Selecting raw materials for sustainable growing media: a protocol to assess the risk for N immobilization

Several materials to be used in growing media were tested for immobilization of mineral N by adding 350 mg N L^-1 substrate as KNO₃ followed by one week of incubation at 37°C. Based on the difference between the theoretical and actual mineral N content after incubation, the % N immobilization was calculated. The robustness and reproducibility of the protocol was tested by changing incubation circumstances, the applied N sources and the amount of N added. The relevance of the indicator was tested (a) for a range of green waste composts, (b) during the composting of four mixtures, and (c) in pot trials with growing media including different composts and lignocellulosic materials. Higher microbial activity in green waste composts (indicated by higher oxygen consumption) and thus a lower compost stability resulted in higher N immobilization. N immobilization decreased due to stabilization during the composting process, with a distinct effect of the specific feedstock composition. The protocol was tested for woody materials and for Miscanthus straw, spent coffee grounds and chopped reed straw. The applied source of mineral N in the test affected the pH of the material, and may thus also affect the microbial activity and thus the related N immobilisation. Growing media including materials with high N immobilization resulted in lower plant growth. We conclude that quality of composts and lignocellulosic materials for use in growing media could be successfully assessed based on this protocol.