Influence of Biogas Digestates on the Composition of Soil Organic Matter

Abstract

The increasing number of biogas plants with various digested organic materials and output of digestates calls for more detailed investigations of their suitability as soil amendments. In a trial in Northern Germany two replicate plots each were treated with fresh or prolonged fermented digestate to investigate the influence of digestates on the composition of soil organic matter (SOM). The relative proportions of relevant SOM compound classes were determined by pyrolysis-field ionization mass spectrometry (Py-FIMS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Using these two methods the influence of digestate on the SOM composition was illustrated by the statistical spectra evaluation by principal component analysis. In both digestate-treated plots, the relative proportions of the slowly degradable alkylaromatic compounds and lipids were larger. In contrast, easily degradable carbohydrates and amides and mostly heterocyclic N-compounds were less than in the control plots. However, these SOM differences due to digestates were less pronounced than differences in the composition of autochthonous SOM in the control plots. In one of the treatments N- and alkylaromatic compounds were more abundant in the aggregate core than on the aggregate surface. In contrast, aliphatic compounds were more abundant on the aggregate surface as disclosed by Py-FI and pyrolysis-gas chromatography-mass spectra (Py-GC/MS). In conclusion, increasing amounts of digestate should be critically monitored according to different digestate feedstock, soil types, ecological factors and crop growth/yield.