Wood ash and biochar addition to forests do not shift soil microbial functional diversity in the short term

Community level physiological profiles (CLPP) performed by the MicroResp method are reliable ecological indicators to assess soil microbial functional diversity. Functional diversity provides a great insight to microbial roles in ecosystems, and it is regulated among others, by soil physicochemical properties. The application of biochar or wood ash to forest soils is often employed to improve soil structure and soil hydraulic properties, increase soil organic matter stocks, neutralise acidity and restore nutrients. We hypothesized that addition of new carbon sources (biochar and wood ashes) may induce a change in the microbial functional diversity. Two experimental sites were established on acidic soils: one on a loamy soil (SOC% 3.9; pH: 4.8) and another on a sandy loam soil (SOC% 10.8; pH: 3.8). CLPP were performed using MicroResp system in surface (0-5 cm) soil samples. The carbon sources used were selected on the basis of their ecological relevance to the soil microbial community. CLLP data of each experimental site was ordinated by principal component analysis. The first two ordination axes (explaining more than 70% of the variance) were selected and their relation with soil hydraulic and nutritional properties, soil structure and microbial biomass carbon was analysed. The major findings of this study may be summarised as follows: a) In the short term, a shift in the microbial substrate use profile for wood ash and biochar additions was not observed, b) the soil microbial community responded most strongly to the addition of carboxylic acids and carbohydrates in both experimental sites, and c) in the loamy soil the first axis of the PCA was related with pH and the second one to the mean weight diameter of soil aggregates, and in the sandy-loam site, the first axis was related to the available water content and the second one to the mean pore diameter