Assessing the suitability of microalgae biomass produced from piggery waste as a fertiliser

The use of microalgae for the recapture of nutrients, water and bioenergy from the treatment of piggery effluent is a promising technology (Astals et al. 2015; Nwoba et al. 2016). The harvested microalgae biomass has the potential to be developed into a high quality, concentrated and balanced fertiliser due to its high nitrogen (N) and phosphorous (P) content that is easy to handle and transport (Mulbry et al. 2005). However, there is still uncertainty regarding the re-use of microalgae in terms of its impact on plant N uptake, dry matter production, N mineralisation, and soil microbial activity. The aims of this study were 2-fold: (1) to investigate the effectiveness of microalgae biomass to supply N to wheat in comparison with a synthetic N fertiliser, ammonium nitrate (NH₄NO₃), and (2) to assess the release of inorganic N (potential N mineralisation rate) following the application of microalgae biomass to soil. Microalgae biomass grown on undiluted, untreated piggery anaerobic digestion effluent was obtained from the Algae R&D Centre, Murdoch University. First, a pot experiment having a 2 × 5 factorial arrangement of treatments was undertaken to test the feasibility of using microalgae as a fertiliser with factors being two N sources (microalgae biomass and NH₄NO₃) applied at five N equivalent levels (0, 10, 20, 40, 80 kg N/ha) for 8 weeks. The trial was planted with wheat (Triticum aestivum L.) and arranged in randomised block design with four replicates per treatment. Second, a laboratory incubation experiment was conducted to quantify the potential N mineralisation of the microalgae using the same experimental design for the pot trial but without the plant. Total dry biomass, root and shoot production were determined using standard methods. Soil mineral N was analysed colourimetrically using a modified hydrazine reduction method. The carbon dioxide (CO₂) production (microbial activity) was measured using an infrared gas analyser.

Utilisation of the microalgae biomass as an alternative fertiliser significantly improved the overall wheat yield and shoot production (Fig. 1), particularly at higher application rates. However, shoot production was slightly lower in the plants receiving the microalgae when compared to the mineral fertiliser NH₄NO₃. The soil N mineralisation rate was positively correlated with the amount of microalgae biomass applied implying that the microalgae provides an available nitrogen source for plants but not to the same extent as NH₄NO₃. This is because a large proportion of the N in microalgae is in an organic form and needs to be mineralised by soil microorganisms before it is plant available. Consequently, an increase in microbial activities (CO₂ production) was observed in the soil amended with the microalgae biomass. Further studies to determine the agronomic and economic value of microalgae as a fertiliser are needed.

Fig. 1.  Effect of mineral fertiliser NH₄NO₃ (○) and microalgae biomass (●) on shoot production (mean ± s.e.m., where n = 3) for wheat when applied at different levels (equivalent to 0 to 80 kg N/ha).