What becomes of nitrogen in your orchard?Research & Extension
Researchers are using nitrogen-tracing technology to help improve apple fruit quality and optimise nitrogen use efficiency by investigating the fate of nitrogen derived from fertiliser over two seasons.
Applying nitrogen fertiliser is a high input cost in apple production. Moreover, nitrogen is often applied at a rate higher than trees demand due to low nitrogen use efficiency and loss via leaching or conversion to nitrous oxide gas.
Application rates vary significantly depending on the site and are often determined by soil nitrogen status, an estimation of nitrogen removed by crop harvest and tree pruning, gut feel and possibly a legacy of how things have always been done. This trial aims to put certainty into understanding nitrogen use by the tree, the efficiency of nitrogen uptake and nitrogen’s fate following application.
A preliminary trial at the Tasmanian Institute of Agriculture (TIA) at the University of Tasmania using nitrogen tracing technology in apple trees grown in pots demonstrated the movement of soil-applied nitrogen within the tree over one growing season. It showed that nitrogen use efficiency was strongly influenced by the timing of application.
Our next step is to conduct a trial in a commercial orchard to reveal the fate of nitrogen within the tree-soil system under different application timings over two growing seasons. This trial will generate a data set that will underpin the Strategic Irrigation and Nitrogen Assessment Tool for Apples (SINATA) that aims to help growers manage their nitrogen and water inputs (see Strategically managing water and nitrogen, Australian Fruitgrower, Oct-Nov 2016).
The field trial
The field trial will compare the impact of three full-rate fertiliser application timings (spring, summer and 50-50 split applications) on nitrogen use efficiency and partitioning of nitrogen through the tree.
Nitrogen will be applied as calcium nitrate fertiliser at a rate of 30g actual nitrogen per tree (equivalent to 50kg N/ha). This fertiliser includes two types or ‘isotopes’ of nitrogen: the stable 15N isotope and the standard 14N isotopic calcium nitrate. This is important from a research perspective because our technology can track the presence of 15N so we can see where it goes after it has been applied to the soil.
Fertiliser will be applied through a drip-irrigation system delivering nitrogen in a similar rate and length of time as a standard fertigation system. The recovery of the 15N isotope from plant organs enables the accurate tracking of how efficient the fertiliser application has been and where the nitrogen has been partitioned within the plant.
The trial has been established at a commercial orchard located at Plenty, in the south-east horticulture region of Tasmania with a mean annual rainfall of 570mm. The apple trees are an eight-year-old Gala cultivar grafted on M26 dwarfing rootstock planted on well-drained, clay loam dermosol soil with a tree spacing of 1.5m by 4m and north-south row orientation.
The nitrogen content within the plant derived from fertiliser taken up will be determined by excavating the whole tree at dormancy in 2018 and then additional trees at commercial harvest in 2019.
Destructive harvest at dormancy will enable us to determine the fate of current season (2017-18) applied nitrogen, and a destructive harvest in 2019 will enable us to determine the nitrogen carried over for remobilisation and early-season growth in the next season (2018-19). This is important because apple trees store significant levels of nitrogen that are used for bloom and shoot growth the following season.
The nitrogen content in the soil and leached below the root zone will be quantified by soil analysis and captured using drainage lysimeters. Drainage lysimeters are essentially long PVC tubes containing a wick that mimics the soil water pressure or soil tension. Moisture exceeding that tension collects at the base of the lysimeter and can be evacuated to determine the amount of water and applied fertiliser that has leached below the roots.
Lysimeters were installed below the root zone of nine trees that will receive 15N in the trial. This information will be used in the development of the SINATA tool.
Soil and leaf samples will also be collected throughout the season at fixed intervals, while fruit will be harvested at commercial harvest to enable the team to conduct nutrient analysis and fruit quality assessment.
Using all this information we hope to develop a fertigation strategy to increase nitrogen use efficiency and optimise fruit yield and quality.
This research is part of the PIPS Orchard Productivity Program, a strategic levy investment under the Hort Innovation Apple and Pear Fund. It is funded by Hort Innovation using the apple and pear levy and funds from the Australian Government.
This research is led by TIA PhD student Bi Tan and is one of the three key research areas of the PIPS Apple fruit and tree nutrition for improving productivity project. This is a four-year collaborative research project between horticulture research scientists at the TIA and Plant and Food Research (PFR) New Zealand led by Dr Nigel Swarts.
The researchers acknowledge Reid Fruits for providing access to its orchard, with special thanks given to orchard managers Mark Salter and Nick Owens, who have provided excellent assistance in setting up the trial. The University of Tasmania provided the scholarship funding to PhD candidate Bi Tan.