Role of organic matter in soil pH change

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Agricultural Sciences, School of Life Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Soil acidification is an important land degradation process in many farming systems. The role of organic matter and its cycling in soil acidification is unclear. The overall aim of this thesis was to investigate the chemical and biological mechanisms of soil pH change resulting from addition of organic compounds. This thesis investigates the impact of added organic matter on soil pH change, nitrogen transformation and microbial mineralisation. Moreover, the thesis identifies the relationships between soil pH change, soil properties and chemical composition of organic matter, and interactions between added organic matter and the native soil organic matter on soil pH change. In a series of laboratory incubation experiments organic compounds commonly found in plant residues were selected based on the type and number of chemical functional groups and were added at various rates (0.1-3 g kg-1 soil) to soils differing in initial pH and organic matter content. The addition of carboxylic (-COOH group) acids (acetic, malic, citric, benzoic and ferulic acid) immediately reduced soil pH due to H+ dissociation. The magnitude of pH decrease depended on the rate of C application, dissociation constant (pKa) of the acids and initial soil pH. During subsequent incubation, pH slowly increased due to the consumption of H+ via microbial decarboxylation of organic anions with associated CO2 release. In contrast, net alkalinisation of soil occurred after organic anion addition. Glucose (OH- group) and glucosamine (-NH2 group) had no initial effect on soil pH, however, during incubation glucose decreased the soil pH in the Podosol (initial pH 4.45) but slightly increased pH in the Tenosol (initial pH 6.20). Soil pH changes after glucose addition were small compared with the other compounds and occur mainly via nitrogen transformation (N mineralisation and uptake). Glucosamine decreased soil pH over time for both Tenosol and Podosol; however the magnitude of decrease was greater in the Tenosol. In the Tenosol, organic acids (malic, citric and ferulic acid) resulted in a final pH which was greater than the initial pH. This generation of additional alkalinity by these compounds was termed alkalinity priming. A further experiment showed that alkalinity priming was greater with malic acid than glucose amendment in a wide range of soils and was reduced in soils with low C content and low initial pH. The fate of glucose and malic acid into soil C pools (mineralised to CO2; immobilised into microbial biomass C; remaining as extractable organic C) was quantified using 14C-labelled compounds. The effect of glucose on alkalinity generation occurs mainly via N cycle while the effect of malic acid via both C and N cycles. The thesis suggests mechanisms for increasing soil alkalinity following organic matter are decarboxylation, microbial NO3 - uptake, ammonification and decomposition of native soil organic matter, depending on soil types and added C.