216 Scharenbroch and Catania: Soil Quality Attributes as Indicators of Urban Tree Performance and chroma) was determined using Munsell soil color charts on wet and air dried soils. Color determinations were made by three individuals per soil and means were calculated. Soil structure type (platy-massive = 0, angular blocky = 1, subangular blocky = 2, granular = 3), structure grade (structureless = 0, weak = 1, moder- ate = 2, strong = 3), and structure size (very fine = 0, fine = 1, me- dium = 2, coarse = 3) were described and scored. Numbers of fine (1 to 2 mm diameter) plant roots were estimated at ten, one cm2 points. Coarse (2 to 10 mm in diameter) roots were estimated at four faces, each 100 cm2 . The areas of redoximorphic concentra- tions and depletions were estimated on the exposed profile face. The soil from the 25 cm × 25 cm × 20 cm hole was sort- ed for earthworms. A hot mustard powder solution (50 g L-1 ) was poured in the excavated hole to extract deeper earthworms (Lawrence and Bowers 2002). The number of adult and juve- nile earthworms was tallied for each excavation, and reported as individuals m-3 . Earthworms were stored on ice in a cooler in petri dishes with damp towels and returned to the laboratory. Earthworms were identified using a dissecting microscope and dichotomous key for the Great Lakes, U.S., region (Hale 2010). For each plot, adult earthworm biomass (ash-free dry mass) was determined for each species (Hale et al. 2004). Lumbricus terres- tris was the only species encountered in the earthworm sampling. Three penetration resistance profiles (0 to 45 cm) were mea- sured per plot using a cone penetrometer (FieldScout SC 900 Soil Penetrometer, Spectrum Technologies, Inc., Plainfield, IL, U.S.). Mean penetration resistance over the 0 to 20 cm depth was calcu- lated. The root restriction depth was calculated by computing the mean penetration resistance profiles for each plot and then identi- fying the depth at which penetration resistance exceeded 2.3 MPa. According to Day and Bassuk (1994), the critical soil strength above which woody plant root elongation is restricted is in the vicinity of 2.3 MPa, depending on soil type and plant species. Infiltration rate was measured at two locations per plot us- ing a double-ring infiltrometer (Turf-Tec International, Tal- lahassee, Florida, U.S.). Sample locations were pre-saturated with 1 L of water. Infiltration rates were measured twice at each sample point and means computed. Volumetric water content of the 0 to 20 cm depth was measured at ten points per plot using a time-domain reflectrometry probe (Field- Scout TDR 300 Soil Moisture Meter, Spectrum Technolo- gies, Inc., Plainfield, Illinois, U.S.). Soil temperature was mea- sured by inserting a 10 cm thermocouple thermometer into the soil (Fluke 52 K/J Thermometer, Everett, Washington, U.S.). Bulk density (ρb ) was measured on undisturbed soil core samples (70 mm wide × 70 mm deep) collected from each plot. Only samples that completely filled the entire core volume were used. The core samples were kept shaded and on ice for trans- port to the lab. Soil was sieved, homogenized, and dried in an oven for 48 hours at 105°C. Material (roots, rock, etc.) greater than 2 mm was removed, and its volume and oven-dry weight determined for bulk density corrections for non-soil material. Ten, 2.5 cm wide × 20 cm deep cores were taken from random plot locations for soil characterization in the laboratory. A uniform sampling depth of 20 cm was adopted since this depth is likely to include mainly the A horizon across all sample plots. The ten soil cores were composited per plot and kept shaded and on ice in a cooler for transport to the lab. In the laboratory, soil sub-samples were weighed, dried for 24 hours at 105°C, and reweighed to calculate gravimetric soil moisture (Topp et al. 2008). Sand, silt, ©2012 International Society of Arboriculture and clay (%) were calculated using the modified pipette method of Kettler et al. (2001). Stability of aggregates (1 to 2 mm) was measured by oscillation of the sample through a height of 37 mm height, 29 times per minute for ten minutes in water (An- gers et al. 2008). The oven-dry weight of water-stable aggregates (WAS) per total oven-dry soil was expressed as a percentage. Soil pH and electrical conductivity (EC) in dS cm-1 were measured in 1:1 (soil:deionized) water pastes (Model Orion 5-Star, Thermo Fisher Scientific Inc., Waltham, Massachusetts, U.S.). Total C and N (%) were determined by automated dry combustion analyzer (Elementar Vario EL III CHNOS, Elemen- measured using a modified indophenol blue meth- od for microplate analyses at absorbance readings of 650 nm (Model ELx 800, Biotek Instruments Inc., Winooski, Vermont, U.S.) (Sims et al. 1995). With Devarda’s alloy, NO3 duced to NH4 , which was then quantified using Sims et al. (1995). Dissolved organic N was reduced to NH4 with persul- + SO4 - was re- + + Particulate organic matter (POM) was measured after shaking 25 g subsamples for 15 hours with sodium hexametaphosphate (NaPO3 +and NO3 sieve, coarse POM on a 250 µm sieve, and fine POM on a 53 )6 . Soil subsamples were fumigated with ethanol-free chloro- form for five days and extracted with 0.5 M K2SO4 µm sieve (Gregorich and Beare 2008). Loss on ignition at 360°C for six hours was used to determine the OM content of the litter SOM, fine POM, and coarse POM fractions (Nelson and Som- mers 1996). The soil fumigation-extraction method (Brookes et al. 1985) was used to determine microbial biomass N in mg kg-1 . Microbial Nitrogen mineralization and microbial respiration were measured using 20-day soil incubations in the dark, at 25°C and with soils adjusted to 60% water-filled-pore space. Carbon dioxide in 0.25 M NaOH traps was precipitated with BaCl2 d-1). Concentrations of NH4 M HCl (standardized) titration to a phenolphthalein endpoint (Parkin et al. 1996), expressed as soil respiration (mg CO2 determined colorimetrically as previously described (Sims et al. 1995). Nitrogen mineralization was determined by subtract- ing inorganic N (NH4 + and NO3 tracts of the incubated soils divided by the incubation period (mg + and NO3 , followed by 0.25 kg-1 - in incubated soils were -) in base extracts from the ex- biomass N was the difference in dissolved organic N between the fumigated and unfumigated baseline samples, using an extraction efficiency factor of k EN = 0.54 (Joergensen and Mueller 1996). and then collecting litter organic matter on a 2000 µm fate and Devarda’s alloy and also measured following Sims et al. (1995). Inorganic N was the sum of extracted NH4 -. (Ca), magnesium (Mg), and sodium (Na) were determined with atomic adsorption spectroscopy (Model A5000, Perkin Elmer Inc., Waltham, MA, U.S.) (Schollenberger and Simon 1945). The sum of these exchangeable bases was expressed as effective cation exchange capacity (eCEC) (Sumner and Miller 1996). Sodium adsorption ratio was computed as the milliequivalent weight of Na divided by the square root of the milliequiva- lent weight of Ca and Mg divided by two. Soil phosphorus (P) was determined with the Olsen extraction and extracts were analyzed colorimetrically at 882 nm on a spectrophotometer (Model UV mini 1240, Shimadzu Inc., Kyoto, Japan) (Olsen and Sommers 1982). Soils were extracted with 0.5 M K2 and NH4 tar, Hanau, Germany). Loss on ignition at 360°C for six hours was used to determine the soil organic matter (SOM) (Nelson and Sommers 1996). Soil sub-samples were extracted with 1 M NH4 OAc (pH 7.0) and mg kg-1 of potassium (K), calcium
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