately 8% of the total soil inorganic fraction content. Soils in Galicia (NW Spain) and in many other humid regions in the world are acidic as a consequence of the high precipitation, grossly exceeding evapotranspiration. Acidic soils are considered to occupy approximately 30% of the Earth's ice-free surface [1]. Aluminium has a complex chemistry and its speciation is of utmost importance in acidic soils. In this type of soils, the occurrence of aluminium in the soil solution, which increases at pH values below 6, and its toxicity to plants can lead to decreased plant yields. In fact, aluminium toxicity is considered to be the most limiting factor for productivity in acidic soils [1]. Most soil Al is bound to the soil mineral structure with only a relatively small proportion released to soil solution. It has been commonly accepted that the Al\({}^{+3}\) ion and monomeric hydroxyl aluminium complexes are the most toxic species of this element [2]. Organocomplexes of Al or combinations with fluoride and sulphate pose lesser concern in regard to phytotoxicity [3]. In Galicia, soils have medium to high exchangeable Al content [4]. Generally, timing agents are added to soils in humid regions with the aim to neutralize the pH. This leads to a decrease of aluminium in the soil solution and in the exchange complex through the formation of organo-mineral complexes or precipitation [5]. Thus, exchangeable Al is rapidly neutralized by lime, while non-exchangeable Al (organo-aluminium complexes, hydroxyl aluminium compounds, polymers) react slower. In the last years, an increasing amount of research has been conducted with the aim to add value to waste and incorporate it to the soil system. Biochar is the solid product produced from the pyrolysis, a thermal process in the absence of oxygen, of waste at moderated temperatures and intended for soil amendment. While the initial focus on biochar research was as a geoengineering approach to increase carbon sequestration [6], a multifaceted research literature has emerged in the last years. Biochars are able to sorb multiple contaminants and can be used in the remediation of organic and inorganic polluted areas [7; 8]. The addition of biochars to soil can result in an improved nutrient cycling [9]. Finally, it is reasonably well established that biochar can be used to increase crop yields, particularly in acidic soils [10; 11]. The reason for this is that biochars tend to be alkaline and part of their benefits in soils, including both, agronomic productivity and soil remediation, has been associated with an amelioration of soil pH [11]. Alkalinity is one of the most important chemical characteristics of biochars. Total biochar alkalinity is comprised of different compartments such as bicarbonate and carbonate (HCO\({}_{3}\)- and CO\({}_{3}\)2-), organic and other inorganic (non-carbonate) alkalinity [12]. As a consequence of the alkalinity promoted by the biochar application, changes may occur in acidic soils, such as an increase in the concentration of available nutrients (especially P), in the cation exchange capacity, as well as in reducing Al toxicity. Hydrothermal carbonization has been explored in the last years as a methodology to transform wet waste biomass into a potential soil amendment named hydrochar. The production of this material can be more cost-effective than biochar, depending on the intrinsic water content of the samples. To date, few articles have attempted to compare the characteristics of hydrochar and biochar (see for example [13; 14]) or its effect upon soil addition [15]. To our knowledge, the potential alleviation of aluminium toxicity and the concomitant changes in the forms of aluminium by biochar or hydrochar has seldom been studied. [PERSON] et al. [16] investigated aluminium phytotoxicity in a hydroponic system using a single biochar.