Exploring the synergistic effects of biochar and arbuscular mycorrhizal fungi on phosphorus acquisition in tomato plants by using gene expression analyses.

Arbuscular mycorrhizal (AM) fungi are symbiotic organisms that contribute significantly to plant mineral nutrition, mainly phosphate. However, their benefits are constricted by the availability of phosphate in the soil, and thus they are recalcitrant as amendment in highly fertilized soils. Biochars are by-products of the pyrolysis of biomass in the absence of oxygen. They can improve soil properties and act as a source of nutrients for plants. However, depending on their origin, the final composition of biochars is extremely variable and thus, their efficiency unpredictable. In order to gain mechanistic insights into how the combined application of biochars and AM fungi contribute to plant phosphate nutrition and growth, we used gene expression analyses of key symbiotic marker genes. We compared for this analysis two biochars originated from very different feedstocks (chicken manure and wheat straw) on tomato plants with or without the AM fungus Rhizophagus irregularis. Our results show that the synergy between AM fungi and biochars as P biofertilizers is greatly governed by the origin of the biochar that determines the speed at which phosphate is released to the soil and absorbed by the plant. Thus, chicken manure biochar quickly impacted on plant growth by readily releasing P, but it turned out detrimental for symbiosis formation, decreasing colonization levels and expression of key symbiotic plant marker genes such as SlPT4 or SlFatM. In contrast, wheat straw biochar was inferior at improving plant growth but stimulated the establishment of the symbiosis, producing plants with the same concentration of phosphate as those with the chicken manure. Taken together, slow P releasing biochars from plant residues appears to be a more promising amendment for long terms experiments in which biofertilizers such as AM fungi are considered. Furthermore, our results indicate that implementing plant transcriptomic analyses might help to mechanistically dissect and better understand the effects of biochars on plant growth in different scenarios.

Screw pyrolysis technology for sewage sludge treatment.

Sewage sludge quantities have grown continuously since the introduction of the European Directive (UWWTD 91/271/EEC) relating to the treatment of urban wastewater. In the present, most of the sewage sludge is combusted in single fuels incineration plants or is co-fired in waste incineration or coal power plants. The combustion of sewage sludge is a proven technology. Other treatments, such as fluidized bed gasification, were successfully adopted to produce suitable syngas for power production. Besides, the number of large wastewater treatment plants is relatively small compared to the local rural ones. Moreover, alternative technologies are arising with the main target of nutrients recovery, with a special focus on phosphorus. The aforementioned issues, i.e. the small scale (below 1MW) and the nutrients recovery, suggest that pyrolysis in screw reactors may become an attractive alternative technology for sewage sludge conversion, recovery and recycling. In this work, about 100kg of dried sewage sludge from a plant in Germany were processed at the newly developed STYX Reactor, at KIT. The reactor combines the advantages of screw reactors with the high temperature filtration, in order to produce particle and ash free vapors and condensates, respectively. Experiments were carried out at temperatures between 350°C and 500°C. The yield of the char decreased from 66.7wt.% to 53.0wt.%. The same trend was obtained for the energy yield, while the maximum pyrolysis oil yield of 13.4wt.% was obtained at 500°C. Besides mercury, the metals and the other minerals were completely retained in the char. Nitrogen and sulfur migrated from the solid to the condensate and to the gas, respectively. Based on the energy balance, a new concept for the decentral production of char as well as heat and power in an externally fired micro gas turbine showed a cogeneration efficiency up to about 40%.