Novel fertilising products from lignin and its derivatives to enhance plant development and increase the sustainability of crop production

Lignin is a by-product of biorefineries and paper mills and is usually discarded or burnt. However, it may represent a source of novel fertilising products, able to support the sustainable intensification of agricultural productions. The aim of this review is to explore the literature regarding the effect of lignin application towards plant growth and nutrient use efficiency. First, we reviewed the biostimulant role of lignin, which was reported to positively perturbing plant hormonal balances or improving the efficiency of photosynthesis and respiration. Also, when added to soils, lignin was shown to enhance nitrogen uptake, as well as the development of beneficial soil microorganisms. Then, we summarised the research related to the chemical modifications of lignin structure devised to boost its bioactivity, an approach opening possibilities to tailor the effects of lignin addition on plant development. We further examined the literature about the use of lignin and its derivatives as starting substrates to produce sustainable materials (chelates, coatings, micro- and nano-materials) for the slow release of plant nutrients. Encouraging results emerged from the summarised articles, suggesting that lignin may replace the currently used synthetic polymers exploited to chelate or entrapping nutrients. We additionally hereby highlighted the role of lignin chemical nature in affecting its biological and release properties, hence pointing out the relevance of thoroughly studying its structure at a molecular level by the most advanced analytical tools. Finally, we suggested the need for researchers to combine their skills and expertise, in order to develop more efficient lignin-inspired fertilisers.