Humic-like bioactivity on emergence and early growth of maize (Zea mays L.) of water-soluble lignins isolated from biomass for energy

Lignin of lignocellulosic residues from biomass for energy can be exploited in sustainable agriculture as plant stimulants. Lignin monomers or their microbial bioproducts are mainly responsible for the plant growth promotion exerted by humic matter in soil. The aim of this work was to verify the humic-like bioactivity of water-soluble lignin isolated from biomass for energy towards plant growth and relate the biostimulation to the lignin molecular structure. Two water-soluble lignins isolated from giant reed (AD) and miscanthus (MG) were characterized for molecular composition by H-1 and P-31 1D-, C-13-H-1 2D-, DOSY-NMR spectroscopy and for conformational structure by size-exclusion chromatography. The effect of different aqueous concentrations of lignin on germination of maize seeds and growth of maize plantlets was assessed in growth-chamber experiments. Both lignins showed humic-like supramolecular structures, but different conformational stability and molecular composition. Their largest bioactivity was revealed at 10 and 50 ppm of lignin organic carbon and both significantly increased length of radicles, lateral seminal roots, and coleoptiles of maize seedlings, as well as total shoot and root dry weights and root length of maize plantlets. However, differences in AD and MG bioactivity were attributed to their conformational stabilities and content of amphiphilic molecules, which may control both the adhesion to plant roots and the release of bioactive molecules upon interactions with plant-exuded organic acids. The humic-like bioactivity of water-soluble lignins indicated that lignocellulosic residues from energy crops may be profitably recycled in agriculture as effective plant growth promoters, thereby increasing the economic and environmental sustainability of energy production from non-food biomasses.