DEVELOPING SUGARCANE - LEGUME COMPANION CROPPING SYSTEMS TO MINIMISE NITROUS OXIDE EMISSIONS Monica Elizabeth Salazar Cajas Agronomist Engineer (Central University of Ecuador) MAppSc. Soil Science (Massey University) A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2018 School of Agriculture and Food Science Abstract Global efforts are underway to reduce greenhouse gas emissions (GHG) from anthropogenic activities. Nitrous oxide (N2O) emissions accounted for 13% of Australia’s National GHG inventory over the period 2016-2017 (NGGI, 2017) with most N2O derived from agricultural soils. Sugarcane soils are high emitters of N2O, and this thesis explores whether legumes, grown as a companion crop with biological N2 fixation (BNF) capacity, can partially replace N fertiliser to lower the emissions of N2O from sugarcane soil. Chapter 2 synthesises published literature on sugarcane intercropping. Most research has focussed on the productivity of sugarcane with intercrops, including legumes. Intercropping can benefit sugarcane yield, have neutral or negative effects. This practice is common in subsistence agriculture, and farm income benefits, but environmental benefits intercropping have not been a research focus. Chapters 3 and 4 explore sugarcane-legume intercropping at three commercial farms in Australia. N2O emissions, soil and crop variables were quantified with different N fertiliser applications and in the presence or absence of legumes. The farms, two Rain-fed, one Irrigated, were located in the dry and wet tropics, and in the subtropics, representing different climate and agronomic settings. Industry-recommended (full) N fertiliser rates were compared with up to 50% reduced N fertiliser rates in the presence or absence of legume and benchmarked against a zero N fertiliser control. We hypothesised that reduced application of N fertiliser limits sugarcane growth and that legumes can alleviate N limitation. However, full and reduced N fertiliser treatments mostly generated similar sugarcane yields, confirming that industry- recommended full N fertiliser rates exceed sugarcane needs. In line with this notion, the reduced N+legume treatments did not improve sugarcane growth. N2O emissions in reduced N+legume were either similar to reduced N fertiliser sugarcane monoculture or higher and similar to the emissions observed with full N fertiliser rates. Soybean strongly benefitted sugarcane yield under N limiting conditions (zero N fertiliser) at one farm, increasing soluble soil N levels and nearly doubling sugarcane yield compared to zero-N sugarcane monoculture, and generating 6-times lower N2O emissions than the full N rate. At the Rain-fed sites, soil nitrate levels explained 81 and 64% of N2O emissions; at the Irrigated site, the interaction of soil nitrate and soil moisture explained 63% of N2O emissions. High N2O emissions factors at the subtropical site were associated with wet, low drainage soil (>70% water filled pore space over summer), conditions that promote denitrification. High N fertiliser rates in Irrigated, well- ii draining soils had lower N2O emissions, possibly shifting N losses from gaseous to leaching. The promising findings observed with soybean under N limitation require further investigation to explore N2O mitigation options with a view of optimising legume facilitation and the ‘tipping point’ for N fertiliser applications. Chapter 5 presents a glasshouse experiment investigating the effect of N fertiliser rate on competition vs facilitation up to peak N accumulation of soybean. No or low N fertiliser rates enhanced soybean BNF but reduced growth of sugarcane. With moderate to high N fertiliser rates, soybean BNF and growth diminished, while sugarcane growth increased which indicatives increased competitive ability. We conclude that N fertiliser rates substantially impact upon the relative performance of sugarcane and legume intercrop, with a trade-off between N fertiliser application and legume BNF. Under the experimental condition, direct N transfer from soybean to sugarcane was negligible. Rather, decomposition and mineralisation of legume biomass are likely to be the main pathway for increased N accumulation in intercropped sugarcane observed in some instances. Chapter 6 synthesises the findings from literature, field and glasshouse experimentation and discusses future directions for sugarcane-legume intercropping research. iii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly- authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, financial support and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my higher degree by research candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis and have sought permission from co- authors for any jointly authored works included in the thesis. iv Publications included in this thesis “No publications included”. Submitted manuscripts included in this thesis “No manuscripts submitted for publication”. Other publications during candidature Conference abstracts: Salazar, M., Robinson, N., Royle, A., DiBella, L., Wang W., Heenan, M., Reeves, S., Schmidt, S. Brackin, R. (2016). Reducing nitrous oxide emissions from sugarcane soil with legume intercropping In: 7th International Nitrogen Initiative Conference (INI 2016): Solutions to improve nitrogen use efficiency for the world. International Nitrogen Initiative Conference, Melbourne, Australia. 4-8 December 2016. Robinson, N., Brackin, R., Paungfoo-Lonhienne, C., Lonhienne, T., Westermann, M., Salazar, M.,Yeoh, Y. K ., Hugenholtz, P., Ragan, M. A., Redding, M., Pratt, C., Wang, W. J., Royle, A., DiBella, L., Lakshmanan, P. and Schmidt, S. (2016). Addressing the nitrogen problem in sugarcane production to reduce pollution of the Great Barrier Reef. In: 7th International Nitrogen Initiative Conference (INI 2016): Solutions to improve nitrogen use efficiency for the world. International Nitrogen Initiative Conference, Melbourne, Australia. 4-8 December 2016. v Contributions by others to the thesis Professor Susanne Schmidt, Dr Richard Brackin and Dr Nicole Robinson designed the field experiments, and contributed to the design of glasshouse experiments. Prof Susanne Schmidt, Dr Richard Brackin, Dr Nicole Robinson and Dr Ryo Fujinuma assisted with data interpretation chapter editing. Further people contributed to this thesis as listed below. Chapter 2 Dr Henrique Coutinho Junqueira Franco (Centro Nacional de Pesquisa em Energia e Materiais, CNPEM, Brazil) and Dr Ovidio Perez (Sugarcane Research Centre Guatemala, CENGICANA) provided reports and papers from Brazil, Guatemala and Mexico on sugarcane intercropping. Chapter 3 and 4 Taleta Bailey, Scott Buckley, Stéphane Guillou, João Carlos de Freitas Junior, Vithya Singh, Zoe Ong and Maren Westermann assisted with data collection and laboratory analyses. Sampling (soil, GHG gases) at the field experiments located in Abergowrie (Wet Tropics) and Burdekin (Dry Tropics) were assisted by technicians Melissa Royle, Minka Ibanez and Dennis Stubbs. Chapter 5 Dr Nicole Robinson assisted with the experimental design, planning and set-up of the glasshouse trial. Dr Richard Brackin, Dr Nicole Robinson and Scott Buckley assisted with the harvest. vi Statement of parts of the thesis submitted to qualify for the award of another degree “No works submitted towards another degree have been included in this thesis”. Research Involving Human or Animal Subjects “No animal or human subjects were involved in this research”. vii Acknowledgements A great thanks to my supervisor Professor Susanne Schmidt whose attention to detail led me finally to learn to punctuate and improve my writing skills. I am so grateful for accepting me in her research team and work in this project. I admire her passion, knowledge, patient guidance, and support throughout the years of my PhD, offering invaluable advice during our long meetings with Dr Richard Brackin, helping me to organise my ideas and asking insightful questions. I much appreciate her guidance for increasing my general knowledge in our weekly team meetings, and the way that she encourages the team to collaborate and help each other. Thousands thanks to my co-supervisor Dr Richard Brackin whose selfless