Development of a Controlled Release Fertiliser by Incorporating Lauric Acid into Microalgal Biomass : Dynamics on Soil Biological Processes for Efficient Utilisation of Waste Resources

Utilisation of microalgae to process large volumes of digestate from the anaerobic digestion of food waste is emerging as a sustainable waste treatment option. A by-product of this treatment is the algal biomass which has potential to be used as an organic fertilizer in a circular economy. However, the microalgae are rapidly mineralized to nitrate when applied to soil which could result in nitrogen loss via leaching, volatilization and runoff. One solution is to combine the microalgae with lauric acid (LA) to slow the release of mineral N. This study aimed to investigate whether combining LA with microalgae to create a new fertilizer product would slow the release of mineral N when applied to soil and whether this product impact on the microbial community structure and activity. The treatments were applied to soil amended with LA combined with either microalgae or urea at rates of 0%, 12.5%, 25% and 50% LA, untreated microalgae or urea and unamended control incubated at 25oC and 40% water holding capacity for 28 days. The impact of treatments on soil chemistry (ammonium, nitrate, pH and EC), soil microbial biomass carbon, CO2 production and bacterial diversity were characterised at 0, 1, 3, 7, 14 and 28 days. The NH4+-N and NO3--N concentration decreased with increasing rate of LA combined microalgae indicating that both N mineralization and nitrification were impacted. A decrease in the predicted nitrification genes amoA/amoB and relative abundance of ammonia oxidizing bacteria (Nitrosomonadaceae) and nitrifying bacteria (Nitrospiraceae) with an increasing rate of LA with microalgae provides further support for possible inhibition of nitrification. Another possible mechanism for the decreased release of N in the LA combined microalgae treatments could be through increased N immobilization. The MBC and CO2 production was higher in the soil amended with LA combined microalgae and there was an increase in the relative abundance of fast-growing heterotrophs. Treating microalgae with LA has the potential to control the release of N by increasing immobilization over nitrification and therefore it might be possible to engineer microalgae to match plant nutrient growth requirements