The recently growing biogas sector generates large amounts of by-products, the digestates, which should be applied to arable land in order to recycle valuable nutrients. However, knowledge about the composition of digestates and their fate in soils, especially under root-caused alkaline conditions, is not sufficient so far. Thus, the aims of this thesis were (1) to examine the elemental composition of different digestates (from sewage sludge, slurry or maize), especially the temporal variability and alteration during fermentation, (2) to investigate the effects of digestates on soil organic matter (SOM) under artificial rhizosphere conditions and (3) to test whether the anion exchange of a model root would increase P availability in a digestate amended soil. These objectives were pursued by a one-year analysis of digestates and their feedstock with subsequent element balancing and by incubation experiments with digestates and soil including anion exchange resins as root models. Digestates showed a great variability depending on their feedstock but also over time. For an element balancing from feedstock to digestate, two, five or ten sampling dates depending on the particular digestate were needed for reliable results. During fermentation, depletions of nitrogen (N), sulphur (S), magnesium (Mg), zinc (Zn) and cadmium (Cd) were determined, probably due to volatilisation or precipitation, while attrition of stirring devices may have resulted in iron (Fe) and manganese (Mn) accumulations. The mineralisation of digestates in soil was increased by resins (the root models) releasing hydrogen carbonate (HCO3-). Ambivalent mineralisation dynamics were observed for digestates from the same biogas plants, but sampled at different times from these plants. Indications for priming, i.e. an extra mineralisation of SOM due to the addition of OM to soil, occurred for digestates from sewage sludge and maize, but not for each biogas plant sampling date. Presumably, digestate stability after fermentation and thus their potential to promote priming in soils vary over time, as well. The release of HCO3- by the resins in exchange for anions from the digestate amended soils resulted in an increase of pH and P mineralisation. Consequently, a peak of P concentration moving away from the resin with time occurred. In conclusion, the heterogeneous composition and stability of digestates over time impede simple predictions of their fate in soils and thus decisions about their use as fertilisers. Moreover, models for P transport to roots should account for additional supply of P by OM mineralisation.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Bioengineering, Environmental engineering, Chemical engineering, Agricultural engineering|
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