Aqueous Nitrogen Dynamics in Irrigated Cropping Systems: Improving precision agriculture and environmental performance for the Australian cotton industry
Creators: name: Latimer, James
Publisher: name: The Australian National University
Year: 2021

description: The Australian cotton industry is an archetypal example of precision agriculture in action, having achieved significant efficiency gains in yield (kg lint ha-1) and water use (kg lint ML-1) over the past 50 years through sustained research and development investment. Unfortunately, nitrogen fertiliser use efficiency (NFUE) has not experienced the same gains over this period, and has instead declined. Australian irrigated cotton production requires high nitrogen (N) inputs to maintain its high yields. N application rates (kg N ha-1) have increased over recent decades due to a range of factors, including low fertiliser costs and grower risk appetites. Average yields have also increased over this period; however, they have not been proportional to the rise in N applications, resulting in steadily declining NFUE. While significant research describing N dynamics in Australian cotton systems already exists, there remain many research gaps to be filled. This thesis aims to address four research gaps to provide additional management levers for the Australian cotton industry to improve NFUE. The four topics explored herein are: (1) the effectiveness of aqueous N application (fertigation or water-run); (2) the mechanisms driving surface runoff N losses in flood irrigation; (3) the reaction rates and residence times of aqueous N; and (4) the degree of plant access to different soil N molecules. A series of field, laboratory, and glasshouse experiments were used to address these questions. Three field experiments measuring fertigation application efficacies were conducted on private farms in the Riverina, New South Wales (NSW) over the 2016-17 summer season. Another field experiment was performed at the Australian Cotton Research Institute (ACRI) in Narrabri, NSW over the 2017-18 season, measuring N runoff variations in alternate furrow irrigation configurations. Two laboratory experiments were performed at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Black Mountain site in Canberra, Australian Capital Territory, measuring the reaction rates and residence times of dissolved urea in soil-water systems. Finally, a glasshouse experiment was performed at CSIRO Black Mountain in January 2019, assessing the N uptake capabilities and preferences of three cotton (Gossypium hirsutum L.) varieties using 15N 13C stable isotope analysis. Chapter 2 aims to answer the question: "How effective are current Australian irrigated cotton fertigation practices at delivering consistent N to crops, and what management levers can be identified to improve outcomes?". Chapter 3 aims to answer the question: "How does N surface runoff vary spatially and temporally at sub-field and intra-irrigation scales respectively, and can this high resolution be used to identify specific mechanistic drivers of N runoff?". Chapter 4 aims to answer the question: "What is the residence time of dissolved N in irrigation water, and how does it vary across the farm environment?". Chapter 5 aims to answer the question: "Which soil N species can commercial cotton (G. hirsutum) directly take up, and what preferences does it exhibit when given a choice?". Improving NFUE represents a triple bottom line opportunity for the Australian cotton industry. Economically, it will save growers the cost of wasted fertiliser, and reduce yield from over-application. Environmentally, it will reduce N2O greenhouse gas emissions, increase soil carbon stocks, and reduce N deep drainage. And socially, it can help to grow Australian cotton's reputation as the most resource efficient in the world, and build its public profile and brand recognition. This thesis aims to improve precision agricultural practices and environmental performance for the Australian cotton industry by providing new information and management tools to increase NFUE.