A Rhizobox-Study Elucidating Biogas-Digestate Fertilization and Soil Compaction Effects on Juvenile Maize Growth and Rhizosphere pH

Biogas digestate (BD) contains nitrogen (N), phosphorus (P), and potassium (K) and is easily and largely available in Germany and other countries in Europe. Nevertheless, few studies compare BD to mineral NPK fertilizer, particularly under soil compaction. The characteristics of these fertilizers and soil compaction may affect rhizosphere pH and root development, thereby affecting nutrient uptake by plants. This 18-day rhizobox study evaluated initial maize growth and root architecture responses to BD (derived from maize silage+chicken manure), mineral NPK, and BD + NPK fertilization under compacted (0–25 cm compacted; 25–55 cm loose) and loose (0–55 cm) substrates. Treatments promoted similar shoot biomass, specific leaf area, and sufficient shoot N, P, and K nutrition. Shoot P content in BD + NPK and NPK was 29–33% higher compared to BD. Root P content in NPK was 26% higher than in BD, independent of compaction, likely favoring root proliferation and explaining the higher root:shoot ratio in NPK. In loose soil, the main root length in NPK was 49% higher compared to BD, but rooting was deeper in BD. Rhizosphere alkalinization measured non-invasively suggested preferential maize N absorption as nitrate. Combining BD with fast-soluble P sources may provide maize performance comparable to mineral NPK.