The application of Kelp (Laminaria digitata) in marginal agroecosystems

Abstract

Lazybeds were a historic agricultural system that used a ridge and furrow approach to improve soils for crop production in marginal land. These Lazybed systems were further characterised by the use of dung/manure and marine macroalgae (in coastal areas) as fertilisers. This thesis aimed to investigate whether Lazybeds could be sustainably re-cultivated and how the use of kelp (Laminaria digitata) and dung/manure fertilisers in the system affect food production. It was important to determine how kelp performed in comparison to more conventionally used organic fertilisers such as animal dung/manures. Further, knowledge gaps existed in understanding how earthworm populations, key to processing nutrients and incorporation of organic matter, would interact with kelp as a food source. To better understand fertiliser and management regimes, climatic variables and earthworm communities, their effects on changes in physico-chemical and biological soil properties and crop yields need to be disentangled. This research primarily investigated how the use of kelp fertilisers on marginal soils could support plant growth, using a combination of laboratory, glasshouse, and field studies. Results show that the effects of organic fertiliser type on crop yields was significant, with fresh kelp increasing yields for some crops when compared to animal manures and commercially available seaweed extract liquid fertiliser. However, changes in soil properties, conventionally associated with improved crop performance, were often found to be similar for both kelp and animal manure-fertilised treatments. Further experimentation found that fresh kelp consistently outperformed kelp at various decompositional stages. This suggests that the mechanism leading to increased crop production in fresh kelp fertilised treatments is rapidly degraded during the decomposition process. Further, kelp fertilisers were found to modify crop growth response to reduced watering, enhancing crop production and increasing resistance to drought more than manure fertilisers. In addition, experiments presented in this thesis found that earthworms decreased the efficacy of kelp fertilisers. This suggests that the mechanisms driving increases in plant growth are partly degraded when kelp is consumed and egested by earthworms. Additional research is required in field settings to evaluate how earthworms selectively feed, and if kelp is avoided, then the crop may benefit from other earthworm activity. Furthermore, kelp and manure-based fertilisers were comparable in their ability to increase yields from continuous to alternate cropping (rotations with rest periods) systems, although yields were greater in the kelp-fertilised, alternate cropping treatment. Determination of Lazybed effects on soil properties and elucidating the mechanisms driving increases in plant growth, could be used to determine land management strategies to enhance sustainable food production, bringing areas of marginal land back into production. However, kelp is a limited natural resource which is already threatened by multiple climatic and anthropogenic pressures and its presence is geographically and temporally variable. As such, there is very limited scope for these agricultural systems to be adopted at scale. However, aspects of the Lazybed system may be most valuable in small-scale coastal systems where economic constraints and environmental concerns make the use of inorganic fertilisers less feasible.