Predicting pH buffering capacity of New Zealand soils from organic matter content and mineral characteristics

Information on the pH buffer capacity of soil is required to estimate changes in pH due to acidic or alkaline inputs, and to model pH-dependent processes within the soil nitrogen (N) cycle. The objective was to determine whether a model based on soil organic matter (SOM) and mineral characteristics (clay content, extractable iron (Fe) and aluminium (Al)) would be adequate to estimate the buffer capacities of New Zealand soils. We measured pH changes in 34 soils, representing a range of SOM and texture, after equilibration with several rates (range 0-15cmol OH(-)kg(-1) soil) of either KOH or Ca(OH)(2). The Ca(OH)(2) method often yielded higher buffer capacity values than the KOH method, possibly because of incomplete reaction of Ca(OH)(2), especially at high addition rates. Buffer capacity (measured using KOH) of the soils was strongly correlated with soil carbon (C) (R-2=0.76), and weakly (but significantly, P<0.05) with clay content, and with dithionite extractable Fe and Al. A regression with soil C, clay, and P-retention (a surrogate for extractable Al and Fe) as independent variables explained 90% of the variability in pH buffering. The role of organic matter was further evaluated by measuring buffer capacity of soil from research plots at Lincoln, Canterbury, New Zealand, that differed in C (21-37gCkg(-1) in the top 7.5cm; 19-26gCkg(-1) in the 7.5-15cm) as a result of the treatments imposed during the 12-year trial period. A substantial decrease in pH buffering (by up to 24% in top 7.5cm) was associated with a decline in SOM following the conversion of permanent pasture (pre-trial land use) to arable cropping. Across all treatments and sampling depths, buffer capacity was linearly related (R-2=0.84, P<0.001) to soil C; the estimated buffer capacity of SOM was 89cmol(c)kg(-1) C pH unit(-1), similar to the value calculated from the previous study with different soil types. After 12 years, treatments with low soil C concentrations tended to be more acidic, possibly partly because of weaker pH buffering.