Arboriculture & Urban Forestry 32(4): July 2006 183 received a value of 2 and those without fertilization received a 0. Each of the six landscape types had four plots with two subplots each for a total of 48 sample points at each sampling date. Site, vegetation, and soil management history were at- tained for each site by personal interviews. Management practices such as fertilization and irrigation were monitored throughout the study. Four 2.5 cm (1 in) diameter by 15 cm (6 in) deep cores were collected within drip lines of trees. Soils were passed througha6mm (0.24 in) wire screen to remove stones and organic debris while minimizing the im- pact on biologic measurements (Ross 1992). After sieving, soil samples were stored at 4°C (39.2°F) until processed. Soil bulk density was determined from soils collected on 17 April 2002. Soil textural analysis was performed by Mid- west Laboratories, Inc. (Omaha, NE) with soils collected on 17 April 2002. Gravimetric soil moisture content was calcu- lated from soil sample weights before and after drying at 105°C (221°F) for 24 hours (Gardner 1965). Soil subsamples were adjusted to 40% water-holding capacity for further analysis (Kaiser et al. 1992; Joergensen and Mueller 1996). A modified soil fumigation–extraction method (Brookes et al. 1985; Sparling et al. 1990; Kaiser et al. 1992; Cabrera and Beare 1993; Joergensen and Mueller 1996) was used to de- termine microbial biomass nitrogen (MBN). Soil subsamples were fumigated with ethanol-free chloroform for 5 days, ex- tracted with 0.5 M K2SO4, and extractable nitrogen was re- duced to NH4 + for colorimetric analysis at 650 nm. Unfumi- gated soil subsamples were analyzed for extractable nitrogen. Microbial biomass nitrogen was represented by the difference in nitrogen between the fumigated and the unfumigated samples using an extraction efficiency factor of KEN0.54 (Joergensen and Mueller 1996). Soil respiration, expressed as potentially mineralizable carbon (PMC), was attained with 20-day soil incubations using NaOH traps. Carbon dioxide sequestered by NaOH was precipitated with BaCl2 followed by 0.25 M HCl titration to a phenolphthalein endpoint (Anderson and Domsch 1978; Parkin et al. 1996; Duriancik 2001). Potentially mineralizable nitrogen (PMN) was deter- mined using 20-day aerobic incubations followed by a nitro- gen extraction with 0.5 M K2SO4 and colorimetric analyses (Drinkwater et al. 1996). Mineral nitrogen (NH4 NO3 −) before incubation was subtracted from nitrogen min- +,NO2 eralized during the 20 days. Mineral nitrogen (MIN) and dissolved organic nitrogen (DON) were determined by extraction with 0.5 M K2SO4 followed by colorimetric analyses at 650 nanometers for am- monium (NH4 +), nitrate (NO3 −), nitrite (NO2 −), and dissolved organic nitrogen (Cabrera and Beare 1993; Sims et al. 1995; Rice et al. 1996; Duriancik 2001). Ammonium concentrations in filtrate extracts were determined. A reduction with Devarda’s alloy and sulfuric acid was used to reduce nitrate and nitrate to ammonium. Mineral nitrogen was represented by NH4 + +NO2 − +NO3 − in extracts; and alkaline persulfate −, and digestion under pressure was used to determine total nitrogen in filtrate extracts. The difference between total extractable nitrogen and mineral nitrogen (NH4 + +NO3 − +NO2 −) was represented as DON. Particulate organic matter fractions were isolated by a modified particle size fractionation method (Elliot and Cambardella 1991; Cambardella and Elliot 1992, 1993; Six et al. 1998; Gill et al. 1999; Bayer et al. 2001; Six et al. 2002; Salas et al. 2003). Soil samples were shaken for 15 hours with sodium hexametaphosphate (NaPO3) and then passed through a series of nested sieves. The organic matter collected on the 2,000, 250, and 53-m sieves was considered litter SOM (lSOM), coarse POM (cPOM), and fine POM (fPOM), re- spectively. Mineral-associated SOM (maSOM) passed through the 53-m sieve and was determined by maSOM total SOM – lSOM – fPOM – cPOM. Loss on ignition (LOI) and automated dry combustion methods were used to determine organic matter, nitrogen, and carbon in each size fraction. Once fractionated, samples were oven dried at 105°C (221°F), homogenized, and weighed. Subsamples of the coarse and fine POM fractions were ana- lyzed for total nitrogen and carbon content by dry combustion with an automated gas combustion analyzer (Vario Max CNS, Mt. Laurel, NJ). Values of nitrogen and carbon in the POM fractions were expressed asgNorCm−2 or as a C/N ratio of those fractions. The remaining portions of the lSOM, cPOM, and fPOM fractions were dried at 105°C (221°F) and burned at 360°C (680°F) for 6 hours to determine ash weight (Schulte et al. 1991; Sikora and Stott 1996). Accuracy of the loss on ignition method was checked with linear regression of urban landscape mean values from all sample dates of the fPOM carbon content determined by automated dry combus- tion and g fPOMm−2 determined by LOI (R20.90). Linear regressions were also performed with values of total SOM determined by the LOI and the Walkley-Black method (Mid- west Laboratories, Omaha, NE) with samples collected on 17 April 2002, 3 October 2002, 11 April 2003, and 10 October 2003, yielding R2 values of 0.69, 0.77, 0.86, and 0.89. The design structure was a repeated measure over time in a nested whole plot. The experimental variables measured were nitrogen pools, the treatments were the urban land- scapes, and the plots were individual trees within urban land- scapes. The model is a fixed-effect model because the urban landscapes studied were not randomly chosen from a larger pool of urban landscapes. Consequently, these results are ap- plicable to these specific types of urban environments. The urban landscapes studied are what we believe to be the most common urban environments; thus, our results are applicable to most of the urban environments an arborist would encoun- ter. Repeated-measures analysis of variance and the general linear model procedure were used with Fisher’s protected least-significant difference to detect seasonal changes and significant differences (P 0.05) among the urban land- ©2006 International Society of Arboriculture
July 2006
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