Fen plant biomass as growing media constituent - reduction of nitrogen immobilization by composting

Peat is the most important constituent for growing media due to its beneficial chemical, physical and biological properties. However, the use of peat is being criticized for environmental reasons. As substitutes of high quality are not sufficiently available, the potential of fen plants, which can be sustainably cultivated on rewetted peatlands, was evaluated. When using chopped plant material without further processing, nitrogen immobilization was identified as the main problem. The aims of this study were (1) to evaluate if composting is a suitable treatment to stabilize nitrogen dynamics of fen plant materials and (2) to test if fen plant materials are free from phytotoxic substances before and after composting. Typha latifolia, Phragmites australis and Phalaris arundinacea were harvested in the period January to March, air-dried and chopped. After the addition of nitrogen (Phragmites: 1000 mg N L^‑1; Typha: 500 mg N L^‑1; Phalaris: 250 mg N L^‑1) and other nutrients, the biomass was composted in small-scale-composters (1.8 m³) for 130 days. The material was turned and moistened on demand and again N-fertilized if necessary. Before, during and after composting, incubation experiments were conducted to quantify the nitrogen immobilization of the plant material. Furthermore, seedling tests with Chinese cabbage were performed at different nitrogen levels to check the absence of phytotoxic substances. Nitrogen immobilization could be reduced from 230 to 59 mg N L^‑1 (Typha), from 1106 to 835 mg N L^‑1 (Phragmites) and from 390 to 155 mg N L^‑1 (Phalaris) within three weeks of composting. At the end of the composting process, also for Phragmites a good stabilization of the nitrogen dynamics was achieved. Irrespective of composting time, Chinese cabbage seedlings in peat reduced growing media containing fen plant material in quantities of 50% by volume showed similar fresh weights than in the peat control if N fertilization was adequate. Hence, no phytotoxic substances were present.