Nitrogen Fertilization of Black Walnut (Juglans nigra L.) During Plantation Establishment. Morphology and Production Efficiency

Grafted, Tippecanoe 1 cultivar black walnut (Juglans nigra L.) trees were planted and grown in an intensively managed plantation in west-central Spain and subjected to six fertilizer treatments (defined as 0, 25, 50, 75, 150, and 300 g tree(-1) nitrogen [N]) delivered via daily fertigation during the 1st year after outplanting. Stem diameter, volume, and mass increased from the unfertilized control to the second fertilizer treatment (50 g N) but showed no gains thereafter. N utilization efficiency (stem volume growth per unit fertilizer) was greatest for the 25 and 50 g tree(-1) N treatments, and fertilizer N use efficiency (gain in stem volume growth over the unfertilized control per unit fertilizer) was greatest at 50 g tree(-1) N. Foliar N concentrations in mid-July provided the best predictors of seasonal stem volume growth. Optimal foliar N was 3.2%, which is higher than values recommended for other black walnut production systems: less than 2.8% was deficient, 3.0% was sufficient, and more than 3.4% suggested toxicity. Branch biomass and branch mass/trunk mass ratio were greater in all fertilizer treatments than in the unfertilized control, indicating that high fertilization rates may shift biomass allocation away from the desired product (stem wood).