Use of anaerobic digestate as biofertilizer: Another step forward in the valorisation of the invasive brown macroalgae Rugulopteryx okamurae

The rapid and aggressive colonization of the invasive macroalgae Rugulopteryx okamurae on the southern coast of Spain has caused an unprecedented environmental, health and economic impact. At present, the scientific community has focused on the valorisation of R. okamurae waste, as tonnes of collected biomass from this species end up in landfills. In this sense, the aim of this study was to assess, for the first time, the viability of the R. okamurae digestate as biofertilizer to growth the Mediterranean forage species Avena strigosa (black oat). Two anaerobic digestates, derived from the anaerobic digestion of the thermally pre-treated (120 degrees C; 45 min) and the zeolite-assisted mechanically pre-treated invasive macroalgae, were used in this study. Both digestates were compared with a commercial inorganic nutritive solution. The effect of different fertilizers and water availability on forage height, aerial, root and seed biomass, as well as photosynthetic activity and macro-and micronutrient forage content were studied. The control treatment showed the lowest height at the end of the experiment (0.38 m) and the digestate from the thermally pre-treated macroalgae treatment had the highest one (0.59 m). Final fresh and dry aerial biomass was separately significantly dependent on both fertilizer type and water stress. Final root and seed biomass were not significantly different among the treatments studied. The lowest photosynthetic activity was obtained from the control treatment (3.61 mu mol m-2 s-1), while the highest was reached by the digestate where R. okamurae was thermally pre-treated (4.96 mu mol m-2 s-1). The results of this study show that both digestates tested can be used effectively as organic fertilizer in the cultivation of black oats, obtaining production efficiency values similar (digestate from mechanically pre-treated macroalgae) or higher (digestate from thermally pre-treated macroalgae) than inorganic fertilizer.