Research Questions/Objectives:
This project aims to investigate how cultivation and insecticide use affect soil macroinvertebrate abundance and diversity, and how these factors correlate with soil carbon in sugarcane farming systems. This will be addressed by the following questions:
Answering these questions aims to provide insights into factors limiting carbon sequestration in a major agricultural industry, which has relevance to possible climate mitigation strategies as well as more sustainable and productive farming practices that maximise biological activity and soil health.
Brief Description of the Project:
Leaving harvested sugarcane waste/trash on the surface as mulch also known as green cane trash blanketing (GCTB) has become common practice in Australian sugar cane systems for the last three decades (Salter, 2024). Despite this, there has not been the expected increase in soil carbon. This issue was recently highlighted in Sugar Research Australia’s and the Queensland Government’s (2022) “Long-term Trash Blanketing and Tillage Trial,” which emphasises the “need to understand the processes limiting improvement in soil carbon and refine trash management to capture greater benefits from this resource.”
My project investigates one potential limiting factor: the impact of farming practices on soil macroinvertebrates and their essential ecosystem services. Practices such as cultivation and insecticide use are well-documented stressors that reduce macroinvertebrate abundance and diversity by disrupting soil ecosystems and eliminating beneficial fauna (Sheibani & Ahangar, 2013; Pankhurst, 2006)). These stressors limit macroinvertebrates who provide critical ecosystem services such as fragmentation of trash and mixing of soil, which are vital for bringing organic matter into the soil. This organic matter can then be broken down by microbes and respired as CO2 or remain in the soil as a more stable form of carbon, promoting long-term carbon sequestration (Frouz, 2018; Nottingham et al., 2022). Increased soil carbon is vital for enhancing soil structure, nutrient retention, and water retention supporting both environmental sustainability and agricultural productivity (Magdoff & Van Es, 2009).
Background and Significance of the Research Question to drought risk, vulnerability, preparedness, or resilience:
Climate change poses a significant challenge to agriculture, with growing pressure to reduce emissions and adapt to climate extremes such as drought (Buck & Palumbo-Compton, 2022). The Australian Government’s net-zero emissions commitment by 2050 exacerbates this need for mitigation strategies, especially in major sectors like agriculture (DCCEEW, 2024). My project aligns with Australia’s climate commitments, by investigating how sugarcane farming practices impact soil macroinvertebrates and their role in carbon sequestration via the incorporation of sugar cane trash within soil. The research aims to provide insights into farming practices that maximise carbon sequestration as a potential climate mitigation strategy.
Additionally, the project is significant for drought resilience and overall soil health. Macroinvertebrates play a crucial role in soil health through their activities including the creation of pores for increased aeration and water infiltration as well as the incorporation of organic matter into the soil (Magdoff & Van Es, 2009; Brady & Weil, 2017). Both soil organic carbon (SOC) and soil organic matter (SOM) are essential for resilient soils due to their contributions to:
Macroinvertebrates and their essential contributions to SOC and SOM promote soils that are well-structured, nutrient-rich, moisture-retentive and stable, making them resilient to climate extremes like drought and improving agricultural productivity.
Academic and research experience relevant to the honours project:
I completed a Bachelor of Science, majoring in Zoology/Ecology, at James Cook University from 2021 to 2024, achieving a GPA of 6.38/7. During my degree, I undertook key subjects such as Tropical Entomology, Soil Properties and Processes for Management, and Field Studies in the Equatorial Tropics. Through my studies, I also assisted with entomology-focused PhD projects, providing me with invertebrate identification and sampling skills. In 2023, I received an Undergraduate Award for Excellence in Soil Science from Soil Science Australia’s QLD Branch, where I presented a project exploring the relationships between soil formation factors, soil properties, and soil fertility, which has provided me with a strong knowledge foundation for this research. Currently, I work casually for the Department of Agriculture, which has further developed my understanding of the interaction between soil health and agricultural productivity and provided me relevant skills such soil sampling. My passion for agricultural science and how it integrates with zoology/ecology, makes me a dedicated researcher for this project.
Principal Supervisor’s skills and experience in relation to this project topic:
Paul Nelson has extensive research experience in sustainable soil management in tropical agricultural systems, including sugarcane. He has supervised 4 PhDs, 3 MPhils and 13 Honours students to successful completion as primary advisor. He has published 5 books, 11 book chapters and 97 peer-reviewed scientific papers, with 8,200 citations. Of his papers, 51 were on soil carbon cycling and soil biology, including one on ants in rainforest soil. He was recently invited to write a book chapter on ‘Sustainable soil management in tropical agriculture’ (Nelson, 2023).
My name is Jorja Harvey, and I am studying a Science Honours, majoring in Zoology/Ecology at JCU in Cairns. I grew up in Newell, a beachside town north of Mossman, where I was fortunate to have an adventurous childhood filled with bike rides, creek swims, and bushwalks. Growing up immersed in nature, I developed a passion for biology, which led me to begin my university journey at UQ studying biomedicine. However, it wasn’t the right path for me, and after a 6-month deferral, I realised that the other side of biology was more fitting. I have always had a love for all animals, whether it was rescuing injured birds or running around with the bug catcher I had as a child. While my insect collection methods have evolved since then, my enthusiasm remains unchanged.
