Examining the potential of duckweed and Azolla species towards attaining bio-circularity in agricultural systems – a systematic literature review

Abstract

To meet the population-driven food demand, intensified livestock production has led to excessive animal manure accumulation exacerbating eutrophication in river systems. As waste management evolve towards resource ‘recovery, recycling, and reuse’, costly and techno-advanced nutrient management technologies like ammonia stripping are often impractical in developing countries. Phytoremediation, particularly by rapidly growing aquatic macrophytes with simple growth requirements, present a cheap and environmentally friendly path towards bio-circularity and sustainability in resource-limited agricultural setups. This review uniquely collates recent research and sheds more light on the role played by duckweed and Azolla spp. in sustainable bio-circular agricultural systems. We particularly zoom-in on the triple utility role of phytoremediation, biomass valorization and potential climate change resilience by duckweed and Azolla species. Following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) framework, 32 peer-reviewed articles (2014–2023) were selected and analysed from a pool of 310 articles. Under specific conditions, duckweed and Azolla species can achieve high relative growth rates (RGRs) of up to 2.3 g/g/day, and potentially yield up to 105 tonnes/hectare/year of dry biomass. Additionally, these plants can accumulate 58% crude protein and 47.2% carbohydrates through efficiently (~100%) remediating specific nitrogen (N) forms and phosphorus (P). Interestingly, Azolla pinnata can sequester CO2 at rates between 150% and 164% of its dry weight in cow manure and urine. By combining bioremediation, biomass valorization and climate mitigation, this review provides practical insights, and a distillation of research gaps, towards sustainable animal manure management, especially in resource-limited agricultural setups such as those of Zimbabwe.