Utilization of Fertilizer N by Walnut Trees (84-wmb3)

The broad objectives' of this project have included aspects of N deficiency including reassessment of the so-called N critical value, determination of tree N content and the amount of annual N uptake, assessment of the periodicity of N transport to leaves and fruit, and the efficiency of fertilizer N recovery using 15N-depleted(NH.hS04' The onset of yield reduction in N-deficient "Serr" and "Hartley" trees corresponded with midsummer leaf N concentrations at or below 2.3% dry weight. The rapidity of onset of N deficiency varied between the "Serr" and "Hartley" orchards and was probably a function of differences in the size of the pool of plant-available N in the soil. Recovery of tree productivity required a minimum of two years following N reapplication to N-deficient trees, although leaf N concentration increased to the level of the fertilized control trees within one year. Trees exhibited a minimum capacity for N uptake during dormancy, and N accumulation by developing catkins (in March) was derived entirely from the N absorbed during previous season(s) and stored over winter in the tree rather than the (labeled) fertilizer N applied to the soil two months earlier. Transport of fertilizer N from the soil to the above-ground tree canopy was initiated following bud break and d~ring pistillate flower maturation and leaf expansion in April. About forty pounds of N was removed from the orchard per ton of nuts harvested. We estimate that an additional 20% (8 lbs N per acre in this instance) was lost from the tree in the abscised leaves and prunings. The "Hartley" trees in our experiment contained nearly 7 lbs N per tree. About 30-40% of this amount ~as replaced from the 'soil annually, and about 75% of that amount (1.80 lbs N per tree) was accounted for in the fruit. Twenty-five and 22% of the labeled N applied in 1~85 was accounted for in the "Serr" and "Hartley" crops, respectively, between 1985 and 1990. An additional 4% of the labeled N applied was accounted for in the abscised leaves. Excavation and analysis in 1991 of two "Hartley" trees (that received labeled N in 1985) indicated that the residual label in the tree represented 4% of the labeled N applied in 1985. Thus, "Hartley" trees recovered an average of 30% of the labeled N applied in 1985, and "Serr" appeared to recover a slightly higher (= 33%) percentage. PROBLEM AND ITS SIGNIFICANCE Of all the essential elements, nitrogen (N) is the element most commonly applied in walnut orchards and'is applied at the greatest rates. We have no information on N usage and fertilizer recovery by mature walnut trees. Characteristics inherent in walnut, as in all fruit trees (i.e., perennial habit, and large woody biomass), make the actual N demand of these organisms difficult to determine. Fertilizer N is typically applied to ensure non-limiting availability, but the