Nitrogen fertilization of intensively managed black walnut plantations in west-central Spain

Rosa Calisto Goodman, Purdue University

Abstract

Efficient fertilization may facilitate high-yielding forest plantations, thereby, sequestering carbon from the atmosphere and reducing pressure on natural forest ecosystems to help meet timber demands. Over-fertilization, however, can bear undue economic costs, cause environmental contamination, alter soil properties, and potentially harm the crop trees. Nitrogen (N) is most often the nutrient of limiting supply and the nutrient to which growth responds most drastically. In this study, grafted (Tippecanoe 1) cultivar black walnut trees were grown in intensively managed plantations (irrigation, weed and pest control) and subjected to five complete fertilizer treatments (defined as 25, 50, 75, 150, and 300 g N tree-1 year -1) and one unfertilized control. Trees were fertigated nightly from 21 May to 23 September during the first growing season and measured for two seasons. Chapter 2 focused on the effects of fertilizer treatments on yield (i.e., stem) and non-yield (i.e., branching, canopies, and fruit production) morphological characteristics. Stem diameter, volume, and mass increased from the unfertilized to the second treatment (50 g N) but showed no significant improvement thereafter. N utilization efficiency was greatest in the lowest two treatments, and fertilizer N use efficiency was greatest in the second treatment. Canopy volume and diameter, total branch biomass, first-order lateral branches, nut production, and incidence of multiple stems increased with increasing fertilization, which may adversely affect stem quality. Allometric equations including diameter at 70 cm and height were the best models to predict stem volume and mass, and diameter at 10 cm and length were the best to predict branch volume and mass. Chapter 3 examined the effects of fertilization on leaflet characteristics and photosynthesis in an effort to determine by what mechanisms N fertilization affected growth. Specific leaf mass and leaflet N and chlorophyll concentrations rose with increasing fertilization, but net photosynthesis at ambient and light-saturated conditions did not increase beyond the lowest treatment (25 g N). Photosynthetic N and chlorophyll use efficiencies decreased with increasing fertilization, but phosphorus use efficiency increased. The chlorophyll index meter (SPAD) had a strong relationship (R2 = 0.8041) with actual chlorophyll concentration and moderate correlation ( R2 =0.4432) with N concentration. Overall, low to moderate fertilizer rates (i.e., 50 g N tree-1 year-1) were most efficient to promote juvenile plantation development of black walnut under this cultural regime. Fertilization beyond sufficiency did not increase stem yield or photosynthetic capacity. Increased branching, foliage, and fruit production seem to be stimulated by higher fertilization rates, which may necessitate additional management costs and/or adversely affect timber quality.

Degree

M.S.

Advisors

Jacobs, Purdue University.

Subject Area

Biogeochemistry|Forestry

COinS