Chemical and biological processes leading to the neutralisation of acidity in soil incubated with litter materials

Plant materials containing high amounts of ash alkalinity can be utilized to increase the pH of acid soils but the chemical and biological processes involved in the release of this alkalinity are not fully understood. In this laboratory study fresh leaf litter from two tree species (Melia azedarach, Castanea sativa) and sugarcane (Saccharum officinarum) trash containing ash alkalinities of 288, 141 and 33 mmolc kg−1 respectively, were mixed at three different rates (4, 16, 32 mg g−1) with acidic topsoil from a Ultic Palexeralf and incubated at 90% WHC and at 25°C for 20 d while monitoring CO2-evolution. Treatment effects were assessed by measuring changes in pH, acid buffering curves and exchangeable cations before and after incubation. Furthermore, soluble organic compounds, mineral N-forms were determined in soil extracts. Immediately after mixing, up to 50% of the added alkalinity was available for acid neutralisation. After incubation, acid neutralisation capacity at pH 4 (ANCpH4) and the pH of the soils with the two higher amendment rates had increased in all treatments. The changes were most pronounced in the Melia amended soils, followed by Castanea and sugarcane and reflected the added amounts of ash alkalinity. In all treatments, soil respiration increased with amendment rate and was closely related to a decline in soluble organic carbon during incubation. Together with the shift from stronger to weaker acidity observed after incubation, this is evidence for the microbial decarboxylation of soluble organic anions.