Fertilizer nitrogen fate in korla fragrant pear orchard using ¹⁵N tracing method

Objectives ¹⁵N isotope tracer technology was used in this paper to study the use and transfer of fertilizer N in Korla fragrant pear trees and the fate of fertilizer N after applied into soil, to increase the understanding of the nitrogen cycle of fertilizer N in soil and tree body.

Methods Six years’old Korla fragrant pear trees were used as the tested materials, a field mini-plot test was conducted in Team 5, Shibagh Village, Chabag Township of Korla City, Xinjiang Uygur Autonomous Region. Conventional urea N 667 g and ¹⁵N-urea N 10 g were applied in each pear tree, 60% of them being applied at the early budding stage and 40% at the early fruit expanding stage. The nitrogen utilization by pear trees, the soil nitrogen residual and nitrogen gas loss were analyzed from the early bud stage to the fruit harvest stage.

Results 1) With the growth stages advancing, the growth center of fragrant pear trees kept changing. Before blooming, the growth center accounted only in roots, and from the full-bloom to the new shoot growing stage, the centers were in both leaves and roots, and from the fruit expansion to fruit harvest stage they were in roots, fruits and leaves. With the development of pear trees, the utilization rate of fertilizer N was increased gradually with the advancing growing stager, with the maximum value of 18.5% at fruit harvest stage. 2) The residual amounts of NO₃^–-N and NH₄⁺-N within the 0–120 cm soil profile decreased gradually, the deeper the less, and the residual amounts of NO₃^–-N and NH₄⁺-N in the 0–60 cm soil layer were much higher than those in 60–120 cm soil layer. With the growth stage, the residual rates of NO₃^–-N and NH₄⁺-N decreased in the soil profile, and the residual rate was the lowest at the fruit harvest stage with 13.9% and 8.41% respectively. The residual rate of inorganic nitrogen was 22.31%. 3) The total amount of ammonia volatilization and N₂O emission accounted only 4.19% for the soil nitrogen loss, and ammonia volatilization was the predominant way.

Conclusions The recovery rate of fertilizer N decreased gradually with the growth stage advancing (except for fruit expanding), and the minimum value was 41.0% at the fruit harvest stage. The loss rate was opposite to the recovery rate, and the loss rate was as high as 59.0% at the fruit harvest stage. The fertilizer N fate was in the order of nitrogen loss>residue in soil>absorption by pear trees.