Effect of Biochar and Hydrochar on Forms of Aluminium in an Acidic Soil

Biochars and hydrochars have a significant effect on soil properties linked to fertility or to carbon cycling and have been proposed as an amendment to increase soil productivity, particularly in acidic soils. Phytotoxic forms of aluminium (Al) are relatively abundant in acidic soils and, for a long period of time, liming has been used to correct this. Ca(OH)2, a pig manure (PM) and two biochars and two hydrochars prepared from pig manure were studied for their effects on Al fractions. Biochars were prepared at 450 °C (BPC450) and 600 °C (BPC600) and hydrochars were obtained using a pig manure solution (ratio 30:70) that was heated at 200 °C (HPC200) and 240 °C (HPC240). A treatment with an amount of Ca(OH)2 necessary to increase soil pH to the same pH value as the average in the treatments BPC450, BPC600, HPC200, HPC240 and PM was used for comparison. The fractionation of Al was studied, with the liming treatment allowing the differentiation between changes in fractionation driven by pH changes from other mechanisms. In relation to the control, all soil amendments presented high capacity of controlling toxic Al, similar to a traditional liming product (Ca(OH)2) and decreased the exchangeable Al extracted by NH4Cl. Both types of materials (biochars and hydrochars) lead to the formation of an increased number of organo-aluminium complexes (OAC). Biochars lead to the formation of OAC of low to medium stability, while hydrochars promoted the formation of OAC of high stability.