86 Werner and Jull: Fertilizer Uptake and Partitioning in Young and Mature Common Hackberry established across the geographic extent of Waukesha. Trees were defined as mature if they were producing fruit and had been established in the landscape for at least twenty years. Young trees had an establishment period of less than five years, according to the City of Waukesha’s forest inventory records, and were not bearing fruit. Three of the young tree plots were planted in 1998, with the remaining plot being planted in 1996. A distance of at least 15.3 m separated all trees within the plots. Fertilizer treat- ments were the sub-plot factor and consisted of three rates of N application, 0 [TRT 1, tap water, <1 mg L-1 N (ppm)], 0.49 (TRT 2), and 1.47 kg N 100 m-2 (TRT 3) of canopy coverage. There were three trees per fertilizer treatment for the young tree plots and two trees per treatment in three of the four ma- ture tree plots. The remaining mature tree plot had a single tree per treatment level because the canopies of the trees extended onto private property and limited access to the potential treat- ment area. A total of 57 trees were used in the study. All young trees were on a structural pruning cycle of 2, 4, and 6 years. All young tree locations were pruned during the same year (Liska, pers. comm.). Mature trees received a similar pruning cycle for the first 6 years following establishment and were then pruned on a 6-year cycle (Liska, pers. comm.). The City of Waukesha did not fertilize any of the study trees prior to the initiation of this work. All young trees received supplemental irrigation in periods of summer drought during the first two years of establishment. Understory vegetation at all mature tree locations consisted isotope (1.5 atom % for both NH4 and NO3 NO3 of a mix of cool season turfgrasses. There was similar understory vegetation at two of the young tree plots, with vegetation at the remaining two plots consisting of annual and perennial herba- ceous weeds. Soils at the plot locations were either silt loams, consisting of the Warsaw and Sawmill soil series, or a loam rep- resented by the Hochheim soil series (USDA 1978). The Warsaw soil series (Typic Argiudoll) is a well-drained silt loam under- lain by a calcareous sand and gravel outwash. The Sawmill soil series (Cumulic Haplaquoll) is a poorly-drained alluvial silt loam. Relative to the Warsaw soil series, the Sawmill series possessed a higher concentration of free carbonates. The Hochheim soil series (Typic Argiudoll) is a well-drained loam atop calcareous glacial till. A composite of four soil samples obtained from ran- dom locations within the area defined by the canopy drip line of each tree was analyzed at the University of Wisconsin–Madison Soil and Plant Analysis Laboratory (Verona, Wisconsin, U.S.) to determine soil pH (1:1 water), soil organic matter (SOM, % loss on ignition), potassium (K), and phosphorus (P, Bray P1). On June 12 and 13, 2001, NH4 double enriched with 15 ) was injected 15.2– N 25.4 cm below the soil surface at 1034 kPa in a 0.6 m × 0.6 m grid extending approximately 1.5 m beyond the canopy drip line. Component tissues from trees were sampled prior to treatment to establish baseline total N and 15 N atom % values (Atkinson et al. 1987). Comparisons of fertilizer N uptake and inferences surround- ing rate of application and age class differences were determined based on changes in the concentration of 15 N within plant tissues. Tissue samples consisted of foliage, current season stem wood (new terminal stem tissue), older stem wood, fruit, and roots. Foliage samples were composed of developing and mature leaves obtained from branches receiving full sunlight located in the upper, mid, and lower portions of the crown. In young trees, cur- rent season stem wood samples consisted of a composite of ter- minal stem tissues from exterior branches, 15–25 cm in length, ©2013 International Society of Arboriculture obtained from the upper, mid, and lower portions of the crown. Mature trees current season stem wood samples were obtained from exterior branches approximately 0.9 m in length that were in crown positions similar to those of young trees. Stem wood samples from young trees consisted of approximately 7–13 cm taken from the base of a limb 1.25 cm in diameter that was removed from the lower one-third of the crown. In mature trees, stem wood samples consisted of 15 cm (6 in) of limb wood obtained from lower branches approximately 7.6 cm in diameter. In young trees, approximately 25–50 g of woody and non-woody roots growing in the upper 25.4 cm of the soil profile were ob- tained equally from two samples beneath the crown. Approxi- mately 150–250 g of woody and non-woody roots growing in the upper 25.4 cm of the soil profile were obtained equally from four locations under the crown of mature trees. In the field, soil was removed from root samples using two, five second rinses with deionized water. Fruit samples from mature trees were obtained during the acquisition of the foliar samples. Tissue samples were collected 14, 30, 60, 90, 120, and 150 days after fertilization and were dried at 60°C until processing in a Wiley mill (Thomas-Wiley Laboratory Mills, Philadelphia, Pennsylva- nia, U.S.) to pass a 1 mm sieve. One gram of the homogenized tissue was pulverized in a Crescent “Wig-L-Bug” Dental Amal- gamator (Crescent Dental Manufacturing, Chicago, Illinois, U.S.) for 120 seconds using two, 5 mm stainless steel ball bear- ings at 5,000 rpm. Bark was included in the processing of current season stem wood and stem wood samples. Approximately 5 mg of pulverized tissue were combusted at 1020°C in a Carlo-Erba Carbon : Nitrogen analyzer (Carlo Erba, Milan, Italy), interfaced with a Europa Scientific TracerMass mass spectrometer (Crew, Cheshire, United Kingdom). Analysis consisted of total N con- centration [N] and percent 15 N on a dry weight basis. Quality control was verified by duplicate analysis every 10–12 samples. Comparisons of fertilizer accumulation in component tissues between mature and young trees were based on changes in the atom percent level of 15 N relative to baseline (pre-fertilization) values and the level of isotopic enrichment in the fertilizer (NDFF %) (Equation 1). Justification for using NDFF % as a measure of fertilizer N recovery is supported by studies using destructive har- vesting techniques (Kraimer et al. 2001). Specifically, fertilizer N recovery is the product of the level of 15 N enrichment (NDFF %) in component tissues and their respective biomasses. The percent nitrogen derived from fertilizer (NDFF) was calculated using a modified form of equations reported by Kraimer et al. (2001). [1] NDFF = (A where A = % 15 ( DC B − − ) ) N in fertilized tissue sample, B = % 15 N in tissues prior to fertilization, C = % 15N in fertilizer, and D = % 15N natural abundance (0.366%, IAEA 1983). The modification was necessitated by City of Waukesha imposed restrictions on the destructive harvesting of study trees. As a result, total tree biomass and component tissue (e.g. leaves, stem wood) biomass could not be determined. Treatment and growth phase effects within a harvest period were determined using mixed model ANOVA procedures in SAS (Littell et al. 2004). Changes over time within the treat- ment structures of each growth phase were evaluated using repeated measures analysis. Treatment level and growth phase were fixed effects and location within a growth phase was a
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