2. Materials and methods
2.1. Biochar properties
Three types of biochar were produced by the VTT Technical Research Centre of Finland using a pilot-scale slow-pyrolysis unit. The pyrolysis process, as well as characteristics and analysis methods of the biochar types are reported in detail in Fagernäs et al., 2014 and Fagernäs and Kuoppala, 2015). The biochar types were produced using Betula sp. wood blocks as a feedstock, subject to three different temperatures: 300 °C, 375 °C and 475 °C; referred to as BC300, BC375 and BC475. The biochar produced were crushed and passed through a 2 mm sieve before addition to the soil (see below). The specific surface area (BET-surface area analysed according to the Standard PANK 2401), water holding capacity (WHC) and other characteristics of the biochar types are presented in Table 1. The H/C and (O + N)/C ratios were calculated to estimate the aromacity and polarity of the biochar types, respectively.