Challenges for the utilization of ammoxidized lignins and wood fibres as a peat substitute in horticultural substrates

This review addresses the research done regarding ammoxidation of lignins and other lignocellulosic materials, as a way to produce nitrogen-rich products. The reported methods of ammoxidation of technical lignins are summarized giving emphasis to the diverse nitrogen structures which may yield and their effect on nitrogen release. The process using ammonia (NH 3 ) under oxygen (O 2 ) atmosphere is the best way to produce nitrogen-rich products. Nitrogen is first incorporated as ammonia in carbonyl and carboxyl groups which then lead to aromatic ring cleavages and formation of amides. The reactions then proceed to form heterocyclic compounds with bound nitrogen. The studies show that most of the nitrogen is incorporated as strongly-bound nitrogen which results in products with long-term release properties. This can cause problems of nitrogen availability in the first growth periods of cultivation tests and lead to higher loads of fertilizers needed to have good performances, which constitutes a disadvantage for its use in horticultural substrates that require mid-term nitrogen availability. This article compiles the main challenges arising from studies carried out in recent years, particularly for the development of nitrogen-rich lignins and wood fibres with nitrogen structures that constitute an effective solution to the problem of nitrogen immobilization. This could enable the development of substrate improvers that are an effective substitute for peat in horticultural substrates, allowing cleaner production by using renewable materials and by-products such as lignin while reducing the use of a non-renewable, greenhouse-gases-emitting material. • Lignin ammoxidation may be crucial for peat substitution with wood fibers • Few studies link lignin's nitrogen groups to nutrient availability in plant tests. • Current methods to evaluate nitrogen structures in lignin are time-consuming • Amide-type nitrogen provides mid-term availability to counter nitrogen immobilization • Further studies are needed to optimize ammoxidation for mid-term available nitrogen