Assessment of soil microbial diversity measurements as indicators of soil functioning in organic and conventional horticulture systems

Soil quality assessment is necessary to detect changes on soil properties among different management practices. Some microbial properties could be useful to predict changes in soil providing an integrated and relevant vision of soil functioning. The aim of this work was to evaluate and compare different methods to assess microbial diversity, such as methyl ester fatty acids (PLEA) and catabolic response profiles (CRP) to act as indicators of soil functioning. The study was carried out in an intensive horticulture production system. Undisturbed soil, 5 years plots, and more than 20 years plots under organic and conventional production were studied. Principal component analysis followed by multivariate discriminate analysis showed that D-glucose, D-glucosamine, alpha-ketobutyric, alpha-ketoglutaric and uric acids were the substrates with the highest sensitivity to separate situations. The same analysis was performed for PLEA, showing that C18:1w9, C13:0, C16:1w9, C14:0, i15:0 and cy19:0 methyl ester fatty acids were the most sensitive. Multivariate analysis of variance of selected substrates and fatty acids showed that CRP and PLEA techniques were both capable to characterize the studied systems. Saturated/monounsaturated (S/M), iso/anteiso (i/a) and cyclopropyl/precursors (cy/pre) microbial stress indicators were higher in plots under conventional management, presenting also these situations the lowest microbial biomass and fungi/bacteria ratio (F/B), especially in plots under conventional management for more than 20 years. Microbial functional diversity, calculated as evenness (E) from CRP was capable to distinguish between all situations and management systems showing the potential of this measurement to act as an integrative indicator of soil functioning.