Arboriculture & Urban Forestry 32(4): July 2006 181 isms (Evangelou 1998). Mineral nitrogen measurements may have value when used in conjunction with biologic indices of soil nitrogen availability (Drinkwater et al. 1996; Qafoku et al. 2001). Indices of soil microbial biomass or activity such as nitro- gen mineralized by microbial enzymes over a given timespan are considered more accurate soil nitrogen availability indices than extractable mineral nitrogen (Stanford and Smith 1972; Keeney 1982; Molina et al. 1983; Drinkwater et al. 1996; Knoepp et al. 2000; Verchot et al. 2001; Anderson et al. 2002; Picone et al. 2002). Measurements of microbial bio- mass and activity have been used to monitor decomposition and mineralization processes from urban soils (Beyer et al. 1995; Pouyat et al. 1995; Beyer et al. 1996; Carreiro et al. 1999). Nitrogen mineralization in urban soils ranged from 0.35 to 2.08 gg−1 d−1 (0.02 to 0.10 lb N 1000 ft−2 d−1) (White and McDonnell 1988). Although considered more ac- curate, current biologic nitrogen assessments are not used because they are rarely offered by commercial soil testing laboratories (Picone et al. 2002). Soil organic matter is a critical determinant of soil physi- cal, chemical, and biologic properties and is often proposed as an index of soil nutrient status (Tisdale and Oades 1982; Burger and Pritchett 1984; Fox et al. 1986; Sikora and Stott 1996; McLaughlin et al. 2000) and nitrogen availability (McLaughlin et al. 2000; Ding et al. 2002). Soil organic matter assessments are commonly offered at relatively low costs by soil commercial testing laboratories (Scharenbroch and Lloyd 2004). Soil organic matter decomposition creates a recalcitrant component that is relatively inactive in nitrogen mineralization (Chefetz et al. 2002; Graham et al. 2002). Recalcitrant organic matter fractions have been found to oc- cupy as much as 60% to 70% of the total soil organic matter (Schnitzer 1986). Turnover rates of 1500 years have been estimated for the stable soil organic matter components (Campbell 1978; Parton et al. 1987). Total soil organic matter changes are slow (Sikora and Stott 1996); thus, seasonal variations in soil nitrogen are difficult to detect with total soil organic matter assessments. An active soil organic matter component, particulate or- ganic matter (POM), has been found to relate soil organic matter dynamics (Paul 1984; Parton et al. 1987; Powlson et al. 1987; Cambardella and Elliott 1992; Janzen et al. 1992; Paustian et al. 1992; Biederbeck et al. 1994; Gregorich et al. 1994; Magid et al. 1996; Chan 1997; Alvarez et al. 1998; Janzen et al. 1998; Six et al. 1998; Graham et al. 2002; Franzlubbers and Stuedemann 2003; Salas et al. 2003). Par- ticulate organic matter has been found to be positively cor- related with soil microbial nitrogen and nitrogen mineraliza- tion (Alvarez et al. 1998; Sohn Lopez-Forment 2000; Duri- ancik 2001; Stinner et al. 2002; Franzlubbers and Stuedemann 2003). Particulate organic matter is indicative of nitrogen availability; but POM has yet to be examined in urban soils. The first goal of this research was to assess the spatial differences in soil nitrogen across an array of urban landscape types. Our hypothesis was that soil nitrogen pools will be significantly different among urban landscapes, thus empha- sizing the need for site-specific soil nitrogen assessment be- fore fertilization. Our second goal was to identify a practical and accurate soil nitrogen assessment method for potential use by urban landscape managers. We propose that POM is an accurate predictor of microbial biomass nitrogen and ni- trogen mineralization rates across an array of urban land- scapes. If POM measurements are well correlated with mi- crobial biomass and activity, then POM measurements have potential to be used as accurate and practical means for urban landscape managers to assess soil nitrogen availability. If POM is found to be indicative of soil nitrogen availability, it can then be examined with indices of plant quality to refine current nitrogen fertilization recommendations for urban landscapes. MATERIALS AND METHODS Study sites were selected within the cities of Moscow, Idaho, 46°44N and 116°58W and Pullman, Washington, 46°43N and 117°11W. Human population density in Moscow, Idaho, is 882 people km−2 (341 people mile−2) and 1069 people km−2 (412 people mile−2) in Pullman, Washington. All urban soils in this study had a silt loam texture with a range of 6% to 26% clay, 54% to 67% silt, and 13% to 33% sand (Scharenbroch et al. 2005). The soils are described as Palouse silt loam with 7% to 25% slopes (USDA 1980, 1981). The soils were formed in loess and volcanic ash and are well to moderately well-drained. The sample region is described as gently sloping to moderately steep silt loam on uplands. The elevation is approximately 790 m (2607 ft). The average an- nual precipitation is 530 to 840 mm (21.2 to 33.6 in); during the study years of 2002 to 2003, precipitation was 760 mm (30.4 in) (Patten 2003). The average frost-free season is 90 to 140 days. During 2002 and 2003, the mean air temperature was 12.1°C (53.8°F) and the temperature in the top 10 cm of soil was approximately 10.1°C (50.2°F) (Patten 2003). Soil samples were collected nine times in 2002 (4/17, 5/01, 5/15, 5/29, 6/12, 6/26, 8/03, 9/05, and 10/03) and nine times in 2003 (4/17, 5/01, 5/15, 5/29, 6/12, 6/26, 8/03, 9/05, and 10/03). We sampled residential yards greater than 50 years old, residential yards less than 10 years old, mulched beds greater than 10 years old, mulched beds less than 3 years old, street tree plantings, and park trees (Table 1). These urban landscapes were assigned a site quality index (SQI) based on landscape age and management practices: SQImean land- scape age + fertilization value (0, 1, or 2) + irrigation value (0, 1, or 2) + grass clipping value (0, 1, or 2) + mulching value (0, 1, or 2). For example, sites fertilized twice annually ©2006 International Society of Arboriculture
July 2006
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
