Monitoring Uptake of Legume N by Apple Tree Using Nitrogen Isotope Discrimination

David  Granatstein; Joan R Davenport; Lee A Kalcsits; Pamela Pavek

doi:10.7273/000002515

Intercropped legumes can supply nitrogen to fruit trees and reduce the need for purchased nitrogen fertilizer, potentially reducing costs and environmental footprint. The project evaluated nitrogen isotope techniques (comparing the 15 N and 14 N signal from the legume N versus other sources) to monitor the degree of uptake by apple trees of legume N grown in the orchard. The project involved two components: 1) a greenhouse study using 15 N-labeled clover residue added to potted apple trees to quantify legume-N uptake and a comparison to values using nitrogen isotope natural abundance; 2) a field study in an existing orchard legume cover crop trial using natural abundance for a qualitative evaluation of legume N and the ability to detect this signal in an ambient field environment. The 15 N enrichment technique is known to be more sensitive than the natural isotope abundance technique, but is not suitable for routine field use by growers or consultants. The objective of the project was to determine the uptake of legume-derived N in a quantitative manner and compare it with a more qualitative method to assess its utility on farms. Lower tree leaf 15 N atom% with legume alleys versus grass was detected in several instances in both trials, indicating that the natural isotope abundance method does have potential use for monitoring the timing of legume uptake by trees. PROJECT DESCRIPTION Objectives. Trial 1. Compare legume-N uptake by potted apple trees using the 15 N enrichment technique versus the natural isotope abundance technique. Trial 2. Monitor natural isotope abundance of legumes, soil, and apple trees in an existing legume cover crop trial to determine the magnitude of the signal difference in the different orchard components. Methods. Trial 1 was repeated using potted trees under shade cloth outdoors, after the initial trial (2014) was compromised by saline water in the greenhouse. Trees ('Golden Delicious'/B.9) were planted on 9 April 2015 in 20 " diameter pots filled with virgin Warden silt loam soil (no history of cropping). Forty-five trees were planted; 20 for each phase of the trial, with 5 extras. The trees were grown through the 2015 season without treatment due to the lack of enough 15 N labeled clover, and then they were placed in cold storage for the winter. There was not enough labeled 'Alice' white clover biomass to do spring 2016 treatments either, so trees and clover were grown through the summer. Fall treatments were applied on 9/13/16 to 20 trees. Prior to this, soil samples (0-10 ") and tree leaves (10 per tree) were collected for a pre-treatment baseline. All trees had received a Hoagland's nutrient solution during 2015 and 2016 in order for them to grow, and there appears to be a small amount of 15 N enrichment in this material that

Monitoring Uptake of Legume N by Apple Tree Using Nitrogen Isotope Discrimination

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