My project examines how sugarcane farming practices (insecticides and cultivation) affect invertebrates and impact soil carbon. I chose this project after developing a keen interest in soil science throughout my degree, with a love of understanding things from the ground up… literally. I also admire the complexity and diversity of soil whose importance is often overlooked. I am passionate about my project as it is a full-circle moment for me, allowing me to research the crop my hometown was built around. I also feel it is an important task to shift the focus from invertebrates as pests to recognising their essential roles. I hope my work will do justice the importance of soil and its many creepy crawly inhabitants.
Future Career Goals:
I aspire to continue agricultural research, emphasising the need for sustainable farming practices that maximise productivity without detriment to the environment. I look forward to deepening my knowledge and sharing my findings.
Background: Despite 30 years of leaving sugarcane trash on the soil surface post-harvest, expected improvements in organic matter and soil carbon have not materialised (SRA, 2022). One potential explanation that remains understudied is the role of soil macroinvertebrates, which facilitate carbon sequestration by fragmenting, burying, and incorporating plant residues into the soil (Jones et al., 1994; Bray et al., 2019). Agricultural practices, particularly cultivation and insecticide use, may negatively impact these beneficial macroinvertebrates through increased mortality, habitat degradation, and displacement (Pretorius et al., 2018; Beaumelle et al., 2023). The specific effects of these factors on macroinvertebrate activity and soil carbon dynamics in sugarcane systems require further investigation.
Study Overview: To address this research gap, a factorial design was utilised, assessing two factors: crop stage (recently cultivated ‘plant’ crop, <6 months vs. established ‘third ratoon’ crop, 3–4 years) and insecticide application (bifenthrin applied at planting vs. untreated). Four treatment combinations were established: Non-Insecticide/Plant, Insecticide/Plant, Non-Insecticide/Third Ratoon, and Insecticide/Third Ratoon, with 4–6 replicates per group. Macroinvertebrate abundance, order level richness, and composition were measured. Additional parameters included surface pore density and total organic and labile carbon at soil depths of 0–5 cm and 5–25 cm. Particular attention was paid to ants and earthworms due to their significant roles in organic matter incorporation.
Key Findings:
Implications and Conclusions: The results indicate that increased time since cultivation positively influences macroinvertebrate abundance, pore creation and labile carbon. Reduced cultivation frequency and intensity through extended cultivation cycles, controlled traffic farming (e.g., aligning row spacing with machinery wheels), and zonal tillage (cultivating only sugarcane rows) are practices that could promote beneficial soil macroinvertebrates and their activities and warrant further investigation (Creighton et al., 2012). Bifenthrin appeared to have minimal negative effects and may even favour resilient generalist macroinvertebrates such as Big-headed ants and springtails.
However, further studies are needed to clarify whether ecosystems dominated by generalist species like Big-headed ants contribute equally to soil carbon storage compared to more diverse macroinvertebrate communities. The observed correlation between increased surface pore density and macroinvertebrate abundance suggests a positive role of these organisms in soil structural maintenance. Future research should explore the long-term impacts of macroinvertebrate activity on stable organic soil carbon, testing paddocks within the same cycle to mitigate possible site-specific differences masking carbon sequestration results.
As challenges facing agriculture such as frequency and intensity of extreme weather events (drought etc.) are further enhanced by climate change, understanding how natural resources such as macroinvertebrates (mainly ants and earthworms) and their contributions to soil health are crucial. These macroinvertebrates improve soil structure, helping soils withstand drought, erosion and compaction. They also aid fertility and carbon storage by breaking down and bringing crop residues into soil. Despite many Australian sugarcane farmers leaving crop residues (trash) on paddocks post-harvest, for more than three decades, soil carbon levels remain lower than expected. This study examined how cultivation and insecticide use influence macroinvertebrates and their beneficial activities in sugarcane.
The study compared newly planted paddocks (<6 months since cultivation) with third-ratoon paddocks (~4 years since cultivation) and assessed the effects of using a bifenthrin-based insecticide at planting. Third-ratoon paddocks had higher macroinvertebrate abundance, particularly ants and earthworms, than newly planted paddocks. Higher abundance aligned with better soil structure (more fauna-made surface pores) and higher labile (easily available) carbon. Insecticide use showed no significant negative effect on overall abundance and was sometimes associated with greater abundance of adaptable, fast-recovering groups such as big-headed ants (Pheidole megacephala) and springtails (Collembola). Insecticide treatments did not change surface pore density but were associated with lower labile carbon. Total soil organic carbon did not differ among treatments over the short term.
Given carbon’s complexity and site-specific nature, longer-term studies comparing crop stages (plant vs. ratoons) within the same paddock and cycle are needed to confirm potential long-term benefits. Additionally, the effect of Bifenthrin should be further studied, comparing more farmers, and to assess if the increase in labile carbon in non-insecticide plots is because of increased microbial activity, more organic input from plants, increased macroinvertebrate activity not displayed by abundance data.
Given my study has confirmed a negative impact of cultivation, I recommend the following practical steps for future research to enhance macroinvertebrate benefits:
Future research should further explore how these strategies impact macroinvertebrate communities, activities (pore creation and carbon sequestration) and crop yield, ultimately aiming to enhance soil health, resilience and productivity.
