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Biogas plants in Germany are producing an increasing amount of biogas residues to be recycled via agricultural crop production. To test whether the wide range of various substrates used in the anaerobic digestion can affect the chemical composition and nutrient availability, seven biogas residues derived from different substrates were investigated with respect to their N supply to ryegrass. Both the short-term and the long-term N availability were studied in a 309-d pot experiment lasting for five successive growth cycles each starting with a fertilizer application. The organic fertilizers were applied based on an equal amount of ammonium-N (300 mg N per pot) and compared to mineral N from ammonium nitrate of equal dosage. Biogas residues varied greatly in their chemical composition (ammonium-N 0.20% to 0.51%, N total 0.36% to 0.75%, and C org 1.85% to 4.75% in fresh matter). After the first growth cycle, the N availability of the biogas residues applied based on ammonium-N was at least equal to that from ammonium nitrate. Differences in N offtake after one fertilizer application were negatively correlated to the C org : N org ratio of the organic fertilizers. After five successive fertilizer applications, the N utilization of most of the organic fertilizers was increased compared to that of the mineral fertilizer. It is concluded that biogas residues provide plant-available N at least corresponding to their ammonium content and that the accumulation of organic N in soil through repeated application of biogas residues contributes to N release.

Although grassland has a significant biomass potential, many different factors can affect the quality of grassland feedstock. Changes in the chemical composition of grassland biomass can lead to a high variation in biogas potential, even within the same plant species. Therefore, four grass species and two legume species were grown in field plots and harvested at sequential stages of maturity in the first three growths. The samples were investigated in order to mathematically describe the relation between their chemical composition and biogas yield. A global biogas yield prediction model for energy crops allowed the differentiation between the samples for biogas yield. However, due to their distinct difference in plant chemical composition, the estimation accuracy was rather low. A model considering lignin, hemicellulose, and crude protein as regressors was most suitable for predicting the biogas yield of grassland plant species, with an accuracy of 31 L/kg volatile solids.

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