Researchers Consider How to Preserve Nitrogen During Organic Waste Recycling

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Apr 12 2022

In a study recently published in the journal Chemosphere, researchers presented a review of the nitrogen cycle and nitrogen loss mitigation measures in organic waste composting.

Study: The nitrogen cycle and mitigation strategies for nitrogen loss during organic waste composting: A review. Image Credit: vchal/Shutterstock.com

Background

Organic waste contamination has become a severe concern in recent years, with negative consequences for the environment and human health. In today's world, the amount of generated organic waste is steadily increasing.

Composting is a potential method for digesting organic waste that is both environmentally friendly and cost-effective. Because mature compost is stable, nutrient-dense, and nontoxic, it can be utilized as a soil amendment and an organic fertilizer. The macronutrient nitrogen is required for biochemical activities and cell growth in the composting process.

The 16S rRNA sequencing method offers detailed information on the dynamics of the microbial community during the composting process. During the composting of organic waste, many nitrifiers and denitrifiers such as Thiobacillus and Pseudomonas have been discovered.

Furthermore, composting period and operating factors have a considerable impact on microbial richness and variety. Several papers on the nitrogen cycle throughout the composting process have recently been examined and discussed. However, there have been no comprehensive studies on the nitrogen cycle of organic waste composting, particularly on nitrogen loss prevention measures.

About the Study

In this study, the authors presented a review to summarize the composting nitrogen cycle. The corresponding operating parameters, enzyme and gene functions, and microbial activity affecting the nitrogen cycle were investigated. The nitrogen loss mitigation options were addressed. The team outlined key aspects of the nitrogen cycle and nitrogen loss in order to aid future composting research in terms of odor management and compost quality.

The researchers provided a comprehensive nitrogen cycle of organic waste composting, summarized the factors affecting the nitrogen cycle of organic waste composting, such as relevant microbial communities, specific enzymes, and gene function, and identified nitrogen loss mitigation strategies with the goal of increasing nitrogen accumulation during the composting process.

In addition, the knowledge gap and future research opportunities were highlighted. Web of Science, Google Scholar, and PubMed were used to compile the bibliographic databases used in this study.

Observations

Oxygen content, moisture, temperature, C/N ratio, and pH were operational characteristics that play an important role in the nitrogen cycle, and altering them was the most straightforward way to limit nitrogen loss. Furthermore, the nitrogen cycle's most important processes, nitrification, and denitrification had a significant impact on microbial community dynamics.

The ammonia-oxidizing bacteria or archaea (AOB/AOA) and nitrite-oxidizing bacteria (NOB), as well as heterotrophic and autotrophic denitrifiers, played a critical part in nitrification and denitrification, with the ammonia monooxygenase (amoA) gene, nitrate reductase genes (narG), and nitrous oxide reductase genes (nosZ).

Additives like struvite salts (MgNH₄PO₄.6H₂O), biochar, and zeolites (clinoptilolite), as well as microbial inoculation with Bacillus cereus (ammonium strain), Pseudomonas donghuensis (nitrite strain), and Bacillus licheniformis (nitrogen fixer), could also assist in limiting nitrogen loss.

The moisture content of the composting process was frequently kept between 55 and 65%, which promoted microbial activity. To aid organic decomposition, the C/N ratio of the original mixture was changed to 20–35. The nitrogen content of compost ranged from 1.93 to 35.6 g/kg. At pH 4.5, archaeal amoA contributed more to the ammonia oxidation process than bacterial amoA.

The ideal oxygen concentration for bacterial growth was between 1% and 10%. Under aerobic conditions, N₂O accounted for more than 70% of total nitrogen loss, but under anaerobic conditions, NH₃ accounted for 47–54% of total nitrogen loss.

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Mineralization, nitrification, anammox, denitrification, and ammonia oxidation were the five key phases in the nitrogen cycle. The nitrogen cycle was influenced by both internal (microbial community, raw compost materials, and porosity) and exterior (moisture, oxygen concentration, temperature, and pH) elements.

In addition, differences in microbial communities (e.g., bacteria, fungus) had a substantial impact on the nitrogen cycle via specific genes and enzymes, with Proteobacteria and Thaumarchaeota as the dominating ones.

Conclusions

In conclusion, this study elucidated that the nitrogen loss can occur throughout the composting process due to the emission of NH₃, N₂O, and N₂, as well as the seeping of NO₃^- in the leachate, with NH₃ and NO₃^- accounting for 30-65% of total nitrogen loss. It was observed that various mitigation measures, such as struvite precipitation and adding additives like zeolites, biochar, and microbial incubation, were used with 20-88% reduced efficiency since nitrogen loss reduces compost quality and creates air pollution.

The authors believe that this analysis offered an overview of the nitrogen cycle in the composting process, which will aid future research into nitrogen loss mitigation and compost quality improvement for field and industrial applications.

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