Composted biowaste digestates as fertilizers: effects on soil chemistry and plant development in a three-year field trial

In einem dreijährigen Feldversuch wurden kompostierte Gärreste aus Bioabfällen hinsichtlich ihrer pflanzenbau­lichen Einsatzmöglichkeiten und ihrer Wirkung auf boden­chemische Parameter geprüft. Zudem wurden die Auf­be­reitungsverfahren Pelletierung und Agglomeratbildung und deren Auswirkung auf die Gärprodukteigenschaften untersucht. Es wurde festgestellt, dass die unmit­telbare Düngewirkung der Produkte aufgrund der niedrigen Stickstoffverfügbarkeit gering ist. Allerdings zeigte sich nach der Anwendung eine deutliche Erhöhung der Kohlenstoffgehalte im Boden, was auf ein relevantes Humusreproduktionspotential durch derartige Gärprodukte schließen lässt. Agglomeratbildung und Pelletierung scheinen die Mineralisierung der Produkte weiter zu verlangsamen, was sich auf die Nährstofffreisetzung negativ, auf eventuelle Kohlenstoffsequenzierung jedoch positiv auswirkt. Problematisch sind speziell bei Bioabfällen hohe Gehalte an Schwermetallen und anderen Störstoffen. Die gesetzlich vorgegebenen Grenzwerte wurden in 50% der untersuchten Chargen überschritten, wobei jedoch nur max. 7% der ausgebrachten Schwermetalle im oberirdischen Pflanzenmaterial wiedergefunden wurden.In a three-year field trial, composted digestates from biowaste were tested for their potential for plant cultivation and their effect on soil chemical parameters. In addition, the preparation processes of pelleting and agglomeration and their effect on the digestate properties were investigated. It has been found that the immediate fertilising effect of the products is low due to the low nitrogen availability. However, there was a significant increase in carbon content in the soil, suggesting a relevant humus repro­duction potential from such digestate products. Agglo­merate formation and pelleting appear to further slow the mineralization of the products, negatively affect­ing nutrient release but positively impacting carbon sequenc­ing. Particularly problematic in biowaste are high levels of heavy metals and other impurities. The statutory limits were exceeded in 50% of the batches exa­mined, but only max. 7% of the discharged heavy metals were recovered in the aboveground plant material

Monitoring N : P Ratio and Cd, Cu, Pb, and Zn Contents in Different Types of Anaerobic Digestates

Depending on the quality of the input substrates, process parameters, and postfermentation treatments, digestates may contain a broad spectrum of potentially toxic elements. We suspected that these contents may vary on a broad scale even under seemingly stable process conditions at the biogas plant. Digestates from four biogas plants were therefore continuously analyzed for their contents of phosphorus, nitrogen, cadmium, copper, lead, and zinc over a period of six years. The input substrates varied between the plants (e.g., cattle and pig slurry and rye and maize silage), but were the same for each plant over the whole period. The N : P ratio of the digestates ranged from 2 to 24, with the digestate coming from cofermentation of pig slurry and energy crops (“DG Pig”) having the widest range of N : P ratio over the years. Heavy metal loads of all digestates and during all evaluations did not exceed the limits set by European or German legislation, but as previously expected, showed a large variability especially if cattle or pig manure were used as substrates. Copper content of Cattle slurry before digestion was 897.7 mg kg−1 DM in one case, and zinc content of DG Pig reached 590.2 mg kg−1 DM also once during the investigation. As a result, we strongly recommend to monitor especially phosphorus, copper, and zinc contents in digestates very closely and in short intervals.Peer Reviewe