Potential of novel organic NP(K)-char fertilizers as part of a circular economy-based phosphorus supply chain – results from a pot experiment

Nutrient density in manure and fermentation residues is too low to economically justify transporting them to regions with phosphorus (P) and nitrogen (N) demands. However, they present a valuable feedstock for the production of commercial P fertilizers via pyrolysis. Phosphorus availability in the resulting chars for plant uptake can be further improved by adding potassium (K) salts to the feedstock prior to pyrolysis, while vinasse can be added for N enrichment. Such recycling-derived N-P(-K doped) fertilizers are expected to release nutrients slowly to the plants and to have positive environmental effects like nitrate retention, reduction of nitrous oxide (N₂O) emissions and carbon (C) sequestration. We conducted a greenhouse pot experiment with celery and spinach cultures to investigate the P fertilization performance and environmental effects of newly developed recycling-derived N-P(-K) char fertilizers. Four novel pyrolysis products were tested: three different N-enriched (vinasse) dry flowable products (pyrolyzed material: dried pelletized chicken manure, digestate, and the dried solid fraction of pig slurry, respectively), and a char from dried pig manure, which had been enriched with potassium acetate prior to pyrolysis. The pot experiment demonstrated that P-char fertilizers, combined with a liquid organic N fertilizer commonly used in organic farming (vinasse), can provide the same P-fertilization as triple super phosphate (TSP), with yields similar to or higher than those obtained with mineral N fertilizers. Furthermore, the char fertilizers, and in particular the K-doped char, reduced nitrate leaching significantly compared to treatments with TSP + calcium ammonium nitrate (CAN) or TSP + vinasse. Liquid vinasse application drastically enhanced N₂O emissions compared to pots where less or no liquid vinasse was applied without char. P-char fertilizers may thus provide a promising building block for a circular economy-based P supply chain while simultaneously reducing environmental costs in the form of less nitrate leaching and lower N₂O emissions and contributing to soil C sequestration in the plough layer.

The project was funded by the Federal Ministry of Food and Agriculture (BMEL) on the basis of a resolution of the German Bundestag. The project was carried out by the Federal Agency for Agriculture and Food (BLE) within the framework of the Innovation Promotion Program. Title of the research project: "Carbon-based fertilizers from phosphorus-rich slurry, manure and fermentation residues by carbonization with nitrogen recovery", grant numbers 2818105215, 2818105015 and 2818107115.