Plant seedling growth and soil respiration responses to seasonal United Kingdom seaweed wrack extracts

Macroalgal extracts offer an alternative option to increase crop yields and plant performance whilst reducing reliance on inorganic chemical fertilisers. Macroalgae have high concentrations of bioactive compounds capable of enhancing crop growth, stress tolerance and pest resistance. This study investigated whether seasonal variation in the chemical composition of three brown seaweeds <i>Ascophyllum nodosum</i>, <i>Fucus serratus</i> and <i>Fucus vesiculosus</i> affected plant growth in <i>Arabidopsis thaliana</i>, <i>Beta vulgaris</i> and <i>Lactuca sativa</i> through different extract concentrations. Crops were treated with 1:20, 1:50 and 1:100 dilutions from macroalgal extracts and compared to controls tap water, deionised water and one commercial macroalgae-based fertiliser made from <i>A. nodosum</i>. Dry weight assessment results revealed that moderately diluted dosages had better effects on plant growth than concentrated dosages, showing that the most suitable concentrations of all macroalgae extracts are 1:50 for <i>Arabidopsis</i>; no trend was detected in <i>B. vulgaris</i> or <i>L. sativa</i> growth. Overall, there were peaks of increased plant growth when treated with extracts harvested in June and August, which correlates with fertility peaks in commercial brown macroalgae in the wild. These results suggest that the optimal harvest for <i>A. nodosum</i>, <i>F. serratus</i> and <i>F. vesiculosus</i> for plant stimulant products should occur between May and August at sustainable harvest levels. Soil respirometry trials using the extracts showed no differences in CO₂ fluxes between the macroalgal species, different harvesting seasons, or correlation with plant biomass. It is therefore likely that macroalgal extracts impact the plant directly and produce minor impact on soil microbiota. Thus, these results support the use of macroalgal fertilizers as a low-cost and environmentally friendly alternative to chemical fertilisers.