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

## TL;DR

This study shows that seaweed extracts can boost plant growth and may replace chemical fertilizers, with best results when harvested between May and August.

## Contribution

The study identifies optimal seaweed harvest times and extract concentrations for enhancing plant growth without affecting soil microbiota.

## Key findings

- Moderately diluted seaweed extracts (1:50) improved Arabidopsis growth more than concentrated doses.
- Seaweed extracts harvested in June and August showed the strongest plant growth stimulation.
- Macroalgal extracts had no significant effect on soil respiration or CO2 fluxes.

## Abstract

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 Ascophyllum nodosum, Fucus serratus and Fucus vesiculosus affected plant growth in Arabidopsis thaliana, Beta vulgaris and Lactuca sativa 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 A. nodosum. 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 Arabidopsis; no trend was detected in B. vulgaris or L. sativa 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 A. nodosum, F. serratus and F. vesiculosus 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.