In the summer of 2022/23 the WasteMINZ Organic Materials Sector Group initiated a research project to determine whether New Zealand can lead in increasing soil carbon stocks by reusing organic waste material. This important research project was funded by Marlborough District Council and was undertaken by Felix Finlayson-Hood, a University of Canterbury student. This summary of Felix Finlayson-Hood’s research paper has been provided by Dr Lisa Hack, Principal Advisor at InSight | Waste & Resources Advisory and Chair of the Organic Materials Sector Group Steering Committee.
2. Key Findings and Points
2.1 Global Interest in Soil Carbon
The paper highlights the growing global interest in raising soil carbon stocks for agricultural and climate-related benefits and suggests that New Zealand has the potential to be a leader in this field by using organic waste as soil amendments.
2.2 Soil Carbon Stocks in New Zealand
The paper discusses New Zealand's soil carbon sequestration potential, the role of different soil orders, and the factors that influence soil carbon storage. The paper explains the two broad categories of soil organic carbon (SOC) components, (MAOC) and particulate organic carbon (POC), and their roles in soil carbon sequestration. POC and MAOC from organic amendments are discussed, including the roles of organic matter leaching, microbial processing, and the C:N ratio of amendments.
The study emphasizes the importance of mineral-associated organic carbon (MAOM), a persistent fraction of organic matter in soil, and its correlation with soil specific surface area (As). Soil's ability to stabilize carbon in the form of MAOM is crucial for carbon sequestration potential.
The paper highlights the factors influencing the persistence of SOC, emphasizing the role of microbial activity, soil aggregation, and mineralogy.
2.3 Benefits of Organic Amendments
The paper calculates the potential benefits of using New Zealand's organic waste materials as soil conditioners, which can increase soil carbon stocks. The study suggests that there may be practical upper limits to SOC increases through organic amendments, and the need for continual application to maintain carbon stocks is discussed. It also notes that the appropriateness of using organic amendments may vary across different sites and types of amendments, highlighting the need for site-specific assessments.
2.4 Trade-off Effects
Organic amendments may result in trade-off effects, such as increased nitrous oxide (N2O) emissions from soil. The complex interactions between organic amendments and N2O emissions are also discussed. The paper explains the potential influence of organic amendments on N2O emissions and how it may offset the positive effects of carbon sequestration due to the high global warming potential of N2O.
The risks associated with biological, chemical, and physical contamination from organic amendments are explored, including pathogens, trace elements, xenobiotics, and microplastics.
The paper concludes that continuous application of organic amendments is a feasible option for increasing carbon stocks within the soil mineralogy limits but may lead to reduced returns over time. It also highlights the importance of addressing N2O emissions and contamination risks.
Overall, this paper provides valuable insights into the potential benefits and challenges of using organic amendments to increase soil carbon stocks in New Zealand, with an emphasis on site-specific considerations and potential trade-offs. However, it should be noted that the research presented in the paper has not undergone peer review in scientific journals.