Soil microbial response to wood ash or lime applied to annual crops and perennial grass in an acid soil of northwestern Alberta

More than 90% of acid soils in western Canada are in Alberta, yet the use of agricultural lime is limited because it is expensive. Wood ash, a by-product of pulp and lumber mills, can be used for liming acid soils. We investigated the effects of amending an acid Luvisol with wood ash or lime on soil microbiological properties at Beaverlodge, Alberta. Both soil amendments were applied at a calcium carbonate rate of 6.72 t ha-1, which was 8.40 t ha-1 for wood ash and 7.47 t ha-1 for lime, in 2002. Soil microbial biomass C (MBC) and the functional diversity and community structures of soil bacteria (indicated by substrate utilization patterns) were measured from 2002 to 2005 under barley (Hordeum vulgare L.), canola (Brassica napus L.), field pea (Pisum sativum L.), and timothy grass (Phleum pratense L.). In the rhizosphere, wood ash increased soil MBC between 2.4-fold in 2002 and 1.3-fold in 2005, and lime increased MBC from 3.2-fold in 2002 to 1.3-fold in 2005. In bulk soil, the increases in MBC ranged from 3.0-fold in 2003 to 1.8-fold in 2005 for wood ash, and from 4.9-fold in 2002 to 2.0-fold in 2005 for lime. Crop effects on MBC were not consistent. Because annual crops were grown in rotation, it is possible that the results obtained in one crop were confounded by effects of the preceding crop. In 2003 and 2004, both amendments increased Shannon index (H’) of bacterial functional diversity in the rhizosphere, and similar results were observed in 2005 in bulk soil. Shifts in the functional structure of bacterial communities due to soil amendment were observed in bulk soil, and shifts due to crop effects were observed in the rhizosphere. In 2003, the average soil pH(CaCl2) increased from 4.91 in control treatments of different crops to 6.60 in lime-amended plots and 6.70 in wood ash-amended plots. In 2004, both wood ash and lime significantly increased soil C mineralization (up to 10 d incubation), but basal respiration (11-24 d incubation) was not affected. The large effect (up to about fivefold) of soil amendments on MBC implies that soil acidity is a major limiting factor for biological processes and the productivity of some Luvisolic soils in Alberta. Wood ash could be used to alleviate these limitations. Key words: C mineralization, microbial diversity, microbial biomass, soil amendment, soil acidity