Abstract
Production of biogas and utilization of the resulting digestates as fertilizers has been increasing in Europe in the last few decades. Depending on the feedstock used for the anaerobic digestion process, these organic fertilizers may be a source of different pollutants, such as trace metals. When applied to soils, digestates may influence the natural metal content and enhance the release of metals to the environment since they can be rich in different trace metals and are usually rich in dissolved organic matter. This study focused on investigating metal presence, seasonal variability of their soluble forms and crop uptake in a two-year field experiment, using two different biogas digestates as fertilizers. The use of digestates as fertilizers was compared to cattle manure, mineral fertilizer and a control without fertilizer addition, with respect to the presence and distribution of the trace metals Cd, Cu, Zn, Ni, Cr, Mn and Mo, as well as Al in the soil and plant. The results of the study showed that both biogas digestates have caused a 10 to 20 % increase in the total soil concentration of Ni, Cd and Cr compared to control plots without fertilizer addition. Application of biogas digestates had only a minor effect on metal uptake in plants. Overall, the selected application rate of 100 kg/ha of plant available nitrogen has had little effect on plant metal uptake and crop quality and the use of biogas digestates was comparable to the use of animal manure.
Highlights
Over the last few decades, the use of different organic residues as fertilizers has increased
The Principal component analysis (PCA) has shown that the water-extractable concentrations of trace metals from plots treated digestate are similar to the ones measured for animal manure
We have reported for the first time results from field experiment conducted through two growing seasons to investigate the application of two different commercial biogas digestates based on food waste and food waste/sewage sludge mixture
Summary
Over the last few decades, the use of different organic residues as fertilizers has increased. Anaerobic digestion for biogas production results in large amounts of liquid digestate, which contains high amounts of nutrients, such as nitrogen, potassium and phosphorus, and micronutrients in plant-available forms. The most common substrates for biogas production are animal manures (Fantozzi and Buratti, 2009), food waste collected from the municipal households and sewage sludge (Maragkaki et al, 2018) or co-digestion of several substrates. Most of the studies have focused on the use of manure or sewage-sludge based digestates with an emphasis on their nutrient and fertilizer value (Ni et al, 2017; Yun et al, 2018). There are few studies with digestates based on source-separated food waste focused on the optimization of digestate quality as fertilizer through co-digestion
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