New approaches to assess stability of wood fiber

In the wake of the decarbonisation of the entire economy, the use of peat in horticulture is under increasing pressure. In Germany, wood fibre is by far the most important substitute in growing media for professional horticulture. However, due to readily microbial degradability and a wide C:N ratio, nitrogen immobilization is a major problem of wood fibre products and considerably limits their use. Concurrently, stability of wood fibre is assessed by incubation experiments in which the change of mineral nitrogen is measured. However, these experiments have three shortcomings: First, they are quite labour and time-consuming as incubation period is up to 21 days. Second, if long-term fertilizers – especially urea-aldehyde condensation products – are applied by the manufacturer, the experiments only provide information about the stability of the N balance, but not about the stability of the material itself. Third, comparability and repeatability of the incubation experiments are rather poor. To overcome these shortcomings new approaches to evaluate stability of wood fibre were tested.

The setup of the first approach is similar to the currently used incubation experiments. However, the incubation period is only five days and additionally to change in mineral nitrogen from the beginning to the end of the experiment, carbon mineralization is analysed continuously by Oxitop®-C measuring heads. The second approach focusses on the characterization of readily decomposable nitrogen and carbon fractions in the wood fibre. On the one hand, hydrolysable nitrogen and carbon is measured and on the other hand, a stepwise thermal fractionation of carbon under pyrolytic conditions is done.

The results of the two approaches are compared to nitrogen immobilization measured in common incubation experiments and further validated by nitrogen balances deduced form short-term pot experiments with Chinese cabbage.