Solubilization of Phosphorus, Silicon, and Calcium and Abundance of Phosphorus-solubilizing Bacteria in Temperate Forest Soils
Giovanni Pastore (01/2017-12/2020)
Support: Marie Spohn
This thesis focuses on the question of how microbes from temperate forest soils influence phosphorus (P), silicon (Si), and calcium (Ca) solubilization from minerals and weathered parental materials as well as to which extent P-solubilizing bacteria contribute to the overall solubilization rates. Despite the increasing awareness of the role of microorganisms in plant nutrition, the potential of microbial communities to release phosphate from mineral phases has not been explored with detail, so far. Moreover, the research on the solubilization of siliceous and calcareous parent materials from forest soils is quite fragmented, regardless of the fact that most experiments have been performed with cultured microorganisms and high doses of organic acids addition. A total of seven deciduous temperate forest soils and two soil depth increments from the mineral horizon were chosen to evaluate how, and to which extent, the microbial communities of different depths affect P solubilization. In the first incubation experiment, microbial net P solubilization rates were determined from primary (hydroxyapatite) and secondary (P-loaded goethite) P-minerals after addition of glucose to five acidic soil extracts (ranging from P-rich to P-poor conditions). Net P solubilization rates were derived from the increase of P concentrations in soil extracts incubated with hydroxyapatite and P-loaded goethite over 14 days. In the second and third incubation experiments, net Si and Ca solubilization rates were determined from four siliceous and two calcareous weathered parent materials, respectively. The net Si and Ca solubilization rates were calculated from the difference between the amounts of Si and Ca measured at the end and at the beginning of the experiment over 14 days. In all incubation experiments, the temporal changes in pH and the concentrations of four organic acids (citric, oxalic, 2-keto-D-gluconic and D-gluconic) were determined in soil extracts, while lactic acid was added to the analyses in extracts from the calcareous soils. Gross P solubilization rates were determined at four acidic and two alkaline forest soils developed on siliceous and calcareous bedrocks. Moreover, the abundance and the taxonomic diversity of P-solubilizing bacteria were carried out from acidic and alkaline soils through a physiological assay in combination with 16s rRNA gene sequencing. The microbial net P solubilization rates were higher in incubations of soil extracts with hydroxyapatite than P-loaded goethite, except one case. The relationship between the pH of soil extracts and microbial net P solubilization rates was negative in incubations with hydroxyapatite and positive in incubations with P-loaded goethite and depended on the different release of protons and organic acids by microbes. In the incubation of goethite, microbes likely downregulated the production of organic acids to prevent acidification, and thus, strong sorption of phosphate ions. Further, the production of monocarboxylic acids coincided significantly with high P release from hydroxyapatite. Altogether, microbial net P solubilization rates from primary and secondary P-minerals increased with the addition of glucose. When weathered siliceous and calcareous parent materials were used, no net P solubilization rates were found as an effect of microbial P immobilization. However, from the net Si and Ca solubilization rates we determined the gross P solubilization rates based on the stoichiometric ratios they had with the P content of bedrocks. The microbial gross P solubilization rates were significantly higher in the incubations of the soil extracts with calcareous than siliceous bedrocks (on average +61%). Carbonates had higher gross P solubilization rates as result of an overall higher microbial activity, as suggested by the amounts of organic acids (up to 4.5 times higher) and microbial biomass (up to 19.1 times higher) in comparison to silicates. This trend was particularly noticeable in the upper soil depth of calcareous soils. Regarding the abundance of P-solubilizing bacteria, our data show that this was significantly higher in calcareous soils than in siliceous soils (on average +46.6%). Also, nonmetric multi-dimensional scaling analyses (nMDS) revealed that the P-solubilizing bacteria in calcareous soils were significantly different from those found in silicate soils. Bacillales and Burkholderiales dominated at the silicate sites, whereas Pseudomonadales and to a much lesser extent Enterobacteriales were the dominant orders at the calcareous sites. In conclusion, P-solubilizing bacteria were more abundant in P-poor soils than in P-rich soils, while solubilization rates were influenced by the mineral chemistry of the bedrocks. In future studies, the extent to which P is released from weathered parent materials may be also “attempted” by determining the net solubilization of the major constituent of the bedrock in relation to the stoichiometric ratio of that element with P, considering that the specific release rates of P are often extremely difficult to measure. Not only is more research on net P solubilization from coniferous and Mediterranean forest soils needed, but also the net P solubilization driven by fungal communities in forest soils warrants further investigation.