2019 ASHS Annual Conference

Fertilizer nitrogen (N) is a key input to the overall N budget in conventional intensive apple production systems. Understanding of uptake and utilization is important as tree N reserves drive bud break and early leaf growth, critical for apple yield and quality. Yet currently nitrogen use efficiency (NUE) is typically less than 40% in these systems. A strategy to optimize the NUE is to match tree N demand to fertilizer N supply. This study investigated timing of application, using ¹⁵N stable isotope, as an approach to optimize NUE in a commercial orchard located in the Derwent Valley region of Tasmania, Australia using ‘Gala’ apple trees grafted on M26 rootstock planted on sandy loam soil. Apple trees were drip-fertigated with 30 g N tree^-1 (approx. 50 kg N ha^-1) of 5% ¹⁵N enriched calcium nitrate at: pre-harvest (three weeks after full-bloom); post-harvest (one week after harvest); 50-50 split (half rate at both pre- and post-harvest) or; unfertigated as a control. Fruit quality was assessed at harvest and 8 weeks post-harvest. To determine NUE, whole trees were excavated at dormancy, separated into different organs and total N and ¹⁵N content of the different organs were measured. The percentage of N derived from fertilizer (NDF) was used to determine partitioning of applied N throughout the tree and overall NUE. No significant differences between unfertigated control and those that received nitrogen treatments were found for fruit yield, size, firmness, starch index, total soluble solid content at harvest or 8-weeks post-harvest. Intensity of red color and red color coverage was significantly reduced, and background color was significantly greener in 50-50 split treatment relative to the other treatments. NUE was highest in trees treated with pre-harvest fertigation (27.8%), followed by post-harvest fertigation (19.5%), and 50-50 split treatment (16.3%). In general, 23% of the pre-harvest fertigated N taken up by apple tree was partitioned to the fruit, while buds of tree that received pre-harvest application contained nearly 4-times the amount of N than buds of trees that received post-harvest applications. The post-harvest N fertigation had significantly greater N allocation to storage (17.5% NDF) in all woody tissue than other treatments (9.7% and 9.3% NDF for pre-harvest and 50-50 split fertigation treatment respectively). The results suggested that pre-harvest fertigation optimized NUE and that N applied pre-harvest was allocated to supporting current season growth whilst post-harvest application was allocated to storage.