Deborah Schönegger successfully defended her master thesis entitled “Phosphorous fertilising potential of fly ashes and effects on soil microbiota and crop” within the framework of our Residue2Heat project under the supervision of Prof. Heribert Insam, Dr. Marina Fernández-Delgado Juárez and Dr. María Gómez Brandón, receiving the maximum qualification at the Institute of Microbiology at the University of Innsbruck.
The production of fast pyrolysis bio oil (FPBO) constitutes one of the newest technologies for gaining a liquid biofuel from woody biomass. During this process fly ashes (FA), rich in minerals and salts, are produced. Many essential macronutrients like P, K, Mg, Ca, S and micronutrients including Fe, Mn, Zn and Cu are retained in the ash. Consequently, biomass ash addition may supply the soil with an ample range of mineral nutrients, among them P. However, FA are often disposed in landfills and their fertilising potential has been underestimated. Therefore, a greenhouse trial was conducted to investigate the impact of FA on soil physico-chemical and (micro)biological properties with a special focus on P, one of the main limiting nutrients in terrestrial ecosystems. The main objective of this study was to evaluate, at a mesocosm level, the impact of FA derived from the fast pyrolysis process on different fractions of soil-P (total, inorganic, plant available and microbial P), as well as on certain enzymatic activities related to the P cycle, and on plant growth after 60 and 100 days of incubation. Furthermore, culture independent methods were used to estimate the composition and abundance of bacterial alkaline phosphatase (phoD), and fungal communities. Overall, ash application increased soil pH and electrical conductivity, and improved soil nutritional status by increasing soil total, inorganic, and plant available P over time. Accordingly, higher plant yields were observed in the ash-treated soils compared to the control. The effect of FA on microbial biomass was time-dependent and increased significantly in plant presence. Acid phosphomonoesterase activity significantly decreased following ash addition. Neither alkaline phosphomonoesterase (ALP) activity nor the abundance and composition of the ALP gene (phoD) harboured by bacteria were affected by FA application. Hence, FA from FPBO production seems to improve soil nutrient status and plant growth without inheriting detrimental effects on soil microbial communities in the mid-term.