Effects of H2SO4 and HNO3 on soil acidification and aluminum speciation in variable and constant charge soils

The difference in effect on acidification and species of aluminum speciation between HNO3 and H2SO4 for two contrasting types of soils in surface charge was investigated. The results show that the effect of H2SO4 on acidification of variable charge soils (Ferric Acrisol and Haplic Acrisol) of subtropical regions was weaker than that of HNO3, due to the specific adsorption of SO42- and the accompanied release of OH-. For two constant charge soils, Haplic Luvisol and Eutric Cambisol, the difference in effect between the two acids is small. The concentrations of total inorganic monomeric aluminum, Al-3+, Al-F complexes and Al-SO4 complexes in the extract from variable charge soils are also lower in H2SO4 systems than those in HNO3 systems, whereas the reverse is true for constant charge soils, except the concentration of Al-3+. For variable charge soils, Al-F complexes are the major species of inorganic monomeric aluminum at high pH but Al-3+ might contribute to a large part at low pH, while for constant charge soils Al-F complexes contribute almost exclusively to the inorganic monomeric aluminum. The presence of a large amount of SO42- in the extract from the constant charge soils leads to a higher proportion of Al-SO4 complexes in total inorganic monomeric aluminum than that from variable charge soil, although the numerical value of proportion is small.